{"id":7165,"date":"2013-03-28T09:57:09","date_gmt":"2013-03-28T13:57:09","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7165"},"modified":"2026-04-18T06:30:33","modified_gmt":"2026-04-18T11:30:33","slug":"hydroboration-of-alkenes-the-mechanism","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2013\/03\/28\/hydroboration-of-alkenes-the-mechanism\/","title":{"rendered":"Hydroboration Oxidation of Alkenes"},"content":{"rendered":"<p><strong>Hydroboration Oxidation of Alkenes<\/strong><\/p>\n<p>In this article we cover one of the most important methods for forming\u00a0<strong>alcohols\u00a0<\/strong>from\u00a0<strong>alkenes<\/strong>, hydroboration-oxidation.<\/p>\n<ul>\n<li>Hydroboration is an\u00a0<strong>addition<\/strong> reaction between an alkene (olefin) and a a borane (neutral species containing a B-H bond). In hydroboration, a C-C pi bond is broken, and a C-H bond as well as a C-B bond is formed. Oxidation of the resulting organoborane with hydrogen peroxide (H<sub>2<\/sub>O<sub>2<\/sub>) then replaces the C-B bond with a C-OH bond.<\/li>\n<li>Typical reagents for hydroboration include borane (BH<sub>3<\/sub>) and its relatives (BH<sub>3<\/sub>\u2022THF, B<sub>2<\/sub>H<sub>6<\/sub>, BH<sub>3<\/sub>\u2022OEt<sub>2<\/sub>) as well as substituted boranes such as disiamyl borane and 9-BBN.<\/li>\n<li>The notable outcome of hydroboration-oxidation is formation of an alcohol on the least substituted carbon of the alkene (&#8220;anti-Markovnikov&#8221; regioselectivity.)\u00a0 This contrasts with the &#8220;Markovnikov&#8221; selectivity for formation of alcohols from alkenes using acid-catalyzed hydration (H<sub>3<\/sub>O+) or oxymercuration.<\/li>\n<li>Hydroboration is stereoselective for\u00a0<em>syn<\/em> addition &#8211; that is,the H and B are delivered to the same face of the alkene. Oxidation of the C-B bond occurs with complete retention of stereochemistry.<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-35386\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/0-Summary-of-hydroboration-oxidation-of-alkenes-with-bh3-borane-and-hydrogen-peroxide.gif\" alt=\"Summary of hydroboration-oxidation of alkenes with bh3 borane and hydrogen peroxide\" width=\"640\" height=\"414\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li style=\"list-style-type: none;\">\n<ol>\n<li><a href=\"#one\">Hydroboration Oxidation of Alkenes<\/a><\/li>\n<li><a href=\"#two\">Hydroboration-Oxidation is Anti Markovnikov Selective<\/a><\/li>\n<li><a href=\"#three\">Hydroboration is Stereoselective for\u00a0<em>syn<\/em> Addition<\/a><\/li>\n<li><a href=\"#four\">The Hydroboration Mechanism<\/a><\/li>\n<li><a href=\"#five\">Mechanism of the Oxidation Step<\/a><\/li>\n<li><a href=\"#six\">Hydroboration is Stereospecific<\/a><\/li>\n<li><a href=\"#seven\">Formation of Enantiomers and Diastereomers<\/a><\/li>\n<li><a href=\"#eight\">Summary<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quiz\">Quiz Yourself!<\/a><\/li>\n<li><a href=\"#references\">(Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<hr \/>\n<h2><a id=\"one\"><\/a>1. Hydroboration Oxidation of Alkenes<\/h2>\n<p>Hydroboration-oxidation is a useful and important method for forming <strong>alcohols<\/strong> from <strong>alkenes<\/strong>. The reaction involves treatment of an alkene (<span style=\"color: #993366;\"><em>also known as an olefin<\/em><\/span>) with a <strong>borane<\/strong> (<span style=\"color: #993366;\"><em>a neutral molecule containing a B-H bond<\/em><\/span>). This results in an organoborane compound, which is generally not isolated but instead immediately treated with an oxidant to give the alcohol product.<\/p>\n<ul>\n<li>In the first step, an alkene is treated with borane (BH<sub>3<\/sub>) or any one of similar reagents. This breaks the C-C pi bond and forms a C-H and C-B bond.<\/li>\n<li>In the second step an oxidant such as H<sub>2<\/sub>O<sub>2<\/sub> is added, usually in the presence of base such as NaOH or KOH. A rearrangement then occurs where the C-B bond is broken and a new C-O bond is formed.<\/li>\n<\/ul>\n<p><img decoding=\"async\" class=\"alignnone wp-image-35328\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/1-hydroboration-oxidation-of-alkenes-with-bh3-or-related-reagents-gives-alcohols-with-hydroxyl-group-on-least-substituted-carbon-of-alkene.gif\" alt=\"hydroboration-oxidation of alkenes with bh3 or related reagents gives alcohols with hydroxyl group on least substituted carbon of alkene\" width=\"640\" height=\"223\" \/><\/a><\/p>\n<p>The borane reagent for hydroboration can come in many specific forms. Like all neutral boron species, borane itself (BH<sub>3<\/sub>) has six valence electrons and an empty p-orbital. In pure form, it is mostly present as diborane (B<sub>2<\/sub>H<sub>6<\/sub>) a species where each boron atom partially achieves a full octet of electrons via sharing of a pair of electrons from a B-H bond. [<span style=\"color: #ff0000;\"><a href=\"#noteone\">Note 1<\/a> <\/span>] .