{"id":12880,"date":"2014-08-18T08:55:23","date_gmt":"2014-08-18T07:55:23","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=12880"},"modified":"2014-08-18T09:28:35","modified_gmt":"2014-08-18T08:28:35","slug":"full-circle-stereoisomeric-transition-states-for-14-pericyclic-shifts","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12880","title":{"rendered":"Full circle. Stereoisomeric transition states for [1,4] pericyclic shifts."},"content":{"rendered":"
\n

This post, the fifth in the series, comes full circle<\/a>. I started off by speculating how to invert the stereochemical outcome of an electrocyclic reaction by inverting a bond polarity. This led to finding transition states for BOTH outcomes<\/a> with suitable substitution, and then seeking other examples. Migration in homotropylium cation<\/a> was one such, with the “allowed\/retention” transition state proving a (little) lower in activation energy than the “forbidden\/inversion” path. Here, I show that with two electrons less, the stereochemical route indeed inverts.\"mob-inva\"<\/a> First, a [1,4] alkyl shift with inversion at the migrating carbon (\u03c9B97XD\/6-311G(d,p)\/SCRF=chloroform); as a four-electron process, this is the “allowed” route.[1]<\/a><\/span> \"mob-inva\"<\/a> The “forbidden” route corresponds to retention of configuration at the migrating carbon.[2]<\/a><\/span> \"mob-retb\"<\/a> The barriers for each process can be seen below from the IRCs. That for inversion<\/strong><\/em> is ~4.5 kcal\/mol lower than retention<\/strong><\/em>. This nicely transposes the values for the six-electron homologue shown in the previous post<\/a>. \"mob-inv\"<\/a>\"mob-ret\"<\/a> There is one more nugget of insight that can be extracted. The start\/end-point for the six-electron process (homotropylium cation) was, as the name implies, homoaromatic. Now, with a four-electron system we also have an inverse. Nominally, we should now end with homo-antiaromaticity (but see [3]<\/a><\/span>). But antiaromaticity is avoided whenever possible<\/a>, and so the homoaromatic bond observed in homotropylium is not formed. It resolutely remains a \u03c3-bond (1.48\u00c5) thus sequestering two electrons, and the remaining two electrons simply form a delocalised allyl cation. With the six-electron homotropylium, reactant\/product were stabilised by that additional (homo)aromaticity, thus inducing a relatively high barrier. With the four-electron system here, no such reactant\/product stabilisation occurs, and hence the reaction barriers are now significantly lower. A rather neat pedagogic example.<\/p>\n

References<\/h2>\n
  1. H.S. Rzepa, \"Gaussian Job Archive for C8H11(1+)\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.1142175<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"Gaussian Job Archive for C8H11(1+)\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.1142174<\/a>\n\n<\/li>\n
  3. C.S.M. Allan, and H.S. Rzepa, \"Chiral Aromaticities. A Topological Exploration of M\u00f6bius Homoaromaticity\", Journal of Chemical Theory and Computation<\/i>, vol. 4, pp. 1841-1848, 2008. https:\/\/doi.org\/10.1021\/ct8001915<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    This post, the fifth in the series, comes full circle. I started off by speculating how to invert the stereochemical outcome of an electrocyclic reaction by inverting a bond polarity. This led to finding transition states for BOTH outcomes with suitable substitution, and then seeking other examples. Migration in homotropylium cation was one such, with […]<\/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_feature_clip_id":0,"_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":[693,1132],"ppma_author":[2661],"class_list":["post-12880","post","type-post","status-publish","format-standard","hentry","category-pericyclic","category-reaction-mechanism-2","tag-activation-energy","tag-reactantproduct"],"yoast_head":"\nFull circle. Stereoisomeric transition states for [1,4] pericyclic shifts. - 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=12880\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Full circle. Stereoisomeric transition states for [1,4] pericyclic shifts. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"This post, the fifth in the series, comes full circle. I started off by speculating how to invert the stereochemical outcome of an electrocyclic reaction by inverting a bond polarity. This led to finding transition states for BOTH outcomes with suitable substitution, and then seeking other examples. Migration in homotropylium cation was one such, with […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12880\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2014-08-18T07:55:23+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2014-08-18T08:28:35+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/08\/mob-inva.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=\"1 minute\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Full circle. 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This molecule is thermally\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":5968,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968","url_meta":{"origin":12880,"position":4},"title":"Quadruple antarafacial delight.","author":"Henry Rzepa","date":"December 18, 2011","format":false,"excerpt":"A feature of many a classic review article is that not only does it organise and rationalise existing literature, but it will predict new chemistry as well. 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