{"id":11856,"date":"2014-01-12T20:51:43","date_gmt":"2014-01-12T20:51:43","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=11856"},"modified":"2014-01-17T20:44:11","modified_gmt":"2014-01-17T20:44:11","slug":"three-for-one-a-pericyclic-brain-teaser","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11856","title":{"rendered":"Three-for-one: a pericyclic brain teaser."},"content":{"rendered":"<div class=\"kcite-section\" kcite-section-id=\"11856\">\n<p>A game one can play with pericyclic reactions is to ask students to identify what type a given example is. So take for example the reaction below.<\/p>\n<p><a href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/p34c.svg\"><img decoding=\"async\" class=\"aligncenter size-full wp-image-11857\" alt=\"p34c\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/p34c.svg\" width=\"400\" \/><\/a><\/p>\n<p>The alternatives are:<\/p>\n<ol>\n<li>A cyclo-elimination reaction (red arrows).<\/li>\n<li>Two concurrent electrocyclic ring openings (blue and magenta arrows)<\/li>\n<li>Two consecutive electrocyclic ring openings<\/li>\n<li>Or could it be a hybrid with characteristics of <strong>both<\/strong> the first two?<\/li>\n<\/ol>\n<p>All the first three are four electron thermal processes; all should occur with involvement of one<em><strong> antarafacial mode<\/strong><\/em> (a\u00a0<em><strong>M\u00f6bius<\/strong><\/em> transition state). But where are those antarafacial modes? Or do all or any of these pericyclic reactions not follow the standard rules? And the rules have nothing to say about whether two separate processes can be concurrent or must be consecutive.<\/p>\n<p>The solution is to concede the limitations of simple electron counting rules, and evaluate the reaction using a quantum mechanical method (\u03c9B97XD\/6-311G(d,p))<\/p>\n<ol>\n<li>The first attempt is to locate a stationary point with symmetry, C<sub>2<\/sub> in this case. Here it is.<span id=\"cite_ITEM-11856-0\" name=\"citation\"><a href=\"#ITEM-11856-0\">[1]<\/a><\/span><img decoding=\"async\" class=\"aligncenter size-full wp-image-11857\" alt=\"p34c\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/2+2.gif\" \/><br \/><img decoding=\"async\" class=\"aligncenter size-full wp-image-11860\" alt=\"2+2E\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/2+2E.svg\" width=\"400\" \/>\n<ul>\n<li>Points of interest include the formation of the cyclo-octatetraene in the valence bond form B rather than A. This points to it being a cyclo-elimination rather than an electrocyclic reaction. For this product, little change occurs for the terminal alkenes (blue or magenta in the reactant), and indeed the C-C length at the stationary point is 1.349\u00c5, only slightly longer than a normal alkene, and not at all the value of ~1.40\u00c5 expected for an aromatic transition state expected of an electrocyclic.<\/li>\n<li>The stereochemistry of this elimination is entirely <em>suprafacial<\/em>, as evidenced by the formation of cis-alkenes. The formal pericyclic rule that disallows such stereochemistry for a 4n-electron thermal reaction is however over-turned by the transition state adopting a <a title=\"Molecular gymnastics in 2+2 cycloadditions.\" href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=5927\" target=\"_blank\">trapezoidal character<\/a>.<\/li>\n<li>The initial sense of rotation of the two pairs of hydrogens show above is however conrotatory (a consequence of the trapezoidal motions), as in fact required of a thermal 4n-electron electrocyclic reaction. It is only well after the transition state is passed that the motion of one pair of these hydrogens reverses itself, and we end up with a cis-alkene after all.<\/li>\n<li>So at the transition state, we see features of both a cycloelimination (trapezoidal geometry) but ALSO of two concurrent electrocyclic ring openings (conrotation). In other words, one gets the characteristics of<strong> three<\/strong> pericyclic reactions in one! Very much a <a href=\"http:\/\/en.wikipedia.org\/wiki\/Chimera_(mythology)\" target=\"_blank\">chimera<\/a>!<\/li>\n<li>There is only one fly in the ointment. This stationary point is in fact a second-order saddle-point and not a transition state.<span id=\"cite_ITEM-11856-1\" name=\"citation\"><a href=\"#ITEM-11856-1\">[2]<\/a><\/span> There are two imaginary frequencies, and the smaller of the two corresponds to desymmetrising the two C-C breaking central bonds<\/li>\n<\/ul>\n<\/li>\n<li>So we turn to the proper transition states in this reaction, the first of which corresponds to the initial of two consecutive electrocyclic ring openings.<span id=\"cite_ITEM-11856-2\" name=\"citation\"><a href=\"#ITEM-11856-2\">[3]<\/a><\/span> <img decoding=\"async\" class=\"aligncenter size-full wp-image-11857\" alt=\"p34c\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/2+2-step1.gif\" \/>\n<ul>\n<li>This looks very different from the preceding pathway. As the (blue) arrows take effect, an antarafacial mode takes hold, ending in the formation of an intermediate bicyclic system with a <em>trans-alkene<\/em> forming. This proper transition state is 8.1 kcal\/mol lower in free energy than the earlier second-order point. The C=C bond length at the transition state becomes 1.392\u00c5, now a typical aromatic value.