Regio- and Chemoselective Michael Addition of Lithium Zincate Species to Nitroolefins with No Additional Solvents: Synthetic and Structural Aspects

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 and imines by other transformations. While the 1,4-conjugated addition of organozinc reagents (R2Zn and RZnX) has been extensively studied, applications of alkali-metal zincates in fundamental organic transformations are still in their infancy. These type of reagents show unique, synergistic chemical characteristics which cannot be replicated by their monometallic (organolithium/organozinc) counterparts. Furthermore, the replacement of the vinylic nitro group by an alkyl group remains a complication encountered when nitrostyrenes are reacted with dialkylzinc compounds in the absence of a Lewis acid. 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. Under optimised reaction conditions (0 °C and with no additional solvents), Michael additions promoted by aliphatic and aromatic lithium organozincates take place with high regio- and chemoselectivity, thereby providing the expected nitroalkanes in yields up to >98% without replacement of the vinylic nitro group by the alkyl group (Scheme 1). Isolation of key intermediates and structural aspects will be discussed as well.