{"id":9105,"date":"2013-01-11T20:22:26","date_gmt":"2013-01-11T20:22:26","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=9105"},"modified":"2013-01-11T21:22:42","modified_gmt":"2013-01-11T21:22:42","slug":"the-benzidine-rearrangement-computed-kinetic-isotope-effects","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9105","title":{"rendered":"The Benzidine rearrangement. Computed kinetic isotope effects."},"content":{"rendered":"
\n

Kinetic isotope effects have become something of a lost art when it comes to exploring reaction mechanisms. But in their heyday they were absolutely critical for establishing the mechanism of the benzidine rearrangement[1]<\/a><\/span>. This classic mechanism proceeds via<\/em> bis<\/em>protonation of diphenyl hydrazine, but what happens next was the crux. Does this species rearrange directly to the C-C coupled intermediate (a concerted [5,5] sigmatropic reaction) or does it instead form a\u00a0\u03c0-complex, as famously first suggested by Michael Dewar[2]<\/a><\/span> [via TS(NN] and only then in a second step [via TS(CC)] form the C-C bond? Here I explore the isotope effects measured and calculated for this exact system.<\/p>\n

\"benzidine-KIE\"<\/p>\n

It boils down to the following. It was supposed that if the mechanism was a concerted [5,5] sigmatropic shift, then both the N-N and the C-C bonds would be breaking\/forming at the transition state and both N and C isotope effects would be expected. However, if a\u00a0\u03c0-complex were formed, then either TS(NN) OR TS(CC) would be the rate determining step, and so either a NN OR a CC isotope effect should manifest, but not BOTH. The experiment carried out by Henry Shine and colleagues was thus expected to be the definitive one.[3]<\/a><\/span> The results (in aqueous ethanol, at 273K) revealed the following: k(2<\/sup>H\/1<\/sup>H) = 0.962, k(14<\/sup>C\/12<\/sup>C)\u2021<\/sup> = 1.013, k(13<\/sup>C\/13<\/sup>C) = 1.013, k(15<\/sup>N\/14<\/sup>N) = 1.041. This might have appeared to prove conclusively that the reaction was concerted, involving both the C-C and N-N bonds; in other words a [5,5] sigmatropic rearrangement.<\/p>\n

The quantum mechanical (closed shell) surface reveals only two separate transition states, TS(NN) and TS(CC), and so at first sight seems to contradict the experimental isotope inference. But the experimental values are very unlikely to be wrong. So how can one reconcile these two methods? Well, the answer is not to give up, but to calculate the isotope effects for BOTH transition states, and see if either of them matches the experimental result. Here are these calculations:<\/p>\n\n\n\n\n\n\n
TS<\/td>\nk(2<\/sup>H\/1<\/sup>H)<\/td>\nk(14<\/sup>C\/12<\/sup>C)<\/td>\nk(13<\/sup>C\/13<\/sup>C)<\/td>\nk(15<\/sup>N\/14<\/sup>N)<\/td>\n<\/tr>\n
CC<\/td>\n\u00a00.946<\/td>\n\u00a01.050<\/td>\n\u00a01.050<\/td>\n\u00a01.032<\/td>\n<\/tr>\n
NN<\/td>\n\u00a01.002<\/td>\n\u00a01.008<\/td>\n\u00a01.007<\/td>\n\u00a01.075<\/td>\n<\/tr>\n
Expt<\/td>\n\u00a00.962<\/td>\n\u00a01.013<\/td>\n\u00a01.013<\/td>\n\u00a01.041<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The match between experiment and theory for TS(CC) is reasonable (given the approximations in both the theory and the difficulty of the experiments and ensuring isotopic purities) but not so for TS(NN). But TS(CC) is a “stepwise-concerted” reaction as a closed shell singlet; as shown in the\u00a0\u00a0IRC computed from TS(CC<\/a>).\u00a0<\/p>\n