<\/li>\n<li>This transition state leads to an intermediate which is 12.7 lower than the preceding transition state, and is then followed by &#8230;<a href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/p34d.svg\"><img decoding=\"async\" class=\"aligncenter size-full wp-image-11864\" alt=\"p34d\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/p34d.svg\" width=\"250\" \/><\/a><\/li>\n<li>a second transition state, with an energy 2.5 kcal\/mol higher than the first (but still 5.6 kcal\/mol lower than the second-order saddle).<sup>\u2021<\/sup> The IRC for this step (below) in effect opens up the second ring in a follow-up electrocyclic. The C=C bond in the second cyclobutene now becomes 1.338\u00c5, which is <b>not<\/b> characteristic of a cyclobutene ring-opening. Notice how again the initial motion of the two hydrogens of the second ring tentatively try a conrotatory motion as before, but this antarafacial motion in fact is soon taken over by rotation of the <em>trans<\/em>-alkene formed in the second step! This reverses the first antarafacial mode, and the net result is that none of these modes survive into the final product\u00a0cyclo-octatetraene which is now in the valence bond form <strong>A<\/strong> rather than B.<span id=\"cite_ITEM-11856-3\" name=\"citation\"><a href=\"#ITEM-11856-3\">[4]<\/a><\/span><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter  wp-image-11857\" alt=\"p34c\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/2+2-2nd.gif\" width=\"384\" height=\"374\" \/><img decoding=\"async\" class=\"aligncenter size-full wp-image-11873\" alt=\"2+2-step2E\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/2+2-step2E.svg\" width=\"300\" \/><br \/><img decoding=\"async\" class=\"aligncenter size-full wp-image-11871\" alt=\"2+2-step2G\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/2+2-step2G.svg\" width=\"300\" \/><\/li>\n<li>This second reaction is clearly part of a pericyclic sequence, but quite unlike any I have ever come across previously. In particular, the morphing of the antarafacial mode away from the cyclobutene ring and onto the <i>trans<\/i>-alkene is indeed quite a novel feature!<\/li>\n<li>The IRC (above) shows the clear presence of a <em><strong>hidden intermediate<\/strong><\/em> (IRC = 2.3), which corresponds to the following (C<sub>2<\/sub>-symmetric) species (which might have biradical or zwitterionic character). The central bond threatens to form, but ultimately does not!<img decoding=\"async\" class=\"aligncenter size-full wp-image-11870\" alt=\"2+2-step2G\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/2+2-step2G.jpg\" width=\"300\" \/><\/li>\n<\/ul>\n<\/li>\n<\/ol>\n<p>So to answer the question posed at the start. Quantum mechanics (but not simply electron counting) suggests the reaction comprises two consecutive electrocyclic ring openings. But the second of these has most unusual features, which perhaps could not have been anticipated. It is not really an electrocyclic, so one could reasonably argue that the answer to the first question posed is in fact <span style=\"background-color: #ffffff; color: #ff0000;\"><strong><i>none of the above<\/i>\u00a0<\/strong><span style=\"color: #000000;\">(and I might add that biradical mechanisms have not been considered either).<\/span><\/span><\/p>\n<p>Perhaps indeed we should start contemplating that the era of simplistic arrow-pushing is coming to an end, and instead we should more routinely start replacing it with quantum mechanical computations. Just a thought!\u00a0<\/p>\n<hr \/>\n<p><sup>\u2021<\/sup>It is perfectly possible that substituents could alter the balance between the cyclo-elimination mode and the two-fold electrocyclic, and resulting in the former being in fact the preferred mode. For example, replacing H by thioformyl (HC=S)\u00a0flips the mechanism to the 2+2 elimination (it removes that second imaginary frequency).<span id=\"cite_ITEM-11856-4\" name=\"citation\"><a href=\"#ITEM-11856-4\">[5]<\/a><\/span><a href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/2+2S.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-11885\" alt=\"2+2S\" src=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/2+2S.gif\" width=\"436\" height=\"373\" \/><\/a><\/p>\n<h2>References<\/h2>\n    <ol class=\"kcite-bibliography csl-bib-body\"><li id=\"ITEM-11856-0\">H.S. Rzepa, \"Gaussian Job Archive for C8H8\", 2014. <a href=\"https:\/\/doi.org\/10.6084\/m9.figshare.899759\">https:\/\/doi.org\/10.6084\/m9.figshare.899759<\/a>\n\n<\/li>\n<li id=\"ITEM-11856-1\">H.S. Rzepa, \"Gaussian Job Archive for C8H8\", 2014. <a href=\"https:\/\/doi.org\/10.6084\/m9.figshare.899760\">https:\/\/doi.org\/10.6084\/m9.figshare.899760<\/a>\n\n<\/li>\n<li id=\"ITEM-11856-2\">H.S. Rzepa, \"Gaussian Job Archive for C8H8\", 2014. <a href=\"https:\/\/doi.org\/10.6084\/m9.figshare.899763\">https:\/\/doi.org\/10.6084\/m9.figshare.899763<\/a>\n\n<\/li>\n<li id=\"ITEM-11856-3\">H.S. Rzepa, \"Gaussian Job Archive for C8H8\", 2014. <a href=\"https:\/\/doi.org\/10.6084\/m9.figshare.899762\">https:\/\/doi.org\/10.6084\/m9.figshare.899762<\/a>\n\n<\/li>\n<li id=\"ITEM-11856-4\">H.S. Rzepa, \"Gaussian Job Archive for C10H8S2\", 2014. <a href=\"https:\/\/doi.org\/10.6084\/m9.figshare.899809\">https:\/\/doi.org\/10.6084\/m9.figshare.899809<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> <!