{"id":6315,"date":"2012-02-26T20:28:01","date_gmt":"2012-02-26T20:28:01","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=6315"},"modified":"2012-02-28T08:50:58","modified_gmt":"2012-02-28T08:50:58","slug":"the-hydroboration-oxidation-mechanism-an-updated-look","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6315","title":{"rendered":"The hydroboration-oxidation mechanism: An updated look."},"content":{"rendered":"
\n

One thing almost always leads to another in chemistry. In the last post, I described how an antiperiplanar migration could compete with an antiperiplanar elimination. This leads to the\u00a0hydroboration-oxidation<\/a> mechanism, the discovery of which resulted in Herbert C. Brown (at least in part) being awarded the Nobel prize in 1979.<\/p>\n

\"\"<\/p>\n

This reaction represents a fairly steep learning curve for new students of organic chemistry. Actually, it turns out its a pretty steep learning curve for most of us.<\/p>\n

    \n
  1. Firstly, it is often described as an\u00a0anti-Markovnikov<\/a>\u00a0\u00a0reaction, with the hydroxyl group attaching to the less-substituted carbon (as above). Unfortunately, the mechanism is revealed as following Markovnikov, rather than being an anti example! Confusing?<\/li>\n
  2. One possible way of representing the mechanism is to show the (nucleophilic) \u03c0-bond attacking the (electrophilic) boron centre of a BH3<\/sub> molecule (itself presumed to be generated from a suitable reagent such as BH3<\/sub>.SMe2<\/sub> in a pre-equilibrium step) to form a carbocation (step 1<\/strong> above). The regiospecificity proceeds so as to form the most stable such carbocation (a tertiary one above). This of course follows the Markovnikov rule and not its negation. The overall reaction might be\u00a0anti-Markovnikov<\/a>\u00a0 but its mechanism is not!<\/li>\n
  3. An updated interpretation (not for the faint hearted!) is nicely explained on this blog<\/a>. This shows that the above “arrow pushing” mechanism, itself based on something called transition state theory<\/em>, is in fact an over-simplification. Instead, it is necessary to resort to molecular dynamics<\/em>[1]<\/a><\/span> rather than transition state theory to explain why the boron attaches to the least substituted<\/strong> carbon of the alkene.<\/li>\n
  4. I think I had better return to more conventional arrow pushing before I loose whatever audience I have. Step 2<\/strong> above transfers a hydrogen from the boron to the carbocation to form an alkyl borane<\/li>\n
  5. In step 3<\/strong>, this species is reacted with hydrogen peroxide.<\/li>\n
  6. In step 4, a proton is transferred in the resulting peroxyalkylborane. Such a step, often simply labelled as PT<\/strong>, is regarded as a freely available equilibrium. In other words, one can move protons around a structure to where-ever is deemed as convenient for the next step (well, within reason).<\/li>\n
  7. Step 5 is the key step, and to highlight this, I have shown the arrow pushing using red arrows<\/span>.\n
      \n
    1. This step now requires the donating bond (shown in blue) and the accepting (anti)bond, shown in green) to be aligned in an antiperiplanar manner. The O-O bond is a very good acceptor, the C-B bond an effective donor, quantified by the E(2) interaction energy in an NBO analysis of 35.5 kcal\/mol and an overlap that looks as below:
      \"\"

      Overlap between the C-B donor and the O-O acceptor. Click for 3D.<\/p><\/div><\/li>\n

    2. Note how the bonding part of the C-B bond (purple) overlaps with the blue of the O-O antibonding orbital, thus forming a new C-O bond. The (blue-red) node along the O-O leads to an entirely cleaved O-O bond, but you can see the genesis of a new B-O bond in the \u00a0orange-red overlap.<\/li>\n<\/ol>\n<\/li>\n
    3. An intrinsic reaction coordinate computed for the reaction is shown below. Note how the evicted water molecule changes direction at the end and makes a bee-line for the boron atom. The end result of this reversal is of course boric acid.\"\"<\/a><\/li>\n<\/ol>\n

      As often happens, it is worth taking a look at a tradition text-book mechanism to see what a modern slant might give it.<\/p>\n

      POSTSCRIPT:<\/strong> \u00a0 You will notice from the comments on this post below, the observation that the hydroboration-oxidation reaction is normally carried out in basic solution. I have therefore repeated the calculations using a deprotonated starting point (\u03c9B97XD\/6-311+G(d,p)\/SCRF=water<\/a>) as shown below.<\/p>\n\n\n\n
      \"\"<\/a><\/td>\n\n

      \"\"<\/a>

      Intrinsic reaction coordinate for deprotonated reactant<\/p><\/div><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

      This is best viewed as below, showing both the energy and the energy gradients as a function of the proceeding reaction.<\/p>\n\n\n\n
      \"\"<\/a><\/td>\n\"\"<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
        \n
      1. Note first the barrier to the migration, which is \u00a0~30 kcal\/mol. This is because OH–<\/sup>\u00a0is an inferior leaving group to H2<\/sub>O (for which the barrier is ~2 kcal\/mol). This would make it a very slow reaction at room temperature.<\/li>\n
      2. Notice how the first prominent action is a rotation of the OH group.<\/li>\n
      3. The O-O bond starts to break before the C-B bond starts to migrate<\/li>\n
      4. At an IRC of +6, a second feature appears, which is the reversal of the trajectory of the evicted OH group in re-attaching itself to the boron (as before).<\/li>\n<\/ol>\n
        As noted in the comments below, the kinetics are a balance between the slow reaction of a deprotonated species in high concentration, and the much faster reaction of a protonated species in lower concentration.<\/div>\n

