{"id":35861,"date":"2024-01-23T13:28:19","date_gmt":"2024-01-23T19:28:19","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=35861"},"modified":"2026-04-18T06:55:07","modified_gmt":"2026-04-18T11:55:07","slug":"alkyne-hydroboration-with-r2bh","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2024\/01\/23\/alkyne-hydroboration-with-r2bh\/","title":{"rendered":"Alkyne Hydroboration With &#8220;R2BH&#8221;"},"content":{"rendered":"<p><strong>Hydroboration-Oxidation of Alkynes<\/strong><\/p>\n<ul>\n<li>As seen in the previous chapter, hydroboration of <strong>alkenes<\/strong> with borane (BH<sub>3<\/sub>) followed by oxidation (NaOH, H<sub>2<\/sub>O<sub>2<\/sub>) gives <strong>alcohols<\/strong> with <strong>anti-Markovnikov<\/strong> regioselectivity (<span style=\"color: #800080;\"><em>See article: <a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/03\/28\/hydroboration-of-alkenes-the-mechanism\/\">Hydroboration-Oxidation of Alkenes<\/a><\/em><\/span>)<\/li>\n<li>Hydroboration of <strong>alkynes<\/strong> is also selective for the anti-Markovnikov product. However, after oxidation with NaOH\/H<sub>2<\/sub>O<sub>2<\/sub> , we generally do not get the simple addition product (enol). Instead we get <strong>aldehydes<\/strong> (with terminal alkynes) or <strong>ketones<\/strong> (with internal alkynes). These are <strong>constitutional isomers<\/strong> of the expected enol.<\/li>\n<li>Enols are in equilibrium with their constitutional isomers (aldehydes or ketones), a phenomenon known as <strong>keto-enol tautomerism<\/strong> (<span style=\"color: #993366;\"><em>See article: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2022\/06\/21\/keto-enol-tautomerism-key-points\/\">Keto-Enol Tautomerism<\/a><\/em><\/span>). Since C-O pi bonds are stronger than C-C pi bonds, equilibrium generally favors the keto form.<\/li>\n<li>Although BH<sub>3<\/sub> <em>can<\/em> be used for the hydroboration of alkynes (<span style=\"color: #800080;\"><em>and many textbooks \/ courses teach this<\/em><\/span>) dialkylboranes such as <strong>di-siamylborane<\/strong> (&#8220;Sia<sub>2<\/sub>BH&#8221;) or <strong>9-BBN<\/strong> (9-Borabicyclo[3.3.1]nonaneare) are better choices. Dialkylboranes (usually abbreviated R<sub>2<\/sub>BH, since they can be considered equivalent for our purposes) generally give higher yields than BH<sub>3<\/sub> in the hydroboration of alkynes.<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-35973\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/0-Summary-hydroboration-oxidation-of-alkynes-to-give-aldehydes-and-ketones-1.gif\" alt=\"Summary-hydroboration oxidation of alkynes to give aldehydes and ketones\" width=\"640\" height=\"653\" \/><\/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 of Alkynes: Not The Products You Expected<\/a><\/li>\n<li><a href=\"#two\">Dialkylboranes (R<sub>2<\/sub>BH) Such As Disiamylborane and 9-BBN Are Often Used Instead of BH<sub>3\u00a0<\/sub>For Hydroboration of Alkynes<\/a><\/li>\n<li><a href=\"#three\">Hydroboration of Alkynes: The Mechanism<\/a><\/li>\n<li><a href=\"#four\">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 of Alkynes: Not The Products You Expected!<\/h2>\n<p>Back in the chapter on alkenes, we learned that alkenes can undergo hydroboration with reagents like borane (BH<sub>3<\/sub>) or diborane (B<sub>2<\/sub>H<sub>6<\/sub>). (<span style=\"color: #993366;\"><em>See article: <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/03\/28\/hydroboration-of-alkenes-the-mechanism\/\">Hydroboration-Oxidation of Alkenes<\/a><\/em><\/span>).\u00a0 In hydroboration, a C-C pi bond is broken, and a C-H and C-B bond are formed. A notable aspect of the reaction is that the C-B bond tends to form on the <strong>least substituted<\/strong> carbon of the alkene.\u00a0 \u00a0After <strong>oxidation<\/strong> of the borane (usually with NaOH\/H<sub>2<\/sub>O<sub>2<\/sub>) we then get an alcohol.