<\/p>\n<p>With its empty p-orbital,\u00a0 BH<sub>3<\/sub> readily accepts a lone pair of electrons even from weak Lewis bases such as diethyl ether (Et<sub>2<\/sub>O), tetrahydrofuran (THF), and dimethyl sulfide (DMS). Various Lewis acid-Lewis base compounds of BH<sub>3<\/sub> (<span style=\"color: #993366;\"><em>known as adducts<\/em><\/span>) are stable enough to allow for their bottling and use as commercially available reagents, although for our purposes they will all be treated as equivalent to BH<sub>3<\/sub>. [<a href=\"#notetwo\"><span style=\"color: #ff0000;\">Note 2<\/span><\/a>]<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-35329\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/2-structure-of-borane-bh3-as-well-as-bh3-thf-bh3-et2o-bh3-dms-for-hydroboration-reactions-all-equivalent.gif\" alt=\"structure of borane bh3 as well as bh3 thf bh3 et2o bh3 dms for hydroboration reactions all equivalent\" width=\"640\" height=\"461\" \/><\/a><\/p>\n<p>Each available B-H bond of a borane is capable of performing a hydroboration reaction. One molar equivalent of borane (BH<sub>3<\/sub>) is therefore capable of performing hydroboration on up to three equivalents of alkene:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35330\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/3-bh3-can-perform-up-to-three-hydroboration-reactions-since-it-has-three-b-h-bonds.gif\" alt=\"bh3 can perform up to three hydroboration reactions since it has three b-h bonds\" width=\"640\" height=\"187\" \/><\/a><\/p>\n<p>There are also many varieties of dialkylborane reagents (R<sub>2<\/sub>BH) such as disiamyl borane or 9-BBN that have more steric hindrance around the boron atom and are often used for hydroboration of alkynes. [<a href=\"#notethree\"><span style=\"color: #ff0000;\">Note 3<\/span><\/a>] <em style=\"color: #993366;\">(See post &#8211; <a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/14\/hydroboration-and-oxymercuration-of-alkynes\/\">Hydroboration of Alkynes<\/a>).\u00a0<\/em><\/p>\n<h2><a id=\"two\"><\/a>2. Hydroboration Oxidation is Anti-Markovnikov Selective<\/h2>\n<p>Hydroboration is an example of an\u00a0<strong>addition<\/strong> reaction, where a C-C pi bond on an alkene is broken, and two new single bonds to C are formed.<\/p>\n<p>Any time addition reactions occur on unsymmetrical alkenes there arises the possibility for forming at least two <strong>constitutional isomers<\/strong>.<\/p>\n<p>When a reaction demonstrates a strong preference for the formation of one constitutional isomer over another, it is said to be <strong>regioselective<\/strong>.<\/p>\n<p>We previously saw, for example, that addition of acids such as HCl to alkenes tends to give the product where the C-<strong>Cl<\/strong> bond forms on the <strong>more substituted<\/strong> carbon of the alkene (<span style=\"color: #993366;\"><em>i.e. the carbon attached to the most carbons and fewest hydrogens<\/em><\/span>) and the C-<strong>H<\/strong> bond forms on the less substituted carbon of the alkene (<span style=\"color: #993366;\"><em>i.e. the carbon attached to the fewest carbons and most hydrogens<\/em><\/span>).\u00a0 Addition of HCl to alkenes, as well as many other reactions are said to have &#8220;<strong>Markovnikov<\/strong>&#8221; regioselectivity. [<span style=\"color: #993366;\"><em>See article: <a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/08\/markovnikovs-rule-1\/\"><span style=\"color: #800080;\">Markovnikov Addition of HCl To Alkenes<\/span><\/a><\/em><\/span>]<\/p>\n<p>Hydroboration swings the other way. Hydroboration reactions generally result in the new C-OH bond being formed on the <strong>less substituted<\/strong> carbon of the alkene with the C-H bond being formed on the <strong>more substituted<\/strong> carbon. This is called, as you might expect,\u00a0 &#8220;anti-Markovnikov&#8221; regioselectivity.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35331\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/4-hydroboration-has-anti-markovnikov-regioselectivity-where-c-oh-bond-forms-on-least-substituted-carbon-of-the-alkene.gif\" alt=\"hydroboration has anti markovnikov regioselectivity where c-oh bond forms on least substituted carbon of the alkene\" width=\"640\" height=\"340\" \/><\/a><\/p>\n<p>When the carbons of the alkene are bonded to the same numbers of hydrogens, hydroboration with BH<sub>3<\/sub> is much less regioselective. [<a href=\"#notefour\"><span style=\"color: #ff0000;\">Note 4<\/span> <\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35332\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/5-when-hydroboration-with-bh3-performed-on-alkenes-with-similar-substitution-pattern-result-is-a-mixture-of-products-poor-regioselectivity.gif\" alt=\"when hydroboration with bh3 performed on alkenes with similar substitution pattern result is a mixture of products - poor regioselectivity\" width=\"640\" height=\"238\" \/><\/a><\/p>\n<p>The anti-Markovnikov regioselectivity of hydroboration complements nicely with the Markovnikov selectivity for the formation of alcohols through acid-catalyzed hydration or oxymercuration:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35333\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/6-acid-catalyzed-hydration-of-alkenes-and-oxymercuration-are-selective-for-the-markovnikov-hydration-of-alkenes.gif\" alt=\"-acid catalyzed hydration of alkenes and oxymercuration are selective for the markovnikov hydration of alkenes\" width=\"640\" height=\"263\" \/><\/a><\/p>\n<p>You may recall, however, that acid-catalyzed hydration can lead to carbocation rearrangements. [<span style=\"color: #993366;\"><em>See article: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/26\/rearrangements-in-alkene-addition-reactions\/\">Rearrangements in Alkene Addition Reactions<\/a><\/em>]<\/span> Oxymercuration is an alternative route for the formation of Markovnikov alcohol products that don&#8217;t have problems with rearrangements.