Reactions of Et₃ZnLi with Ketones:  Electronic and Steric Effects¹

Abstract

Toluene solutions of composition Et(3)ZnLi react rapidly with aldehydes and ketones to form addition products. Et(3)ZnNa and Et(3)ZnK solutions react readily with the same substrates although metalation, as well as addition, is significant with substrates having alpha-hydrogens. The Et(3)ZnM solutions react with 2-cyclohexenone to give mainly the 1,4-addition product. Relative rates of addition of Et(3)ZnLi to substituted acetophenones give a Hammett rho of 2.78. Addition of Et(3)ZnLi to acetophenone is slowed significantly by alpha and ortho methyl substituents; relative rates of addition to acetophenone, o-methylacetophenone, and tert-butyl phenyl ketone are 1.00, 0.012, and 0.003.

Homoleptic Zincate‐Promoted Room‐Temperature Halogen–Metal Exchange of Bromopyridines

“…The TMEDA signals were found dramatically shielded in n Bu 3 ZnLi⋅TMEDA and n Bu 4 ZnLi 2 ⋅TMEDA, in particular the methylene protons (from 2.05 to 1.37 ppm). Such chemical shifts indicated a profound change in coordination and were characteristic of TMEDA‐chelated metal9 (entries 4 and 5). Taking into account the identical TMEDA shifts for both reagents, a similar coordination mode was likely.…”

Regioselective SN2 Opening of Vinylic Epoxides with Trialkylzincates and Trialkylaluminates

“…We thought that with an unbiased system we would no longer observe products arising from an S N 2 substitution at the terminal carbon atom of the epoxide. (5Z,7Z)-5,7-Dodecadiene monoepoxide (29) and (6E,8E)-6,8-tetradecadiene monoepoxide (30) were selected as representative substrates. We observed a slight difference of reactivity between both substrates.…”

3,5-Bis(trifluoromethyl)acetophenone

“…Et 3 ZnLi has been shown to be an effective reducing agent of aldehydes and ketones to form the corresponding addition products. 6 3,5-Bis(trifluoromethyl)acetophenone has been used in competitive experiments to study the mechanism and the electronic effects on additions of Et 3 ZnLi to different acetophenones. Relative rates of addition of Et 3 ZnLi to substituted acetophenones give a Hammet ρ of 2.78, a much larger value than those reported for most similar reactions of other polar organometallic reagents and comparable to that observed for the addition of t-BuMgCl to benzophenones, which points to an electron-transfer mechanism in the process.…”