SYNTHESIS OF A NOVEL TYPE OF CHIRAL PHOSPHINOCARBOXYLIC ACIDS - THE PHOSPHINE-PALLADIUM COMPLEXES CATALYZED ASYMMETRIC ALLYLIC ALKYLATION

A novel type of chiral cycloalkylphosphines bearing the carboxy group at the beta-position were developed, and used for palladium catalyzed asymmetric alkylation of allylic substrates such as 2-cyclohexenylacetate and 1,3-disubstituted-propenyl acetates (R1CH = CHCH(OAc)R2: R1 = R2 = Ph; R1 = Ph, R2 = (CH2)4OAc; R1 = Ph, R2 = (CH2)6OAc; R1 = Ph, R2 = (CH2)10OAc). Reaction of the propenyl acetates with soft carbon nucleophiles such as triethyl sodiophosphonoacetate and sodiomalonic acid esters in the presence of a palladium catalyst prepared in situ from Pd(OAc)2 and chiral (2-diphenylphosphino)cycloalkanecarboxylic acids (7a,b) gave high yields of alkylation products (PhCH = CHCH(X)Ph: > 77 %ee for X = CH(CO2Et)P(O)(OEt)2 and > 72 %ee for X = CH(CO2Me)2). The alkylation products 15 and 28a-c were converted into optically active alpha-methylene-gamma-lactone and alpha-methylene macrolide derivatives. The high stereoselectivity demonstrated by the chiral phosphinocarboxylic acid-palladium catalyzed allylic alkylation suggested to be caused by an electronic repulsion between the carboxy group on the ligand and the incoming soft carbon nucleophile, which directs the nucleophilic attack on one of the pi-allyl carbons.