Distannylations and Silastannylations of In Situ Generated Allenes

Abstract

Owing to their high versatility, organometallic compounds have become extremely valuable building blocks and have found a wide range of applications in organic synthesis, especially in modern cross-coupling chemistry. This development has been possible through the tremendous progress over the past 40 years in the synthesis and applicability of organometallic substrates. Besides the typically used monometalated compounds, also dimetalated derivatives are of enormous interest, especially if different organometallic centers can made to undergo reactions selectively. In this context, Mitchell et al. reported on palladiumcatalyzed distannylations and silastannylations of alkynes and allenes. The catalytic dimetalation of allenes has the advantage that products are formed that have a vinylic and an allylic metal group as well. Both functionalities can be coupled independently and selectively, which makes the products of the catalytic dimetalation of allenes highly valuable building blocks for organic synthesis. Besides diborations, the addition of stannagermanes and silaboranes to alkenes and alkynes have been reported as well. Disilylations of allenes are also possible, but in general this reaction requires higher reaction temperatures. In the last few years, Cheng et al. have reported on palladium-catalyzed carbostannylations, carbosilylations, and acyl metalations. Our group has also been involved for several years in hydroand distannylations of alkynes and allenes, especially in the presence of molybdenum and tungsten catalysts. Depending on the catalyst used, propargyl acetateA can selectively be converted either into the vinyl stannane B or the dimetalated allyl acetate C (Scheme 1). The molybdenum-catalyzed hydrostannylation of allenyl alcohols D provided allyl stannanes E in good yields. Herein we present a new protocol towards dimetalated alkenes starting from alkynes or vinyl stannanes. This reaction came into our focus during our investigations of the influence of different catalysts on the regioselectivity of hydrostannylations. While Bu3SnH addition towards the alkyne 1a gave rise to the expected vinyl stannane 2a in the presence of the molybdenum catalyst [Mo(CO)3(CNtBu)3] (MoBI3), the palladium-catalyzed reaction surprisingly provided the distannylated product 3a (Scheme 2). At the beginning it was unclear how this product could be formed, but we speculated that also the palladium-catalyzed reaction provided 2a as an intermediate, which might undergo elimination to an allene under the reaction conditions. Therefore, we investigated the