Novel, Highly Efficient and Selective Ruthenium Catalysts for the Synthesis of Vinyl Esters from Carboxylic Acids and Alkynes
An efficient and highly selective synthesis of vinyl esters was developed that employed carboxylic acids and terminal alkynes as reagents, and novel tethered (η1:η6-phosphanoarene)ruthenium complexes as catalyst precursors. With these catalyst systems, the reactions were somewhat slower than when using the best catalysts reported to date, but the selectivities were excellent with up to 98 % of the Markovnikov isomer. In addition, with a turnover number (TON) of 5000, these catalysts greatly outperformed their competitors. Minute amounts of water had a beneficial impact on the reaction rates, while maintaining the same selectivities. Furthermore, microwave heating at 160 °C also considerably accelerated the reactions, albeit to the detriment of the selectivity. Finally, it should also be noted that formation of dimerization products of 1-alkynes was completely suppressed by using these tethered (η1:η6-phosphanoarene)ruthenium catalytic systems.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)