{"id":10518,"date":"2013-05-20T10:11:31","date_gmt":"2013-05-20T09:11:31","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=10518"},"modified":"2020-07-08T11:19:39","modified_gmt":"2020-07-08T10:19:39","slug":"woodwards-symmetry-considerations-applied-to-electrocyclic-reactions","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10518","title":{"rendered":"Woodward’s symmetry considerations applied to electrocyclic reactions."},"content":{"rendered":"
\n

Sometimes the originators of seminal theories in chemistry write a personal and anecdotal account of their work. Niels Bohr[1]<\/a><\/span> was one such<\/a> and four decades later Robert Woodward wrote “The conservation of orbital symmetry<\/em>” (Chem. Soc. Special Publications (Aromaticity), 1967<\/strong>, 21<\/em>, 217-249; it is not online and so no doi can be given). Much interesting chemistry is described there, but (like Bohr in his article), Woodward lists no citations at the end, merely giving attributions by name. Thus the following chemistry (p 236 of this article) is attributed to a Professor Fonken, and goes as follows (excluding the structure in red):<\/p>\n

\"wood\"<\/p>\n

A search of the literature reveals only one published article describing this reaction[2]<\/a><\/span> by Dauben and Haubrich, published some 21 years after Woodward’s description (we might surmise that Gerhard Fonken never published his own results). In fact this more recent study was primarily concerned with 193-nm photochemical transforms (they conclude that “the Woodward-Hoffmann rules of orbital symmetry are not followed”) but you also find that the thermal outcome of heating 4<\/strong> is a 3:2 mixture of compounds 5<\/strong> and 6<\/strong>, and that only 6<\/strong> goes on to give the final product 7<\/strong>. It does look like a classic and uncomplicated example of Woodward-Hoffmann rules.<\/p>\n

So let us subject this system to a “reality check” (\u03c9B97XD\/6-311G(d,p) calculations). The transform of 4 \u2192 5<\/strong> rotates the two termini of the cleaving bond in a direction that produces the stereoisomer 5, with a trans<\/em> alkene straddled by two cis<\/em>-alkenes[3]<\/a><\/span>. The two carbon atoms that define the termini of the newly formed hexatriene are ~ 4.7\u00c5 apart; too far to be able to close to form 7<\/strong>.<\/p>\n\n\n\n\n
\u00a04 \u2192 5<\/strong><\/td>\n\u00a04 \u2192 6<\/strong><\/td>\n<\/tr>\n
\"8\"<\/td>\n \"8\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

But with any electrocyclic reaction, two directions of rotation are always possible, and it is a rotation in the other direction\u00a0<\/strong>that gives 4 \u2192 6<\/strong>[4]<\/a><\/span>, ending up with a hexatriene with the trans<\/em>-alkene at one end and not the middle (for which the free energy of activation is 3.1 kcal\/mol higher in energy). Now the two termini of the hexatriene end up ~3.0\u00c5 apart, much more amenable to forming a bond between them to form 7<\/strong>.<\/p>\n

It is at this point that the apparently uncomplicated nature of this example starts to unravel. If one starts from the 3.0\u00c5\u00a0end-point<\/strong> of the above reaction coordinate and systematically contracts the bond between these two termini, a transition state is found leading not to\u00a07<\/strong> but to the (endothermic) isomer 8<\/strong>.[5]<\/a><\/span>This form has\u00a0a six-membered ring with a trans<\/strong>-alkene motif (which explains why it is so endothermic).\u00a0<\/p>\n\n\n\n\n\n
\"wood1\"<\/td>\n<\/tr>\n
6 \u21a0 8<\/th>\n<\/tr>\n
\"8\"<\/td>\n\"wood2\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Before discussing the implications of this transition state, I illustrate another isomerism that 6<\/strong> can undertake; a low-barrier atropisomerism<\/a>[6]<\/a><\/span> to form 9<\/strong>, followed by another reaction with a relatively low barrier, 9 \u21a0\u00a0 7<\/strong>[7]<\/a><\/span>to give the product that Woodward gives in his essay.<\/p>\n\n\n\n\n\n\n
6\u00a0\u21a0 9<\/th>\n<\/tr>\n
\"6-atrop\"<\/td>\n\"6-atrop\"<\/td>\n<\/tr>\n
9 \u21a0 7<\/th>\n<\/tr>\n
\"9to7a\"<\/td>\n\"9to7a\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

