Improved Protocols for the Molybdenum-Catalyzed Hydrostannation of Alkynes

Abstract

In the molybdenum-catalyzed hydrostannation of alkynes the addition mode of tributyltin hydride has a strong effect on the outcome of the reaction. Slow addition increases the yield, even if the amount of tributyltin hydride is reduced to only one equivalent. Microwave irradiation speeds up the reaction dramatically, sometimes at the cost of regioselectivity.From a synthetic point of view, transition-metal-catalyzed reactions are extremely interesting and important, not least because many of these reactions proceed under very mild conditions. 1 In general, they also tolerate a wide variety of functional groups and are therefore suitable for the synthesis of complex molecules. Of the various transition metals used, palladium reigns supreme and is commonly used for cyclizations, allylic alkylations, cross-couplings, and many other important reactions. 2 In particular, Stilletype couplings are extremely popular 3 using aryl or vinyl stannanes as organometallic coupling partners. The latter can easily be obtained by hydrostannation of alkynes, 4 which can be carried out under different reaction conditions. Besides a radical pathway, a palladium-catalyzed version of the hydrostannation has also been developed during the last few years. The major drawback of both methods results from difficulties in controlling the regioselectivity of the tributyltin hydride addition to unsymmetrical alkynes. A comparison of these two protocols shows that the transition-metal-catalyzed version has the advantage of a clean cis addition, which results from the mechanism of the reaction. Therefore, only two products are formed during this process, though radical tributyltin hydride additions generate E/Z mixtures of the corresponding stannylated alkenes. Most examples described so far employ palladium complexes as catalysts. However, other transition metals, such as rhodium complexes can also be used. 5 Guibé et al. described an application of a molybdenum catalyst, 6 and recently, Chiu et al. applied a copper complex to this type of reaction. 7