Cyclisation of acetylenic carboxylic acids and acetylenic alcohols to oxygen-containing heterocycles using cationic rhodium(I) complexes

Square planar cationic rhodium(I) dicarbonyl complexes [{Rh((mim)(2)CH2)(CO )(2)}+BPh4-] (1) and [{Rh((mBnzim)(2)CH2)(CO)(2)}+BPh4-] (2) [mim =N-methyl imidazol-2-yl, mBnzim = N-methylbenzimidazol-2-yl] are catalysts for the cy clisation of alkynoic acids to lactones. The unsaturated acids, 4-pentynoic acid, 4-hexynoic acid and 5-hexynoic acid were cyclised to gamma-methylene -gamma-butyrolactone, E-5-ethylidenetetrahydro-2-furanone and 6-methylidene tetrahydo-2-pyrone, respectively. Cyclisation of 4-hexynoic acid proceeds s tereoselectively with exclusive formation of the E-isomer of 5-ethylidenete trahydro-2-furanone. Complexes 1 and 2 also catalyse cyclisation of acetyle nic alcohols to oxygen-containing heterocycles. (C) 2000 Elsevier Science S .A. All rights reserved.