Morita-Baylis-Hillman Reactions Between Conjugated Nitroalkenes or Nitrodienes and Carbonyl Compounds

Abstract

Morita-Baylis-Hillman (MBH) reactions between conjugated nitroalkenes or nitrodienes and various carbonyl compounds such, as glyoxylate, trifluoropyruvate, pyruvaldehyde, oxomalonate, ninhydrin, and formaldehyde have been extensively investigated. The reactions proceeded smoothly in the presence of DMAP (40-100 mol-%) in acetonitrile and in some cases also in that of imidazole (100 mol-%) in CHCl3 or THF, to provide the multifunctional adducts in good to excellent yields. The reactions catalyzed by DMAP in acetonitrile were superior to the imidazole-catalyzed reactions both in terms of the rate of reaction and in terms of the isolated yields of the MBH adducts. The catalytic roles played by DMAP and imidazole in these reactions, vis-à-vis other MBH catalysts such as DABCO, are attributed primarily to resonance stabilization of the initial zwitterionic intermediates. Whereas the E isomers are the major or exclusive products in the cases of glyoxylate, pyruvaldehyde, and formaldehyde, the Z isomers predominate in the cases of trifluoropyruvate and ninhydrin. Interestingly, oxomalonate forms E isomers with aromatic nitroalkenes and Z isomers with aliphatic ones. These selectivities and the formation of unusual deconjugated products in the case of certain β-alkyl-nitroethylenes have been explained.