Rhodium catalyzed alkynoic acid cyclizations

Abstract

The synthesis and characterization of a novel indenylrhodium(I) complex has been investigated. The reaction of two equivalents 1,3,5-triaza-7-phosphaadamantane added to one equivalent di(cyclooctene)indenylrhodium(I) dissolved in THF resulted in a 67% yield of indenylbis(1,3,5-triaza-7-phosphaadamantane)rhodium(I). The catalytic activity of the complex was assessed using the intramolecular alkynoic acid cyclizations of 4-pentynoic acid and 5hexynoic acid. Addition of 5 mol% of the catalyst resulted in complete conversion from 4pentynoic acid to γ-methylene-γ-butyrolactone at room temperature in only 24 hours. Furthermore, 2 mol% of the catalyst facilitated the complete conversion of 4-pentynoic acid in under 2 hours with moderate heating. The addition of 5 mol% of the catalyst to a solution of 5-hexynoic acid resulted in a partial conversion to 6-methylidenetetrahydo-2-pyrone after 6 hours with heating.