{"id":7740,"date":"2012-09-19T14:19:42","date_gmt":"2012-09-19T13:19:42","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=7740"},"modified":"2012-09-19T14:47:06","modified_gmt":"2012-09-19T13:47:06","slug":"the-direct-approach-is-not-always-the-best-butadiene-plus-dichlorocarbene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7740","title":{"rendered":"The direct approach is not always the best: butadiene plus dichlorocarbene"},"content":{"rendered":"
\n

The four-electron thermal cycloaddition (in reverse a cheletropic elimination) of dichlorocarbene to ethene is a classic example of a forbidden pericyclic process taking a roundabout route<\/a> to avoid directly violating the Woodward-Hoffmann rules. However, a thermal six-electron process normally does take the direct route, as in for example the Diels-Alder cycloaddition\u00a0as Houk and co have recently showed using molecular dynamics[1]<\/a><\/span>. So can one contrive a six-electron cycloaddition involving dichlorocarbene?<\/p>\n

\"\"<\/p>\n

Surely, it should now form the two new C-C bonds at the same time (synchronously)? Well, here comes a \u03c9B97XD\/6-311G(d,p)\/SCRF=dichloromethane<\/a> intrinsic reaction coordinate calculation:<\/p>\n

\"\"

Butadiene + dichlorocarbene.<\/p><\/div>\n

    \n
  1. The reaction starts at IRC -5,\u00a0<\/li>\n
  2. and proceeds with only a small barrier to the transition state (IRC =0.0)\u00a0<\/li>\n
  3. At IRC +4, the potential flattens out and the gradients drop, with formation of the first C-C bond completed. But the gradients do not quite go to zero, which would have implied the formation of a discrete intermediate such as:\"\"<\/li>\n
  4. The concerted reaction continues and by IRC ~ +11, the two chlorine atoms now exhibit quite different C-Cl lengths. The one that is orthogonal to the second forming C-C bond is normal (1.815Å), whereas the one antiperiplanar to the C-C bond is 1.92Å. There are some interesting stereoelectronic alignments involved.<\/li>\n
  5. Coincidentally perhaps, but these phenomena of an intermediate almost forming in a system containing a CCl2<\/sub> group with concomitant lengthening of one C-Cl bond compared to the other, was also observed in my IRC for the addition of thiolate to a dichlorobuteneone<\/a>. For that system, \u00a0Dan Singleton’s work had shown that molecular dynamics is necessary to obtain a more complete picture, and that may well be also true for the example here! \u00a0Perhaps Ken Houk might give it a go!<\/li>\n
  6. The second C-C bond then completes at around IRC +16.\n

    \"\"<\/p>\n

    \"\"<\/p>\n<\/li>\n<\/ol>\n

    Well, this shows that a reaction only modestly removed from the classical six-electron Diels-Alder can change character dramatically from the synchrony expected of the latter. I am hunting for a simple explanation of this phenomenon, but perhaps participation of the C-Cl bonds makes this different from a simple cycloaddition. Or possibly, the explanation will only properly emerge when the molecular dynamics is studied?<\/p>\n

    References<\/h2>\n
    1. K. Black, P. Liu, L. Xu, C. Doubleday, and K.N. Houk, \"Dynamics, transition states, and timing of bond formation in Diels\u2013Alder reactions\", Proceedings of the National Academy of Sciences<\/i>, vol. 109, pp. 12860-12865, 2012. https:\/\/doi.org\/10.1073\/pnas.1209316109<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      The four-electron thermal cycloaddition (in reverse a cheletropic elimination) of dichlorocarbene to ethene is a classic example of a forbidden pericyclic process taking a roundabout route to avoid directly violating the Woodward-Hoffmann rules. However, a thermal six-electron process normally does take the direct route, as in for example the Diels-Alder cycloaddition\u00a0as Houk and co have […]<\/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":[912,915,2650,843],"ppma_author":[2661],"class_list":["post-7740","post","type-post","status-publish","format-standard","hentry","tag-asynchronous","tag-houk-and-co","tag-pericyclic","tag-reaction-mechanism"],"yoast_head":"\nThe direct approach is not always the best: butadiene plus dichlorocarbene - 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=7740\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The direct approach is not always the best: butadiene plus dichlorocarbene - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The four-electron thermal cycloaddition (in reverse a cheletropic elimination) of dichlorocarbene to ethene is a classic example of a forbidden pericyclic process taking a roundabout route to avoid directly violating the Woodward-Hoffmann rules. However, a thermal six-electron process normally does take the direct route, as in for example the Diels-Alder cycloaddition\u00a0as Houk and co have […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7740\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-09-19T13:19:42+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2012-09-19T13:47:06+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/09\/carbene-b.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":"The direct approach is not always the best: butadiene plus dichlorocarbene - 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=7740","og_locale":"en_GB","og_type":"article","og_title":"The direct approach is not always the best: butadiene plus dichlorocarbene - Henry Rzepa's Blog","og_description":"The four-electron thermal cycloaddition (in reverse a cheletropic elimination) of dichlorocarbene to ethene is a classic example of a forbidden pericyclic process taking a roundabout route to avoid directly violating the Woodward-Hoffmann rules. 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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":9,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9","url_meta":{"origin":7740,"position":1},"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":11642,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11642","url_meta":{"origin":7740,"position":2},"title":"Avoided (pericyclic) anti-aromaticity: Reactions of t-butyl-hydroxycarbene.","author":"Henry Rzepa","date":"November 13, 2013","format":false,"excerpt":"Not long ago, I described a cyclic carbene in which elevating the carbene lone pair into a \u03c0-system transformed it from a formally 4n-antiaromatic \u03c0-cycle into a 4n+2 aromatic \u03c0-cycle. From an entirely different area of chemistry, another example of this behaviour emerges; Schreiner's trapping and reactions of t-butyl-hydroxycarbene, as\u2026","rel":"","context":"In "pericyclic"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"H-mig","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/11\/H-mig.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5927,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927","url_meta":{"origin":7740,"position":3},"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":22774,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22774","url_meta":{"origin":7740,"position":4},"title":"Trimerous pericyclic reactions.","author":"Henry Rzepa","date":"October 8, 2020","format":false,"excerpt":"I occasionally spot an old blog that emerges, if only briefly, as \"trending\". In this instance, only the second blog I ever wrote here, way back in 2009 as a follow up to this article. With something of that age, its always worth revisiting to see if any aspect needs\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":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/10\/10-1024x671.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":25633,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25633","url_meta":{"origin":7740,"position":5},"title":"A new type of bispericyclic reaction: Cyclopropanone + butadiene.","author":"Henry Rzepa","date":"September 30, 2022","format":false,"excerpt":"The term bispericyclic reaction was famously coined by Caramella et al in 2002 to describe the unusual features of the apparently innocuous dimerisation of cyclopentadiene. It shows features of two paths for different pericyclic reactions, comprising a 2+4 cycloaddition in the early stages, but evolving into a (degenerate) pair of\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/09\/anomalous.gif?resize=350%2C200&ssl=1","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\/7740","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=7740"}],"version-history":[{"count":10,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7740\/revisions"}],"predecessor-version":[{"id":7755,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7740\/revisions\/7755"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7740"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7740"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7740"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=7740"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}