{"id":20354,"date":"2018-12-21T11:04:47","date_gmt":"2018-12-21T11:04:47","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=20354"},"modified":"2019-01-07T10:39:32","modified_gmt":"2019-01-07T10:39:32","slug":"epoxidation-of-ethene-a-new-substituent-twist","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=20354","title":{"rendered":"Epoxidation of ethene: a new substituent twist."},"content":{"rendered":"
\n

Five years back,\u00a0I speculated<\/a> 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 involved.<\/p>\n

Below are two representations of the mechanism. The synchronous mechanism involves five “curly arrows”, two of which are involved in forming a bond between oxygen and carbon, and three of which transfer a proton to the group X (X=O). The second variation asynchronously stops at the half way stage to form a pseudo ion-pair (the “hidden intermediate”) and the proton transfer only occurs in the second stage. If the ethene is substituted with deuterium, experimentally an inverse kinetic isotope effect is observed, which provides strong evidence that at the transition state, no proton transfer is occurring<\/p>\n

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

Before I go on, I should say that you will not find the mechanism as shown in either variation above in very many text books, which tend to practice “curly arrow economy” by employing only four arrows. I will not pursue this aspect here, except to note that as drawn above, the synchronous mechanism resembles that of a pericyclic reaction<\/strong> in a variation known as coarctate, as I noted in the original post (DOI:\u00a010.14469\/hpc\/4807<\/a>).<\/p>\n

Now I introduce a veritable variation into this reaction, known as Payne epoxidation[1]<\/a><\/span>,†<\/sup> which replaces the peracid with a reagent generated by adding hydrogen peroxide to a nitrile to generate a transient species which can be represented by X=NH above. How does this change things? The model below also uses propene rather than ethene (M062X\/Def2-TZVPPD\/SCRF=dichloromethane).\u2021<\/sup> This transition state (\u0394G298<\/sub> 31.3 kcal\/mol) shows two C-O bond formations, and as before the proton is clearly not yet transferred to the nitrogen (X=NH). Because of this asynchrony, the reaction could also be called a coarctate pseudo-pericyclic<\/em> reaction.<\/p>\n

\"\"

Asynchronous concerted mechanism. Click for 3D<\/p><\/div>\n

However, the proton transfer is nonetheless part of a concerted mechanism, as shown by the IRC profile. \"\"<\/a><\/p>\n

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

The gradient norm most clearly shows the “hidden ion-pair intermediate” at IRC = -1, and the proton transfer only occurs after this point is passed.\"\"<\/a><\/p>\n

This is even more spectacularly illustrated with a plot of dipole moment along the IRC; \"\"<\/a><\/p>\n

In truth, no real differences are yet revealed between the Payne reagent and the peracid. In fact, this is a real surprise, since the NH of the Payne reagent should be very much more basic than the carbonyl oxygen of the peracid. But more exploration of the potential energy surface reveals another transition state!<\/p>\n

\"\"

Stepwise mechanism. Click for 3D<\/p><\/div>\n

\"\"<\/a>This is seen forming the two C-O bonds AFTER the proton transfer from oxygen to nitrogen. It is 4.2 kcal\/mol lower<\/strong> than the first transition state, which corresponds to the scheme below.<\/p>\n

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

The new ion-pair shown above is 7.1 kcal\/mol higher than the previous reactant, but is so much more basic than before that the overall activation energy is indeed lowered. Two distinctly separate IRCs can be constructed for this alternative, the first a pure proton transfer (not shown) and the second a pure C-O bond forming process (below). This second step is both concerted and almost purely synchronous.<\/p>\n

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

So now we see how a small change to the reactant molecules (X=O to X=NH) can induce a reaction for which two quite different mechanisms can operate, an asynchronous one albeit with a hidden intermediate and a fully stepwise one in which a quite different, but this time real, intermediate is involved. Nevertheless for both the peracid mechanism and the peroxyimine variation shown here, the proton transfer is NOT involved in the rate limiting step. So for this variation too, inverse kinetic isotope effects would be expected.<\/p>\n

\n

\u2021<\/sup>FAIR data for the calculations at DOI: 10.14469\/hpc\/4909<\/a> †<\/sup>Thanks Ed for pointing this out.<\/p>\n

References<\/h2>\n
  1. G.B. PAYNE, P.H. DEMING, and P.H. WILLIAMS, \"Reactions of Hydrogen Peroxide. VII. Alkali-Catalyzed Epoxidation and Oxidation Using a Nitrile as Co-reactant\", The Journal of Organic Chemistry<\/i>, vol. 26, pp. 659-663, 1961. https:\/\/doi.org\/10.1021\/jo01062a004<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    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 involved. Below are two representations […]<\/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_feature_clip_id":0,"_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":[2542,984,1395,1459,2617,1453,2506,1410,2499,368,1442,2505,142,1594],"ppma_author":[2661],"class_list":["post-20354","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-chemical-kinetics","tag-chemical-reaction","tag-chemistry","tag-deuterium","tag-isotope-effect","tag-kinetic-isotope-effect","tag-natural-sciences","tag-organic-chemistry","tag-overall-activation-energy","tag-pericyclic-reaction","tag-physical-organic-chemistry","tag-physical-sciences","tag-potential-energy-surface","tag-rearrangement-reactions"],"yoast_head":"\nEpoxidation of ethene: a new substituent twist. - 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=20354\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Epoxidation of ethene: a new substituent twist. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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 involved. Below are two representations […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=20354\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2018-12-21T11:04:47+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2019-01-07T10:39:32+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2018\/12\/imino.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":"Epoxidation of ethene: a new substituent twist. - 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=20354","og_locale":"en_GB","og_type":"article","og_title":"Epoxidation of ethene: a new substituent twist. - Henry Rzepa's Blog","og_description":"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”. 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That collection\u00a0 is still available and is viewable\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":14070,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14070","url_meta":{"origin":20354,"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":9105,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9105","url_meta":{"origin":20354,"position":3},"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":2423,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2423","url_meta":{"origin":20354,"position":4},"title":"The oldest reaction mechanism: updated!","author":"Henry Rzepa","date":"September 14, 2010","format":false,"excerpt":"Unravelling reaction mechanisms is thought to be a 20th century phenomenon, coincident more or less with the development of electronic theories of chemistry. Hence electronic\u00a0arrow pushing as a term. But here I argue that the true origin of this immensely powerful technique in chemistry goes back to the 19th century.\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\/09\/wheland.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":14112,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14112","url_meta":{"origin":20354,"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","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\/20354","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=20354"}],"version-history":[{"count":26,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/20354\/revisions"}],"predecessor-version":[{"id":20393,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/20354\/revisions\/20393"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=20354"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=20354"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=20354"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=20354"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}