{"id":5655,"date":"2011-11-28T15:18:23","date_gmt":"2011-11-28T15:18:23","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=5655"},"modified":"2011-11-28T15:35:53","modified_gmt":"2011-11-28T15:35:53","slug":"a-modern-take-on-pericyclic-cycloaddition-dimerisation-of-cis-butene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5655","title":{"rendered":"A modern take on pericyclic cycloaddition. Dimerisation of cis-butene"},"content":{"rendered":"
\n

The \u03c02<\/sub> + \u03c02<\/sub> cyclodimerisation of cis<\/em>-butene<\/a> is the simplest cycloaddition reaction with stereochemical implications. I here give it the same treatment as I did previously for electrocyclic pericyclic reactions<\/a>.<\/p>\n

\"\"<\/a>
\nThe photochemical reaction is known<\/a> to give a mixture of two tetramethylcyclobutanes in the ratio of 1.3:1.0, with the all-cis<\/em> isomer apparently predominating. The key geometry is the conical intersection<\/strong><\/em>, at which the energies of the S1<\/sub> and S0<\/sub> states coincide. This geometry has a typical trapezoidal appearance, with suprafacial<\/em><\/strong> addition accross both components. The exo<\/em> addition<\/a>\u00a0is calculated to be about 1 kcal\/mol lower in total energy than the endo<\/a>\u00a0(at the CASSCF(12,8)\/6-31G(d) level), which implies that the latter should be the minor and not the major form. However, these CASSCF energies are not corrected for thermal (entropic) or dynamic correlation components, and moreover the active space orbitals are probably not identical either (I demonstrated in another post <\/a>how the orbitals of the alkene interact with those of the methyl groups, and its quite likely that the endo<\/em> and exo<\/em> orientations will result in slightly different interactions) which makes such comparisons non trivial. These calculations do support the idea that both isomers should form (which at first sight might be counter-intuitive given the apparent steric constraints of the endo<\/em> isomer).<\/p>\n\n\n\n
\"\"

2+2 exo addition. click for 3D<\/p><\/div><\/td>\n

\"\"

2+2 endo addition. Click for 3D.<\/p><\/div><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The thermally activated reaction is not known for this alkene, and the calculations support this with an enormous barrier to reaction (> 68 kcal\/mol).<\/p>\n

\"\"<\/a><\/p>\n

As a 4n-electron pericyclic, the selections rules require there to be one antarafacial<\/em><\/strong> component present, and the IRC for this reaction<\/a> illustrates this very nicely. The formation of two pairs of C-C bonds is very asynchronous. Only when the first bond is almost complete does the second\u00a0C=C start to rotate. The second C-C bond only starts to form after this rotation (the antarafacial<\/em> component) is essentially complete, forming a product where one methyl group is on the opposite face of the ring to the other three. Note in particular that the rhs alkene has the two C-H hydrogens synplanar<\/em>\u00a0to start with, but that they are exactly antiperiplanar<\/em> in the product.<\/p>\n

\"\"

2a + 2s cycloaddition showing IRC. Click for 3D<\/p><\/div>For this small system the two critical points, a conical intersection and a transition state, could not be more different. But they do capture the essential features of pericyclic reactions and their selection rules.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

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 isomer apparently predominating. The key […]<\/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":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[],"tags":[24,775,2650,373],"ppma_author":[2661],"class_list":["post-5655","post","type-post","status-publish","format-standard","hentry","tag-energy","tag-methyl","tag-pericyclic","tag-tutorial-material"],"yoast_head":"\nA modern take on pericyclic cycloaddition. Dimerisation of cis-butene - 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=5655\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"A modern take on pericyclic cycloaddition. Dimerisation of cis-butene - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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 isomer apparently predominating. The key […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5655\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2011-11-28T15:18:23+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2011-11-28T15:35:53+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/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=\"2 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"A modern take on pericyclic cycloaddition. 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I suggested it boiled down to electronic interactions between the methyl groups and the central alkene resulting in the formation of a H...H \"topological\" bond, rather than attraction between the H...H region to form\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/10\/cis-butene2.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5927,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927","url_meta":{"origin":5655,"position":1},"title":"Molecular gymnastics in 2+2 cycloadditions.","author":"Henry Rzepa","date":"December 14, 2011","format":false,"excerpt":"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\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\/2011\/12\/14d1.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":4592,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4592","url_meta":{"origin":5655,"position":2},"title":"The stereochemistry of [8+2] pericyclic cycloadditions.","author":"Henry Rzepa","date":"July 10, 2011","format":false,"excerpt":"Steve Bachrach has blogged on the reaction shown below. 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 "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\/2011\/07\/2%2B8.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10611,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10611","url_meta":{"origin":5655,"position":3},"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":[]},{"id":22881,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22881","url_meta":{"origin":5655,"position":4},"title":"Trimerous pericyclic reactions: what is the effect of changing the electron count by two?","author":"Henry Rzepa","date":"November 2, 2020","format":false,"excerpt":"In an earlier post, I pondered on how the \"arrow pushing\" for the thermal pericyclic reactions of some annulenes (cyclic conjugated hydrocarbons) could be represented in terms of either two separate electrocyclic reactions or of one cycloaddition reaction. Each reaction is governed by selection rules which can be stated in\u2026","rel":"","context":"In "pericyclic"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/11\/C2-1024x620.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":5968,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968","url_meta":{"origin":5655,"position":5},"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 "Interesting chemistry"","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":[]}],"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","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London."}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5655","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=5655"}],"version-history":[{"count":36,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5655\/revisions"}],"predecessor-version":[{"id":5714,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5655\/revisions\/5714"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5655"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5655"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5655"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=5655"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}