{"id":11995,"date":"2014-03-02T06:47:11","date_gmt":"2014-03-02T06:47:11","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=11995"},"modified":"2024-02-06T07:43:10","modified_gmt":"2024-02-06T07:43:10","slug":"explaining-the-anti-markovnikov-addition-of-borane-to-but-2-ene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11995","title":{"rendered":"The wrong trousers: the anti-Markovnikov addition of borane to 2-methylpropene."},"content":{"rendered":"
\n

A staple of introductory undergraduate teaching in organic chemistry is Markovnikov’s rule<\/a>, which states: “the addition of a protic acid HX to an alkene results in the acid hydrogen (H) becoming attached to the carbon with fewer alkyl substituents and the halide (X) group to the carbon with more alkyl substituents<\/em>“. Shortly thereafter,\u00a0students are exposed to the “anti-Markovnikov” addition of borane to e.g.<\/em> 2-methylpropene. In order to achieve a consistent explanation for both reactions, I normally show students the following mechanism.\u00a0Here I introduce a “reality check” to the first component of that mechanism (for the oxidative step, see this post<\/a>).<\/p>\n

\"butene+BH3\"<\/a><\/p>\n

The premise of this mechanism is that electronically at least, both the Markovnikov and the anti-Markovnikov additions actually arise from the same effect, which is to produce predominantly the more highly substituted intermediate carbocation. So the issue now is whether this intermediate, as might be invoked above for hydroboration, is real or whether it is simply an expediency to enable students to recognise the single electronic origin of both types of reaction (?<\/strong> above). Enter the\u00a0\u03c9B97XD\/6-311G(d,p)\/SCRF=thf procedure. Firstly,\u00a0I note that the reaction is normally done with “stabilized” borane, via<\/em> pre-coordination to thf.<\/p>\n

Starting from either Markovnikov or anti-Markovnikov orientations, only one transition state could be found[1]<\/a><\/span>, meaning that the\u00a0anti-Markovnikov preference is determined entirely by this transition state. The exit trajectories can presumably lead to either product, but favouring\u00a0anti-Markovnikov. The transition state shown here was\u00a0also used by Singleton[2]<\/a><\/span> for a molecular dynamics study of the hydroboration of propene with BH3<\/sub>.thf. There, dynamics were invoked to explain why 10% of the product goes Markovnikov.<\/p>\n

\"Click

Transition state for addition of borane to butene. Click for 3D<\/p><\/div>\n

The reaction with thf clearly occurs in two stages.[3]<\/a><\/span> The first stage subsumes the transition state\u00a0and is an SN<\/sub>2 like nucleophilic displacement at boron by the alkene, liberating thf as the leaving group. The second stage reveals an excellent example of a “hidden intermediate<\/em>“, which could be described as the zwitterionic carbocation shown in the scheme above. Perhaps instead it is less classical than this, being closer to a non-classical bridged species involving an asymmetric \u03c0-complex between the borane and the alkene. At any rate, at\u00a0IRC ~8, this hidden intermediate now decides to transfer a hydride to form the alkyl borane rather than to become an explicit intermediate.\u00a0<\/p>\n

\"borane\"<\/a>\"borane\"<\/a>\"boraneG\"<\/a><\/p>\n

\"\"<\/a><\/p>\n

So\u00a0?<\/strong>\u00a0turns out to be a hidden intermediate rather than an explicit one in this model. This hidden intermediate plays the role of the conventional transition state in its presumed determination of the regioselectivity of the hydroboration reaction. The IRC of course is a single trajectory; molecular dynamics will give a more statistical indication of the product distribution. Certainly, these “hidden intermediates” as key structures in mechanistic pathways are starting to turn up<\/a> in more and more classical reactions.<\/p>\n

