New Strategies for Stereo- and Regiocontrol in Allene Hydrosilylation.

Abstract

Controlling the regiochemistry in additions of silanes across units of unsaturation is a significant challenge in route to important stereo- and regiodefined alkenyl- and allylsilane intermediates. In many cases the absence of substrate biases or appended directing groups, these reactions are unselective affording mixtures of inseparable constitutional isomers. With the aim to develop catalyst-controlled strategies to find solutions for these challenges, we investigated allene hydrosilylations due to the possibility of accessing either allyl- or alkenylsilanes from one substrate class. Regioselective allene hydrosilylations with N-heterocyclic carbene (NHC) complexes of nickel and palladium have been developed and are described. In this reaction manifold, regioselectivity is governed primarily by choice of metal as alkenylsilanes are produced via nickel catalysis with larger NHC ligands and allylsilanes are produced via palladium catalysis with smaller NHC ligands. These complementary methods allow either regioisomeric product to be obtained with exceptional regiocontrol. Additional complementary access to the corresponding alkenylsilanes is accomplished with palladium catalysis via a novel ligand-controlled strategy with a class of large NHC ligands. In this approach, modification of NHC ligand structure from a common palladium pre-catalyst permits either regioisomeric product to be afforded in excellent regiocontrol via the reversal of a palladium hydrometalation to a silylmetalation. As an extension of this discovery, a one-pot hydroarylation protocol has been elucidated to afford functionalized branched alkene isomers. This process involves regioselective allene hydrosilylation in tandem with a palladium-catalyzed cross-coupling reaction to furnish exclusively functionalized 1,1-disubstituted olefins. Regio- and stereocontrolled additions to 1,3-disubstituted allenes affording either E-allylsilanes with palladium NHC catalysts or Z-alkenylsilanes with nickel NHC catalysts have been discovered. It was found that a variety of symmetrically and unsymmetrical 1,3-disubstituted allenes were tolerated with the optimized reaction conditions. The NHC nickel catalyst developed selectively affords only the Z-stereoisomer, which is typically the opposite stereochemistry experienced in normal mode transition-metal catalyzed hydrometalations. In addition, when the nickel reaction was conducted with unsymmetrical allenes differentiated by steric encumbrance result in only one isomer was afforded in high yields. These developments provide access to important stereo- and regiodefined, metalated intermediates useful in a variety of synthetic transformations.