Structural features of benzylic carbanions. A theoretical study

Abstract

Benzylic anions were studied with the semiempirical MNDO molecular orbital method. Structural changes were analyzed as metal counterions and solvents were allowed to interact with the benzylic carbon. Generally, benzylic carbanions were found to be trigonal planar but became pyramidal or bent when metals were included in the calculation.*A referee has criticized us for regarding the results of our calculations as the absolute truth and that no effort to evaluate the reliability of the approximate computational method with hifh level ab initio calculations has been made. The re lability of MNDO with respect to anions is discussed in reference 12. Generally, the MNDO deficiencies in computed geometries are that CC bonds are too short, CH bonds too long and carbanion centers too flat. Additionally, it can be seen that MNDO underestimates C-Li bond-lengths when compared to the a b initio split-valence basis sets: Clark, T.; Spitznagel, G. W.; Klose, R. and Schleyer P.v.R. in J. Am. Chem. SOC., 106 (16), 4412 (1984). Nonetheless the speed and accuracy of MNDO facilitate the study of larger anions: Dietrich, H.; Waruno, M.; Wilhelm, D.; Clark, T. and Schleyer, P. v. R., Angew. Chem. Int. Ed. EngL 23 (8), 621 (1984) and references therein.lecular orbital methods, the structural features of benzylic anions as isolated molecules, as salts and, in the case of benzyllithium, as solvated species. COMPUTATIONAL METHODS*All calculations were performed on a VAX-11/780 using the MNDO Hamil.tonian in Stewart's MOPAC' or on a CDC Cyber 855 using the MNDO Hamiltonian in Thiel's MNDOC.' Unless specified, all geometries were completely optimized without imposing symmetry constraints, and, all systems were assumed to exist as ground state singlets. The original lithium parameters of Thiel were used.' These parameters closely reproduced the allyllithium structure reported by Boche3 who used an unpublished, refined set of lithium parameters from Schleyer. RESULTS AND DISCUSSIONThe molecules we have considered in this study are depicted in Figures 1 and 2. Although the diphenylmethyl anion has recently been studied by Bank4 we repeat these calculations with the incorporation of counter ions. Also, solvation of organolithiums was restricted to the parent benzyllithium because of the enormous amount of CPU time involved.