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35334\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/7-carbocation-rearrangements-can-occur-in-the-acid-catalyzed-hydration-of-alkenes-with-h2so4-and-water-but-do-not-occur-with-oxymercuration.gif\" alt=\"carbocation rearrangements can occur in the acid catalyzed hydration of alkenes with h2so4 and water but do not occur with oxymercuration\" width=\"640\" height=\"324\" \/><\/a><\/p>\n<p>The hydride and alkyl shifts that can happen during acid-catalyzed hydration reactions are <strong>never<\/strong> observed during hydroboration reactions, which are a good clue that hydroboration doesn&#8217;t pass through a carbocation intermediate.<\/p>\n<p>Now for some practice. Knowing that hydroboration gives anti-Markovnikov alcohols, draw the products of these reactions:<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"35295\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"35295\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"35295\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"35295\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-35295\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-35295 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"35295\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-uq4ra\" data-id=\"uq4ra\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2543-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2543-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<p>Here&#8217;s another one:<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"35296\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"35296\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"35296\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"35296\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-35296\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-35296 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"35296\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-f9xdo\" data-id=\"f9xdo\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2544-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2544-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<h2><a id=\"three\"><\/a>3. Hydroboration-Oxidation is Stereoselective for <em>syn<\/em> Addition<\/h2>\n<p>In addition to being <strong>regioselective\u00a0<\/strong>for formation of anti-Markovnikov alcohols, hydroboration is <strong>stereoselective<\/strong> for <em>syn\u00a0<\/em>addition products.<\/p>\n<p><em>Syn<\/em> addition means that the two new single bonds form on the <strong>same<\/strong> face of the alkene. (<span style=\"color: #993366;\"><em>Other examples of reactions that perform syn<\/em><em> addition are catalytic hydrogenation and dihydroxylation<\/em><\/span>).<\/p>\n<p>Addition to 1-methylcyclopentene, for example, produces <strong>only<\/strong> the products where C-OH and C-H have added to the same face of the pi bond. No products of <em>anti<\/em> addition are observed.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35335\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/10-hydroboration-always-occurs-with-syn-stereoselectivity-where-the-two-new-bonds-form-on-the-same-face-of-the-pi-bond.gif\" alt=\"hydroboration always occurs with syn stereoselectivity where the two new bonds form on the same face of the pi bond\" width=\"640\" height=\"380\" \/><\/a><\/p>\n<p>[<a href=\"#notefive\"><span style=\"color: #ff0000;\">Note 5<\/span><\/a>]<\/p>\n<p>Try drawing the product of this reaction:<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"35297\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"35297\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"35297\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"35297\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-35297\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-35297 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"35297\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-4usai\" data-id=\"4usai\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2545-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2545-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<p>Alternatively, try working backwards from the final product. <span style=\"color: #993366;\"><em>(But be careful! )<\/em><\/span><\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"35298\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"35298\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"35298\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"35298\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-35298\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-35298 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"35298\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-sboky\" data-id=\"sboky\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2546-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2546-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<h2><a id=\"four\"><\/a>4. The Hydroboration Mechanism<\/h2>\n<p>OK. So hydroboration is observed to have anti-Markovnikov regioselectivity and is stereoselective for the <em>syn<\/em> product. It also doesn&#8217;t give rearranged products.