{"id":8426,"date":"2012-11-30T09:09:04","date_gmt":"2012-11-30T09:09:04","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=8426"},"modified":"2013-02-14T08:10:24","modified_gmt":"2013-02-14T08:10:24","slug":"a-pericyclic-dichotomy","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8426","title":{"rendered":"A pericyclic dichotomy."},"content":{"rendered":"
\n

A dichotomy is a division into two mutually exclusive, opposed, or contradictory groups. <\/em>Consider the reaction below. <\/em>The bicyclic pentadiene on the left could in principle open on heating to give the monocyclic [12]-annulene (blue or red)\u00a0via<\/em> what is called an electrocyclic reaction<\/strong><\/em> as either a six (red<\/span>) or eight (blue<\/span>) electron process. These two possibilities represent our dichotomy; according to the Woodward-Hoffmann (WH) pericyclic selection rules, they represent contradictory groups. Depending on the (relative) stereochemistry at the ring junctions, if one reaction is allowed by the WH rules, the other must be forbidden, and of course vice-versa<\/em>. It is a nice challenge to ask students to see if the dichotomy can be reconciled.<\/p>\n

\"\"<\/p>\n

I start the process by pondering the relationship between the two forms of the [12]annulene shown on the right. Are the representations shown in red or blue just resonance isomers<\/strong><\/em> (analogous to the Kekule forms of benzene), or something else? If the former, then they truly represent the same species; they are just different ways of representing the contributions to the wavefunction, and the dichotomy stands. But if they are in fact different species, then we can start to eliminate the apparent contradiction by stating that the red and the blue arrows actually represent different reactions, leading to different (albeit isomeric) products. In this scenario, the red and blue forms of the [12]-annulene are NOT resonance isomers but distinct valence bond isomers<\/strong><\/em>, with a positive energy activation barrier to their interconversion.\u00a0To find out, let us start with the transition states for both processes:<\/p>\n\n\n\n\n
C2<\/sub> symmetry<\/a><\/th>\nCs<\/sub> symmetry<\/a><\/th>\n<\/tr>\n
\n
\"\"

Transition state for blue arrows. Click for 3D.<\/p><\/div>\n<\/td>\n

\n
\"\"

Transition state for red arrows. Click for 3D.<\/p><\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    \n
  1. \u00a0The blue arrows<\/span>\u00a0(representing 4n,n=2 electrons) result in a transition state with an axis of symmetry<\/strong><\/em><\/li>\n
  2. with the bond forming\/cleaving from the bottom face of one terminus of the rhs-conjugated system to the top face of the other terminus, in other words an antarafacial<\/strong><\/em> bond,\u00a0<\/li>\n
  3. with conrotation<\/strong><\/em> of the groups at the termini, resulting in<\/li>\n
  4. all the bonds in the 8-ring being approximately 1.4\u00c5 in length (other than the central bond), whilst those in the 6-ring alternate strongly. The 8-ring is (M\u00f6bius) aromatic and the 6-ring is (M\u00f6bius)\u00a0anti-aromatic.<\/li>\n
  5. Contrary-wise, the red arrows<\/span>\u00a0(representing 4n+2,n=1 electrons) result in a transition state with a plane of symmetry<\/strong><\/em><\/li>\n
  6. with the bond forming from the same bottom face of the lhs-conjugated termini, in other words a suprafacial<\/strong> <\/em>bond,\u00a0<\/li>\n
  7. with disrotation<\/strong><\/em> of the groups at the termini, resulting in\u00a0<\/li>\n
  8. all the bonds in the 6-ring being approximately 1.4\u00c5 in length, whilst those in the 8-ring alternate strongly. The 6-ring is now (H\u00fcckel) aromatic and the 8-ring is (H\u00fcckel)\u00a0anti-aromatic.<\/li>\n
  9. The transition state with C2<\/sub> symmetry is in fact 10.1 kcal\/mol lower<\/strong> in free energy than the one with Cs<\/sub> symmetry.<\/li>\n<\/ol>\n
    So the arrows follow the aromaticity (or vice versa<\/em>), and this determines the stereochemistry (axis or plane of symmetry) and ultimately the nature of the product of each reaction. Are these annulenes indeed different? Shown below are the final outcomes of following an IRC (intrinsic reaction coordinate) from the transition state of the red<\/span><\/a><\/span> and the blue<\/a> reaction downhill to the [12]-annulenes. Not only is the outcome valence bond isomers, but they are also atropisomers<\/a>.<\/div>\n
    \n\n\n\n\n
    Product, C2 (axis)<\/th>\nProduct, Cs (plane)<\/th>\n<\/tr>\n
    \"\"<\/td>\n\"\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

