An Air- and Water-Stable Hydrogen-Bond-Donor Catalyst for the Enantioselective Generation of Quaternary Carbon Stereocenters by Additions of Substituted Cyanoacetate Esters to Acetylenic Esters.
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abstract
The chiral enantiopure cobalt(III) complex -[Co((S,S)-dpen)3 ]3+ 2Cl- B(C6 F5 )4 - (-(S,S)-23+ 2Cl- B(C6 F5 )4 - ; dpen=1,2-diphenylethylenediamine) is an effective catalyst, together with pyridine (10mol% each), for enantioselective additions of substituted cyanoacetate esters NCCH(R)CO2 R' to acetylenic esters R''CCCO2 R'''. In the resulting adducts NC(R'O2 C)C(R)CR''C=CHCO2 R''', C=C isomers in which the CO2 R''' moiety is trans to the new carbon-carbon bond dominate (avg. ratio 98:2). These are obtained in 70-98% ee (avg. 86%; data for optimum R' and R'''), as determined by 1 HNMR with the chiral solvating agent -(S,S)-23+ 2I- B(3,5-C6 H3 (CF3 )2 )4 - . NMR experiments show that the cyanoacetate and acetylenic esters and pyridine can hydrogen bond to certain NH groups of the catalyst. Rates are zero order in the cyanoacetate and acetylenic esters as well as the catalyst, and implications are discussed.