\u00a0 So how does it work?<\/p>\n<p>Our best theory that fits all the evidence for the mechanism of hydroboration is that it happens via a concerted, cyclic transition state:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35336\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/13-mechanism-for-hydroboration-reaction-with-bh3-and-transition-state-involves-concerted-syn-addition-lining-up-partial-charges.gif\" alt=\"-mechanism for hydroboration reaction with bh3 and transition state involves concerted syn addition lining up partial charges\" width=\"640\" height=\"309\" \/><\/a><\/p>\n<p>This mechanism accounts for<\/p>\n<ul>\n<li>the exclusive observation of\u00a0<em>syn<\/em> products (C-B and C-H are formed at the same time)<\/li>\n<li>the lack of any observed carbocation rearrangements (<span style=\"color: #993366;\"><em>the reaction proceeds in one step, without an intermediate<\/em><\/span>)<\/li>\n<li>the observed &#8220;anti-Markovnikov&#8221; selectivity<\/li>\n<\/ul>\n<p>More on that last point. What&#8217;s the origin of this &#8220;anti-Markovnikov&#8221; selectivity we&#8217;ve been discussing, anyway?<\/p>\n<p>Well, in the transition state drawn above, note that the most electronegative (\u03b4<sup>\u2013\u00a0<\/sup>) atom ends up forming a bond to the carbon <strong>best<\/strong> able to stabilize positive charge (<span style=\"color: #993366;\"><em>i.e. the <strong>more<\/strong> substituted carbon)<\/em><\/span> and the most electron-poor atom (\u03b4<sup>+<\/sup>\u00a0 ) ends up bonded to the carbon <strong>less<\/strong> able to stabilize positive charge (<span style=\"color: #993366;\"><em>i.e. the <strong>least<\/strong> substituted carbon<\/em><\/span>).<\/p>\n<p>If you stop to think about that for a moment, that explanation doesn&#8217;t sound so different from the explanation for Markovnikov selectivity in the addition of HCl (and similar species) to alkenes, now does it? (<span style=\"color: #993366;\"><em>See post: <span style=\"color: #800080;\"><a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/08\/markovnikovs-rule-1\/\">Markovnikov selectivity<\/a><\/span>]<\/em><\/span><\/p>\n<p><strong>That is correct.<\/strong> The explanation for &#8220;anti-Markovnikov&#8221; selectivity in hydroboration is <strong>essentially the same<\/strong> as the explanation for &#8220;Markovnikov&#8221; selectivity in addition of hydrogen halides.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35337\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/14-spider-man-meme-markovnikov-anti-markovnikov.gif\" alt=\"spider-man-meme-markovnikov-anti-markovnikov\" width=\"640\" height=\"484\" \/><\/a><\/p>\n<p>What gives?<\/p>\n<ul>\n<li>In both cases the more electronegative atom ( <span style=\"color: #0000ff;\">\u03b4<sup>\u2013<\/sup><\/span>) lines up with the more substituted carbon. But the <strong>polarity<\/strong> of the <strong>hydrogen<\/strong> is different in each case.<\/li>\n<li>In the B-H bond, the most electronegative atom (\u03b4<sup>\u2013\u00a0<\/sup>) is<strong>\u00a0<\/strong>hydrogen (2.2) and the least electronegative atom (\u03b4<sup>+<\/sup>\u00a0 ) is boron (2.0). In\u00a0 HCl, the most electronegative atom is Cl (<span style=\"color: #ff0000;\">3.16<\/span>) and the least electronegative atom is H (2.0).<\/li>\n<\/ul>\n<p>In other words, <strong>hydrogen<\/strong> \u00a0is <strong>more<\/strong> <strong>electronegative<\/strong> (<span style=\"color: #0000ff;\">\u03b4<sup>\u2013 <\/sup><\/span>) relative to <strong>boron<\/strong> \u00a0(<span style=\"color: #ff0000;\">\u03b4<sup>+<\/sup><\/span>\u00a0 ) , but\u00a0<strong>less electronegative<\/strong> (<span style=\"color: #ff0000;\">\u03b4<sup>+<\/sup><\/span><sup>\u00a0<\/sup>) \u00a0relative to <strong>halides<\/strong> (<span style=\"color: #0000ff;\">\u03b4<\/span><sup><span style=\"color: #0000ff;\">\u2013<\/span> <\/sup>). It just so happens that when H is bonded to boron, it is the more electronegative atom and will therefore line up with the carbon best able to stabilize positive charge (i.e. the more substituted carbon).<\/p>\n<p>So the observed regioselectivity isn&#8217;t so &#8220;weird&#8221; after all.<\/p>\n<p>As for the oxidation step&#8230;. OK, that <em>is<\/em> a little bit weird.<\/p>\n<h2><a id=\"five\"><\/a>5. Mechanism of the Oxidation Step<\/h2>\n<p>After hydroboration is complete, the resulting organoborane needs to be treated with an oxidant like H<sub>2<\/sub>O<sub>2<\/sub> to obtain an alcohol.<\/p>\n<p>(<span style=\"color: #993366;\"><em>Probably best not to try to isolate the organoborane unless you are familiar with inert atmosphere (Schlenk-line) techniques, as organoboranes have a tendency to spontaneously combust in air. Very pretty green flame, though<\/em><\/span>).<\/p>\n<p>This is usually done under <strong>basic<\/strong> conditions.