Yamabe and co[4]<\/a><\/span> have come to similar conclusions (their model used a dication rather than an ion-pair).\u00a0In the latest twist,\u00a0Ghigo et al<\/em>[5]<\/a><\/span>\u00a0used the same model as here (HCl to provide protonation as an ion-pair) but identified biradical (radical-cation) character at the transition state. The latter group also calculated kinetic isotope effects[6]<\/a><\/span> for the open shell biradical TS, finding an even better match with experiment than above.<\/p>\n

So this see-saw mechanism has oscillated between a stepwise\u00a0\u03c0-complex, then a\u00a0direct [5,5] rearrangement and\u00a0in more recent times using computational modelling, a concerted [5,5] sigmatropic proceeding via<\/em> an initially formed \u03c0-complex, and (finally) via<\/em> a multi-step mechanism proceeding through a biradical\u00a0\u03c0-complex and involving radical coupling, which nevertheless appears to behave in some aspects as a concerted [5,5] rearrangement. It is fascinating that a simple diprotonation of a hydrazine could so readily induce biradical character, and\u00a0that such an apparently simple reaction could have so many twists and turns!<\/p>\n

\n

\u2021<\/sup> For one 14<\/sup>C-12<\/sup>C pair.<\/p>\n

References<\/h2>\n
  1. H.J. Shine, H. Zmuda, K.H. Park, H. Kwart, A.G. Horgan, and M. Brechbiel, \"Benzidine rearrangements. 16. The use of heavy-atom kinetic isotope effects in solving the mechanism of the acid-catalyzed rearrangement of hydrazobenzene. The concerted pathway to benzidine and the nonconcerted pathway to diphenyline\", Journal of the American Chemical Society<\/i>, vol. 104, pp. 2501-2509, 1982. https:\/\/doi.org\/10.1021\/ja00373a028<\/a>\n\n<\/li>\n
  2. M. Dewar, and H. McNicoll, \"Mechanism of the benzidine rearrangement\", Tetrahedron Letters<\/i>, vol. 1, pp. 22-23, 1959. https:\/\/doi.org\/10.1016\/s0040-4039(01)82765-9<\/a>\n\n<\/li>\n
  3. W. Subotkowski, L. Kupczyk-Subotkowska, and H.J. Shine, \"The benzidine and diphenyline rearrangements revisited. 1-14C and 1,1'-13C2 kinetic isotope effects, transition state differences, and coupled motion in a 10-atom sigmatropic rearrangement\", Journal of the American Chemical Society<\/i>, vol. 115, pp. 5073-5076, 1993. https:\/\/doi.org\/10.1021\/ja00065a018<\/a>\n\n<\/li>\n
  4. S. Yamabe, H. Nakata, and S. Yamazaki, \"\u03c0 Complexes in benzidine rearrangement\", Organic & Biomolecular Chemistry<\/i>, vol. 7, pp. 4631, 2009. https:\/\/doi.org\/10.1039\/b909313c<\/a>\n\n<\/li>\n
  5. G. Ghigo, A. Maranzana, and G. Tonachini, \"A change from stepwise to concerted mechanism in the acid-catalysed benzidine rearrangement: a theoretical study\", Tetrahedron<\/i>, vol. 68, pp. 2161-2165, 2012. https:\/\/doi.org\/10.1016\/j.tet.2012.01.014<\/a>\n\n<\/li>\n
  6. G. Ghigo, S. Osella, A. Maranzana, and G. Tonachini, \"The Mechanism of the Acid\u2010Catalyzed Benzidine Rearrangement of Hydrazobenzene: A Theoretical Study\", European Journal of Organic Chemistry<\/i>, vol. 2011, pp. 2326-2333, 2011. https:\/\/doi.org\/10.1002\/ejoc.201001636<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Kinetic isotope effects have become something of a lost art when it comes to exploring reaction mechanisms. But in their heyday they were absolutely critical for establishing the mechanism of the benzidine rearrangement. This classic mechanism proceeds via bisprotonation of diphenyl hydrazine, but what happens next was the crux. Does this species rearrange directly to […]<\/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":[4],"tags":[982,591,843,981,983],"ppma_author":[2661],"class_list":["post-9105","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-henry-shine","tag-michael-dewar","tag-reaction-mechanism","tag-tscc","tag-yamabe-and-co"],"yoast_head":"\nThe Benzidine rearrangement. Computed kinetic isotope effects. - 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=9105\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The Benzidine rearrangement. Computed kinetic isotope effects. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Kinetic isotope effects have become something of a lost art when it comes to exploring reaction mechanisms. But in their heyday they were absolutely critical for establishing the mechanism of the benzidine rearrangement. This classic mechanism proceeds via bisprotonation of diphenyl hydrazine, but what happens next was the crux. 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Some benzidine rearrangements are indeed thought to go through the [3,3] route. The topic has been reviewed here.