Chemoselective Michael Additions of Alkali-metal Zincates to Nitrostyrenes in Deep Eutectic Solvents and Under Neat Conditions

The conjugate addition of organometallics to nitroalkenes provides a useful method for nitro-alkylation. This type of addition is much pursued in organic synthesis as the nitro group can be easily transformed into various functional groups including carbonyl derivatives by the Nef reaction, amines by reduction, nitriles, imines by other transformations1. While 1,4-conjugated additions of organozinc reagents (R2Zn and RZnX) have been extensively studied2, applications of alkali-metal zincates in fundamental organic transformations are still in their infancy3. This type of reagents show unique, synergistic chemical characteristics which cannot be replicated by their monometallic (organolithium/organozinc) counterparts. In this Communication, we compare the kinetic reactivity of two different alkali-metal zincates (namely triorgano- and tetraorganozincates) in Deep Eutectic Solvents and under neat conditions towards variously substituted nitroalkenes. It will be shown that, under optimized reaction conditions (0 °C and with no added solvents) Michael additions promoted by aliphatic and aromatic organozincates take place with high regio- and chemoselectivity providing the expected nitroalkanes in yields to up to >98%, and with no replacement of the vinylic nitro group by the alkyl group, which often remains a complication. Isolation of key intermediates, and stereochemical aspects will be discussed as well.