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

The diprotic, tridentate O,S,O-ligands LH2 {[RC(=O)CH2](2)S; R=tBu (3a) or Ph (3b)}, comprising hard (0) 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=3a-2H(+) (4a) or 3b-2H(+) (4 b); tmeda,= N,N,N',N'-tetramethylethylenediamine], [LZn(bipy)] [L=3a-2H+ (5a); bipy=2,2'-bipyridine], [LZn(phen)] [L= 3a-2H(+) (6a); phen=1,10-phenanthroline], and [LZn(dabco)] [L=[3a-2H+] (7a); dabco=1,4-diazabicyclo[2.2.2]octanel. These complexes are accessible by simple Bronsted acid base reaction of 3a or 3b 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% (4a), 53% (4b), 58% (5a), and 61% (6a) yields, whereas 7a (74% yield) is isolated as colorless powder. The zinc center in 4a, 4b, 5a, and 6a has trigonal bipyramidal coordination, as proven by single-crystal X-ray diffraction analysis. Remarkably, 4a shows the highest catalytic activity hitherto reported for zinc-catalyzed hydrosilylation over a wide range of substrates with a turnover frequency of 970 Furthermore, a catalytic mechanism is proposed.