The solid-state structure of (+)-(i)-menthyl 2-formamido-4, 4,4-trifluorobutanoate and the hydrogenation of the 2-butenoate precursor

Hydrogenation of Z-(-)-(1′R, 3′R, 4′S)-menthyl 2-formamido-4,4,4-trifluoro-2-butenoate catalyzed by Pd/C was performed at atmospheric pressure to yield a mixture of (2R, 1′R, 3′R, 4′S)- and (2 S, 1′R, 3′R, 4′S)-menthyl 2-formamido-4,4,4-trifluorobutanoate diastereomers in a 55:45 ratio, respectively. Repeated fractional crystallization from ethyl acetate and vapor diffusion of petroleum ether afforded (+)-(2 S, 1′R, 3′R, 4′S)-menthyl 2-formamido-4,4,4-trifluorobutanoate as clear colorless, crystalline prisms which were subjected to single-crystal X-ray diffraction analysis. The crystals belong to the orthorhombic system P2₁2₁2₁, and at 213 K:a=5.054(1), b= 10.000(2), c=32.707(1) Å, V=1652.9(4) ų, Z=4, R(F)=0.040, and R_w(F)=0.037. The finding of the (2 S)-configuration for the formamido-acid portion of the (+)-ester enabled the configurational assignment of the asymmetric hydrogenation products of Z-methyl 2-formamido-4, 4,4-trifluoro-2-butenoate catalyzed by chiral diphosphine/rhodium(I) complexes.