We can now analyse the two transformations 6\u00a0\u21a0 8<\/strong> and 9 \u21a0\u00a0 7<\/strong>. The first involves antarafacial bond formation (blue arrows<\/span>) at the termini and an accompanying 180\u00b0 twisting about the magenta bond<\/span> which creates a second antarafacial component<\/strong><\/em>[8]<\/a><\/span>.<\/em>\u00a0So this is a thermally allowed six-electron (4n+2) electrocyclisation with a double-M\u00f6bius twist[9]<\/a><\/span>. The second reaction is a more conventional purely suprafacial version[10]<\/a><\/span> (red arrows<\/span>) of the type Woodward was certainly thinking of; it is 18.0 kcal\/mol lower in free energy than the first (the transition state for\u00a06\u00a0\u21a0 9 is 10.8 kcal\/mol lower than that for\u00a09 \u21a0 7).<\/p>\n

I hope that this detailed exploration of what seems like a pretty simple example at first sight shows how applying a “reality-check” of computational quantum mechanics can cast (some unexpected?) new light on an old problem. We may of course speculate on how to inhibit the pathway\u00a06\u00a0\u21a0 9\u00a0\u21a0 7<\/strong> to allow only\u00a06\u00a0\u21a0 8 <\/strong>to proceed (the reverse barrier from 8<\/b> is quite low, so 8<\/strong> would have to be trapped at very low temperatures).\u00a0<\/p>\n

References<\/h2>\n
  1. N. Bohr, \"Der Bau der Atome und die physikalischen und chemischen Eigenschaften der Elemente\", Zeitschrift f\ufffdr Physik<\/i>, vol. 9, pp. 1-67, 1922. https:\/\/doi.org\/10.1007\/bf01326955<\/a>\n\n<\/li>\n
  2. W.G. Dauben, and J.E. Haubrich, \"The 193-nm photochemistry of some fused-ring cyclobutenes. Absence of orbital symmetry control\", The Journal of Organic Chemistry<\/i>, vol. 53, pp. 600-606, 1988. https:\/\/doi.org\/10.1021\/jo00238a023<\/a>\n\n<\/li>\n
  3. H.S. Rzepa, \"Gaussian Job Archive for C10H14\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.704833<\/a>\n\n<\/li>\n
  4. H.S. Rzepa, \"Gaussian Job Archive for C10H14\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.704834<\/a>\n\n<\/li>\n
  5. H.S. Rzepa, \"Gaussian Job Archive for C10H14\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.704755<\/a>\n\n<\/li>\n
  6. H.S. Rzepa, \"Gaussian Job Archive for C10H14\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.704754<\/a>\n\n<\/li>\n
  7. H.S. Rzepa, \"Gaussian Job Archive for C10H14\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.704844<\/a>\n\n<\/li>\n
  8. H.S. Rzepa, \"Gaussian Job Archive for C10H14\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.704841<\/a>\n\n<\/li>\n
  9. H.S. Rzepa, \"Double-twist M\u00f6bius aromaticity in a 4n+ 2 electron electrocyclic reaction\", Chemical Communications<\/i>, pp. 5220, 2005. https:\/\/doi.org\/10.1039\/b510508k<\/a>\n\n<\/li>\n
  10. H.S. Rzepa, \"Gaussian Job Archive for C10H14\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.704995<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Sometimes the originators of seminal theories in chemistry write a personal and anecdotal account of their work. Niels Bohr was one such and four decades later Robert Woodward wrote “The conservation of orbital symmetry” (Chem. Soc. Special Publications (Aromaticity), 1967, 21, 217-249; it is not online and so no doi can be given). Much interesting […]<\/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":[559],"tags":[774,24,835,40,1066,2651,553,2650,744,843,1065,1051],"ppma_author":[2661],"class_list":["post-10518","post","type-post","status-publish","format-standard","hentry","category-pericyclic","tag-electrocyclic","tag-energy","tag-final-product","tag-free-energy","tag-gerhard-fonken","tag-historical","tag-niels-bohr","tag-pericyclic","tag-professor","tag-reaction-mechanism","tag-robert-woodward","tag-woodward"],"yoast_head":"\nWoodward's symmetry considerations applied to electrocyclic reactions. - 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=10518\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Woodward's symmetry considerations applied to electrocyclic reactions. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Sometimes the originators of seminal theories in chemistry write a personal and anecdotal account of their work. 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Niels Bohr was one such and four decades later Robert Woodward wrote “The conservation of orbital symmetry” (Chem. Soc. Special Publications (Aromaticity), 1967, 21, 217-249; it is not online and so no doi can be given). 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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":5763,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5763","url_meta":{"origin":10518,"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 "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":5632,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5632","url_meta":{"origin":10518,"position":2},"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":9,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9","url_meta":{"origin":10518,"position":3},"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":8426,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8426","url_meta":{"origin":10518,"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":11856,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11856","url_meta":{"origin":10518,"position":5},"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":[]}],"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\/10518","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=10518"}],"version-history":[{"count":47,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10518\/revisions"}],"predecessor-version":[{"id":22540,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10518\/revisions\/22540"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10518"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10518"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10518"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=10518"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}