Palladium-catalyzed enantioselective allylic alkylation of thiocarboxylate ions: asymmetric synthesis of allylic thioesters and memory effect/dynamic kinetic resolution of allylic esters.
The palladium-catalyzed allylic alkylation of KSAc and KSBz with racemic cyclic and acyclic allylic esters by using N,N'-(1R,2R)-1,2-cyclohexandiylbis[2-(diphenylphosphino)-benzamide] as ligand frequently gave the corresponding allylic thioesters with high ee values and yields. The reaction of the cyclic allylic carbonates with KSAc in the presence of H(2)O was accompanied by a partial palladium-catalyzed enantioselective "hydrolysis" of the substrates with formation of the corresponding enantioenriched allylic alcohols. The degree of the "hydrolysis" was strongly dependent on the solvent and the thiocarboxylate ion. Highly selective kinetic resolutions (KRs) were observed in the palladium-catalyzed reaction of the racemic cyclohexenyl and cycloheptenyl acetates with KSAc. While the KR of the cyclohexenyl acetate is characterized by a selectivity factor S = 72 +/- 19, that of the cycloheptenyl acetate afforded (R)-cycloheptenyl acetate of >or=99% ee in 48% yield and (S)-cycloheptenyl thioacetate of 98% ee in 50% yield. The palladium-catalyzed reaction of the racemic cyclopentenyl acetate with KSAc showed a strong "memory effect" (ME), that is, both enantiomers reacted with different enantioselectivities. The ME was probed by studying the palladium-catalyzed reactions of both the matched acetate of >or=99% ee and the mismatched acetate of >or=99% ee with KSAc. The acetates not only reacted with different enantioselectivities and rates but also suffered an unexpected and concomitant palladium-catalyzed racemization in the presence of the chiral ligand. This led in the case of the mismatched acetate to a temporary dynamic kinetic resolution (DKR) that featured a racemization of the mismatched acetate by the chiral catalyst. Studies of the palladium-catalyzed reaction of the racemic cyclopentenyl acetate, carbonate, and naphthoate with KSAc in the presence of the chiral ligand also showed the ME to be strongly dependent on the nucleofuge. This also allowed the synthesis of (S)-cyclopentenyl thioacetate of 92% ee in high yield from the racemic cyclopentenyl naphthoate.
Lüssem
Bernhard J
BJ
Institut für Organische Chemie der Rheinisch-Westfälischen Technischen Hochschule (RWTH) Aachen, Prof.-Pirlet-Str. 1, 52056 Aachen, Germany.
Gais
Hans-Joachim
HJ