{"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":"2011-12-15T17:05:23","modified_gmt":"2011-12-15T17:05:23","slug":"molecular-gymnastics-in-22-cycloadditions","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927","title":{"rendered":"Molecular gymnastics in  2+2 cycloadditions."},"content":{"rendered":"<div class=\"kcite-section\" kcite-section-id=\"5927\">\n<p>In <a href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=5655\" target=\"_blank\">this earlier post<\/a>, I described how the stereochemistry of \u03c0<sub>2<\/sub>+\u03c0<sub>2<\/sub> cycloadditions occurs <em>suprafacially<\/em> if induced by light, and how one <em>antarafacial<\/em> component appears if the reaction is induced by heat alone. I <a href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=5834\" target=\"_blank\">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\u03c0<sub>2<\/sub>+\u03c0<sub>2<\/sub>\u00a0cycloaddition can indeed avoid an <em>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).<br \/>\n<a href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/2+2.svg\"><img decoding=\"async\" class=\"aligncenter size-full wp-image-5929\" title=\"2+2\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/2+2.svg\" alt=\"\" \/><\/a><\/p>\n<table style=\"margin-left: auto; margin-right: auto;\" border=\"0\">\n<tbody>\n<tr>\n<td><a href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/14de.svg\"><img decoding=\"async\" class=\"aligncenter size-full wp-image-5934\" title=\"14de\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/14de.svg\" alt=\"\" width=\"200\" \/><\/a><\/td>\n<td><a href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/14dg.svg\"><img decoding=\"async\" class=\"aligncenter size-full wp-image-5936\" title=\"14dg\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/14dg.svg\" alt=\"\" width=\"200\" \/><\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>The <a href=\"http:\/\/hdl.handle.net\/10042\/to-11331\" target=\"_blank\">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 <em>suprafacial.<\/em> At IRC ~-4, the nature of motion changes into a slide of one C=C axis along the other, converting the <a href=\"http:\/\/dx.doi.org\/10.1039\/A805668D%20\" target=\"_blank\">trapezoidal<\/a> geometry into a square. By doing so, a full frontal &#8220;violation&#8221; is changed into a more subtle sideways insertion.<\/p>\n<p><div id=\"attachment_5933\" style=\"width: 288px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-5933\" class=\" wp-image-5933 \" title=\"14d1\" onclick=\"jmolInitialize('..\/Jmol\/');jmolSetAppletColor('orange');jmolApplet([450,450],'load wp-content\/uploads\/2011\/12\/14d.log;frame 15; zoom 100;connect (atomno=4) (atomno=1) partial;connect (atomno=2) (atomno=3) partial;vectors on;vectors 4;vectors scale 5.0; color vectors yellow; vibration 20;animation mode loop;measure 3 20;measure 1 2;');\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/14d1.gif\" alt=\"\" width=\"278\" height=\"170\" \/><p id=\"caption-attachment-5933\" class=\"wp-caption-text\">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 \u03c0<sub>2<\/sub>+\u03c0<sub>2<\/sub>\u00a0cycloaddition. That both the thermal AND the photochemical pathways have essentially the same geometry is indeed a violation of WH!<\/p>\n<!-- kcite active, but no citations found -->\n<\/div> <!-- kcite-section 5927 -->","protected":false},"excerpt":{"rendered":"<p>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 [&hellip;]<\/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":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[4],"tags":[152,2650,373],"ppma_author":[2661],"class_list":["post-5927","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-animation","tag-pericyclic","tag-tutorial-material"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Molecular gymnastics in 2+2 cycloadditions. - Henry Rzepa&#039;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=5927\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Molecular gymnastics in 2+2 cycloadditions. - Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"og:description\" content=\"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 [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"article:published_time\" content=\"2011-12-14T13:34:03+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2011-12-15T17:05:23+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/2+2.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":"Molecular gymnastics in 2+2 cycloadditions. - Henry Rzepa&#039;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=5927","og_locale":"en_GB","og_type":"article","og_title":"Molecular gymnastics in 2+2 cycloadditions. - Henry Rzepa&#039;s Blog","og_description":"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 [&hellip;]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927","og_site_name":"Henry Rzepa&#039;s Blog","article_published_time":"2011-12-14T13:34:03+00:00","article_modified_time":"2011-12-15T17:05:23+00:00","og_image":[{"url":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/2+2.svg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"1 minute"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Molecular gymnastics in 2+2 