\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":"NCI surface. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/benzidinenci.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":9186,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9186","url_meta":{"origin":9105,"position":1},"title":"The \u03c0-complex in the benzidine rearrangement: a molecular orbital analysis.","author":"Henry Rzepa","date":"January 18, 2013","format":false,"excerpt":"Michael Dewar famously implicated a so-called\u00a0\u03c0-complex in the benzidine rearrangement, back in the days when quantum mechanical calculations could not yet provide a quantitatively accurate reality check. Because this\u00a0\u03c0-complex actually remains a relatively unusual species to encounter in day-to-day chemistry, I thought I would try to show in a simple\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":[]},{"id":14070,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14070","url_meta":{"origin":9105,"position":2},"title":"Natural abundance kinetic isotope effects: expt. vs theory.","author":"Henry Rzepa","date":"June 3, 2015","format":false,"excerpt":"My PhD thesis involved determining kinetic isotope effects (KIE) for aromatic electrophilic substitution reactions in an effort to learn more about the nature of the transition states involved. I learnt relatively little, mostly because a transition state geometry is defined by 3N-6 variables (N = number of atoms) and its\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":20354,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=20354","url_meta":{"origin":9105,"position":3},"title":"Epoxidation of ethene: a new substituent twist.","author":"Henry Rzepa","date":"December 21, 2018","format":false,"excerpt":"Five years back,\u00a0I speculated about the mechanism of the epoxidation of ethene by a peracid, concluding that kinetic isotope effects provided interesting evidence that this mechanism is highly asynchronous and involves a so-called \"hidden intermediate\". Here I revisit this reaction in which a small change is applied to the atoms\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.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2018\/12\/imine2.gif?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":15295,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15295","url_meta":{"origin":9105,"position":4},"title":"I\u2019ve started so I\u2019ll finish. Mechanism and kinetic isotope effects for protiodecarboxylation of indoles.","author":"Henry Rzepa","date":"January 2, 2016","format":false,"excerpt":"Another mechanistic study we\u00a0started in\u00a01972 is\u00a0here 40+ years on\u00a0subjected to quantum mechanical scrutiny. The kinetics are again complex, the mechanism involving protonation\u2021 of the indole carboxylate (by a general acid), followed by the presumption of a zwitterionic Wheland intermediate that then loses carbon dioxide in a second step (blue arrows).\u2026","rel":"","context":"In "Historical"","block_context":{"text":"Historical","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=565"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":14112,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14112","url_meta":{"origin":9105,"position":5},"title":"Natural abundance kinetic isotope effects: mechanism of the Baeyer-Villiger reaction.","author":"Henry Rzepa","date":"June 10, 2015","format":false,"excerpt":"I have blogged before about the mechanism of this classical oxidation reaction. Here I further explore computed models, and whether they match the observed kinetic isotope effects (KIE) obtained using the natural-abundance method described in the previous post. There is much previous study of this rearrangement, and the issue can\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"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\/9105","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=9105"}],"version-history":[{"count":11,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9105\/revisions"}],"predecessor-version":[{"id":9113,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9105\/revisions\/9113"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=9105"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=9105"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=9105"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=9105"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}