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 pi 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.