<\/p>\n<p>(<span style=\"color: #993366;\"><em>First, the conjugate base of H<sub>2<\/sub>O<sub>2<\/sub> is more nucleophilic than H<sub>2<\/sub>O<sub>2<\/sub> itself, and secondly, a base like NaOH helps with hydrolysis of the boronic ester that results after rearrangement<\/em><\/span>).<\/p>\n<p>The first thing to happen (after deprotonation of H<sub>2<\/sub>O<sub>2<\/sub>) is addition of the peroxide anion to boron.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35338\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/15-oxidation-step-in-hydroboration-mechanism-part-1-and-2-coordination-of-peroxide-to-boron-with-h2o2.gif\" alt=\"oxidation step in hydroboration mechanism part 1 and 2 - coordination of peroxide to boron with h2o2\" width=\"640\" height=\"378\" \/><\/a><\/p>\n<p>Now comes the fun part. In the next step, the pair of electrons in the C-B bond migrates over to oxygen (form C-O, break C-B). The O-O bond breaks, liberating the hydroxide ion HO(-) as a leaving group.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35339\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/16-mechanism-of-the-key-rearrangement-step-in-hydroboration-oxidation-involves-migration-of-c-b-bond-to-form-a-new-c-o-bond-with-breakage-of-o-o.gif\" alt=\"mechanism of the key rearrangement step in hydroboration oxidation involves migration of c-b bond to form a new c-o bond with breakage of o-o\" width=\"640\" height=\"333\" \/><\/a><\/p>\n<p>This probably looks weird. It might help to think of the C-B bond as as a nucleophile, attacking the electrophilic oxygen and liberating a hydroxide leaving group.<\/p>\n<p><span style=\"color: #993366;\"><em>Normally, oxygen is not the type of atom that undergoes attack by a nucleophile, but the oxygen-oxygen bond is quite weak (<span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/doi.org\/10.1021\/acs.jpca.0c02859\">roughly 45 kcal\/mol<\/a><\/span>); blame it on all those electron pairs repelling each other! These types of rearrangements are not uncommon in peroxide chemistry. [A similar rearrangement is found in a reaction we often encounter in Org 2 known as the<span style=\"color: #993366;\"> <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2018\/10\/08\/nitration-baeyer-villiger\/\">Baeyer-Villiger Oxidation<\/a>]<\/span>\u00a0<\/em><\/span><\/p>\n<p>For accounting purposes, it can be helpful to draw the ugly version first, just keeping track of the bonds that form and break.<\/p>\n<p>After migration, the resulting compound (a &#8220;boronic ester&#8221;) then undergoes <strong>hydrolysis<\/strong> by hydroxide ion to eventually form boric acid and the alcohol.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35340\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/17-basic-hydrolysis-steps-in-hydroboration-oxidation-formation-of-final-alcohol.gif\" alt=\"basic hydrolysis steps in hydroboration oxidation formation of final alcohol\" width=\"640\" height=\"236\" \/><\/a><\/p>\n<p>Note that <strong>stereochemistry is conserved\u00a0<\/strong>throughout the rearrangement process. The C-B bond is converted to C-O with perfect fidelity.<\/p>\n<h2><a id=\"six\"><\/a>6. Hydroboration is Stereospecific<\/h2>\n<p>Hydroboration is an example of a\u00a0<strong>stereospecific\u00a0<\/strong>reaction (<span style=\"color: #993366;\"><em>a feature it shares with several other reactions of alkenes, including dihydroxylation, epoxidation, and cyclopropanation among others<\/em><\/span>).<\/p>\n<p>Sometimes the word &#8220;stereospecific&#8221; is used to describe reactions that are super-dee-duper 100% selective for a certain outcome (<span style=\"color: #993366;\"><em>e.g. saying hydroboration is inherently &#8220;stereospecific&#8221; for the syn product due to its concerted mechanism<\/em><\/span>).<\/p>\n<p>A more IUPAC-approved definition of stereospecific refers to the fact that you can take two <strong>stereoisomeric <\/strong>reactants that differ only in the configuration at <strong>one<\/strong> carbon (e.g. the (<em>E<\/em>) and (<em>Z<\/em>) alkenes, below) and the products themselves will also be <strong>stereoisomers.\u00a0<\/strong><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35341\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/18-definition-of-a-stereospecific-reaction-is-one-where-two-molecules-differing-only-in-their-configuration-are-converted-into-stereoisomeric-products-IUPAC.gif\" alt=\"definition of a stereospecific reaction is one where two molecules differing only in their configuration are converted into stereoisomeric products-IUPAC\" width=\"640\" height=\"542\" \/><\/a><\/p>\n<p>So, in addition to being 100% selective for the <em>syn<\/em> product, hydroboration also fits this definition of stereospecificity.<\/p>\n<h2><a id=\"seven\"><\/a>7. Formation of Enantiomers and Diastereomers<\/h2>\n<p>Being planar, alkenes can undergo <strong>addition<\/strong> <strong>reactions <\/strong>from either face of the pi bond.<\/p>\n<p>Depending on the structure of the alkene, this may lead to the creation of a new chiral center. In the absence of any chiral influences (<span style=\"color: #993366;\"><em>e.g. by starting with an optically enriched starting material or using an optically enriched reagent<\/em><\/span>) formation of a new chiral center will result in a racemic mixture.