Carbonyl compounds, rhodium catalyzed hydrosilylation

Asymmetric Hydrosilylation with Dihydrosilanes. Chiral alkoxysilanes have also been obtained in the rhodium catalyzed hydrosilylation of carbonyl compounds (68, 78, 79) (eq. [29]). 

Yu and co-workers have expanded upon Ojima s work through development of an effective Rh-catalyzed protocol for the cyclization/hydrosilylation of allenyl carbonyl compounds to form silylated vinylcycloalkanols and heterocyclic alcohols.For example, reaction of tosylamide 44 (X = NTs, R = H, n= ) and triethylsilane catalyzed by Rh(acac)(GO)2 (1 mol%) under GO (10 atm) at 70 °G for 8h gave the silylated vinyl pyrrolidinol 45 (X = NTs, R = H, n= ) in 74% yield with exclusive formation of the m-diastereomer (Equation (29)). The rhodium-catalyzed reaction was also effective for the cyclization of alleneones and for the formation of carbocycles, oxygen heterocycles, and six-membered cyclic alcohols (Equation (29)). However, Rh-catalyzed cyclization/hydrosilylation of allenyl carbonyl compounds that possessed substitution on an allenyl carbon atom was not established (Equation (29)). The efficiency of the Rh-catalyzed reaction of allenyl carbonyl compounds depended strongly on GO pressure. Reactions run under 10 atm GO were more efficient than were... 

Hydrosilylation of various carbonyl compounds, enones and related functional groups catalyzed by Group VIII transition metal complexes, especially phosphine-rhodium complexes, have been extensively studied1,3, and the reactions continue to serve as useful methods in organic syntheses. 

A number of metal complexes catalyze the hydrosilylation of various carbonyl compounds by triethylsilane. Stereoselectivity is observed in the hydrosilylation of ketones as in the reactions of 4-t-butylcyclohexanone and triethylsilane catalyzed by ruthenium, chromium, and rhodium metal complexes (eq 4). Triethylsilane and Chlorotris(triphenylphosphine)rho-dium(I) catalyst effect the regioselective 1,4-hydrosilylation of Q ,/3-unsaturated ketones and aldehydes. Reduction of mesityl oxide in this manner results in a 95% yield of product that consists of 1,4- and 1,2-hydrosilylation isomers in a 99 1 ratio (eq 5). This is an exact complement to the use of phenylsilane, where the ratio of respective isomers is reversed to 1 99. ... 

First, for reasons of clarity, the currently-accepted mechanism of transition-metal complex catalyzed-hydrosilylation reactions will be described briefly. Furthermore, consideration of selective, if not asymmetric, reduction of certain carbonyl compounds by way of rhodium(I)-catalyzed hydrosilylation (Section 4) is included in this review because the catalytic process and stereochemical course of this reaction correlate closely with those of their asymmetric reduction under similar conditions that will be described in the succeeding section. 

SELECTIVE REDUCTION OF CARBONYL COMPOUNDS via HYDROSILYLATION CATALYZED BY A RHODIUM(I) COMPLEX... 

In 1958, Russian chemists [31] reported that chloroplatinic acid-catalyzed hydrosilylation of a,/3-unsaturated carbonyl compounds takes place in a 1,4-fashion. Recently, it has been disclosed [35] that highly selective 1,2- as well as 1,4-addition of hydrosilanes to ajS-unsaturated terpene ketones can be achieved by using chloro-tris(triphenylphosphine)rhodium(I) (7), the selectivity depending markedly on the nature of the hydrosilane employed as described in Section 4.1. This achievement has resulted in studies on two kinds of selective asymmetric hydrosilylation of a, unsaturated carbonyl compounds by making use of either selective 1,4-addition or 1,2-addition the 1,4-addition induces asymmetry on a jS-carbon to afford optically active saturated carbonyl compounds, while the 1,2-addition gives optically active allylic alcohols. 

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