cycloadditions.","datePublished":"2011-12-14T13:34:03+00:00","dateModified":"2011-12-15T17:05:23+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927"},"wordCount":252,"commentCount":4,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/2+2.svg","keywords":["animation","pericyclic","Tutorial material"],"articleSection":["Interesting chemistry"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927","name":"Molecular gymnastics in 2+2 cycloadditions. - 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If it were a pericyclic cycloaddition, both new bonds would form simultaneously, as shown with the indicated arrow pushing. Ten electrons would be involved, and in theory, the transition state would have 4n+2 aromaticity. In fact\u00a0Fernandez, Sierra and Torres have reported\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","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\/2011\/07\/2%2B8.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5968,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968","url_meta":{"origin":5927,"position":1},"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. I have already noted Woodward and Hoffmann's (WH)\u00a0review as achieving the former, and here I take a (sideways) look at one of their\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","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\/12\/286.svg","width":350,"height":200},"classes":[]},{"id":5655,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5655","url_meta":{"origin":5927,"position":2},"title":"A modern take on pericyclic cycloaddition. Dimerisation of cis-butene","author":"Henry Rzepa","date":"November 28, 2011","format":false,"excerpt":"The \u03c02 + \u03c02 cyclodimerisation of cis-butene is the simplest cycloaddition reaction with stereochemical implications. I here give it the same treatment as I did previously for electrocyclic pericyclic reactions. The photochemical reaction is known to give a mixture of two tetramethylcyclobutanes in the ratio of 1.3:1.0, with the all-cis\u2026","rel":"","context":"In \"energy\"","block_context":{"text":"energy","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=energy"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/2%2B2-exo.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6977,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6977","url_meta":{"origin":5927,"position":3},"title":"The direct approach is not always the best: ethene + dichlorocarbene","author":"Henry Rzepa","date":"June 12, 2012","format":false,"excerpt":"The reaction between a carbene and an alkene to form a cyclopropane is about as simple a reaction as one can get. But I discussed before how simple little molecules (cyclopropenyl anion) can hold surprises. So consider this reaction: The reaction is a 4-electron pericyclic process, and so is subject\u2026","rel":"","context":"In \"limbo dancer\"","block_context":{"text":"limbo dancer","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=limbo-dancer"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/06\/dcc.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5834,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5834","url_meta":{"origin":5927,"position":4},"title":"Violations. There are none!","author":"Henry Rzepa","date":"December 11, 2011","format":false,"excerpt":"Thus famously wrote Woodward and Hoffmann (WH) in their\u00a0classic monograph about the conservation of orbital symmetry in pericyclic reactions. But they also note that the \"fantastic\" hydrocarbon (number 85 in their review) shown below presents a situation of great interest in having a half life of ~30 minutes at 353K\u2026","rel":"","context":"In \"electrocyclic reaction\"","block_context":{"text":"electrocyclic reaction","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=electrocyclic-reaction"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/85.svg","width":350,"height":200},"classes":[]},{"id":10611,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10611","url_meta":{"origin":5927,"position":5},"title":"Another Woodward pericyclic example dissected: all is not what it seems.","author":"Henry Rzepa","date":"May 22, 2013","format":false,"excerpt":"Here is another example gleaned from that Woodward essay of 1967 (Chem. Soc. Special Publications (Aromaticity), 1967, 21, 217-249), where all might not be what it seems. Woodward notes that the reaction between the (highly reactive) 1 does not occur. This is attributed to it being a disallowed \u03c06 +\u2026","rel":"","context":"In \"free energy barrier\"","block_context":{"text":"free energy barrier","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=free-energy-barrier"},"img":{"alt_text":"w2+2+2","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/05\/w2%2B2%2B2.gif?resize=350%2C200","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\/5927","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=5927"}],"version-history":[{"count":18,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5927\/revisions"}],"predecessor-version":[{"id":5946,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5927\/revisions\/5946"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5927"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5927"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5927"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=5927"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}