\u00a0 (<span style=\"color: #993366;\"><em><a href=\"https:\/\/www.masterorganicchemistry.com\/tutoring\/\">Matt<\/a> calls this &#8220;preservation of optical inactivity&#8221;. )<\/em><\/span><\/p>\n<p>If you&#8217;re already familiar with solving &#8220;enantiomers, diastereomers, or the same&#8221; problems, you should be fine. [<span style=\"color: #800080;\"><em>See article: <a href=\"https:\/\/www.masterorganicchemistry.com\/2019\/03\/08\/enantiomers-diastereomers-or-the-same-1-using-models\/\">Enantiomers, Diastereomers or The Same?<\/a><\/em><\/span>]\u00a0 So long as you are able to successfully apply the proper pattern of <strong>regioselectivity<\/strong> (<em>anti-Markovnikov<\/em>) and <strong>stereoselectivity\u00a0<\/strong>(<em>syn<\/em>) on your starting alkene, it&#8217;s much better to just\u00a0<strong>draw out the products<\/strong> and analyze their relationships from there rather than try to memorize all the specific cases that could come up.<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"35299\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"35299\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"35299\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"35299\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-35299\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-35299 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"35299\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-yeive\" data-id=\"yeive\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2547-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2547-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<p>One good thing to keep in mind, however, is that the presence of a pre-existing chiral center will render the two faces of the alkene non equivalent and lead to the formation of a mixture of <strong>diastereomers<\/strong>. (<span style=\"color: #993366;\"><em>with each diastereomer being present as a racemic mixture<\/em><\/span>).<\/p>\n<p>Diasteromers will generally\u00a0<strong>not<\/strong> be formed in equal amounts since one approach of the alkene is often favored over the other due to differing <strong>steric hindrance <\/strong>encountered by the electrophile as it approaches the alkene.<\/p>\n<p>A classic example is the hydroboration of norbornene, a vaguely tent-shaped molecule.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35342\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/20-hydroboration-of-bicyclic-alkenes-with-pre-existing-chiral-centers-will-result-in-mixture-of-diastereomers.gif\" alt=\"hydroboration of bicyclic alkenes with pre existing chiral centers will result in mixture of diastereomers\" width=\"640\" height=\"317\" \/><\/a><\/p>\n<p>Hydroboration occurs exclusively on the outside of the tent to give only one product (as a mixture of enantiomers). The other diastereomer is formed in such tiny quantities that it is not generally observed.<\/p>\n<p>Here is another example. See if you can predict the major product.<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"35300\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"35300\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"35300\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"35300\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-35300\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-35300 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"35300\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-rwn7e\" data-id=\"rwn7e\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2548-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2548-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<h2><a id=\"eight\"><\/a>8. Summary<\/h2>\n<ul>\n<li>Hydroboration-oxidation is an addition reaction that results in the formation of alcohols.<\/li>\n<li>A classic reagent for hydroboration is borane (BH<sub>3<\/sub>) and related reagents (e.g. BH<sub>3<\/sub>\u2022THF) which can perform hydroborations on up to three molar equivalents of alkene. Substituted organoboranes such as disiamyl borane and 9-BBN have a single B-H bond and can be useful for the hydroboration of alkynes.<\/li>\n<li>In hydroboration-oxidation a new C-OH bond is generally formed on the less substituted carbon of the alkene. This is referred to as &#8220;anti-Markovnikov&#8221; regioselectivity. Hydroboration is stereoselective, giving exclusively <em>syn<\/em> addition; the C-H and C-B bonds are formed on the same face of the alkene. Oxidation with hydrogen peroxide (H<sub>2<\/sub>O<sub>2<\/sub>) occurs with complete retention of stereochemistry on the C-B bond.<\/li>\n<li>The key step of the oxidation reaction is the shifting of a pair of electrons in the C-B bond to form a new C-O bond, with breakage of the relatively weak (45 kcal\/mol) O-O bond. The resulting boronic ester is then hydrolyzed with base to give an alcohol.