-- kcite-section 11856 -->","protected":false},"excerpt":{"rendered":"<p>A game one can play with pericyclic reactions is to ask students to identify what type a given example is. So take for example the reaction below. The alternatives are: A cyclo-elimination reaction (red arrows). Two concurrent electrocyclic ring openings (blue and magenta arrows) Two consecutive electrocyclic ring openings Or could it be a hybrid [&hellip;]<\/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":[4,559,1086],"tags":[],"ppma_author":[2661],"class_list":["post-11856","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","category-pericyclic","category-reaction-mechanism-2"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Three-for-one: a pericyclic brain teaser. - Henry Rzepa&#039;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=11856\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Three-for-one: a pericyclic brain teaser. - Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"og:description\" content=\"A game one can play with pericyclic reactions is to ask students to identify what type a given example is. 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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 &quot;Curly arrows&quot;","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":5763,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5763","url_meta":{"origin":11856,"position":1},"title":"So near and yet so far. The story of the electrocyclic ring opening of a cyclohexadiene.","author":"Henry Rzepa","date":"December 6, 2011","format":false,"excerpt":"My previous three posts set out my take on three principle categories of pericyclic reaction. Here I tell a prequel to the understanding of these reactions. In 1965, Woodward and Hoffmann in their theoretical analysis (submitted Nov 30, 1964) for which the Nobel prize (to Hoffmann only of the pair,\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","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\/ht.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":12825,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12825","url_meta":{"origin":11856,"position":2},"title":"Using a polar bond to flip: on the knife-edge!","author":"Henry Rzepa","date":"August 10, 2014","format":false,"excerpt":"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,\u2026","rel":"","context":"In &quot;pericyclic&quot;","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":5632,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5632","url_meta":{"origin":11856,"position":3},"title":"A modern take on the pericyclic electrocyclic ring opening of cyclobutene.","author":"Henry Rzepa","date":"November 26, 2011","format":false,"excerpt":"Woodward and Hoffmann published their\u00a0milestone article\u00a0 \"Stereochemistry of Electrocyclic Reactions\" in 1965. This brought maturity to the electronic theory of organic chemistry, arguably started by the proto-theory of Armstrong some 75 years earlier. Here, I take a modern look at the archetypal carrier of this insight, the ring opening of\u2026","rel":"","context":"In \"Adam\"","block_context":{"text":"Adam","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=adam"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/con-open.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8426,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8426","url_meta":{"origin":11856,"position":4},"title":"A pericyclic dichotomy.","author":"Henry Rzepa","date":"November 30, 2012","format":false,"excerpt":"A dichotomy is a division into two mutually exclusive, opposed, or contradictory groups. Consider the reaction below. The bicyclic pentadiene on the left could in principle open on heating to give the monocyclic [12]-annulene (blue or red)\u00a0via what is called an electrocyclic reaction as either a six (red) or eight\u2026","rel":"","context":"In \"M\u00f6bius\"","block_context":{"text":"M\u00f6bius","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=mobius"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":5655,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5655","url_meta":{"origin":11856,"position":5},"title":"A modern take on pericyclic cycloaddition. Dimerisation of cis-butene","author":"Henry Rzepa","date":"November 28, 2011","format":false,"excerpt":"The \u03c02 + \u03c02 cyclodimerisation of cis-butene is the simplest cycloaddition reaction with stereochemical implications. I here give it the same treatment as I did previously for electrocyclic pericyclic reactions. The photochemical reaction is known to give a mixture of two tetramethylcyclobutanes in the ratio of 1.3:1.0, with the all-cis\u2026","rel":"","context":"In \"energy\"","block_context":{"text":"energy","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=energy"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/2%2B2-exo.jpg?resize=350%2C200","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\/11856","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=11856"}],"version-history":[{"count":16,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11856\/revisions"}],"predecessor-version":[{"id":11950,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11856\/revisions\/11950"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=11856"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=11856"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=11856"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=11856"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}