{"id":22541,"date":"2020-07-21T09:35:19","date_gmt":"2020-07-21T08:35:19","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=22541"},"modified":"2020-07-22T07:06:17","modified_gmt":"2020-07-22T06:06:17","slug":"the-willgerodt-kindler-reaction-mechanistic-reality-check-1","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22541","title":{"rendered":"The Willgerodt-Kindler Reaction: mechanistic reality check 1."},"content":{"rendered":"
\n

The Willgerodt reaction[1]<\/a><\/span>, discovered in 1887 and shown below, represents a transformation with a once famously obscure mechanism. A major step in the elucidation of that mechanism came[2]<\/a><\/span> using the then new technique of 14<\/sup>C radio-labelling, shortly after the atom bomb projects during WWII made 14<\/sup>CO2<\/sub> readily available to researchers. Here I am going to start the process of applying the far more recent technique of quantitative quantum mechanical modelling to see if some of the proposed mechanisms stand up to its scrutiny.<\/p>\n

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

In the classic experiment, it was shown that if acetophenone labelled on the carbonyl group with 14<\/sup>C was subjected to Willgerodt conditions, almost 100% of the label ended up in the benzylic carbon(red dot).[2]<\/a><\/span> Why was this considered remarkable? Because it was effectively the oxygen of the carbonyl (via<\/em> the proxy of the N in the intermediate enamine) that appeared to be migrating along the C2<\/sub> carbon chain, rather than the phenyl group which is a known very effective migrator! The rather harsh conditions of the Willgerodt reaction were replaced in 1923 by the somewhat milder Kindler modification,[3]<\/a><\/span> using morpholine + S8<\/sub> as the catalyst\u00a0instead of ammonia + S8<\/sub>. The mechanism below is adapted from a typical source of named organic reactions<\/a>; the Wikipedia page<\/a> is a rather more elaborate version of this. In essence these mechanisms suggest that the aziridine species labelled Int2<\/strong> below is an undetected intermediate accounting for the radio-labelling experiment.<\/p>\n

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

The computational reality check can be undertaken by calculating the relative free energies of the species labelled above, setting that of the reactant to\u00a0\u0394\u0394G = 0.0. The model used is B3LYP+GD3+BJ\/Def2-TZVPP\/SCRF=water (FAIR data DOI: 10.14469\/hpc\/7294<\/a>). For the reaction to be reasonably fast at 403K, the highest species on this pathway should be no higher than ~30 kcal\/mol above the reactant. The calculations reveal that TS3<\/strong> is around 42.6 kcal\/mol above the reactant, with a very flat potential energy surface in the region of the transition state, in which C-N cleavage preceeds 1,2-hydrogen migration.<\/p>\n

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

\"\"<\/a>\"\"<\/a>The value of this barrier height suggests an alarm bell ringing. Protonating the species (via<\/em> tosic acid) does not help. So we conclude that the mechanism needs \u00a0“optimising” to try to find a lower energy pathway to product.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n
Species<\/th>\n\u0394\u0394G<\/th>\n<\/tr>\n
Reactant<\/td>\n0.0<\/td>\n<\/tr>\n
TS1<\/td>\n27.9 (44.4)a<\/sup><\/td>\n<\/tr>\n
Int1<\/td>\n5.8 (13.0)a<\/sup><\/td>\n<\/tr>\n
TS2<\/td>\n16.9 (22.4)a<\/sup><\/td>\n<\/tr>\n
Int2<\/td>\n14.7<\/td>\n<\/tr>\n
TS3<\/td>\n42.6<\/span><\/td>\n<\/tr>\n
Product<\/td>\n-21.2<\/td>\n<\/tr>\n
Int3<\/td>\n-0.4<\/td>\n<\/tr>\n
a<\/sup>Protonated on S<\/small><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

A hint of how this might be done comes from the energy of the species labelled Int3<\/strong>, which is a thiirane rather than an aziridine intermediate. \u00a0Such thiiranes will be explored in part two of this theme. It may also be that explicit base catalysis of TS3 via<\/em> proton abstraction may be more facile than a direct [1,2] hydrogen shift. Much like organic syntheses, where reaction yields have to be optimised by often long and arduous explorations, so too on occasion do reaction mechanisms!<\/p>\n

