{"id":5927,"date":"2011-12-14T13:34:03","date_gmt":"2011-12-14T13:34:03","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=5927"},"modified":"2011-12-14T13:34:03","modified_gmt":"2011-12-14T13:34:03","slug":"molecular-gymnastics-in-22-cycloadditions","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=5927","title":{"rendered":"Molecular gymnastics in 2+2 cycloadditions."},"content":{"rendered":"
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In this earlier post<\/a>, I described how the stereochemistry of \u03c02<\/sub>+\u03c02<\/sub> cycloadditions occurs suprafacially<\/em> if induced by light, and how one antarafacial<\/em> component appears if the reaction is induced by heat alone. I also noted<\/a> how Woodward and Hoffmann (WH) explained that violations to their rules were avoided by mandating a change in mechanism requiring stepwise pathways with intermediates along the route. Here I illustrate how the stereochemistry of a thermal\u00a0\u03c02<\/sub>+\u03c02<\/sub>\u00a0cycloaddition can indeed avoid an antarafacial<\/em> component by performing appropriate gymnastic contortions instead of a mechanistic change\u00a0(a WH violation certainly in the letter of their law, if not their spirit).
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The IRC for the reaction<\/a> (R=CH=S) shows the two molecules moving towards each other in a trapezoidal motion, with the axes of the C=C bonds parallel and the faces clearly suprafacial.<\/em> At IRC ~-4, the nature of motion changes into a slide of one C=C axis along the other, converting the trapezoidal<\/a> geometry into a square. By doing so, a full frontal “violation” is changed into a more subtle sideways insertion.<\/p>\n

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Animation of a 2+2 thermal cycloaddition with no antarafacial components. Click for 3D.<\/p><\/div>This trapezoidal motif for a thermal reaction does require the two R groups to have electron withdrawing nature, but it is noteworthy that the same motif was found for the photochemically induced \u03c02<\/sub>+\u03c02<\/sub>\u00a0cycloaddition. That both the thermal AND the photochemical pathways have essentially the same geometry is indeed a violation of WH!<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

In this earlier post, I described how the stereochemistry of \u03c02+\u03c02 cycloadditions occurs suprafacially if induced by light, and how one antarafacial component appears if the reaction is induced by heat alone. I also noted how Woodward and Hoffmann (WH) explained that violations to their rules were avoided by mandating a change in mechanism requiring […]<\/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":[6],"tags":[32,1529,1527],"class_list":["post-5927","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-animation","tag-pericyclic","tag-tutorial-material"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-1xB","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5927","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=5927"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5927\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5927"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5927"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5927"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}