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}⁺BPh ^- ₄] (1) and [{Rh((mBnzim)₂CH₂)-(CO)₂}⁺BPh ^- ₄] (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.