{"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":"2013-11-14T07:26:43","modified_gmt":"2013-11-14T07:26:43","slug":"avoided-pericyclic-anti-aromaticity-reactions-of-t-butyl-hydroxycarbene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/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_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},"jetpack_post_was_ever_published":false},"categories":[559,1086],"tags":[20],"ppma_author":[2661],"class_list":["post-11642","post","type-post","status-publish","format-standard","hentry","category-pericyclic","category-reaction-mechanism-2","tag-steve-bachrach"],"yoast_head":"\nAvoided (pericyclic) anti-aromaticity: Reactions of t-butyl-hydroxycarbene. - 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=11642\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Avoided (pericyclic) anti-aromaticity: Reactions of t-butyl-hydroxycarbene. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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. 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[…]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11642\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2013-11-13T19:49:47+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2013-11-14T07:26:43+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/11\/schreiner.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=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Avoided (pericyclic) anti-aromaticity: Reactions of t-butyl-hydroxycarbene. - 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=11642","og_locale":"en_GB","og_type":"article","og_title":"Avoided (pericyclic) anti-aromaticity: Reactions of t-butyl-hydroxycarbene. - Henry Rzepa's Blog","og_description":"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. 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Formulating an analogous mechanism for such reaction of an alkyne\u00a0sounds straightforward, but one gradually realises that it requires raiding knowledge from several other areas of (perhaps slightly more advanced) chemistry to achieve a joined up\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":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/1.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":15048,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15048","url_meta":{"origin":11642,"position":1},"title":"I’ve started so I’ll finish. 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The BBC program became known\u00a0for the\u00a0catch phrase\u00a0I've started so I'll finish;\u00a0here I will try to follow this precept with\u2026","rel":"","context":"In "Historical"","block_context":{"text":"Historical","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=565"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":18897,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18897","url_meta":{"origin":11642,"position":2},"title":"Dispersion-induced triplet aromatisation?","author":"Henry Rzepa","date":"January 3, 2019","format":false,"excerpt":"There is emerging interest in cyclic conjugated molecules that happen to have triplet spin states and which might be expected to follow a 4n rule for aromaticity. The simplest such system would be the triplet state of cyclobutadiene, for which a non or anti-aromatic singlet state is always found to\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\/2019\/01\/CBD-1024x717.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":8174,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8174","url_meta":{"origin":11642,"position":3},"title":"Mechanisms of carbon monoxide insertion reactions: A reality check on carbonylation of methyl manganese pentacarbonyl","author":"Henry Rzepa","date":"November 4, 2012","format":false,"excerpt":"When methyl manganese pentacarbonyl is treated with carbon monoxide in e.g. di-n-butyl ether, acetyl manganese pentacarbonyl is formed. This classic experiment conducted by Cotton (of quadruple bond fame) and Calderazzo in 1962 dates from an era when chemists conducted extensive kinetic analyses to back up any mechanistic speculations. Their suggested\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.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/11\/CO%2Bethene.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":22996,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22996","url_meta":{"origin":11642,"position":4},"title":"An interesting aromatic molecule found in Titan’s atmosphere: Cyclopropenylidene","author":"Henry Rzepa","date":"November 7, 2020","format":false,"excerpt":"Cyclopropenylidene must be the smallest molecule to be aromatic due to \u03c0-electrons, with just three carbon atoms and two hydrogen atoms. It has now been detected in the atmosphere of Titan, one of Saturn's moons and joins benzene, another aromatic molecule together with the protonated version of cyclopropenylidene, C3H3+ also\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":"","width":0,"height":0},"classes":[]},{"id":24129,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=24129","url_meta":{"origin":11642,"position":5},"title":"Sterically stabilized cyclopropenylidenes. An example of Octopus publishing?","author":"Henry Rzepa","date":"August 15, 2021","format":false,"excerpt":"Whilst I was discussing the future of scientific publication in the last post, a debate was happening behind the scenes regarding the small molecule cyclopropenylidene. This is the smallest known molecule displaying\u00a0\u03c0-aromaticity, but its high reactivity means that it is unlikely to be isolated in the condensed phase. A question\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/08\/Screenshot-814-1024x792.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"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\/11642","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=11642"}],"version-history":[{"count":16,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11642\/revisions"}],"predecessor-version":[{"id":11667,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11642\/revisions\/11667"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=11642"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=11642"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=11642"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=11642"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}