Theoretical Studies of the Gas-Phase Identity Nucleophilic Substitution Reactions of Cyclopentadienyl Halides

Abstract

The gas phase identity nucleophilic substitution reactions of halide anions (X = F, Cl, Br) with cyclopentadienyl halides (I) are investigated at the B3LYP/6-311+G**, MP2/6-311+G** and G2(+)MP2 levels involving five reaction pathways: σ attack S N 2, β-S N 2'-syn, β-S N 2'-anti, γ-S N 2'-syn and γ-S N 2'-anti paths. In addition, the halide exchange reactions at the saturated analogue, cyclopentyl halides (2), and the monohapto circumambulatory halide rearrangements in 1 are also studied at the same three levels of theory. In the σ-attack S N 2 transition state for 1 weak positive charge develops in the ring with X = F while negative charge develops with X = Cl and Br leading to a higher energy barrier with X = F but to lower energy barriers with X = Cl and Br than for the corresponding reactions of 2. The π-attack β-S N 2' transition states are stabilized by the strong n C - π* C = C charge transfer interactions, whereas the π-attack γ-S N 2' transition states are stabilized by the strong n C -σ* C - X interactions. For all types of reaction paths, the energy barriers are lower with X = F than Cl and Br due to the greater bond energy gain in the partial C-X bond formation with X = F. The β-S N 2' paths are favored over the γ-S N 2' paths only with X = F and the reverse holds with X = Cl and Br. The σ-attack S N 2 reaction provides the lowest energy barrier with X = Cl and Br, but that with X = F is the highest energy barrier path. Activation energies for the circumambulatory rearrangement processes are much higher (by more than 18 kcal mol - 1 ) than those for the corresponding S N 2 reaction path. Overall the gas-phase halide exchanges are predicted to proceed by the σ-attack S N 2 path with X = Cl and Br but by the β-S N 2'-anti path with X = F. The barriers to the gas-phase halide exchanges increase in the order X = F < Br < Cl, which is the same as that found for the gas-phase identity methyl transfer reactions.