{"id":5632,"date":"2011-11-26T11:26:43","date_gmt":"2011-11-26T11:26:43","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=5632"},"modified":"2011-11-28T15:21:04","modified_gmt":"2011-11-28T15:21:04","slug":"a-modern-take-on-the-pericyclic-ring-opening-of-cyclobutene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5632","title":{"rendered":"A modern take on the pericyclic electrocyclic ring opening of cyclobutene."},"content":{"rendered":"
\n

Woodward and Hoffmann published their\u00a0milestone article<\/a>\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<\/a> some 75 years earlier. Here, I take a modern look at the archetypal carrier of this insight, the ring opening of dimethylcyclobutene.<\/p>\n

\"\"<\/a><\/p>\n

The thermal (\u0394) reaction is defined by the transition state<\/strong><\/em>. The remarkable feature noted by Woodward and Hoffmann was the stereospecificity of pericyclic reactions. In this example, the breaking \u03c3-bond<\/strong>\u00a0in the transition state is defined by its connectivity to the top face of one terminus (the red arrow) and the bottom face of the other terminus (the green arrow). The technical name for this is antarafacial<\/em><\/strong>, and this is also associated with a C2<\/sub> axis of symmetry for (this particular) example. The modern theoretical explanation for this is a M\u00f6bius-aromatic<\/a>\u00a0transition state\u00a0resulting from a total of 4n<\/strong> circulating electrons (note the methyl flags<\/a> waving).<\/p>\n\n\n\n\n
\"\"<\/a><\/td>\n\"\"<\/a><\/td>\n<\/tr>\n
\"\"

IRC for Electrocylic ring opening of dimethyl cyclobutene. Click for 3D.<\/p><\/div><\/td>\n

\"\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The concept of aromatic (or anti-aromatic) transition states is a very useful one for thermal reactions, but this transition state becomes a little less helpful for the photochemical (h\u03bd) version. Instead, a new concept is introduced of a conical intersection<\/a> between the (thermal) ground state and the (photochemical) excited state. Think of it in terms of the famous painting showing God (in an exalted state) touching Adam (very much on the ground).<\/p>\n

\"\"<\/a>

A schematic conical intersection.<\/p><\/div>\n

The conical intersection is the geometry at which a photochemically excited molecule leaves the S1<\/sub> state and returns to the ground S0<\/sub> state. It is this point that determines the resulting stereochemistry. That for dimethyl-cyclobutene (casscf(12,8)\/6-31g(d,p) model<\/a>) is shown below. On the right hand side of the molecule, the\u00a0\u03c3-bond region looks very similar to that of the thermal transition state shown above; the bond is associated with the bottom<\/strong> face of the molecule. However, the left hand side is rotated clockwise relative to the thermal reaction, and this rotation now presents the bottom face for connection to the \u03c3-bond (rather than the top face as for the thermal case). The\u00a0\u00a0\u03c3-bond is thus connected suprafacially<\/strong><\/em>.<\/p>\n

\"\"

The conical intersection for the photochemical reaction of dimethylcyclobutene. Click for 3D<\/p><\/div>When the rules are presented to students, the photochemical case is often defined as the inverse of the thermal rule. Thus for the same electron count (4n in this case), a thermal reaction is antarafacial<\/strong><\/em> and the photochemical one suprafacial<\/strong><\/em>. A more fascinating question is whether the aromaticity associated with the key geometry should also be inverted. Thus the thermal reaction corresponds to a M\u00f6bius-aromatic<\/em><\/strong> (topological linking number<\/a> =1\u03c0) transition state. Should the photochemical reaction rule be inverted to refer to a H\u00fcckel-aromatic<\/strong><\/em>(topological linking number = 0\u03c0) conical intersection?<\/p>\n

I am unaware of any formal studies of the aromaticity of conical intersections<\/em> specifically, but it would be nice to know if this analogy has any reality. Watch this space.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

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 dimethylcyclobutene. The thermal (\u0394) reaction […]<\/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":[],"tags":[773,772,770,774,2651,2650,373],"ppma_author":[2661],"class_list":["post-5632","post","type-post","status-publish","format-standard","hentry","tag-adam","tag-archetypal-carrier","tag-conical-intersections","tag-electrocyclic","tag-historical","tag-pericyclic","tag-tutorial-material"],"yoast_head":"\nA modern take on the pericyclic electrocyclic ring opening of cyclobutene. - 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=5632\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"A modern take on the pericyclic electrocyclic ring opening of cyclobutene. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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 dimethylcyclobutene. The thermal (\u0394) reaction […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5632\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2011-11-26T11:26:43+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2011-11-28T15:21:04+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/electrocyclic.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 modern take on the pericyclic electrocyclic ring opening of cyclobutene. - 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=5632","og_locale":"en_GB","og_type":"article","og_title":"A modern take on the pericyclic electrocyclic ring opening of cyclobutene. - Henry Rzepa's Blog","og_description":"Woodward and Hoffmann published their\u00a0milestone article\u00a0 “Stereochemistry of Electrocyclic Reactions” in 1965. 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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 "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\/ht.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10611,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10611","url_meta":{"origin":5632,"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":9,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9","url_meta":{"origin":5632,"position":2},"title":"A Disrotatory 4n+2 electron anti-aromatic M\u00f6bius transition state for a thermal electrocyclic reaction.","author":"Henry Rzepa","date":"April 2, 2009","format":false,"excerpt":"Mauksch and Tsogoeva have recently published an article illustrating how a thermal electrocyclic reaction can proceed with distoratory ring closure, whilst simultaneously also exhibiting 4n electron M\u00f6bius-aromatic character. Why is this remarkable? Because the simple Woodward-Hoffmann rules state that a disrotatory thermal electrocyclic reaction should proceed via a H\u00fcckel-aromatic 4n+2\u2026","rel":"","context":"In "pericyclic"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"Electrocylization of [14] annulene","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/p322.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":22774,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22774","url_meta":{"origin":5632,"position":3},"title":"Trimerous pericyclic reactions.","author":"Henry Rzepa","date":"October 8, 2020","format":false,"excerpt":"I occasionally spot an old blog that emerges, if only briefly, as \"trending\". 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 "Curly arrows"","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":11856,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11856","url_meta":{"origin":5632,"position":4},"title":"Three-for-one: a pericyclic brain teaser.","author":"Henry Rzepa","date":"January 12, 2014","format":false,"excerpt":"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\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":"p34c","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/01\/2%2B2-2nd.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":31375,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31375","url_meta":{"origin":5632,"position":5},"title":"A breakthrough in Molecular Solar Thermal (MOST) energy storage – Dewar Pyrimidone.","author":"Henry Rzepa","date":"May 14, 2026","format":false,"excerpt":"MOST is a chemical method of converting photonic or light energy into storable thermal energy which can be released on demand. A recent breakthrough in such methods has been reported in which a pyrimidone molecule is efficiently converted by 310nm light into the isomeric Dewar pyrimidone. This molecule is thermally\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":"","width":0,"height":0},"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\/5632","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=5632"}],"version-history":[{"count":17,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5632\/revisions"}],"predecessor-version":[{"id":5713,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5632\/revisions\/5713"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5632"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5632"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5632"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=5632"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}