title: Chiroptical Properties of the Ketene and Diazo Chromophores. 1. Conformation and Optical Activity of 1-Alken-1-ones and 1-Diazoalkanes vs Aldehydes.
abstract: 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 alpha-hydrogen is the global minimum of the ketenes and diazoalkanes, unlike the aldehydes where eclipsing by an alpha-alkyl group is preferred over a hydrogen. In all three classes of compounds, the optical rotational strength of the n-pi transition is greatest for conformers with the dihedral angle, CCCX, in the range 80-100 degrees. Within this range, the signs of the n-pi rotational strengths for the ketenes and diazoalkanes are opposite to ones for the aldehydes. As a whole, the torsional dependencies of the n-pi rotational strength of the ketene and diazo chromophores can be explained within the framework of a model of "through-bond" perturbation. CD spectra of (S)-alpha-methyl-substituted butanal (7), 1-penten-1-one (8), and 1-diazobutane (9) were obtained experimentally. The calculated chiroptical properties of equilibrium mixtures of conformers of compounds 7-9 are in agreement with experimental data.
authors.affiliation: Department of Chemistry, The University of Calgary, Calgary, Alberta T2N 1N4, Canada.
authors.forename: Gennady V.
authors.initials: GV
authors.lastname: Shustov
authors.forename: Fang
authors.initials: F
authors.lastname: Sun
authors.forename: Ted S.
authors.initials: TS
authors.lastname: Sorensen
authors.forename: Arvi
authors.initials: A
authors.lastname: Rauk