<\/li>\n<\/ul>\n<hr \/>\n<h2><strong><a id=\"notes\"><\/a>Notes<\/strong><\/h2>\n<div class=\"related-articles\"><p><strong>Related Articles<\/strong><\/p><ul><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/11\/markovnikovs-rule-2-why-it-works\/\" class=\"\"><span>Alkene Hydrohalogenation Mechanism And How It Explains Markovnikov\u2019s Rule<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/14\/hydroboration-and-oxymercuration-of-alkynes\/\" class=\"\"><span>Hydroboration and Oxymercuration of Alkynes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/04\/02\/epoxidation-hydroxylation-cyclopropanation-alkene-mechanism\/\" class=\"\"><span>Alkene Addition Pattern #3: The \u201cConcerted\u201d Pathway<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2014\/01\/21\/synthesis-reactions-of-alkenes\/\" class=\"\"><span>Synthesis (4) \u2013 Alkene Reaction Map, Including Alkyl Halide Reactions<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/02\/26\/rearrangements-in-alkene-addition-reactions\/\" class=\"\"><span>Rearrangements in Alkene Addition Reactions<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2019\/03\/08\/enantiomers-diastereomers-or-the-same-1-using-models\/\" class=\"\"><span>Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/07\/04\/the-sn2-mechanism\/\" class=\"\"><span>The SN2 Mechanism<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/organic-chemistry-practice-problems\/alkene-reactions-practice-problems\/\" class=\"\"><span>Alkene Reactions Practice Problems (MOC Membership)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/03\/20\/alkene-addition-pattern-2-the-three-membered-ring-pathway\/\" class=\"\"><span>Alkene Addition Pattern #2: The \u201cThree-Membered Ring\u201d Pathway<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2023\/10\/18\/cyclopropanation-of-alkenes\/\" class=\"\"><span>Cyclopropanation of Alkenes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/11\/25\/palladium-on-carbon-pdc\/\" class=\"\"><span>Palladium on Carbon (Pd\/C) for Catalytic Hydrogenation<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2010\/07\/02\/stereoselective-stereospecific\/\" class=\"\"><span>Stereoselective and Stereospecific Reactions<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/01\/22\/alkene-addition-regioselectivity-syn-anti\/\" class=\"\"><span>Alkene Addition Reactions: \u201cRegioselectivity\u201d and \u201cStereoselectivity\u201d (Syn\/Anti)<\/span><\/a><\/li><\/ul><\/div>\n<p><em>This is an updated version of two older articles (Hydroboration-Oxidation of Alkenes, and Hydroboration of Alkenes &#8211; The Mechanism) which have been combined into one larger article. <\/em><\/p>\n<p><strong><a id=\"one\"><\/a>Note 1<\/strong>. This is an example of a <a href=\"https:\/\/en.wikipedia.org\/wiki\/Three-center_two-electron_bond\">three-center, two-electron bond<\/a>, where two bonding electrons are shared between two atoms. The bridging B-H bonds are significantly longer (1.31 \u00c5) than the non-bridging B-H bonds. (1.19 \u00c5)<\/p>\n<p><strong><a id=\"notetwo\"><\/a>Note 2<\/strong>. The decision to use one reagent or the other (e.g. BH<sub>3<\/sub>\u2022OEt<sub>2<\/sub> vs BH<sub>3<\/sub>\u2022SMe<sub>2<\/sub> vs B<sub>2<\/sub>H<sub>6<\/sub>) is often based more on factors like price, availability,\u00a0 shelf life, and convenience rather than their reactivity profiles.<\/p>\n<p><strong><a id=\"notethree\"><\/a>Note 3<\/strong>. Structures of 9-BBN and disiamylborane.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35387\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/F-1-structures-of-9-bbn-and-disiamylborane.gif\" alt=\"F-1-structures of 9-bbn and disiamylborane\" width=\"640\" height=\"326\" \/><\/a><\/p>\n<p><strong><a id=\"notefour\"><\/a>Note 4<\/strong>. This 57:43 ratio can be improved significantly, however, by using bulkier reagents for hydroboration (such as 9-BBN or disiamylborane, above). With disiamylborane the ratio is 97:3 and with 9-BBN it is over 99:1. [Page 12 in the PDF of <a href=\"https:\/\/www.nobelprize.org\/uploads\/2018\/06\/brown-lecture.pdf\">Ref]<\/a><\/p>\n<p><strong><a id=\"notefive\"><\/a>Note 5.\u00a0<\/strong>Note that while <em>syn<\/em> and\u00a0<em>cis<\/em> are sometimes used interchangeably, they are actually distinct.\u00a0<em>cis-<\/em> and\u00a0<em>trans-<\/em> refer to the relative orientations of two groups about a multiple bond or ring where rotation is impossible (e.g. cis and trans alkenes) whereas <em>syn<\/em> and\u00a0<em>anti<\/em> is defined by the <strong>dihedral angle<\/strong> between two bonds A-B and C-D in a four-atom system A-B-C-D.\u00a0 Dihedral angles, and therefore <em>syn<\/em> and <em>anti<\/em> relationships can change with bond rotation.\u00a0 [<span style=\"color: #993366;\"><em>See article &#8211; <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2020\/02\/28\/staggered-vs-eclipsed-conformations-of-ethane\/\">Staggered vs. Eclipsed Conformations of Ethane<\/a><\/em><\/span>] When we say &#8220;syn&#8221; addition we are referring to the relative orientation of the two new bonds in the transition state.<\/p>\n<p><strong>Note 6.\u00a0<\/strong>In olden days, BH<sub>3<\/sub> was used for the reduction of the carbonyl group (such as aldehydes to alcohols). Hydroboration was serendipitously discovered during one of these reactions by H.C.Brown&#8217;s co-worker Subba Rao, who noticed that reduction of an ester consumed &#8220;extra&#8221; BH3 beyond the two equivalents expected per ester.