Theoretical calculations for neighboring group participation in gas-phase elimination kinetics of 2-hydroxyphenethyl chloride and 2-methoxyphenethyl chloride

Theoretical calculation of the kinetics and mechanisms of gas-phase elimination of 2-hydroxyphenethyl chloride and 2-methoxyphenethyl chloride has been carried out at the MP2/6-31G(d,p), B3LYP/6-31G(d,p), B3LYP/6-31 +G(d,p), B3PW91/6-31G(d,p) and CCSD(T) levels of the theory. The two substrates undergo parallel elimination reactions. The first process of elimination appears to proceed through a three-membered cyclic transition state by the anchimeric assistance of the aromatic ring to produce the corresponding styrene product and HCl. The second process of elimination occurs through a five-membered cyclic transition state by participation of the oxygen of o-OH or the o-OCH₃ to yield in both cases benzohydrofuran. The B3PW91/6-31G(d,p) method was found to be in good agreement with the experimental kinetic and thermodynamic parameters for both substrates in the two reaction channels. However, some differences in the performance of the different methods are observed. NBO analysis of the pyrolysis of both phenethyl chlorides implies a C-Cl bond polarization, in the sense of C^{δ +} Cl^{δ -}, which is a rate-determining step for both parallel reactions. Synchronicity parameters imply polar transition states of these elimination reactions.