Steric Effects. A Study of a Rationally Designed System

Article abstract of DOI:10.1021/ja00537a036

The principles for the rational design of systems suitable for the study of steric effects are defined.A suitable molecular framework <6-(2-X-phenyl)-5-Y-1,1-dimethylindan, 1> was synthesized and a study of internal rotaition by dynamic NMR spectroscopy (DNMR) 0f 33 derivatives, differing principally in the nature of the molecular fragment (X), showed the following. (i) For 1 (Y = Me; X = halogen) the rotaition barriers (ΔG excit.) increase smoothly and monotonically with the van der Waals radius of X (r_x), which permits the estimation of effective r_x for fragments of lower symmetry. (ii) The rotational barriers are the sum of additive contributions, designated interference values (I^H-X), which can be used to predict the rotational barriers in 2,2'-disubstituted biphenyls. (iii) A simple geometrical parameter, apparent overlap (r*), which is related to the distortion of the framework in the transition state, is proposed and found to have an excellent linear correlation with the barrier to rotation in 2,2'-disubstituted biphenyls. (iv) This correlation can be used for a semiquantitative estimation of rotational barriers in biaryls and other systems.