The effect of steric crowding on the structures of solvolytically reactive arylvinyl systems

Abstract

The structures of 1-anisyl-2,2-diphenylvinyl bromide (4a), α-bromo- and α-tosyloxy-9-(p-methoxy-benzylidene) anthrone, (5a) and (5b), and p-acetamido-α-bromostyrene, (6), were determined by X-ray crystallography. Most of the torsional angles of the aryl groups with the double bond are 44.3 ± 3.3°, except that for the angle of the β-ring cis to Br in (4a) which is 68.6″ and that for the α-ring in (5a) which is 75.6°. The central rings of the anthronylidene moiety in (5a) and (5b) are puckered. The C–Br bond lengths in (4a)(1.927 Å) and (5a)(1.923 Å) are longer than in (6)(1.907 Å) and than the mean value for other C–Br bonds. The XCC angle is wider for (5a)(X = Br) than for (5b)(X = O). The geometric parameters obtained from the crystal structure determination were discussed in relation to the contribution of steric crowding to the relative solvolysis rates and k_Br/k_OTs ratios of the corresponding compounds.