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)2CH2)(CO)2}+BPh4−] (1) and [{Rh((mBnzim)2CH2)(CO)2}+BPh4−] (2) [mim=N-methylimidazol-2-yl, mBnzim=N-methylbenzimidazol-2-yl] are catalysts for the cyclisation of alkynoic acids to lactones. The unsaturated acids, 4-pentynoic acid, 4-hexynoic acid and 5-hexynoic acid were cyclised to γ-methylene-γ-butyrolactone, E-5-ethylidenetetrahydro-2-furanone and 6-methylidenetetrahydo-2-pyrone, respectively. Cyclisation of 4-hexynoic acid proceeds stereoselectively with exclusive formation of the E-isomer of 5-ethylidenetetrahydro-2-furanone. Complexes 1 and 2 also catalyse cyclisation of acetylenic alcohols to oxygen-containing heterocycles.