Structural aspects of the mechanical and thermal dissociation of the central bond in 2,2′-bis(2,3,4-triarylchromenyl)s
Abstract
2,2′-Bis(2,3,4-triphenylchromenyl)(1a) has been shown to undergo reversible homolytic cleavage of the C(2)–C(2′) bond to give green-coloured chromenyl radicals when subjected to mechanical force or heating in the solid state. 2,4-Bis-(p-chlorophenyl)(1b), 2,4-bis-(p-tolyl)(1c), 2,4-bis-(p-methoxyphenyl)(1d), and 2,4-bis-(p-bromophenyl)(1e) derivatives have been synthesized, and substituent effects on the radical dissociation investigated. Upon pressing 1b exhibits the highest degree of dissociation, while thermal dissociation is facilitated by electron-releasing methyl (1c) and methoxy (1d) groups. Dissociation enthalpies (ΔH) for 1a–e in dichloromethane solutions are evaluated to be 12–15 kcal mol–1. The small values of ΔH are considered to be mainly due to overcrowding around the C(2)–C(2′) bond. MMP2 calculation predicts that meso-1a is more stable than the (±)-isomer and the lowest-energy conformation of meso-1a is a centrosymmetric anti form. In the optimized geometry of meso-1a C(2)–C(2′) the bond length is calculated to be 1.584 Å, which is rather longer than a normal Csp3–Csp2 bond. AM1 calculations give similar results and suggest that through-bond interactions between the lone-pairs on the oxygen atoms and/or 2- and 2′-aryl groups are not significant compared with the steric repulsion.