{"id":16942,"date":"2016-10-09T17:35:50","date_gmt":"2016-10-09T16:35:50","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=16942"},"modified":"2017-07-14T13:54:03","modified_gmt":"2017-07-14T12:54:03","slug":"%cf%83-or-%cf%80-nucleophilic-reactivity-of-imines-a-mechanistic-reality-check-using-substituents","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16942","title":{"rendered":"\u03c3 or \u03c0 nucleophilic reactivity of imines? A mechanistic reality check using substituents."},"content":{"rendered":"<div class=\"kcite-section\" kcite-section-id=\"16942\">\n<p><a href=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=16902\">Previously<\/a>, a mechanistic twist to the oxidation of imines using peracid had emerged. Time to see how substituents respond to this mechanism.<\/p>\n<p><a href=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/10\/azir-X.svg\"><img decoding=\"async\" class=\"aligncenter size-large wp-image-16944\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/10\/azir-X.svg\" alt=\"azir-x\" width=\"350\" \/><\/a><\/p>\n<p>With\u00a0X = NO<sub>2<\/sub> 100% oxaziridine and no nitrone is obtained experimentally; with\u00a0X =\u00a0NMe<sub>2\u00a0<\/sub>, the population is inverted with nitrone as the dominant product at\u00a078%.<span id=\"cite_ITEM-16942-0\" name=\"citation\"><a href=\"#ITEM-16942-0\">[1]<\/a><\/span> Calculations (\u03c9B97XD\/Def2-TZVPP\/SCRF=dichloromethane), data collection \u00a0DOI: <a href=\"http:\/\/doi.org\/10.14469\/hpc\/1743\">10.14469\/hpc\/1743<\/a><span id=\"cite_ITEM-16942-1\" name=\"citation\"><a href=\"#ITEM-16942-1\">[2]<\/a><\/span> are summarised in the table.\u00a0The initial model employs the simpler peracetic acid as oxidant (R=Me) and we see here a computed\u00a0preference of 4.2\u00a0kcal\/mol for oxiziridine when the aryl substituent\u00a0X = NO<sub>2\u00a0<\/sub>(a ratio of\u00a01024:1 in its favour) but reduced to 1.4 kcal\/mol when \u00a0X =\u00a0NMe<sub>2<\/sub>. \u00a0This hardly changes when the acid is changed from ethanoic\u00a0to mCPBA (meta-chloroperbenzoic acid), the oxidant actually used in the experiments.<\/p>\n<table border=\"1\">\n<tbody>\n<tr>\n<th>Substituents<\/th>\n<th>\u03c0<\/th>\n<th>\u03c3<\/th>\n<\/tr>\n<tr>\n<td>R=Me, X=NO<sub>2<\/sub><\/td>\n<td><!-- -1067.915713-->-4.2<\/td>\n<td><!-- -1067.909085-->0.0\u00a0<\/td>\n<\/tr>\n<tr>\n<td>R=Me, X=NMe<sub>2<\/sub><\/td>\n<td><!-- -997.304632-->-1.4<\/td>\n<td><!-- -997.302438-->0.0<\/td>\n<\/tr>\n<tr>\n<td>R=m-Cl-phenyl, X=NO<sub>2<\/sub><\/td>\n<td><!-- - -->-4.1<\/td>\n<td><!-- -1719.214040 -->0.0\u00a0<\/td>\n<\/tr>\n<tr>\n<td>R=m-Cl-phenyl, X=NMe<sub>2<\/sub><\/td>\n<td>-1.3<\/td>\n<td>0.0<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>You can see from the transition state structures that\u00a0\u03c0 attack is helped by stacking between the aryl face of the m-chloroperbenzoic acid and the aryl group on the imine, whereas\u00a0\u03c3 is not.\u00a0<\/p>\n<p><img decoding=\"async\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/10\/4-1024x855.jpg\" alt=\"4\" width=\"220\" \/><img decoding=\"async\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/10\/3-1024x511.jpg\" alt=\"3\" width=\"220\" \/><\/p>\n<p>These results show that our proposed mechanism can reproduce the selectivity for formation of oxaziridine when the aryl group bears\u00a0X=NO<sub>2\u00a0<\/sub> but misses the mark of predicting nitrone formation when\u00a0X=NMe<sub>2<\/sub>. Experimentally nitrone is favoured by \u0394\u0394G<sub>298<\/sub> 0.75 kcal\/mol, whereas the calculation disfavours this by -1.3 kcal\/mol. Is this discrepancy enough to sink this mechanistic model? \u00a0Or might yet another variation on the mechanism, such shifting the proton from peracid to the\u00a0X=NMe<sub>2\u00a0<\/sub>do the trick?\u00a0<\/p>\n<p>What \u00a0I have tried to show here is how one can iterate towards a realistic mechanism by gradually refining the models so that more and more experimental observations are correctly predicted. \u00a0Sometimes of course, it might be the experiment itself that has to be repeated and refined, although we have not quite reached that point yet with this example.