<\/p>\n<p>In contrast to acid-catalyzed hydration of alkenes, which forms alcohols on the <strong>more<\/strong> <strong>substituted<\/strong> carbon of the alkene (&#8220;Markovnikov&#8221; regioselectivity)\u00a0 [<span style=\"color: #993366;\"><em>See article &#8211; <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2023\/09\/15\/hydration-alkenes-acid\/\">Hydration of Alkenes With Aqueous Acid<\/a><\/em><\/span>], hydroboration gives us an alcohol on the <strong>least substituted<\/strong> carbon of the alkene (&#8220;<strong>anti-Markovnikov<\/strong>&#8221; regioselectivity). [<span style=\"color: #993366;\"><em>See article &#8211; <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/03\/28\/hydroboration-of-alkenes-the-mechanism\/\">Hydroboration of Alkenes<\/a><\/em><\/span>]<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-35944\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/1-alkenes-undergo-hydroboration-to-give-alcohols-with-anti-markovnikov-selectivity-after-oxidation.gif\" alt=\"alkenes undergo hydroboration to give alcohols with anti markovnikov selectivity after oxidation\" width=\"640\" height=\"288\" \/><\/a><\/p>\n<p>If a reaction works on alkenes, it makes sense to try to apply it to alkynes. After all, there&#8217;s only one extra pi bond. What difference could just one extra pi bond make? \u00a0[<span style=\"color: #993366;\"><em><strong>Narrator<\/strong>: This is organic chemistry. Of course one extra pi-bond can make a huge difference, mwa-ah-ah (evil laugh)<\/em><\/span>].<\/p>\n<p>If hydroboration follows the same pattern of bonds formed\/ broken as that for alkenes, along with anti-Markovnikov selectivity, we should expect to end up with the &#8220;alkene-alcohol&#8221; (usually referred to as an &#8220;enol&#8221;) below after oxidation.<\/p>\n<p>When terminal alkynes undergo hydroboration, we don&#8217;t get this product. Instead, we get&#8230; an aldehyde!<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-35945\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/2-hydroboration-of-terminal-alkynes-does-not-result-in-enols-instead-it-gives-aldehydes.gif\" alt=\"hydroboration of terminal alkynes does not result in enols - instead it gives aldehydes\" width=\"640\" height=\"373\" \/><\/a><\/p>\n<p>What the&#8230;?\u00a0 You might (rightly) be shaking your head or insisting that this is a misprint.\u00a0 Who ordered this?<\/p>\n<p>How did we end up with a <strong>C\u2013O<\/strong> <strong>pi bond<\/strong> instead of a C\u2013C pi bond?<\/p>\n<p>The only thing that seems consistent here is that the C-O bond forms on the least substituted end of the alkyne (anti-Markovnikov regioselectivity).<\/p>\n<p>Likewise, when internal alkynes undergo hydroboration, the products aren&#8217;t enols either. They&#8217;re <strong>ketones<\/strong>.<\/p>\n<p><em>(<span style=\"color: #993366;\">When internal alkynes are attached to groups of similar size, there is no &#8220;least substituted&#8221; side of the alkyne and therefore no &#8220;anti-Markovnikov&#8221; selectivity.\u00a0 So we get <strong>mixtures<\/strong> of ketones<\/span>).\u00a0<\/em><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35946\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/3-hydroboration-of-internal-alkynes-gives-mixtures-of-ketones.gif\" alt=\"hydroboration of internal alkynes gives mixtures of ketones\" width=\"640\" height=\"417\" \/><\/a><\/p>\n<p>So what&#8217;s going on here? We expected enols, and end up with aldehydes (or ketones) instead. How?<\/p>\n<p>If you look closely and squint, you may notice that the observed products (aldehydes and ketones) \u00a0are\u00a0<strong>constitutional isomers<\/strong> of our expected (&#8220;enol&#8221;) products. (<span style=\"color: #993366;\"><em>Same molecular formula, different connectivity<\/em><\/span>).