{"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:\/\/rzepa.net\/blog\/2012\/09\/19\/the-direct-approach-is-not-always-the-best-butadiene-plus-dichlorocarbene\/","title":{"rendered":"The direct approach is not always the best: butadiene plus dichlorocarbene"},"content":{"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<\/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[cite]10.1073\/pnas.1209316109[\/cite]. 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.<\/figcaption><\/figure>\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","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":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[229,1199,1884,2076],"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"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/7740","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/comments?post=7740"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/7740\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=7740"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=7740"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=7740"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}