<\/p>\n<p>&nbsp;<\/p>\n<h2>References<\/h2>\n    <ol class=\"kcite-bibliography csl-bib-body\"><li id=\"ITEM-16942-0\">D.R. Boyd, P.B. Coulter, N.D. Sharma, W. Jennings, and V.E. Wilson, \"Normal, abnormal and pseudo-abnormal reaction pathways for the imine-peroxyacid reaction\", <i>Tetrahedron Letters<\/i>, vol. 26, pp. 1673-1676, 1985. <a href=\"https:\/\/doi.org\/10.1016\/s0040-4039(00)98582-4\">https:\/\/doi.org\/10.1016\/s0040-4039(00)98582-4<\/a>\n\n<\/li>\n<li id=\"ITEM-16942-1\">H. Rzepa, \"\u00cf\u0083 or \u00cf\u0080 nucleophilic reactivity of imines\", 2016. <a href=\"https:\/\/doi.org\/10.14469\/hpc\/1743\">https:\/\/doi.org\/10.14469\/hpc\/1743<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> <!-- kcite-section 16942 -->","protected":false},"excerpt":{"rendered":"<p>Previously, a mechanistic twist to the oxidation of imines using peracid had emerged. Time to see how substituents respond to this mechanism. With\u00a0X = NO2 100% oxaziridine and no nitrone is obtained experimentally; with\u00a0X =\u00a0NMe2\u00a0, the population is inverted with nitrone as the dominant product at\u00a078%. Calculations (\u03c9B97XD\/Def2-TZVPP\/SCRF=dichloromethane), data collection \u00a0DOI: 10.14469\/hpc\/1743 are summarised in [&hellip;]<\/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":[1412,1888,1889,1898,1887],"ppma_author":[2661],"class_list":["post-16942","post","type-post","status-publish","format-standard","hentry","category-reaction-mechanism-2","tag-functional-groups","tag-imine","tag-nitrone","tag-nme2","tag-oxaziridine"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>\u03c3 or \u03c0 nucleophilic reactivity of imines? A mechanistic reality check using substituents. - Henry Rzepa&#039;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=16942\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"\u03c3 or \u03c0 nucleophilic reactivity of imines? A mechanistic reality check using substituents. - Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"og:description\" content=\"Previously, a mechanistic twist to the oxidation of imines using peracid had emerged. Time to see how substituents respond to this mechanism. With\u00a0X = NO2 100% oxaziridine and no nitrone is obtained experimentally; with\u00a0X =\u00a0NMe2\u00a0, the population is inverted with nitrone as the dominant product at\u00a078%. Calculations (\u03c9B97XD\/Def2-TZVPP\/SCRF=dichloromethane), data collection \u00a0DOI: 10.14469\/hpc\/1743 are summarised in [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16942\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"article:published_time\" content=\"2016-10-09T16:35:50+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2017-07-14T12:54:03+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/10\/azir-X.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=\"2 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"\u03c3 or \u03c0 nucleophilic reactivity of imines? 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A mechanistic twist emerges.","author":"Henry Rzepa","date":"September 28, 2016","format":false,"excerpt":"The story so far. Imines react with a peracid to form either a nitrone (\u03c3-nucleophile) or an oxaziridine (\u03c0-nucleophile). The balance between the two is on an experimental\u00a0knife-edge, being strongly influenced by substituents on the imine. Modelling these reactions using the \"normal\" mechanism for peracid oxidation did not reproduce this\u2026","rel":"","context":"In &quot;reaction mechanism&quot;","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"6ts-irc1","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/09\/6TS-IRC1.