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

Abstract

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 k^{CBS-QB3/Eckart}_404-606K = 6.46 × 10¹² exp(−19944.6/T) and k^{CBS-QB3/Eckart}_404-606K = 1.26 × 10¹⁰T^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(C₁) associated with the pseudoradical center on the ipso-carbon atom by means of Fold (F^†) type catastrophe, and (ii) transformation of disynaptic basin V(C₁,O) into the monosynaptic basin V₃(O) associated with the heterolytic rupture of the C₁–O bond by means of elliptic umbilic (E) catastrophe, and merging of V₃(S) and V(C₁) monosynaptic basins into the new disynaptic basin of V(C₁,S) associated with the formation of the C₁–S bond via usual sharing of the non-bonding electron densities (C₁-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.