{"id":8588,"date":"2012-12-09T08:56:23","date_gmt":"2012-12-09T08:56:23","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=8588"},"modified":"2023-03-10T08:23:37","modified_gmt":"2023-03-10T08:23:37","slug":"why-is-the-sharpless-epoxidation-enantioselective-part-1-a-simple-model","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8588","title":{"rendered":"Why is the Sharpless epoxidation enantioselective? Part 1: a simple model."},"content":{"rendered":"
\n

Sharpless epoxidation converts a prochiral allylic alcohol into the corresponding chiral epoxide with > 90% enantiomeric excess[1]<\/a><\/span>,[2]<\/a><\/span>. Here is the first step in trying to explain how this magic is achieved.<\/p>\n

\"\"<\/p>\n

The scheme above shows how (achiral) prop-2-enol is converted using the asymmetric catalyst\u00a0(R,R)-diethyl tartrate \u00a0and t<\/em>-butyl hydroperoxide as oxidant into the (S)-chiral epoxide. The first step is to try to construct a simple model for the reaction, and in this post I will start by using one titanium as the core of the stage on which these actors will perform. This is the\u00a0mononuclear model<\/strong><\/em>\u2020<\/sup>. One can simply envisage that a molecule of tartrate displaces two i<\/sup>PrOH molecules from Ti(Oi<\/sup>Pr)4<\/sub> in an ester exchange to form a Ti(Oi<\/sup>Pr)2<\/sub>(tartrate) complex. The remaining two iso-propanols\u00a0are then replaced by one molecule each of prop-2-enol and\u00a0t<\/sup>Bu-OOH. Now we have the species Ti(OOt<\/sup>Bu)(O-CH2<\/sub>CH=CH2<\/sub>)(tartrate) as the starting point from which a transition state for oxygen transfer to the alkene to form the (S) epoxide (for R,R tartrate) can be constructed\u00a0(\u03c9B97XD\/6-311G(d,p)\/SCRF=dichloromethane model).<\/p>\n\n\n\n\n\n
\n
\"\"

Mononuclear TS for S-epoxide. Click for 3D.<\/p><\/div>\n<\/td>\n

\n
\"\"

Mononuclear TS for R-epoxide. Click for 3D.<\/p><\/div>\n<\/td>\n<\/tr>\n

IRC for mononuclear model showing oxygen atom transfer<\/span><\/th>\n<\/tr>\n
\"\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The transition state leading to (S)<\/a> epoxide emerges as 0.86 kcal\/mol higher in \u0394G\u2021<\/sup> than the (R)<\/a>, contrary to the experimental result where (S) is formed with high specificity[1]<\/a><\/span>. Inspecting the model, it is clear that the allylic alcohol substrate sits in a very open pocket un-encumbered by any nearby groups (bottom right in the animation above) and so the lack of \u03c0-facial selectivity is perhaps not surprising.<\/p>\n

To elaborate the model, I will turn to a crystal structure determined for a Ti complex bearing a t<\/em>-butyl peroxy group[3]<\/a><\/span>, showing it to be a binuclear complex\u00b6<\/sup> (magenta arrows indicate the peroxy groups)\u00a0with bridging oxygen atoms.\u2021<\/sup><\/p>\n

\"\"

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

In the follow-up post, \u00a0we will see whether these binuclear models can do better at explaining the enantioselectivity of the Sharpless reaction.<\/p>\n

\n

\u2020<\/sup> See this post<\/a> for an example of such “single-site” catalysis using Mg or this article for an example using silver[4]<\/a><\/span>.<\/p>\n

\u00b6<\/sup>A binuclear Zn catalyst with similar oxy-bridges is used to co-polymerise epoxides themselves with carbon dioxide[5]<\/a><\/span>. Many such binuclear complexes are known.<\/p>\n

\u2021<\/sup> The other element for which a number of examples of such t<\/em>-butyl peroxy bonding are known is oddly enough, lithium.[6]<\/a><\/span><\/p>\n

\"\"

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

\n

Postscript:<\/strong> Two lower energy conformations for the S<\/a> and R<\/a> transition states have been found, the latter being 1.6 kcal\/mol lower in free energy.\u00a0<\/p>\n\n\n\n\n
S<\/th>\nR<\/th>\n<\/tr>\n
\"S-new\"<\/td>\n\"R-new\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

