{"id":26272,"date":"2023-08-25T11:43:07","date_gmt":"2023-08-25T10:43:07","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=26272"},"modified":"2023-08-25T11:43:07","modified_gmt":"2023-08-25T10:43:07","slug":"pre-mechanism-for-the-swern-oxidation-formation-of-chlorosulfonium-chloride","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=26272","title":{"rendered":"Pre-mechanism for the Swern Oxidation: formation of chlorodimethylsulfonium chloride."},"content":{"rendered":"
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The Swern oxidation[1]<\/a><\/span> is a class of “activated” dimethyl sulfoxide<\/a> (DMSO) reaction in which the active species is a chlorodimethylsulfonium chloride salt. The mechanism of this transformation as shown in e.g. Wikipedia is illustrated below.\u2021<\/sup> However, an interesting and important aspect of chemistry is not apparent in this schematic mechanism and to rectify this, a full computed mechanism is laid out below, for which the FAIR data has a DOI: 10.14469\/hpc\/13151<\/a>
\n\"\"<\/a><\/p>\n

\n\"\"
\nThe first step involves attack of the oxygen of the DMSO on one carbon of the oxalyl chloride, which can be considered as an addition\/elimination<\/em>\u2021<\/sup> substitution at the carbon. The departing chloride anion ends up loosely associated with the sulfur centre. The net result is that the trigonal bipyramidal sulfur is axially coordinated by the chlorine, but equatorially coordinated by the oxygen. The transition state for this step (TS1), shown at IRC = 0.0 in the above energy profile, has a relatively low activation barrier. Click on any animation to view 3D model<\/strong>.<\/p>\n

\"TS1\"<\/p>\n

The key step is what is called a pseudorotation at the sulfur centre (TS2), which transforms the ax\/eq<\/em> relationship of the Cl\/O atoms at the sulfur into an ax\/ax<\/em> one (TS at IRC +8.5 above). This is the energy high point along the reaction path.<\/p>\n

\"TS2\"<\/p>\n

The S-O bond length response during this transformation is shown below. As the chlorine moves into this di-axial relationship, the S-O bond begins to weaken, from 1.666\u00c5 at the start, 1.746\u00c5 at the TS and 2.152\u00c5 at the end.<\/p>\n

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

\nThis prepares the system for the final step (TS3), which is cleavage of the already weakened S-O bond (TS at IRC = 13.0 below, TS = 0.0 being the pseudorotation), accompanied by extrusion of CO, CO2<\/sub> and Cl–<\/sup>. The liberated “ionic” chloride anion ends up loosely associated with the sulfur (2.88\u00c5), whilst the “covalent” chlorine which had helped to evict the oxygen is 2.06\u00c5.<\/p>\n

\"\"<\/p>\n

\"TS3\"<\/p>\n

So to conclude, the mechanism of the formation of chlorodimethylsulfonium chloride is perhaps better illustrated as shown below involving the extra pseudorotation step, which as it happens is actually the rate determining step for this reaction. This pre-mechanism to the\u00a0Swern oxidation is given little attention in most representations, such as the one at\u00a0Wikipedia.\u00a0But it actually contains a multitude of interesting (stereoelectronic) effects and is well worth teaching!<\/p>\n

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

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\u2021<\/sup> Well, not quite. The Wiki version<\/a> does not show the eliminating chloride anion in the first step (which is implied). The resulting curly arrows in the Wikipedia version are unbalanced and hence not formally correct! The double-headed arrow included in the representation above indicates an addition\/elimination<\/em> mechanism, which can be tracked by monitoring the carbonyl C=O bond length. It starts at 1.183\u00c5, reaches a maximum of 1.197\u00c5 just after the TS, then drops back to 1.191\u00c5 at the end as the chloride anion eliminates.<\/small><\/p>\n

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Citing this blog post: DOI 10.14469\/hpc\/13156<\/strong><\/p>\n

\n

References<\/h2>\n
  1. K. Omura, and D. Swern, \"Oxidation of alcohols by \u201cactivated\u201d dimethyl sulfoxide. a preparative, steric and mechanistic study\", Tetrahedron<\/i>, vol. 34, pp. 1651-1660, 1978. https:\/\/doi.org\/10.1016\/0040-4020(78)80197-5<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The Swern oxidation is a class of “activated” dimethyl sulfoxide (DMSO) reaction in which the active species is a chlorodimethylsulfonium chloride salt. The mechanism of this transformation as shown in e.g. Wikipedia is illustrated below.\u2021 However, an interesting and important aspect of chemistry is not apparent in this schematic mechanism and to rectify this, a […]<\/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,"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":[2336,1085],"tags":[],"class_list":["post-26272","post","type-post","status-publish","format-standard","hentry","category-curl-arrows","category-reaction-mechanism-2"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-6PK","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/26272","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=26272"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/26272\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=26272"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=26272"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=26272"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}