{"id":11642,"date":"2013-11-13T19:49:47","date_gmt":"2013-11-13T19:49:47","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=11642"},"modified":"2013-11-13T19:49:47","modified_gmt":"2013-11-13T19:49:47","slug":"avoided-pericyclic-anti-aromaticity-reactions-of-t-butyl-hydroxycarbene","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=11642","title":{"rendered":"Avoided (pericyclic) anti-aromaticity: Reactions of t-butyl-hydroxycarbene."},"content":{"rendered":"
\n

Not long ago, I described a cyclic carbene<\/a> in which elevating the carbene lone pair into a \u03c0-system transformed it from a formally 4n-antiaromatic \u03c0-cycle into a 4n+2 aromatic \u03c0-cycle. From an entirely different area of chemistry, another example of this behaviour emerges; Schreiner’s[1]<\/a><\/span> trapping and reactions of t-butyl-hydroxycarbene, as described on Steve Bachrach’s blog<\/a>. A point I often make is that chemistry is all about connections, and so here I will discuss such a connection.\"schreiner\"<\/a><\/p>\n

The essence of Schreiner’s[1]<\/a><\/span> work is that once generated, t-butyl hydroxycarbene could rearrange in three different ways:<\/p>\n

    \n
  1. A [1,2] hydrogen migration (blue, formally a pericyclic sigmatropic reaction)<\/li>\n
  2. A [1,2] methyl migration (magenta, formally a pericyclic sigmatropic reaction)<\/li>\n
  3. A C-H carbene insertion<\/a> (red, formally a pericyclic cycloaddition).<\/li>\n<\/ol>\n

    As pericyclic reactions, all three would be four electron thermal processes, and again all would require an antarafacial component to be present somewhere. So I pose the question here: are they indeed true pericyclic reactions, and if so are they true four-electron ones (with an implied antarafacial component somewhere)?<\/p>\n

    I will show a computed IRC for each (\u03c9B97XD\/6-311G(d,p)). First, the [1,2] hydrogen migration.[2]<\/a><\/span> This emerges as a proton transfer, with a base (the \u03c3-carbene) abstracting a proton from an acid (the \u03c3-O-H bond). Crucially, the C=O bond is hardly involved in the process. Put simply, the non-involvement of that C=O means the process is not pericyclic. So there is no need for an antarafacial mode.<\/p>\n

    \"H-mig\"<\/p>\n

    Next, the [1,2] methyl migration (if a true 4-electron pericyclic, one might imagine it would migrate with inversion of configuration). Here, one can see two distinct phases to the migration:<\/p>\n

    \"Me-mig\"<\/p>\n

      \n
    1. The first phase involves a methyl migration (with retention) to form a bond by donating into the empty p-\u03c0-orbital on the carbene (the same into which two electrons were promoted to invert antiaromaticity<\/a>). This is a two-electron process, analogous to a [1,2] migration in a carbocation. This phase requires no antarafacial mode.<\/li>\n
    2. This forms, if you like, a zwitterionic resonance, leaving behind a carbocation, and forming a carbanion, as below.\"me-mig\"<\/a><\/li>\n
    3. The second pair of electrons now come into play (the carbene pair), ending up forming the C=C bond. Crucially, this occurs AFTER the first pair have been used to migrate the methyl group. Because the sequence is now separated, this process too does not require an antarafacial mode; it effectively comprises two consecutive 2-electron processes, which overall constitute an asynchronous pericyclic process. There are no actual intermediates along the IRC (hardly a hint of even a hidden one), so it is a concerted process overall, and the zwitterionic species implied above does not actually form.\"me-mig-IRC\"<\/a><\/li>\n<\/ol>\n

      \u00a0Now for the third mode, the insertion of the carbene into a C-H bond.[3]<\/a><\/span> This too occurs in two phases:<\/p>\n

      .\"Me-ins\"<\/p>\n

        \n
      1. The first phase involves the transfer of the two electrons from a C-H\u00a0\u03c3-bond into\u00a0the empty p-\u03c0-orbital on the carbene (a hydride transfer).<\/li>\n
      2. This forms a carbocation\/carbanion zwitterionic resonance. The pyramidal carbon then inverts (umbrella mode. Is this an antarafacial mode?)<\/li>\n
      3. and the carbanion then ring closes onto the carbocation to form a cyclopropane. As before,\u00a0\u00a0the sequence is now separated, and again does not require an antarafacial mode (?). The IRC profile (below) does appear to show a hidden intermediate (IRC = 2.9) but in fact this is the rotation of the O-H bond, and does not involve any bond formation.<\/li>\n<\/ol>\n

        \"me-ins\"<\/a><\/p>\n

        \"me-ins-IRC\"<\/p>\n

        So in the end, all three apparently pericyclic thermal transformations of t-butyl hydroxycarbene avoid 4-electron cyclic antiaromaticity by either becoming acyclic, or by timing the development of the two electron pairs so that they occur sequentially and not concurrently. None of the three is a true pericyclic!\u00a0<\/p>\n

        References<\/h2>\n
        1. D. Ley, D. Gerbig, and P.R. Schreiner, \"Tunneling control of chemical reactions: C\u2013H insertion versus H-tunneling in tert-butylhydroxycarbene\", Chem. Sci.<\/i>, vol. 4, pp. 677-684, 2013. https:\/\/doi.org\/10.1039\/c2sc21555a<\/a>\n\n<\/li>\n
        2. H.S. Rzepa, \"Gaussian Job Archive for C5H10O\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.848560<\/a>\n\n<\/li>\n
        3. H.S. Rzepa, \"Gaussian Job Archive for C5H10O\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.848613<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

          Not long ago, I described a cyclic carbene in which elevating the carbene lone pair into a \u03c0-system transformed it from a formally 4n-antiaromatic \u03c0-cycle into a 4n+2 aromatic \u03c0-cycle. From an entirely different area of chemistry, another example of this behaviour emerges; Schreiner’s trapping and reactions of t-butyl-hydroxycarbene, as described on Steve Bachrach’s blog. […]<\/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,"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":[564,1085],"tags":[348],"class_list":["post-11642","post","type-post","status-publish","format-standard","hentry","category-pericyclic","category-reaction-mechanism-2","tag-steve-bachrach"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-31M","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11642","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=11642"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11642\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=11642"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=11642"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=11642"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}