Catalytic Enantioselective Hydrosilylation of Ketones with Rhodium-Phosphite Complexes Containing a TADDOLate and a Dihydrooxazole Unit
作者:Dieter K. Heldmann、Dieter Seebach
DOI:10.1002/(sici)1522-2675(19990707)82:7<1096::aid-hlca1096>3.0.co;2-i
日期:1999.7.7
New types of chiral phosphorus/nitrogen ligands, capable of forming six-membered-ring metal chelates have been prepared from alpha,alpha,alpha',alpha'-tetraaryl-1,2-dioxolane-4,5-dimethanols (TADDOLs), PCl3, and dihydrooxazole alcohols (from amino acids) (7 in Scheme 1). The X-ray crystal structure of a Rh complex of one of these ligands, 8b, has been determined (Scheme 2 and Fig.). Enantioselective hydrosilylations of dialkyl and aryl alkyl ketones with Ph2SiH2/0.01 equiv. Rh-1.7 have been studied and found to provide secondary alcohols in enantiomer ratios of up to 97: 3 (Scheme 3 and Table). The ligand prepared from (R,R)-TADDOL and the (R)-valine-derived (R)-alpha,alpha-dimethyl-4-isopropyl-4,5-dihydro gives better results than the (R,R,S)-isomer (7d vs. 7c in Scheme 3), and an i-Pr group on the 4,5-dihydrooxazole ring gives rise to a slightly better selectivity than a Ph group. With the (R,R,R)-ligands the hydrogen transfer occurs from the Re face of the oxo groups (Sclleme 4).