Silylchromium-Catalyzed Selective Semihydrogenation ofUndirected Allenes by Sila-metalation and beta-Silyl Elimination

The semihydrogenation of simple allenes remains an issue because of the difficulty in control of selectivity without coordination assistance of directing groups, wherein over-hydrogenated alkanes and E/Z-mixed stereo- and regio-isomers can be potentially formed. We report here an electron-rich cyclic (alkyl)(amino)carbene (CAAC)-phosphine-ligated chromium(III) complex serving as precatalyst in promoting the selective semihydrogenation of undirected allenes by forming active silylchromium species. The semihydrogenation allows the selective addition of H2 to the less substituted and electron-rich alkyl-substituted C=C bonds of allenes with enabling the suppression of competitive over-hydrogenation, proving general to form trisubstituted E-alkenes in high chemo-, regio- and stereoselectivity. By this CAAC-phosphine-Cr-catalyzed selective semihydrogenation, unsaturated nitro, nitrile, alkenyl and hydrogenation-sensitive bromide, chloride and fluoride were compatible with various substitution environments. The in-situ forming silylchromium as active catalyst by reaction of low-valent chromium complex with chlorosilane was established by studying the catalytic and stoichiometric reactivity of this species in the semihydrogenation. Deuterium labeling experiments, HRMS, XPS, EPR and magnetic susceptibility analysis of the related intermediates and theoretical studies support a pathway involving sila-metalation of allene and b-silyl elimination processes with silyl as non-innocent ligand, leading to the E-alkene products.