    So at the end we see that there is no actual dichotomy. The reactions above (red or blue arrows) give different products, with different symmetries, and differently aromatic transition states. But in doing so, they encapsulate the selection rules for pericyclic reactions very nicely indeed.\u00a0For more details of this, see this citation [1]<\/a><\/span>.<\/p>\n

    References<\/h2>\n
    1. H.S. Rzepa, \"The Aromaticity of Pericyclic Reaction Transition States\", Journal of Chemical Education<\/i>, vol. 84, pp. 1535, 2007. https:\/\/doi.org\/10.1021\/ed084p1535<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      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 (blue) electron process. These two […]<\/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":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[],"tags":[410,2650,971,949,843,373],"ppma_author":[2661],"class_list":["post-8426","post","type-post","status-publish","format-standard","hentry","tag-mobius","tag-pericyclic","tag-pericylic","tag-positive-energy-activation-barrier","tag-reaction-mechanism","tag-tutorial-material"],"yoast_head":"\nA pericyclic dichotomy. - Henry Rzepa'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=8426\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"A pericyclic dichotomy. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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 (blue) electron process. These two […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8426\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-11-30T09:09:04+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2013-02-14T08:10:24+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/11\/dichotomy.svg\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"A pericyclic dichotomy. - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8426","og_locale":"en_GB","og_type":"article","og_title":"A pericyclic dichotomy. - Henry Rzepa's Blog","og_description":"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 (blue) electron process. 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There are none!","author":"Henry Rzepa","date":"December 11, 2011","format":false,"excerpt":"Thus famously wrote Woodward and Hoffmann (WH) in their\u00a0classic monograph about the conservation of orbital symmetry in pericyclic reactions. But they also note that the \"fantastic\" hydrocarbon (number 85 in their review) shown below presents a situation of great interest in having a half life of ~30 minutes at 353K\u2026","rel":"","context":"In \"electrocyclic reaction\"","block_context":{"text":"electrocyclic reaction","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=electrocyclic-reaction"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/85.svg","width":350,"height":200},"classes":[]},{"id":10611,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10611","url_meta":{"origin":8426,"position":1},"title":"Another Woodward pericyclic example dissected: all is not what it seems.","author":"Henry Rzepa","date":"May 22, 2013","format":false,"excerpt":"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 +\u2026","rel":"","context":"In \"free energy barrier\"","block_context":{"text":"free energy barrier","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=free-energy-barrier"},"img":{"alt_text":"w2+2+2","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/05\/w2%2B2%2B2.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5927,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5927","url_meta":{"origin":8426,"position":2},"title":"Molecular gymnastics in 2+2 cycloadditions.","author":"Henry Rzepa","date":"December 14, 2011","format":false,"excerpt":"In this earlier post, I described how the stereochemistry of \u03c02+\u03c02 cycloadditions occurs suprafacially if induced by light, and how one antarafacial component appears if the reaction is induced by heat alone. I also noted how Woodward and Hoffmann (WH) explained that violations to their rules were avoided by mandating\u2026","rel":"","context":"In "Interesting chemistry"","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\/14d1.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5968,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5968","url_meta":{"origin":8426,"position":3},"title":"Quadruple antarafacial delight.","author":"Henry Rzepa","date":"December 18, 2011","format":false,"excerpt":"A feature of many a classic review article is that not only does it organise and rationalise existing literature, but it will predict new chemistry as well. I have already noted Woodward and Hoffmann's (WH)\u00a0review as achieving the former, and here I take a (sideways) look at one of their\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/12\/286.svg","width":350,"height":200},"classes":[]},{"id":5716,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5716","url_meta":{"origin":8426,"position":4},"title":"A modern take on pericyclic sigmatropic migrations.","author":"Henry Rzepa","date":"November 29, 2011","format":false,"excerpt":"Another common type of pericyclic reaction is the migration of hydrogen or carbon along a conjugated chain, as in the [1,3] migration of a carbon as shown below. As before, I explore the stereochemistry of the thermal and photochemical reactions. The reaction is known to proceed thermally\u00a0with inversion of configuration\u2026","rel":"","context":"In \"pericyclic\"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=pericyclic"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/s.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5655,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5655","url_meta":{"origin":8426,"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","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London."}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8426","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=8426"}],"version-history":[{"count":22,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8426\/revisions"}],"predecessor-version":[{"id":9552,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8426\/revisions\/9552"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=8426"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=8426"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=8426"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=8426"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}