{"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":"2012-09-19T14:19:42","modified_gmt":"2012-09-19T13:19:42","slug":"the-direct-approach-is-not-always-the-best-butadiene-plus-dichlorocarbene","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/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_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":[1],"tags":[911,914,1529,1530],"class_list":["post-7740","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-asynchronous","tag-houk-and-co","tag-pericyclic","tag-reaction-mechanism"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-20Q","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7740","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=7740"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7740\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7740"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7740"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7740"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}