ASYMMETRIC CYCLOPROPANATIONS BY RHODIUM(II) N-(ARYLSULFONYL)PROLINATECATALYZED DECOMPOSITION OF VINYLDIAZOMETHANES IN THE PRESENCE OF ALKENES - PRACTICAL ENANTIOSELECTIVE SYNTHESIS OF THE 4 STEREOISOMERS OF 2-PHENYLCYCLOPROPAN-1-AMINO ACID

The rhodium N-(arylsulfonyl)prolinate catalyzed decomposition of vinyl diazomethanes in the presence of alkenes leads to a very general metho d for the synthesis of functionalized cyclopropanes in a highly diaste reoselective and enantioselective mode. A detailed study was undertake n to determine the key factors that control the enantioselectivity of this process. The highest levels of enantioselectivity were obtained u sing cyclic N-(arylsulfonyl)amino acids as ligands for the dirhodium c atalyst, and the optimized catalyst was 4-dodecylphenyl)sulfonyl]-(L)- prolinato]dirhodium. The carbenoid structure has a critical effect on the degree of asymmetric induction, and the combination of a small ele ctron-withdrawing group such as a methyl ester and an electron-donatin g group such as vinyl or phenyl resulted in the highest levels of enan tioselectivity. The use of electron neutral alkenes and pentane as sol vent also enhanced the enantioselectivity of the process. The syntheti c utility of this chemistry was illustrated by its application to the synthesis of ail four stereoisomers of 2-phenylcyclopropan-1-amino aci d. The occurrence of the highly stereoselective cyclopropanations was rationalized by a model in which the ligands were considered to adopt a D-2 symmetric arrangement.