ENANTIOSELECTIVE INTRAMOLECULAR CYCLOPROPANATIONS OF ALLYLIC AND HOMOALLYLIC DIAZOACETATES AND DIAZOACETAMIDES USING CHIRAL DIRHODIUM(II) CARBOXAMIDE CATALYSTS
Mp. Doyle et al., ENANTIOSELECTIVE INTRAMOLECULAR CYCLOPROPANATIONS OF ALLYLIC AND HOMOALLYLIC DIAZOACETATES AND DIAZOACETAMIDES USING CHIRAL DIRHODIUM(II) CARBOXAMIDE CATALYSTS, Journal of the American Chemical Society, 117(21), 1995, pp. 5763-5775
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.