New methods for the generation of organozinc reagents and application to organic synthesis. Special emphasis on the reaction and formation of nitro olefins.

Abstract

The insertion of zinc metal into primary alkyl chlorides, bromides, phosphates and sulfonates in a polar solvent in the presence of a catalytic amount of LiI (0.2 equiv.) provides new organozinc reagents of the type RZnX (X = Cl, Br, OSO$\sb2$R, OP(O)(OR)$\sb2$) in excellent yields. After transmetallation to the corresponding copper reagent RCu(CN)ZnX using CuCN$\cdot$2LiCl, addition to electrophiles such as Michael acceptors affords the 1,4-adducts. Similarly, various new allylic and benzylic zinc reagents are prepared without the formation of any Wurtz-coupling side-product. Also, intermediate allylic chromium(III) reagents can be generated from the corresponding allylic mesylates or phosphates and their reaction with aldehydes is found to proceed with high diastereoselectivity. Several zinc and copper $\beta$-metallated alkylphosphonates are prepared by the insertion of zinc to dialkyl $\beta$-bromoalkylphosphonates, followed by transmetallation with copper. This new class of d$\sp2$ reagents reacts in excellent yields with a broad range of electrophiles such as acyl chlorides, aldehydes, enones, acetylenic esters, allylic and alkynyl halides, trialkyltin halides and nitro olefins. The addition of the copper-zinc reagents RCu(CN)ZnX to a variety of nitro olefins produces polyfunctional nitroalkanes in high yields. The intermediate zinc or copper nitronates can be directly submitted to a Nef reaction (O$\sb3$, $-$78$\sp\circ$C) and converted to functional ketones in a one-pot procedure. The addition of RCu(CN)ZnX to nitro olefins bearing a leaving group (RSO$\sb2$, RS) in the $\beta$-position provides nitro olefins with 100% E selectivity. The reaction is applied to the stereoselective preparation of 1,3-nitrodienes and to a Diels-Alder reaction precursor.