{"id":10611,"date":"2013-05-22T09:13:19","date_gmt":"2013-05-22T08:13:19","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=10611"},"modified":"2013-05-22T09:13:19","modified_gmt":"2013-05-22T08:13:19","slug":"another-woodward-pericyclic-example-dissected-all-is-not-what-it-seems","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10611","title":{"rendered":"Another Woodward pericyclic example dissected: all is not what it seems."},"content":{"rendered":"
\n

Here is another example gleaned from that Woodward essay of 1967 (Chem. Soc. Special Publications<\/em> (Aromaticity), 1967<\/strong>, 21<\/em>, 217-249), where all might not be what it seems.<\/p>\n

\"W\"<\/p>\n

Woodward notes that the reaction between the (highly reactive) 1<\/strong> does not occur. This is attributed to it being a disallowed \u03c06<\/sub> + \u03c02<\/sub> cycloaddition (blue<\/span> + magenta<\/span> arrows) rather than an allowed \u03c04\u00a0+ \u03c02<\/sub>\u00a0cycloaddition\u00a0(red<\/span> + magenta<\/span> arrows). So what does quantum mechanics say? Well, a disallowed reaction can be broken down into several stages, each involving fewer electrons, and this is what happens. The first of these stages becomes instead an electrocyclic ring opening (green<\/span> arrows) in which one \u03c3-bond from the cyclobutene is “borrowed” to form a bis<\/em>-allene intermediate, before being returned to the original bond in the second stage.<\/p>\n\n\n\n\n\n\n\n\n
Electrocyclic ring opening[1]<\/a><\/span><\/th>\n<\/tr>\n
\"we\"<\/td>\n<\/tr>\n
\"w-e\"<\/td>\n\"w-eG\"<\/td>\n<\/tr>\n
2+2+2 cycloaddition[2]<\/a><\/span><\/th>\n<\/tr>\n
\"w2+2+2\"<\/td>\n<\/tr>\n
\"w2+2+2\"<\/td>\n\"w2+2+2G\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The first transition state for ring opening proceeds in the appropriate Woodward-Hoffmann conrotatory mode, and has a free energy barrier of ~ 45 kcal\/mol. This is still 46.1 kcal\/mol lower than the very unfavourable second step, which involves a 2+2+2 cycloaddition. Both are formally symmetry-allowed<\/em> reactions, they just have very high barriers to reaction which accounts for the non-occurance experimentally. Of course, one interpretation of the WH rules is that any pericyclic with a high barrier could be regarded as forbidden, but in this case not on the grounds of symmetry.<\/p>\n

References<\/h2>\n
  1. H.S. Rzepa, \"Gaussian Job Archive for C8H10\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.705831<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"Gaussian Job Archive for C8H10\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.705830<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    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 + \u03c02 cycloaddition (blue + magenta […]<\/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":[],"tags":[206,2651,843,373],"ppma_author":[2661],"class_list":["post-10611","post","type-post","status-publish","format-standard","hentry","tag-free-energy-barrier","tag-historical","tag-reaction-mechanism","tag-tutorial-material"],"yoast_head":"\nAnother Woodward pericyclic example dissected: all is not what it seems. - 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=10611\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Another Woodward pericyclic example dissected: all is not what it seems. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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 + \u03c02 cycloaddition (blue + magenta […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10611\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2013-05-22T08:13:19+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/05\/W.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":"Another Woodward pericyclic example dissected: all is not what it seems. - 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=10611","og_locale":"en_GB","og_type":"article","og_title":"Another Woodward pericyclic example dissected: all is not what it seems. - Henry Rzepa's Blog","og_description":"Here is another example gleaned from that Woodward essay of 1967 (Chem. 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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":6977,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6977","url_meta":{"origin":10611,"position":1},"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":5968,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968","url_meta":{"origin":10611,"position":2},"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":[]},{"id":5087,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5087","url_meta":{"origin":10611,"position":3},"title":"cis-Butene: a reaction coordinate dissected and methyl flags.","author":"Henry Rzepa","date":"October 12, 2011","format":false,"excerpt":"In two previous posts, I have looked at why cis-butene adopts conformation (a) rather than (b). 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":9,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9","url_meta":{"origin":10611,"position":4},"title":"A Disrotatory 4n+2 electron anti-aromatic M\u00f6bius transition state for a thermal electrocyclic reaction.","author":"Henry Rzepa","date":"April 2, 2009","format":false,"excerpt":"Mauksch and Tsogoeva have recently published an article illustrating how a thermal electrocyclic reaction can proceed with distoratory ring closure, whilst simultaneously also exhibiting 4n electron M\u00f6bius-aromatic character. Why is this remarkable? Because the simple Woodward-Hoffmann rules state that a disrotatory thermal electrocyclic reaction should proceed via a H\u00fcckel-aromatic 4n+2\u2026","rel":"","context":"In "pericyclic"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"Electrocylization of [14] annulene","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/p322.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1158,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1158","url_meta":{"origin":10611,"position":5},"title":"Mechanistic M\u00e9nage \u00e0 trois","author":"Henry Rzepa","date":"November 18, 2009","format":false,"excerpt":"Curly arrow pushing is one of the essential tools of a mechanistic chemist. Many a published article will speculate about the arrow pushing in a mechanism, although it is becoming increasingly common for these speculations to be backed up by quantitative quantum mechanical and dynamical calculations. These have the potential\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":"Oxygen-nitrogen exchange between three nitrosonium cations","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/11\/no-exchange.jpg?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\/10611","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=10611"}],"version-history":[{"count":19,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10611\/revisions"}],"predecessor-version":[{"id":10640,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10611\/revisions\/10640"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10611"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10611"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10611"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=10611"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}