<\/p>\n<p>And it turns out that the enols <strong>do<\/strong> form, at least as the initial product. But alkenes attached to OH groups (enols) can readily undergo conversion to a constitutional isomer containing a C-O pi bond\u00a0 instead of a C-C pi bond. These constitutional isomers are in equilibrium with each other, and equilibrium tends to favor the form with the C-O (pi) bond. This is known as &#8220;<strong>keto-enol tautomerism<\/strong>&#8220;. [<span style=\"color: #993366;\"><em>See article: <span style=\"color: #993366;\"><a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2022\/06\/21\/keto-enol-tautomerism-key-points\/\">Keto-enol Tautomerism<\/a><\/span><\/em><\/span>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35947\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/4-hydroboration-oxidation-of-alkynes-gives-enols-which-tautomerize-into-ketones.gif\" alt=\"hydroboration oxidation of alkynes gives enols which tautomerize into ketones\" width=\"640\" height=\"311\" \/><\/a><\/p>\n<p>It&#8217;s not magic. It&#8217;s just that hydroboration-oxidation of alkynes is a gateway to enols, and a fundamental property of enols is that they are in equilibrium with their keto form. We&#8217;ll cover this in more detail in Org 2.<\/p>\n<p><span style=\"color: #993366;\"><em>(In my opinion, it&#8217;s these kinds of unexpected diversions that make organic chemistry is fun! Note: your definition of &#8220;fun&#8221; may vary).<\/em><\/span><\/p>\n<h2><a id=\"two\"><\/a>2. Dialkylboranes (R<sub>2<\/sub>BH) Such As Disiamylborane and 9-BBN Are Often Used Instead of BH<sub>3\u00a0<\/sub>For Hydroboration of Alkynes<\/h2>\n<p>Although BH<sub>3<\/sub> <em><strong>can<\/strong><\/em> be used for the hydroboration of alkynes, and it often appears in class notes and exams, in practice it&#8217;s more common to use <strong>dialkylboranes<\/strong> (R<sub>2<\/sub>BH) for hydroboration of alkynes. In a dialkylborane, two of the B-H bonds have been replaced by bonds to an alkyl group (<strong>R<\/strong>).\u00a0 The remaining B-H is perfectly capable of performing hydroboration reactions.\u00a0 [<a href=\"#noteone\"><span style=\"color: #ff0000;\">Note 1<\/span><\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35974\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/5-in-hydroboration-of-alkynes-R2BH-such-as-disamylborane-and-9-BBN-are-better-than-BH3-1.gif\" alt=\"\" width=\"640\" height=\"556\" \/><\/a><\/p>\n<p><span style=\"color: #000000;\">One of the main advantages of using R<sub>2<\/sub>BH is that it has better anti-Markovnikov selectivity. The better selectivity arises since the bulky R groups prefer to be close to the alkyne C\u2013H for steric reasons. (<span style=\"color: #800080;\"><em>If you make a model of what a &#8220;Markovnikov-selective&#8221; hydroboration would look like with a dialkylborane, you&#8217;d find that the alkyl groups on the borane can bump into the alkyl group adjacent to the triple bond<\/em><\/span>).\u00a0<\/span><\/p>\n<p>Three reagents commonly used for hydroboration of alkynes include <strong>disiamylborane<\/strong>, <strong>9-BBN <\/strong><span style=\"color: #993366;\"><em>(9-Borabicyclo[3.3.1]nonane)<\/em><\/span>, and <strong>catecholborane<\/strong>.<\/p>\n<p>While these reagents do have their subtle differences, <em>for our purposes<\/em> these reagents can all considered to be equivalent, and we often just abbreviate them as &#8220;R<sub>2<\/sub>BH&#8221; instead of drawing them out.<\/p>\n<p>Here are some examples of hydroboration of alkynes with disiamylborane and 9-BBN , which we will just abbreviate as &#8220;R<sub>2<\/sub>BH&#8221;):<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35949\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/6-examples-of-hydroboration-with-dialkylboranes-R2BH.gif\" alt=\"examples of hydroboration with dialkylboranes R2BH\" width=\"640\" height=\"464\" \/><\/a><\/p>\n<p>See if you can draw the product in this case:<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"35953\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"35953\"] {\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=\"35953\"] {\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=\"35953\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-35953\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-35953 