Insights into the kinetics and molecular mechanism of the Newman-Kwart rearrangement

The kinetic and thermodynamic parameters of Newman-Kwart rearrangement (NKR) of N,N-dimethyl O-arylthiocarbamate into N,N-dimethyl S-arylcarbamate have been evaluated using the Minnesota density functionals (M06-2X and MN15-L) conjugated with the aug-cc-pVTZ basis set and compared with benchmark CBS-QB3 results. Environments of diphenyl ether and o-dichlorobenzene solvents were simulated implicitly by means of the SMD solvation model. The employed temperatures 505 K in diphenyl ether and 413 K in o-dichlorobenzene were more than two times larger than the cross-over temperature, implying shallow tunneling. The evaluated kinetic and energetic parameters using the CBS-QB3 method were in excellent agreement with the experimentally supplied data. The rate constant in diphenyl ether can be expressed by original and modified Arrhenius equations kCBS-QB3/Eckart(404-606K) = 6.46 x 10(12) exp(-19944.6/T) and kCBS-QB3/Eckart(404-606K) = 1.26 x 10(10)T(0.865) exp(-19516.8/T), respectively. Topological evolution along the reaction has been studied at the B3LYP/CBSB7 level in an o-dichlorobenzene environment using the quantum chemical topology tools. The electron density in C-O and C-S bonds was strongly polarized toward the oxygen and sulfur atoms, respectively, in such a manner that carbonyl and thiocarbonyl bonds cannot be described by Lewis's bonding model. Topological and chemical events along the reaction may be described as: (i) formation of a new monosynaptic basin V(C-1) associated with the pseudoradical center on the ipso-carbon atom by means of Fold (F-dagger) type catastrophe, and (ii) transformation of disynaptic basin V(C-1,O) into the monosynaptic basin V-3(O) associated with the heterolytic rupture of the C-1-O bond by means of elliptic umbilic (E) catastrophe, and merging of V-3(S) and V(C-1) monosynaptic basins into the new disynaptic basin of V(C-1,S) associated with the formation of the C-1-S bond via usual sharing of the non-bonding electron densities (C-1-to-S coupling of pseudoradical centers) by means of a Cusp (C) type catastrophe. The detailed picture of chemical events along the NKR reveals that the electron density flow does not take place in a cyclic and one-way curve, suggesting a non-concerted mechanism.

Automated summary

The kinetic and thermodynamic parameters of the Newman-Kwart rearrangement were evaluated using Minnesota density functionals with the aug-cc-pVTZ basis set and compared with benchmark results. The study showed excellent agreement between the evaluated parameters and experimental data, with topological and chemical events occurring during the reaction process.