Silylchromium‐Catalyzed Selective Semihydrogenation of Undirected Allenes by Sila‐Metalation and β‐Silyl Elimination

The semihydrogenation of simple allenes remains an issue because of the difficulty in controlling of selectivity without coordination assistance from 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 while enabling the suppression of competitive over-hydrogenation, proving generally 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 β-silyl elimination processes with silyl as noninnocent ligand, leading to the E-alkene products.