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"35953\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-9bs6t\" data-id=\"9bs6t\">\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\/2663-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\/2663-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 of Alkynes: The Mechanism<\/h2>\n<p>Let&#8217;s take a closer look into the mechanism of the hydroboration of alkynes with R<sub>2<\/sub>BH. How does this reaction work?<\/p>\n<p>The mechanism is essentially the same as that in the hydroboration of alkenes, except that we have to draw a mechanism for keto-enol tautomerism of the resulting enol.<\/p>\n<p>In the first step, a concerted hydroboration reaction happens such that the new C-B bond is formed on the carbon bearing the most hydrogens (i.e. the terminal carbon of the alkyne) and a new C-H bond is formed on the more substituted carbon of the alkyne (<strong>Step 1)<\/strong>. Both the C-H and C-B bonds are formed on the same side of the alkyne, an example of\u00a0<em>syn<\/em> addition.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35950\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/8-mechanism-for-hydroboration-of-alkynes-part-1-hydroboration.gif\" alt=\"mechanism for hydroboration of alkynes part 1 - hydroboration\" width=\"640\" height=\"284\" \/><\/a><\/p>\n<p>As discussed in the article on hydroboration of alkenes [<a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/03\/28\/hydroboration-of-alkenes-the-mechanism\/\">link<\/a>], the main reason for anti-Markovnikov selectivity in hydroboration is that the carbon best able to support partial positive charge (the &#8220;more substituted&#8221; carbon) will be most stabilized when it lines up with the atom bearing a partial negative charge. Since H is more electronegative (2.2) than B (2.0), it&#8217;s actually the <strong>H<\/strong> that forms a bond to the more substituted carbon, while the partially positive boron forms a bond with the terminal carbon of the alkyne.<\/p>\n<p><span style=\"color: #993366;\"><em>Steric effects can also be important, particularly with alkynes. anti-Markovnikov selectivity can be increased by using R<sub>2<\/sub>BH instead of BH<sub>3<\/sub>. The idea is that the large, bulky alkyl groups will have fewer steric interactions when they are adjacent to the terminal C-H bond on the alkyne<\/em>.\u00a0\u00a0<\/span><\/p>\n<p>After hydroboration, the resulting alkenylborane (sometimes, &#8220;vinylborane&#8221;) is then treated with an oxidant &#8211; commonly, hydrogen peroxide (H<sub>2<\/sub>O<sub>2<\/sub>) in the presence of aqueous base (NaOH\/H<sub>2<\/sub>O). Base deprotonates hydrogen peroxide, which attacks the boron (<strong>Step 2<\/strong>, form B\u2013O).<\/p>\n<p>The oxygen-oxygen bond in peroxides is quite weak (about 40 kcal\/mol) . What happens next is that the pair of electrons in the C\u2013B bond act as a <strong>nucleophile<\/strong>, attacking oxygen (<strong>Step 3,<\/strong> form C\u2013O, break C\u2013B) and breaking the O\u2013O bond. The curved arrow we draw to show electron flow might remind you of the mechanism we see in <strong>hydride shifts<\/strong>\u00a0and <strong>alkyl shifts<\/strong>. (<em><span style=\"color: #993366;\">See article &#8211; <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/15\/rearrangement-reactions-1-hydride-shifts\/\">Rearrangement Reactions &#8211; Hydride Shifts<\/a><\/span><\/em>)<\/p>\n<p>Throughout this process, configuration on the alkene is <strong>conserved<\/strong>. That is, the C-O bond that is formed here remains on the same side of the alkene as the C\u2013H bond.