Dae Dong Sung

Theoretical Studies of the Nucleophilic Substitution of Halides and Amine at a Sulfonyl Center

Gas-phase nucleophilic substitution reactions, F(-) + CH(3)SO(2)F, Cl(-) + CH(3)SO(2)Cl, Cl(-) + CH(3)SO(2)F, and NH(3) + CH(3)SO(2)Cl, have been investigated at the B3LYP/6-311+G** and MP2/6-31+G* levels of theory. A very shallow well for the reaction intermediate in a triple-well potential energy surface (PES) was observed for the identity fluoride exchange, but double well PESs were obtained for the other three reactions with three different PES profiles. NBO analyses of the transition states showed substantial charge transfer interactions in all cases which provided a much larger amount of stabilization energy compared with the corresponding species at the carbon center of methyl halides. This difference is primarily caused by the strong electropositive nature of the sulfur center. The F-S-F axial linkage in the distorted TBP type intermediate in the identity fluoride exchange reaction exhibited a weak three-center, four-electron omega-bonding, which is considered to provide stability of the intermediate. All the reactant (RC) and product complexes (PC) have Cs symmetry. The symmetry plane bisects angles HCH (of methyl group), OSO (of sulfonyl group), and HNH (of ammonia). Vicinal charge transfer interactions between the two out-of-plane C-H, S-O, and N-H bonds provide extra stabilization to the ion-dipole complexes together with H-bond formation of in-plane H atom with the nucleophile and/or leaving group.

Methanolyses of para-substituted benzoyl chlorides in isodielectric binary mixtures

The kinetics and mechanism of solvolysis of p-substituted benzoyl chlorides have been investigated in three isodielectric systems: methanol–acetonitrile, methanol–nitrobenzene, and methanol–nitromethane mixtures. Only SN1–SN2 processes are favoured by electron-donating substituents in all binary systems, but carbonyl addition processes become dominant in the low basicity (β) region of binary mixtures, especially in methanol–nitromethane for compounds with electron-withdrawing substituents. The rates for unsubstituted substrates as well as those substitutes with electron-withdrawing groups are higher than those for p-methyl substituted substrates, suggesting the contribution of a combined SN1–SN2 and carbonyl addition pathway, rather than the involvement of a concerted displacement with variable transition state.

Ethyl anomaly in the nucleophilic substitution at a series of β-methylated alkyl RCH2Z and carbonyl RCOZ carbon centers for R = Me, Et, i-Pr, t-Bu, and Z = LG.

We have carried out DFT studies to explore the cause of anomalously fast reaction rates of ethyl group (R = Et) in the gas-phase S(N)2 reactions of RCH(2)Cl+Cl(-) and RCH(CN)Cl+Cl(-), and also for those in the cationic forms of RCH(2)(+) and RCH(CN)(+) with R = Me, Et, i-Pr, and t-Bu. The TS stabilization by hyperconjugative donor-acceptor vicinal charge transfers (CTs) from R to the major NBOs at the reaction center carbon in the S(N)2 TSs were estimated using natural bond orbital (NBO) analyses. In all cases the hyperconjugative CT stabilization increases in the order R = t-Bu < i-Pr < Me < Et in agreement with the experimental as well as theoretical rate orders, exhibiting an ethyl anomaly. We have also determined the reorganization energies and hyperconjugative CTs from R to the two major NBOs, C-O(-) and C-N(+), in the tetrahedral intermediate formed with five water molecules, T(5w), by transformation of sp(2) to sp(3) centers in the reactions of RC(═O)OC(6)H(5) with NH(3). The reorganization energy is the lowest and CT stabilization is the strongest with R = Et in line with the fastest experimental rate. We conclude that C-H is a better donor than C-C bond orbital and hyperconjugative vicinal σ chain extension leads to a stronger CT stabilization in the TS. The stronger CT stabilization for R = Et rather than Me is achieved by enhanced hyperconjugative CT to the reaction center in the TS as a result of narrower energy gap and greater overlap brought about by long-range orbital mixing as the C-H σ-chain is extended from n = 2 for Me to n = 3 for Et. We find that CT properties of the all-trans vicinal hyprconjugative C-H σ-chains are closely analogous to the corresponding conjugative polyene π-chains although skeletal patterns of bridge bonds are different and the stabilization energy gained by extension of the σ-chain is much weaker than that gained by the π-chain.