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":16844,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16844","url_meta":{"origin":16942,"position":1},"title":"\u03c3 or \u03c0? The ambident nucleophilic reactivity of imines: crystallographic and computational reality checks.","author":"Henry Rzepa","date":"September 21, 2016","format":false,"excerpt":"Nucleophiles are species\u00a0that seek to react with an electron deficient centre by donating a lone or a \u03c0-bond pair of electrons. The ambident variety has two or more such possible sources in the same molecule, an example of which might be hydroxylamine\u00a0or\u00a0H2NOH. I previously discussed how for this example, the\u2026","rel":"","context":"In &quot;crystal_structure_mining&quot;","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"imine2","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/09\/imine2.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":16402,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16402","url_meta":{"origin":16942,"position":2},"title":"The mechanism of silylether deprotection using a tetra-alkyl ammonium fluoride.","author":"Henry Rzepa","date":"May 25, 2016","format":false,"excerpt":"The substitution of a nucleofuge (a good leaving group) by a nucleophile at a carbon centre\u00a0occurs with inversion\u00a0of configuration at the carbon, the mechanism being known by\u00a0the term\u00a0SN2\u00a0(a story I have also told\u00a0in this post). Such displacement at silicon famously proceeds by a quite different mechanism, which\u00a0I here quantify with\u2026","rel":"","context":"In &quot;reaction mechanism&quot;","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":22578,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22578","url_meta":{"origin":16942,"position":3},"title":"The Willgerodt-Kindler Reaction: mechanistic reality check 2.","author":"Henry Rzepa","date":"August 14, 2020","format":false,"excerpt":"Continuing an exploration of the mechanism of this reaction, an alternative new mechanism was suggested in 1989 (having been first submitted to the journal ten years earlier!). 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 &quot;reaction mechanism&quot;","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":12895,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12895","url_meta":{"origin":16942,"position":4},"title":"Computationally directed synthesis:  2,3-dimethyl-2-butene + NO(+).","author":"Henry Rzepa","date":"September 6, 2014","format":false,"excerpt":"In the previous posts, I explored reactions which can be flipped between two potential (stereochemical) outcomes. This triggered a memory from Alex, who pointed out this article from 1999 in which the nitrosonium cation as an electrophile can have two outcomes A or B when interacting with the electron-rich 2,3-dimethyl-2-butene.\u2026","rel":"","context":"In &quot;pericyclic&quot;","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"NOa","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/09\/NOa.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":25662,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25662","url_meta":{"origin":16942,"position":5},"title":"Nitroaryls- A less-toxic alternative reagent for ozonolysis: modelling the final step to form carbonyls.","author":"Henry Rzepa","date":"October 8, 2022","format":false,"excerpt":"Sometimes you come across a reaction which is so simple in concept that you wonder why it took so long to be accomplished in practice. In this case, replacing toxic ozone O3\u00a0as used to fragment an alkene into two carbonyl\u00a0compounds (\"ozonolysis\") by a relatively non-toxic simple nitro-group based reagent, ArNO2\u2026","rel":"","context":"In &quot;reaction mechanism&quot;","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/10\/ozr.gif?resize=350%2C200&ssl=1","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\/16942","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=16942"}],"version-history":[{"count":25,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/16942\/revisions"}],"predecessor-version":[{"id":18629,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/16942\/revisions\/18629"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=16942"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=16942"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=16942"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=16942"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}