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35951\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/9-mechanism-for-hydroboration-of-alkynes-part-2-oxidation.gif\" alt=\"mechanism for hydroboration of alkynes part 2- oxidation\" width=\"640\" height=\"361\" \/><\/a><\/p>\n<p>This gives us a molecule where the oxygen remains bonded to boron (a boronic ester). In the presence of aqueous base (NaOH\/H2O), the O-B bond is cleaved (<strong>Step 4<\/strong>), resulting in a species with a carbon-carbon pi bond attached to a negatively charged oxygen. This is the conjugate base of an enol, and is known as an <strong>enolate<\/strong>.<\/p>\n<p>In the presence of water, the enolate undergoes protonation at\u00a0<strong>carbon<\/strong>, (<strong>Step 5<\/strong>) resulting in formation of the C-O pi bond and the final aldehyde product. (We&#8217;ll have a lot more to say about enols and enolates in Org 2).<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-35952\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/10-mechanism-for-hydrolysis-of-vinyl-boronic-ester-and-keto-enol-tautomerism.gif\" alt=\"mechanism for hydrolysis of vinyl boronic ester and keto enol tautomerism\" width=\"640\" height=\"293\" \/><\/a><\/p>\n<h2><a id=\"four\"><\/a>4. Summary<\/h2>\n<p>Hydroboration of alkynes gives us a new method for the synthesis of aldehydes (with terminal alkynes) and ketones (with internal alkynes).<\/p>\n<p>Previously, we&#8217;ve only been able to access aldehydes and ketones through <strong>oxidative cleavage\u00a0<\/strong>of alkenes with ozone (O<sub>3<\/sub>) (<span style=\"color: #800080;\"><a style=\"color: #800080;\" href=\"https:\/\/www.masterorganicchemistry.com\/2013\/04\/23\/alkene-reactions-ozonolysis\/\"><em>See article: Ozonolysis of Alkenes<\/em><\/a><\/span>)<\/p>\n<p>The reaction to form ketones is generally only useful if we&#8217;re starting with a symmetrical ketone. Otherwise, we will often end up with mixtures!<\/p>\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\/03\/28\/hydroboration-of-alkenes-the-mechanism\/\" class=\"\"><span>Hydroboration Oxidation of Alkenes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/14\/hydroboration-and-oxymercuration-of-alkynes\/\" class=\"\"><span>Oxymercuration of Alkynes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2022\/06\/21\/keto-enol-tautomerism-key-points\/\" class=\"\"><span>Keto-Enol Tautomerism<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/15\/rearrangement-reactions-1-hydride-shifts\/\" class=\"\"><span>Rearrangement Reactions (1) \u2013 Hydride Shifts<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/08\/22\/rearrangement-reactions-2-alkyl-shifts\/\" class=\"\"><span>Carbocation Rearrangement Reactions (2) \u2013 Alkyl Shifts<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/08\/19\/lindlars-catalyst-partial-cis-reduction\/\" class=\"\"><span>Partial Reduction of Alkynes With Lindlar\u2019s Catalyst<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/08\/nanh3-partial-reduction-of-alkynes\/\" class=\"\"><span>Partial Reduction of Alkynes With Na\/NH3 To Obtain Trans Alkenes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/05\/01\/acetylide-formation-alkylation\/\" class=\"\"><span>Acetylides from Alkynes, And Substitution Reactions of Acetylides<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/06\/11\/alkynes-via-elimination-reactions\/\" class=\"\"><span>Alkenes To Alkynes Via Halogenation And Elimination Reactions<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2014\/01\/29\/synthesis-5-reactions-of-alkynes\/\" class=\"\"><span>Synthesis (5) \u2013 Reactions of Alkynes<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1. <\/strong>The first attempt to get terminal alkynes to undergo hydroboration resulted in an intractable mess. Details here. It was later found that the main problem is that BH<sub>3<\/sub> does a <strong>second<\/strong> hydroboration, and we end up with the diborane, below.