D. A. Evans Oxymercuration EXamples Chem 206 Oxymercuration via Hemiketals Hemiacetals Proposed Mechanism J. L Leighton et al, Org. Lett. 2000, 2, 3197-3199 I Lewis acid catalyzes formation of hemiketal General Reaction: diastereoselection >10: 1 R HgCIOAC Yb(×2 a The Oxymercuration Step(Kinetic Phase) HocIOAc Yb(X2) Mechanistic Observations. rate-determining H + HgCIOAC HgCIOAC Me acetone, 2h rt HgCl do 1: 1-mixture of diastereomers Yb(X2) much recovered starting material that mercurium ion formation is rate-determining under A)Lewis acid would promote the formation of the putative hemiketal imtermediate tures and longer reaction times the products are shown to B)Lewis acid would promote reversability of the oxymercuration prooes M OAc, 5% Yb(oYtl3 5%Yb(oYt)3 1. 1-mixture of diastereomers H Me MM-2 H x‖ 93% yield Erel=+5.2 kcal/molR OH O H R' HgClOAc 5% Yb(OYt)3 R OH O Me Me HgClOAc 5% Yb(OYt)3 R OH O Me Me HgClOAc Lewis acid addends were surveyed. the logic for this step was two-fold: (A) Lewis acid would promote the formation of the putative hemiketal imtermediate. (B) Lewis acid would promote reversability of the oxymercuration process Me3C OH O Me Me HgClOAc 5% Yb(OYt)3 O O H Me Me Me H Me O O H Me Me Me Me H MM-2 Me3C O O HgCl Me Me Me3C O O HgCl Me Me R O O HgCl Me Me R O O HgCl Me Me R O O HgCl R' HOAc, 5% Yb(OYt)3 O O H R Me Me H Yb(X2 ) O O H R Me Me H Yb(X2) O O H R Me Me H ClHg OH H R H HgClOAc HgClOAc O Me Me Yb(X2 ) HOAc, 5% Yb(OYt)3 O O H R Me Me H Yb(X2 ) Hg X O O H R Me Me H Yb(X2 ) Hg X 5% Yb(OYt)3 Hg Cl YbX3 O O H R Me Me H Hg H Cl YbX3 O O H R Me Me H Yb(X2 ) O O H R Me Me H H O O H R Me Me H HgCl –OAc D. A. Evans Oxymercuration Examples Chem 206 Oxymercuration via Hemiketals & Hemiacetals ■ Lewis acid catalyzes formation of hemiketal + J. L. Leighton et. al, Org. Lett. 2000, 2, 3197-3199 + ■ General Reaction: diastereoselection >10:1 ■ Mechanistic Observations: + ❉ ~1:1-mixture of diastereomers Product formed in low yield. much recovered starting material acetone, 2h rt + ❉ acetone, 2 min 0 °C ~1:1-mixture of diastereomers 93% yield Proposed Mechanism ■ The Oxymercuration Step (Kinetic Phase) rate-determining step low diastereoselectivity Erel = 0 Erel = +5.2 kcal/mol Leighton presumes that mercurium ion formation is rate-determining under kinetic conditions. At higher temperatures and longer reaction times the products are shown to interconvert