1,4-Addition of TMSCCl(3) to Nitroalkenes: Efficient Reaction Conditions and Mechanistic Understanding

Improved synthetic conditions allow preparation of TMSCCl(3) in good yield (70 %) and excellent purity. Compounds of the type NBu(4)X [X=Ph(3)SiF(2) (TBAT), F (tetrabutylammonium fluoride, TBAF), OAc, Cl and Br] act as catalytic promoters for 1,4-additions to a range of cyclic and acyclic nitroalkenes, in THF at 0–25 °C, typically in moderate to excellent yields (37–95 %). TBAT is the most effective promoter and bromide the least effective. Multinuclear NMR studies ((1)H, (19)F, (13)C and (29)Si) under anaerobic conditions indicate that addition of TMSCCl(3) to TBAT (both 0.13 M) at −20 °C, in the absence of nitroalkene, leads immediately to mixtures of Me(3)SiF, Ph(3)SiF and NBu(4)CCl(3). The latter is stable to at least 0 °C and does not add nitroalkene from −20 to 0 °C, even after extended periods. Nitroalkene, in the presence of TMSCCl(3) (both 0.13 M at −20 °C), when treated with TBAT, leads to immediate formation of the 1,4-addition product, suggesting the reaction proceeds via a transient [Me(3)Si(alkene)CCl(3)] species, in which (alkene) indicates an Si⋅⋅⋅O coordinated nitroalkene. The anaerobic catalytic chain is propagated through the kinetic nitronate anion resulting from 1,4 CCl(3)(−) addition to the nitroalkene. This is demonstrated by the fact that isolated NBu(4)[CH(2)−NO(2)] is an efficient promoter. Use of H(2)C−CH(CH(2))(2)CH−CHNO(2) in air affords radical-derived bicyclic products arising from aerobic oxidation.