References<\/h2>\n
  1. J.M. Klunder, S.Y. Ko, and K.B. Sharpless, \"Asymmetric epoxidation of allyl alcohol: efficient routes to homochiral .beta.-adrenergic blocking agents\", The Journal of Organic Chemistry<\/i>, vol. 51, pp. 3710-3712, 1986. https:\/\/doi.org\/10.1021\/jo00369a032<\/a>\n\n<\/li>\n
  2. R.M. Hanson, and K.B. Sharpless, \"Procedure for the catalytic asymmetric epoxidation of allylic alcohols in the presence of molecular sieves\", The Journal of Organic Chemistry<\/i>, vol. 51, pp. 1922-1925, 1986. https:\/\/doi.org\/10.1021\/jo00360a058<\/a>\n\n<\/li>\n
  3. G. Boche, K. M\u00f6bus, K. Harms, and M. Marsch, \"[((\u03b7<sup>2<\/sup>-<i>tert<\/i>-Butylperoxo)titanatrane)<sub>2<\/sub>\u00b7 3 Dichloromethane]:\u2009 X-ray Crystal Structure and Oxidation Reactions\", Journal of the American Chemical Society<\/i>, vol. 118, pp. 2770-2771, 1996. https:\/\/doi.org\/10.1021\/ja954308f<\/a>\n\n<\/li>\n
  4. J.L. Arbour, H.S. Rzepa, J. Contreras\u2010Garc\u00eda, L.A. Adrio, E.M. Barreiro, and K.K.(. Hii, \"Silver\u2010Catalysed Enantioselective Addition of O\uf8ffH and N\uf8ffH Bonds to Allenes: A New Model for Stereoselectivity Based on Noncovalent Interactions\", Chemistry \u2013 A European Journal<\/i>, vol. 18, pp. 11317-11324, 2012. https:\/\/doi.org\/10.1002\/chem.201200547<\/a>\n\n<\/li>\n
  5. A. Buchard, F. Jutz, M.R. Kember, A.J.P. White, H.S. Rzepa, and C.K. Williams, \"Experimental and Computational Investigation of the Mechanism of Carbon Dioxide\/Cyclohexene Oxide Copolymerization Using a Dizinc Catalyst\", Macromolecules<\/i>, vol. 45, pp. 6781-6795, 2012. https:\/\/doi.org\/10.1021\/ma300803b<\/a>\n\n<\/li>\n
  6. W. Uhl, M. Reza\u2005Halvagar, and M. Claesener, \"Reducing Ga\uf8ffH and Ga\uf8ffC Bonds in Close Proximity to Oxidizing Peroxo Groups: Conflicting Properties in Single Molecules\", Chemistry \u2013 A European Journal<\/i>, vol. 15, pp. 11298-11306, 2009. https:\/\/doi.org\/10.1002\/chem.200900746<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Sharpless epoxidation converts a prochiral allylic alcohol into the corresponding chiral epoxide with > 90% enantiomeric excess,. Here is the first step in trying to explain how this magic is achieved. The scheme above shows how (achiral) prop-2-enol is converted using the asymmetric catalyst\u00a0(R,R)-diethyl tartrate \u00a0and t-butyl hydroperoxide as oxidant into the (S)-chiral epoxide. The […]<\/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":[4],"tags":[152,956,1138,955,40,960,843,373],"ppma_author":[2661],"class_list":["post-8588","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-animation","tag-asymmetric-epoxidation","tag-catalysis","tag-enantioselective","tag-free-energy","tag-lower-energy-conformations","tag-reaction-mechanism","tag-tutorial-material"],"yoast_head":"\nWhy is the Sharpless epoxidation enantioselective? 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The […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8588\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-12-09T08:56:23+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2023-03-10T08:23:37+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/sharpless.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":"Why is the Sharpless epoxidation enantioselective? 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Here in part 2, I investigate whether a binuclear model might have more success.\u00a0The new model is constructed using two units of\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":"WAWBUR. Click for 3D","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/WAWBUR.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":12308,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12308","url_meta":{"origin":8588,"position":1},"title":"Enantioselective epoxidation of alkenes using the Shi Fructose-based catalyst. An undergraduate experiment.","author":"Henry Rzepa","date":"April 15, 2014","format":false,"excerpt":"The journal of chemical education can be a fertile source of ideas for undergraduate student experiments. Take this procedure for asymmetric epoxidation of an alkene. When I first spotted it, I thought not only would it be interesting to do in the lab, but could be extended by incorporating some\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":8733,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8733","url_meta":{"origin":8588,"position":2},"title":"Non covalent interactions in the Sharpless transition state for asymmetric epoxidation.","author":"Henry Rzepa","date":"December 19, 2012","format":false,"excerpt":"The Sharpless epoxidation of an allylic alcohol had a big impact on synthetic chemistry when it was introduced in the 1980s, and led the way for the discovery (design?) of many new asymmetric catalytic systems. Each achieves its chiral magic by control of the geometry at the transition state for\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":8850,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8850","url_meta":{"origin":8588,"position":3},"title":"Sharpless epoxidation, enantioselectivity and conformational analysis.","author":"Henry Rzepa","date":"January 3, 2013","format":false,"excerpt":"I return to this reaction one more time. Trying to explain why it is enantioselective for the epoxide product poses peculiar difficulties. Most of the substituents can adopt one of several conformations, and some exploration of this conformational space is needed. Amongst the conformational possibilities are the two rotations shown\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":11065,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11065","url_meta":{"origin":8588,"position":4},"title":"Experimental evidence for “hidden intermediates”? Epoxidation of ethene by peracid.","author":"Henry Rzepa","date":"August 25, 2013","format":false,"excerpt":"The concept of a \"hidden intermediate\" in a reaction pathway has been promoted by Dieter Cremer and much invoked on this blog. When I used this term in a recent article of ours, a referee tried to object, saying it was not in common use in chemistry. The term clearly\u2026","rel":"","context":"In "Curly arrows"","block_context":{"text":"Curly arrows","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2327"},"img":{"alt_text":"peracid+alkene1","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/08\/peracid%2Balkene1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":2857,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2857","url_meta":{"origin":8588,"position":5},"title":"Gravitational fields and asymmetric synthesis","author":"Henry Rzepa","date":"November 20, 2010","format":false,"excerpt":"Our understanding of science mostly advances in small incremental and nuanced steps (which can nevertheless be controversial) but sometimes the steps can be much larger jumps into the unknown, and hence potentially more controversial as well. More accurately, it might be e.g. relatively unexplored territory for say a chemist, but\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\/11\/isophorone.jpg?resize=350%2C200","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\/8588","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=8588"}],"version-history":[{"count":58,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8588\/revisions"}],"predecessor-version":[{"id":25954,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/8588\/revisions\/25954"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=8588"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=8588"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=8588"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=8588"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}