Rhodium- and iridium-catalyzed double hydroalkoxylation of alkynes, an efficient method for the synthesis of O,O-acetals: Catalytic and mechanistic studies

An efficient method for the synthesis of O,O-acetals via metal-catalyzed double hydroalkoxylation of alkynes was developed using the Ir(I) and Rh(I) complexes [Ir(PyP)(CO)₂]BPh₄ (1) and [Rh(bim)(CO) ₂]BPh₄ (2), where PyP = 1-[2-(diphenylphosphino)ethyl] pyrazole and bim = bis(N-methylimidazol-2-yl)-methane, as catalysts for the consecutive addition of two alcohol functional groups to terminal and nonterminal alkynes to form O,O-acetals. When the catalyzed cyclization of alkynols was performed in the presence of an excess amount of methanol as a cosolvent, a molecule of methanol was incorporated into the acetal product. The catalyzed cyclization of alkynols in the absence of an alcoholic solvent led to cyclization with incorporation of a second molecule of substrate in the final acetal product. Complexes 1 and 2 were also effective as catalysts for the cyclization of alkyne diols to form bicyclic O,O-acetals. The iridium complex 1 was more efficient than the rhodium complex 2 in promoting the reactions of aliphatic alkyne diols. On the other hand, the rhodium complex 2 was more effective for promoting the reactions of aromatic substrates. Mechanistic investigation using low-temperature NMR spectroscopy showed that the catalytic cycle proceeded via π coordination of the alkyne of the substrate to the metal center followed by the sequential addition of two hydroxyl groups to form O,O-acetals. Deuteration studies and analysis of reaction intermediates supported the proposed mechanism.