{"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.rzepa.net\/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_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":[165,1528,1530,1527],"class_list":["post-10611","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-free-energy-barrier","tag-historical","tag-reaction-mechanism","tag-tutorial-material"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-2L9","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10611","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=10611"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10611\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10611"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10611"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10611"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}