References<\/h2>\n
  1. C. Willgerodt, \"Ueber die Einwirkung von gelbem Schwefelammonium auf Ketone und Chinone\", Berichte der deutschen chemischen Gesellschaft<\/i>, vol. 20, pp. 2467-2470, 1887. https:\/\/doi.org\/10.1002\/cber.18870200278<\/a>\n\n<\/li>\n
  2. W.G. Dauben, J.C. Reid, P.E. Yankwich, and M. Calvin, \"The Mechanism of the Willgerodt Reaction<sup>1<\/sup>\", Journal of the American Chemical Society<\/i>, vol. 72, pp. 121-124, 1950. https:\/\/doi.org\/10.1021\/ja01157a034<\/a>\n\n<\/li>\n
  3. K. Kindler, \"Studien \u00fcber den Mechanismus chemischer Reaktionen. Erste Abhandlung. Reduktion von Amiden und Oxydation von Aminen\", Justus Liebigs Annalen der Chemie<\/i>, vol. 431, pp. 187-230, 1923. https:\/\/doi.org\/10.1002\/jlac.19234310111<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The Willgerodt reaction, discovered in 1887 and shown below, represents a transformation with a once famously obscure mechanism. A major step in the elucidation of that mechanism came using the then new technique of 14C radio-labelling, shortly after the atom bomb projects during WWII made 14CO2 readily available to researchers. Here I am going to […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_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}},"categories":[1086],"tags":[],"ppma_author":[2661],"class_list":["post-22541","post","type-post","status-publish","format-standard","hentry","category-reaction-mechanism-2"],"yoast_head":"\nThe Willgerodt-Kindler Reaction: mechanistic reality check 1. - 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=22541\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The Willgerodt-Kindler Reaction: mechanistic reality check 1. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The Willgerodt reaction, discovered in 1887 and shown below, represents a transformation with a once famously obscure mechanism. 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Here the key intermediate proposed is a thiirenium cation (labelled 8 in the article) and labelled\u00a0Int3 below. The model chosen is the same\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":9105,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9105","url_meta":{"origin":22541,"position":1},"title":"The Benzidine rearrangement. Computed kinetic isotope effects.","author":"Henry Rzepa","date":"January 11, 2013","format":false,"excerpt":"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\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":22694,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22694","url_meta":{"origin":22541,"position":2},"title":"The Willgerodt-Kindler reaction. Completing the Box set.","author":"Henry Rzepa","date":"September 7, 2020","format":false,"excerpt":"These four posts (the box set) set out to try to define the energetics for a reasonable reaction path for the Willgerodt-Kindler reaction. The rate of this reaction corresponds approximately to a free energy barrier of ~30 kcal\/mol. Any pathway found to be >10 kcal\/mol at its highest point above\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":6543,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6543","url_meta":{"origin":22541,"position":3},"title":"The Dieneone-phenol controversies.","author":"Henry Rzepa","date":"April 30, 2012","format":false,"excerpt":"During the 1960s, a holy grail of synthetic chemists was to devise an efficient route to steroids. R. B. Woodward was one the chemists who undertook this challenge, starting from compounds known as dienones (e.g. 1) and their mysterious conversion to phenols (e.g. 2 or 3) under acidic conditions. This\u2026","rel":"","context":"In "Chemical IT"","block_context":{"text":"Chemical IT","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/04\/patha.svg","width":350,"height":200},"classes":[]},{"id":3003,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3003","url_meta":{"origin":22541,"position":4},"title":"Janus mechanisms (the past and the future): Reactions of the diazonium cation.","author":"Henry Rzepa","date":"December 11, 2010","format":false,"excerpt":"Janus was the mythological Roman god depicted as having two heads facing opposite directions, looking simultaneously into the past and the future. Some of the most ancient (i.e. 19th century) known reactions can be considered part of a chemical mythology; perhaps it is time for\u00a0a Janus-like look into their future.\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\/2010\/12\/diazonium.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":14601,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14601","url_meta":{"origin":22541,"position":5},"title":"Yes, no, yes. Computational mechanistic exploration of (nickel-catalysed) cyclopropanation using tetramethylammonium triflate.","author":"Henry Rzepa","date":"October 1, 2015","format":false,"excerpt":"A fascinating re-examination has appeared of a reaction first published in 1960 by Wittig and then repudiated by him in 1964 since it could not be replicated by a later student. According to the new work, the secret to a successful replication\u00a0seems to be\u00a0the presence of traces of a nickel\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\/22541","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=22541"}],"version-history":[{"count":22,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/22541\/revisions"}],"predecessor-version":[{"id":22570,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/22541\/revisions\/22570"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=22541"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=22541"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=22541"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=22541"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}