Michael reactions, allylic anions, sulfones, nitroolefins Abatractz Michael addition of allylsulfone carbanion~ to nitroolefins where shown to be sensitive to substitution in both the donor and the acceptor. While sultbne 10 gave predominantly products of 7-addition with aromatic nitrcoletW, addition to aliphatic nitmolefins produced almost exclusively u-adducts. In the presence of an OH fiction in the sulfone (see 13) only (Z) padducts were observed, while a bromine substituent reversed the regiochemistry. The Michael addition is a powerful method for the formation of single C-C bonds, hence the current interest to achieve stereocontrol, in reactions of acyclic precursors,~ and regiocontrol, when allylic anions are involved as donors. 4 In this context a recent study on conjugate additions of allylic sulfones to cyclic enones revealed the preferential formation of vinylic sulfone adducts via r-addition, rationalized by the steric constraints of a cyclic transition statePThere are no reports, however, on the outcome of analogous reactions with nitroolefins, which are highly effective Michael acceptors?
We report herewith some preliminary findings on conjugate additions of allylic sulfones to nitroolefms which emphasize the influence of substitution in both reactants on the stereo-and regiochemical outcome of the reactions. When lithiated methallyl sulfone 10 was reacted with 8nitrostyrene 1 (Table, entry 1)' preferential y-addition occurred (83%) to give vinylic sulfones (lla,b), with diastereomeric 12a,b as the minor products of a-addition.6 Electron-releasing (p-OMe) or withdrawing (p-Cl) groups caused a varying decrease of regioselectivity (entries 3,4,6). The major vinylic sulfone stereoisomer had an (E) geometry9 (ratio E/z: 90-95:10-5), rationalized by the cisoid configuration of the carbanion.4 When hexamethylphosphoramide (HMPA) was added to the reaction mixture10 (entries 2 and 5) similar amounts of (E) and (Z) isomers were formed. These results were consistent with a weakening of coordination of the cisoid carbanion with the counterion, which brought as well to the relative increase in a-addition.
A reversal of regiochemical preference occurred when lithiated methallylsulfone 10 was reacted with aliphatic instead of aromatic nitrooletins under identical conditions (Table, entries &11)1 With nitropropene as the acceptor, a-addition occurred exclusively, to give diastereomers 12 in a syiwmti ratio of 84~16.11 The degree of diastereoselectivity at the vicinal centers of allylic sulfone products 12 decreased with increase of the bulk of the aliphatic nitroolefin, but the regioselectivity remained high even with hindered nitroolefins, hence the regioreversal cannot be attributed to steric factors.
Even more dramatic regio-and stereochemical preferences were observed when substituents were introduced in the methyl group of methallyl sulfone. Thus lithiation of hydroxysulfone 1312 (2.2 equiv. LDA) and addition of an aromatic nitroolefin' led to the formation of a single, regio-and stereohomogeneous product (15 -17, Scheme) by exclusive r-addition in high yieldJ3J4
Conversely, reaction's of bromosulfone 1416 with the same nitroolefins occurred regioselectively to give, by a-addition, at the sulfone substituted terminus, the products 18 -19, with vinylic sulfones being