        References<\/h2>\n
        1. Y. Oyola, and D.A. Singleton, \"Dynamics and the Failure of Transition State Theory in Alkene Hydroboration\", Journal of the American Chemical Society<\/i>, vol. 131, pp. 3130-3131, 2009. https:\/\/doi.org\/10.1021\/ja807666d<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

          One thing almost always leads to another in chemistry. In the last post, I described how an antiperiplanar migration could compete with an antiperiplanar elimination. This leads to the\u00a0hydroboration-oxidation mechanism, the discovery of which resulted in Herbert C. Brown (at least in part) being awarded the Nobel prize in 1979. This reaction represents a fairly […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[],"tags":[373],"ppma_author":[2661],"class_list":["post-6315","post","type-post","status-publish","format-standard","hentry","tag-tutorial-material"],"yoast_head":"\nThe hydroboration-oxidation mechanism: An updated look. - Henry Rzepa's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6315\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The hydroboration-oxidation mechanism: An updated look. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"One thing almost always leads to another in chemistry. In the last post, I described how an antiperiplanar migration could compete with an antiperiplanar elimination. This leads to the\u00a0hydroboration-oxidation mechanism, the discovery of which resulted in Herbert C. Brown (at least in part) being awarded the Nobel prize in 1979. This reaction represents a fairly […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6315\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-02-26T20:28:01+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2012-02-28T08:50:58+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/02\/HB.svg\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"4 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"The hydroboration-oxidation mechanism: An updated look. - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6315","og_locale":"en_GB","og_type":"article","og_title":"The hydroboration-oxidation mechanism: An updated look. - Henry Rzepa's Blog","og_description":"One thing almost always leads to another in chemistry. In the last post, I described how an antiperiplanar migration could compete with an antiperiplanar elimination. This leads to the\u00a0hydroboration-oxidation mechanism, the discovery of which resulted in Herbert C. Brown (at least in part) being awarded the Nobel prize in 1979. 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The mechanism is described in all text books of organic chemistry as involving an alkyl migration. Here I take a look at the scheme described by\u00a0Alvarez-Idaboy, Reyes\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/05\/bv1.svg","width":350,"height":200},"classes":[]},{"id":26272,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=26272","url_meta":{"origin":6315,"position":2},"title":"Pre-mechanism for the Swern Oxidation: formation of chlorodimethylsulfonium chloride.","author":"Henry Rzepa","date":"August 25, 2023","format":false,"excerpt":"The Swern oxidation is a class of \"activated\" dimethyl sulfoxide (DMSO) reaction in which the active species is a chlorodimethylsulfonium chloride salt. The mechanism of this transformation as shown in e.g. Wikipedia is illustrated below.\u2021 However, an interesting and important aspect of chemistry is not apparent in this schematic mechanism\u2026","rel":"","context":"In "Curly arrows"","block_context":{"text":"Curly arrows","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2327"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":6279,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6279","url_meta":{"origin":6315,"position":3},"title":"E2 elimination vs ring contraction: anti-periplanarity in action.","author":"Henry Rzepa","date":"February 20, 2012","format":false,"excerpt":"The anti-periplanar principle permeates organic reactivity. Here I pick up on an example of the antiperiplanar E2 elimination (below, blue) by comparing it to a competing reaction involving a [1,2] antiperiplanar migration (red). The relative rates of these two processes will depend on several factors such as the ability of\u2026","rel":"","context":"In \"conformational analysis\"","block_context":{"text":"conformational analysis","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=conformational-analysis"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/02\/Ring.svg","width":350,"height":200},"classes":[]},{"id":2207,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2207","url_meta":{"origin":6315,"position":4},"title":"Tunable bonds looked at in a different way","author":"Henry Rzepa","date":"July 11, 2010","format":false,"excerpt":"The title of this post merges those of the two previous ones. The tunable C-Cl bond brought about in the molecule tris(amino)chloromethane by anomeric effects will be probed using the Laplacian of the electronic density. The figure above shows the Laplacian for a conformation of tris(amino)chloromethane with one of the\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/07\/app-1-0.67.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":16671,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16671","url_meta":{"origin":6315,"position":5},"title":"A periodic table for anomeric centres.","author":"Henry Rzepa","date":"August 6, 2016","format":false,"excerpt":"In the last few posts, I have explored the anomeric effect as it occurs at an atom centre X. Here I try to summarise the atoms for which the effect is manifest in crystal structures. The effect is defined by X bearing two substituents, one of which Z is a\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6315","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=6315"}],"version-history":[{"count":29,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6315\/revisions"}],"predecessor-version":[{"id":6333,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6315\/revisions\/6333"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6315"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6315"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6315"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=6315"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}