{"id":12825,"date":"2014-08-10T12:05:58","date_gmt":"2014-08-10T11:05:58","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=12825"},"modified":"2014-08-10T12:05:58","modified_gmt":"2014-08-10T11:05:58","slug":"using-a-polar-bond-to-flip-on-the-knife-edge","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2014\/08\/10\/using-a-polar-bond-to-flip-on-the-knife-edge\/","title":{"rendered":"Using a polar bond to flip: on the knife-edge!"},"content":{"rendered":"

In my\u00a0first post<\/a> on the topic, I discussed how inverting the polarity of the C-X bond from X=O to X=Be (scheme below) could flip the stereochemical course of the electrocyclic pericyclic reaction of a divinyl system. This was followed up<\/a> by exploring what\u00a0happens at the half way stage, i.e.<\/em> X=CH2<\/sub>, the answer being that\u00a0one gets an antarafacial<\/em><\/strong> pathway as with X=O. Here I fill in another gap, X=BH to see if a metaphorical microscope can be used to view the actual region of the “flip” to a suprafacial<\/em><\/strong> mode.\"divinylketon\"<\/a> This time, uniquely, it proved possible to locate TWO transition states for this process, one suprafacial<\/em>[cite]10.6084\/m9.figshare.1133933[\/cite] and one antarafacial<\/em>[cite]10.6084\/m9.figshare.1133934[\/cite], this latter being 10.5 kcal\/mol lower in \u0394G\u2020<\/sup> (\u03c9B97XD\/6-311G(d,p)\/SCRF=dichloromethane). It is quite rare to be able to find BOTH stereochemical outcomes of a thermal pericyclic reaction.\u2021<\/sup><\/p>\n

First, the antarafacial IRC (X=BH)[cite]10.6084\/m9.figshare.1133936[\/cite]. There are several interesting features. Note at IRC = -8, the divinyl compound appears as a Hidden Intermediate<\/strong> <\/em>(HI), having formed from a compound where the HB=C substituent has ring opened from a cyclobutene-like precursor (initial electrocyclic). If you watch the animation, you can see the antarafacial bond\u00a0forming from the bottom<\/strong> face of the vinyl group on the left to the top<\/strong> face of the vinyl group in the HI on the right (antarafacial=conrotation). Because the entire process is concerted (no real intermediates participate), we have here an unusual pericyclic cascade<\/em> where one electrocyclic reaction is immediately followed by another quite different one.\"BH-antaraa\"<\/a> \"BH-antara\"<\/a> \"BH-antaraG\"<\/a> Now for the suprafacial IRC[cite]10.6084\/m9.figshare.1134015[\/cite]. It is pretty similar to the previous path, but again if you inspect very carefully you will see that it is the TOP<\/strong> face of the vinyl group on the left forming the bond to the TOP<\/strong> face of the vinyl group on the right (suprafacial\/disrotation). \"BH-supraa\"<\/a>\"BH-supra\"<\/a>\"BH-supraG\"<\/a> You might ask if the molecules used here are realistic, i.e.<\/em> could they form the basis of real reactions to be conducted in a laboratory? Well, the C=B-C fragment has 9 hits in the CCDC crystal database (none for C=B-H). One example is cited here.[cite]10.1002\/anie.199513401[\/cite]. So, yes, possibly a realistic system, except the barriers do look too high. Perhaps suitable substituents might help? But even if this could not be carried out in a test-tube,\u00a0it does teach one about pericyclic reactions and how one might manipulate them.<\/p>\n

\n

\u2021<\/sup>One such is the [1,6] sigmatropic shift in homotropylium cation involving migration of a Me2<\/sub>C+<\/sup> group, where the “allowed” process in which the migrating group retains its configuration has a barrier of 17.7 kcal\/mol and the “forbidden” route where the migrating group inverts its configuration with a barrier of 38.6 kcal\/mol (thanks to Alex Genaev; I will strive to make the coordinates available via<\/em> the repository shortly).<\/p>\n","protected":false},"excerpt":{"rendered":"

In my\u00a0first post on the topic, I discussed how inverting the polarity of the C-X bond from X=O to X=Be (scheme below) could flip the stereochemical course of the electrocyclic pericyclic reaction of a divinyl system. This was followed up by exploring what\u00a0happens at the half way stage, i.e. X=CH2, the answer being that\u00a0one gets […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[12,13],"tags":[117,1143,1145],"class_list":["post-12825","post","type-post","status-publish","format-standard","hentry","category-pericyclic","category-reaction-mechanism-2","tag-alex-genaev","tag-hawaii","tag-hb"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/12825","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=12825"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/12825\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=12825"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=12825"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=12825"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}