High Efficiency in Catalytic Hydrosilylation of Ketones with Zinc‐Based Precatalysts Featuring Hard and Soft Tridentate O,S,O‐Ligands

Abstract

AbstractThe diprotic, tridentate O,S,O‐ligands LH2 {[RC(=O)CH2]2S; R=tBu (3 a) or Ph (3 b)}, comprising hard (O) and soft (S) donor atoms, have been employed for the first time in zinc‐mediated hydrosilylation of various ketones, giving, after protolytic workup, the corresponding alcohols. The respective precatalysts used are novel thiobis(enolato) zinc complexes, [LZn(tmeda)] [L=3 a−2H+ (4 a) or 3 b−2H+ (4 b); tmeda= N,N,N′,N′‐tetramethylethylenediamine], [LZn(bipy)] [L=3 a−2H+ (5 a); bipy=2,2′‐bipyridine], [LZn(phen)] [L=3 a−2H+ (6 a); phen=1,10‐phenanthroline], and [LZn(dabco)] [L=[3 a−2H+] (7 a); dabco=1,4‐diazabicyclo[2.2.2]octane]. These complexes are accessible by simple Brønsted acid–base reaction of 3 a or 3 b with dimethylzinc in a 1:1 molar ratio in the presence of tmeda, bipy, phen, or dabco as auxiliary ligands. The first four complexes are isolated as yellow or colorless crystals in 76 % (4 a), 53 % (4 b), 58 % (5 a), and 61 % (6 a) yields, whereas 7 a (74 % yield) is isolated as colorless powder. The zinc center in 4 a, 4 b, 5 a, and 6 a has trigonal bipyramidal coordination, as proven by single‐crystal X‐ray diffraction analysis. Remarkably, 4 a shows the highest catalytic activity hitherto reported for zinc‐catalyzed hydrosilylation over a wide range of substrates with a turnover frequency of 970 h−1. Furthermore, a catalytic mechanism is proposed.