Chiroptical properties of the ketene and diazo chromophores. Part 2.1 Dissymmetric perturbing influence of α-alkyl substituents on the n–π* transitions in cyclic dialkylketenes and 2-diazoalkanes vs. cyclic ketones

Abstract

The relationship between the n–π* optical activity and the molecular geometry has been studied for structurally related ketones, ketenes, and diazoalkanes by theoretical and experimental methods. Ketones (4a–e), ketenes (5a–e), and diazoalkanes (6a–e) have a rigid bicyclo[2.2.1]heptane skeleton while ketenes (7a,b) have the less rigid cyclohexyl framework. Geometries were optimized at the B3LYP/6-31G* level and chiroptical properties were calculated by the CIS/6-31+G* method. The theoretical signs of the n–π* Cotton effects are in agreement with available experimental measurements: ketones 4a–e (literature data), ketenes 5d,e (this work), and diazoalkanes 6d,e (this work). All signs are correctly predicted from additive torsional dependencies of the n–π* optical rotational strengths {[R]^n–π*(ϕ) models}. The models reflect the dissymmetric perturbing influence of α-alkyl substituents while taking some account of the relative strain of the C_α–C_β bonds. Analysis of the chiroptical properties of (2S,6S)-(2,6-dimethylcyclohex-ylidene)methanone 7b by means of the ketene [R]^n–π*(ϕ) model and of the ab initio calculations show that this compound exists preferentially in the form of a chair conformer, for which a fast degenerate ring conversion is typical.