CHIROPTICAL PROPERTIES OF THE KETENE AND DIAZO CHROMOPHORES. 1. CONFORMATION AND OPTICAL ACTIVITY OF 1-ALKEN-1-ONES AND 1-DIAZOALKANES VS ALDEHYDES

Ab initio methods have been employed to study the conformational behavior and chiroptical properties of acyclic, structurally related aldehydes, ketenes, and diazoalkanes of the type MeCHRCH=XY. The study involved aldehydes 1, 4, and 7 (XY = O, R = H, Me, Et, correspondingly), ketenes (1-alken-1-ones) 2, 5, and 8 (XY = CO, R = H, Me, Et), and 1-diazoalkanes 3, 6, and 9 (XY = NN, R = H, Me, Et). Geometries were optimized at the B3LYP/6-31G* level, stationary points were characterized by vibrational frequency analysis, and final energies of 7−9 were obtained at the B3LYP/6-311+G**//B3LYP/6-31G* level. The chiroptical properties were calculated by the CIS/6-31+G* method. It was found that rotational barriers of the functional group (CHXY) about the CC bond are lowered in the following order:  aldehydes (1.5−2.1 kcal mol-1) > ketenes (1.3−1.6) > diazoalkanes (0.5−0.7). A conformer with the C=X bond eclipsed by an α-hydrogen is the global minimum of the ketenes and diazoalkanes, unlike the aldehydes where ecli...