References<\/h2>\n
  1. H.S. Rzepa, \"Gaussian Job Archive for C8H19BO\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.949678<\/a>\n\n<\/li>\n
  2. 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
  3. H.S. Rzepa, \"Gaussian Job Archive for C8H19BO\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.949689<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    A staple of introductory undergraduate teaching in organic chemistry is Markovnikov’s rule, which states: “the addition of a protic acid HX to an alkene results in the acid hydrogen (H) becoming attached to the carbon with fewer alkyl substituents and the halide (X) group to the carbon with more alkyl substituents“. Shortly thereafter,\u00a0students are exposed […]<\/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":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[1086],"tags":[],"ppma_author":[2661],"class_list":["post-11995","post","type-post","status-publish","format-standard","hentry","category-reaction-mechanism-2"],"yoast_head":"\nThe wrong trousers: the anti-Markovnikov addition of borane to 2-methylpropene. - 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=11995\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The wrong trousers: the anti-Markovnikov addition of borane to 2-methylpropene. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"A staple of introductory undergraduate teaching in organic chemistry is Markovnikov’s rule, which states: “the addition of a protic acid HX to an alkene results in the acid hydrogen (H) becoming attached to the carbon with fewer alkyl substituents and the halide (X) group to the carbon with more alkyl substituents“. 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A case for doing so might be my post of about a year ago, addressing why borane reduces a carboxylic acid, but not its ester, where I\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":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/10\/acyloxy1-page001.svg","width":350,"height":200},"classes":[]},{"id":16402,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16402","url_meta":{"origin":11995,"position":1},"title":"The mechanism of silylether deprotection using a tetra-alkyl ammonium fluoride.","author":"Henry Rzepa","date":"May 25, 2016","format":false,"excerpt":"The substitution of a nucleofuge (a good leaving group) by a nucleophile at a carbon centre\u00a0occurs with inversion\u00a0of configuration at the carbon, the mechanism being known by\u00a0the term\u00a0SN2\u00a0(a story I have also told\u00a0in this post). Such displacement at silicon famously proceeds by a quite different mechanism, which\u00a0I here quantify with\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":"","width":0,"height":0},"classes":[]},{"id":10073,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10073","url_meta":{"origin":11995,"position":2},"title":"The mechanism of ester hydrolysis via alkyl oxygen cleavage under a quantum microscope","author":"Henry Rzepa","date":"April 2, 2013","format":false,"excerpt":"My previous dissection of the mechanism for ester hydrolysis dealt with the acyl-oxygen cleavage route (red bond). There is a much rarer alternative: alkyl-oxygen cleavage (green bond) which I now place under the microscope. Here, guanidine is used as a general acid\/base, which results in a reasonable activation barrier for\u2026","rel":"","context":"In \"acetic acid\"","block_context":{"text":"acetic acid","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=acetic-acid"},"img":{"alt_text":"alkylg","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/03\/alkylg.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6315,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6315","url_meta":{"origin":11995,"position":3},"title":"The hydroboration-oxidation mechanism: An updated look.","author":"Henry Rzepa","date":"February 26, 2012","format":false,"excerpt":"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.\u2026","rel":"","context":"In \"Tutorial material\"","block_context":{"text":"Tutorial material","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=tutorial-material"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/02\/HB.svg","width":350,"height":200},"classes":[]},{"id":6618,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6618","url_meta":{"origin":11995,"position":4},"title":"The mechanism of the Baeyer-Villiger rearrangement.","author":"Henry Rzepa","date":"May 7, 2012","format":false,"excerpt":"The Baeyer-Villiger rearrangement was named after its discoverers, who in\u00a01899\u00a0described the transformation of menthone into the corresponding lactone using Caro's acid (peroxysulfuric acid). The mechanism is described in all text books of organic chemistry as involving an alkyl migration. 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The mechanism involves an initial equilibrium between React and Int1, followed by capture of the hydrogen by Int1 to form a 5-coordinate borane intermediate (Int2 below, as\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":"","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\/11995","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=11995"}],"version-history":[{"count":49,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11995\/revisions"}],"predecessor-version":[{"id":26793,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11995\/revisions\/26793"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=11995"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=11995"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=11995"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=11995"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}