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35388\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/F-2-Hydroboration-discovery-H-C-Brown-Nobel-Lecture.png\" alt=\"F-2-Hydroboration discovery H C Brown Nobel Lecture\" width=\"640\" height=\"711\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/F-2-Hydroboration-discovery-H-C-Brown-Nobel-Lecture.png 1028w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/F-2-Hydroboration-discovery-H-C-Brown-Nobel-Lecture-270x300.png 270w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/F-2-Hydroboration-discovery-H-C-Brown-Nobel-Lecture-922x1024.png 922w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/F-2-Hydroboration-discovery-H-C-Brown-Nobel-Lecture-768x853.png 768w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/08\/F-2-Hydroboration-discovery-H-C-Brown-Nobel-Lecture-684x760.png 684w\" sizes=\"(max-width: 640px) 100vw, 640px\" \/><\/a><\/p>\n<hr \/>\n<h2><strong><a id=\"quiz\"><\/a>Quiz Yourself!<\/strong><\/h2>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/2556-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/2557-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/0617-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3049-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3050-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n<hr \/>\n<h2><strong><a id=\"references\"><\/a>(Advanced) References and Further Reading<\/strong><\/h2>\n<p>One of the best overall references that covers the basics is <strong>Organic Syntheses via Boranes <\/strong>(Wiley, 1975) by H. C. Brown. It&#8217;s available for 1 hour loans on\u00a0 archive.org . <a href=\"https:\/\/archive.org\/details\/organicsyntheses0000brow\/\">Link<\/a>.<\/p>\n<p>H.C. Brown&#8217;s <a href=\"https:\/\/www.nobelprize.org\/uploads\/2018\/06\/brown-lecture.pdf\"><strong>Nobel Lecture<\/strong> <\/a>is also well worth reading for the story of how his research program on boron chemistry evolved. Some of the earliest techniques for working with air-sensitive reagents were developed in Brown&#8217;s lab.<\/p>\n<ol>\n<li><strong>A STEREOSPECIFIC CIS HYDRATION OF THE DOUBLE BOND IN CYCLIC DERIVATIVES<\/strong><br \/>\nHerbert C. Brown and George Zweifel<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1959<\/strong> 81 (1), 247-247<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja01510a059\">10.1021\/ja01510a059<\/a><\/li>\n<li><strong>(\u2212)-ISOPINOCAMPHEOL<br \/>\n<\/strong>George Zweifel and H.C. Brown<br \/>\n<em>Org. Synth.<\/em> <strong>1972<\/strong>, 52, 59<br \/>\n<strong>DOI:<\/strong> <a href=\"https:\/\/orgsyn.org\/demo.aspx?prep=cv6p0719\">10.15227\/orgsyn.052.0059\u00a0<\/a><\/li>\n<li><strong>XXIV. Directive Effects in the Hydroboration of Some Substituted Styrenes<br \/>\n<\/strong>Herbert C. Brown and Richard L. Sharp<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1966<\/strong> <em>88<\/em> (24), 5851-5854<br \/>\n<strong>DOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/abs\/10.1021\/ja00976a029\">10.1021\/ja00976a029<\/a><br \/>\nA very nice Physical Organic study of the hydroboration of styrenes, involving a Hammett plot (a classic tool in physical organic chemistry) to determine a relationship between the stereochemistry of the reaction and the electron density of the alkene.<\/li>\n<li><strong>A SIMPLE AND CONVENIENT METHOD FOR THE OXIDATION OF ORGANOBORANES USING SODIUM PERBORATE: (+)-ISOPINOCAMPHEOL<br \/>\n<\/strong>George W. Kabalka, John T. Maddox, Timothy Shoup, and Karla R. Bowers<br \/>\n<em>Organic Syntheses<\/em>, Coll. Vol. 9, p.522 (1998); Vol. 73, p.116 (1996)<br \/>\n<strong>DOI: <\/strong><a href=\"http:\/\/DOI: 10.15227\/orgsyn.073.0116\">15227\/orgsyn.073.0116<\/a><br \/>\nA Hydroboration-oxidation procedure where the oxidizing agent is sodium perborate, a cheap, safe, easily handled oxidant commonly used in laundry detergents.<\/li>\n<li><strong>Hydroboration with Pyridine Borane at Room Temperature<br \/>\n<\/strong>Julia M. Clay and and Edwin Vedejs<strong><br \/>\n<\/strong><em>Journal of the American Chemical Society<\/em> <strong>2005<\/strong> <em>127<\/em> (16), 5766-5767<br \/>\n<strong>DOI: <\/strong><a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja043743j\">1021\/ja043743j<\/a><br \/>\nA modern method for doing hydroboration at room temperature using pyridine-borane, which usually requires heating to 75-100 \u00b0C to liberate the borane.<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"<p>Hydroboration Oxidation of Alkenes In this article we cover one of the most important methods for forming\u00a0alcohols\u00a0from\u00a0alkenes, hydroboration-oxidation. Hydroboration is an\u00a0addition reaction between an alkene <\/p>\n","protected":false},"author":1,"featured_media":35386,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1418],"tags":[554,906,550,908,553,552,907,251,528],"post_folder":[],"class_list":["post-7165","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-alkene-reactions","tag-anti-markovnikoff","tag-anti-markovnikov","tag-bh3","tag-concerted","tag-h2o2","tag-hydroboration","tag-organoborane","tag-oxidation","tag-syn"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Hydroboration Oxidation of Alkenes &#8211; Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"What&#039;s the mechanism of hydroboration-oxidation? Why is it syn? Why is it anti-Markovnikov? 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