ENANTIOSELECTIVE INTRAMOLECULAR CYCLOPROPANATIONS OF ALLYLIC AND HOMOALLYLIC DIAZOACETATES AND DIAZOACETAMIDES USING CHIRAL DIRHODIUM(II) CARBOXAMIDE CATALYSTS

Diazo decomposition of allylic and homoallylic diazoacetates 10a-p and 22a-j catalyzed by chiral dirhodium(II) tetrakis[methyl 2-pyrrolidone -5(S)-carboxylate], Rh-2(5S-MEPY)(4) (7), and its enantiomer, Rh-2(5R- MEPY)(4) (8), produces the corresponding intramolecular cyclopropanati on products 11a-p and 23a-j in good to excellent yields and with excep tional enantioselectivity. Higher enantiocontrol is observed with ally lic diazoacetates than with their homoallylic counterparts, but allyli c diazoacetates are subject to greater variations in enantioselectivit ies with changes in substitution patterns on the carbon-carbon double bond. For example, the enantioselectivities in the intramolecular cycl opropanations of 3-alkyl/aryl-2(Z)-alken-1-yl diazoacetates are genera lly greater than or equal to 94%, whereas the cyclizations of the homo logous 4-alkyl/aryl-3(Z)-alken-1-yl diazoacetates are typically in the range of 70-90% ee. The corresponding 3-alkyl/aryl-2(E)-alken-1-yl an d 4-alkyl/aryl-3(E)-alken-1-yl diazoacetates undergo cyclization with slightly lower ee's (54-85%). Although the Rh-2(5S-MEPY)(4)-catalyzed cyclization of the 2-methallyl diazoacetate 10c proceeds with only 7% ee, alternative chiral dirhodium(II) catalysts, including those with m ethyl N-acylimidazolidin-2-one-4(S)-carboxylate ligands such as Rh-2(4 S-MACIM)(4) (14) and Rh-2(4S-MPAIM)(4) (15), may be employed to increa se the level of enantiocontrol to 78 and 65%, respectively. Some allyl ic diazoacetamides also undergo highly enantioselective cyclization to form cyclopropyl lactams as illustrated by the diazo decomposition of N-allyl diazoacetamide (19) in the presence of dirhodium(II) tetrakis [methyl 2-oxazolidinone-4(S)-carboxylate], Rh-2(4S-MEOX)(4), to give t he 3-azabicydo[3.1.0]hexan-2-one 20 in 98% ee. The absolute configurat ion and the level of enantiocontrol in these intramolecular cyclopropa nations have been interpreted by a transition state model in which the important determinants are (i) the preferred conformation about the r hodium-carbon bond; (ii) the trajectory of approach of the double bond to the metallocarbene center; and (iii) the orientation of the double bond with respect to the chiral face of the catalyst.