Theoretical study of methoxy group influence in the gas-phase elimination kinetics of methoxyalkyl chlorides

The unimolecular gas-phase elimination kinetics of 2-methoxy-1- chloroethane, 3-methoxy-1-chloropropane, and 4-methoxyl-1-chloroburane has been studied by using density functional theory (DFT) methods to propose the most reasonable mechanisms of decomposition of the aforementioned compounds. Calculation results of 2-methoxy-1-chloroethane and 3-methoxy-1-chloropropane suggest dehydrochlorination through a concerted nonsynchronous four-centered cyclic transition state (TS) to give the corresponding olefin. In the case of 4-methoxyl-1-chloroburane, in addition to the 1,2-elimination mechanism, the anchimeric assistance by the methoxy group, through a polar five-centered cyclic TS, provides additional pathways to give 4-methoxy-butene, tetrahydrofuran and chloromethane. The bond polarization of the C-Cl, in the direction of C ^δ+···Cl ^{δ -}, is the limiting step of these elimination reactions. The significant increase in rate together with the formation of a cyclic product tetrahydrofuran in the gas-phase elimination of 4-methoxyl-1-chloroburane is attributed to neighboring group participation of the oxygen of the methoxy group in the TS. The theoretical calculations show a good agreement with the reported experimental results.