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-36007\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/F1-double-hydroboration-of-terminal-alkynes-occurs-with-BH3-and-oxidation-works-poorly.gif\" alt=\"double hydroboration of terminal alkynes occurs with BH3 and oxidation works poorly\" width=\"640\" height=\"408\" \/><\/a><\/p>\n<p>To make a long story short, yields for converting this into aldehydes are not very high. The major product from hydroboration of 1-hexyne was 1-hexanol, which formed when mild acid (water!) was added to the di-borylated product. [see <a href=\"#refthree\">Ref<\/a>]<\/p>\n<p>A significant amount (10%)\u00a0 of 1,2-hexanediol was also formed, indicating that regioselectivity for the terminal alkyne was not very good (about 9:1 for each hydroboration = about 80% overall).<\/p>\n<p>This was much improved with disiamylborane, which gives good yields of aldehydes and only performs a single hydroboration on the terminal alkyne. [<a href=\"#reftwo\">Ref<\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-36008\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/F2-disiamylborane-is-an-excellent-reagent-for-mono-hydroboration-of-terminal-alkynes.gif\" alt=\"disiamylborane is an excellent reagent for mono hydroboration of terminal alkynes\" width=\"640\" height=\"407\" \/><\/a><\/p>\n<p>Another useful reagent for hydroboration of alkynes is catecholborane. This cleanly gives the vinyl boronic esters, below. [<a href=\"#reffour\">Ref<\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-36009\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/01\/F3-catecholborane-is-useful-for-mono-hydroboration-of-terminal-alkynes-and-resulting-alkenyl-boranes-can-undergo-suzuki-reaction.gif\" alt=\"catecholborane is useful for mono hydroboration of terminal alkynes and resulting alkenyl boranes can undergo suzuki reaction\" width=\"640\" height=\"508\" \/><\/a><\/p>\n<p>These boronic esters undergo an important reaction with alkenyl halides in the presence of a palladium catalyst known as\u00a0<strong>cross-coupling.<\/strong> The variant with boron has become known as the Suzuki reaction, which has become one of the most commonly used reactions in modern organic chemistry.<\/p>\n<p>So hydroboration of alkynes is important not just for the production of aldehydes from alkynes, also for the formation of alkenyl boronic esters.<\/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\/2666-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\/1653-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\/1654-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\/0700-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\/0705-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\/0712-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>Some online books (some available through Archive.org, some not)<\/p>\n<ul>\n<li><a href=\"https:\/\/archive.org\/details\/boranesinorganic0000brow\/page\/n13\/mode\/2up\">Boranes in Organic Chemistry<\/a><\/li>\n<li><a href=\"https:\/\/archive.org\/details\/organicsyntheses0000brow\/mode\/2up\">Organic Syntheses via Boranes<\/a><\/li>\n<\/ul>\n<ol>\n<li><strong>Nobel Lecture. <\/strong>H.C. Brown (1979). [<a href=\"https:\/\/www.nobelprize.org\/uploads\/2018\/06\/brown-lecture.pdf\">Link<\/a>] H.C. Brown won the 1979 Nobel Prize (with George Wittig) for the development of boron reagents in organic chemistry.<\/li>\n<li><strong><a id=\"reftwo\"><\/a>Hydroboration. XI. The Hydroboration of Acetylenes\u2014A Convenient Conversion of Internal Acetylenes into cis-Olefins and of Terminal Acetylenes into Aldehydes<\/strong><br \/>\nHerbert C. Brown and George Zweifel<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1961<\/strong> 83 (18), 3834-3840<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja01479a024\">10.1021\/ja01479a024\u00a0<\/a><br \/>\nFrom the abstract: &#8220;The treatment of internal acetylenes, such as 3-hexyne, with the theoretical quantity of hydroborating reagent results in the formation of the corresponding trivinylborane. However, under the same conditions, terminal acetylenes such as 1-hexyne undergo dihydroboration predominantly.&#8221;<\/li>\n<li class=\"p1\"><strong><a id=\"refthree\"><\/a>Germinal Organometallic Compounds. I. The Synthesis and Structure of 1,1-Diborohexane<\/strong><br \/>\nG. Zweifel and H. Arzoumanian<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1967<\/strong> 89 (2), 291-295<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja00978a022\">10.1021\/ja00978a022<\/a><br \/>\nFrom the abstract: &#8220;The hydroboration of 1-hexyne with diborane in a 3:1 ratio results in the formation of a polymeric dihydroborated product. Oxidation of this material with alkaline hydrogen peroxide produces 80% 1-hexanol and only 10-12% 1,2-hexanediol. This points to an initial rapid hydrolysis of the dihydroboration intermediate&#8230;.The use of either [disiamylborane] or dicyclohexylborane as the hydroborating reagents gives the 1,1-diboro- derivatives in 90-96% yield.&#8221;<\/li>\n<li class=\"p1\"><strong><a id=\"reffour\"><\/a>Hydroboration. XXXIX. 1,3,2-Benzodioxaborole (catecholborane) as a new hydroboration reagent for alkenes and alkynes. General synthesis of alkane- and alkeneboronic acids and esters via hydroboration. Directive effects in the hydroboration of alkenes and alkynes with catecholborane<\/strong><br \/>\nHerbert C. Brown and S. K. Gupta<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1975<\/strong> 97 (18), 5249-5255<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja00851a038\">10.1021\/ja00851a038<\/a><\/li>\n<li class=\"p1\"><strong>A convenient stereoselective synthesis of substituted alkenes via hydroboration-iodination of alkynes<\/strong><br \/>\nGeorge. Zweifel, Henri. Arzoumanian, and Charles C. Whitney<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1967<\/strong> 89 (14), 3652-3653<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja00990a061\">10.1021\/ja00990a061<\/a><\/li>\n<li><strong>Hydroboration. 50. Hydroboration of representative alkynes with 9-borabicyclo[3.3.1]nonane &#8211; a simple synthesis of versatile vinyl bora and gem-dibora intermediates<\/strong><br \/>\nHerbert C. Brown, Charles G. Scouten, and Ronald Liotta<br \/>\n<em>Journal of the American Chemical Society<\/em> <strong>1979<\/strong> 101 (1), 96-99<br \/>\n<strong>DOI<\/strong>: <a href=\"https:\/\/pubs.acs.org\/doi\/10.1021\/ja00495a016\">10.1021\/ja00495a016\u00a0<\/a><\/li>\n<li><strong>PALLADIUM-CATALYZED REACTION OF 1-ALKENYLBORONATES WITH VINYLIC HALIDES: (1Z,3E)-1-PHENYL-1,3-OCTADIENE<\/strong><br \/>\nMiayura, N.; Suzuki, A.<br \/>\n<em>Org. Synth<\/em>. <strong>1990<\/strong>, 68, 130<br \/>\n<strong>DOI:<\/strong><a href=\"http:\/\/www.orgsyn.org\/Result.aspx\">15227\/orgsyn.068.0130<\/a><br \/>\nA procedure by Nobel Laureate Akira Suzuki for the hydroboration of an alkyne with catecholborane. The resulting product can then be subsequently used in a Pd-catalyzed Suzuki coupling reaction.<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"<p>Hydroboration-Oxidation of Alkynes As seen in the previous chapter, hydroboration of alkenes with borane (BH3) followed by oxidation (NaOH, H2O2) gives alcohols with anti-Markovnikov regioselectivity <\/p>\n","protected":false},"author":1,"featured_media":35958,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1419],"tags":[14881,889,477,14883,14882,14880,552,14884,283],"post_folder":[],"class_list":["post-35861","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-alkyne-reactions","tag-9-bbn","tag-addition-reactions","tag-alkynes-2","tag-boronic-ester","tag-catecholborane","tag-disiamylborane","tag-hydroboration","tag-suzuki-reaction","tag-tautomerism"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Alkyne Hydroboration With &quot;R2BH&quot; &#8211; Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"Alkynes can be converted to aldehydes or ketones through hydroboration-oxidation with &quot;R2BH&quot; (e.g. disiamylborane or 9-BBN). 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