title: The molecularity of the Newman-Kwart rearrangement.
abstract: It was recently reported that the venerable Newman-Kwart rearrangement (1→2) proceeds via mixed first- and second-order kinetics. Prior to this, the rearrangement had been considered to proceed exclusively via an intramolecular O(Ar)→S(Ar) migration. A new bimolecular pathway, possibly involving an 8-membered cyclic transition state, was proposed to account for reaction rates that increased disproportionately with substrate concentration under microwave heating conditions. We report a reanalysis of the kinetics and molecularity of the rearrangement of N,N-dimethyl O-(p-nitrophenyl)thiocarbamate 1a in N,N-dimethylacetamide solvent. Using HPLC, isotopic labeling ((2)H, (18)O, (34)S), and ESI-ICRMS methods, we show that there is no evidence for a bimolecular pathway en route to 2a, with near-perfect exponential decay in 1a at concentrations ranging from 0.11 to 4.70 M. Instead, it is demonstrated that under the microwave heating conditions, a delayed negative feedback signal to the microwave power balancing loop results in oscillatory reaction overheating. Due to higher tan δ in the solute, the amplitude of this oscillation increases with the concentration of 1a, and this phenomenon best accounts for the kinetic behavior previously misinterpreted as being mixed first- and second-order in nature.
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
authors.forename: Matthew
authors.initials: M
authors.lastname: Burns
authors.forename: Guy C
authors.initials: GC
authors.lastname: Lloyd-Jones
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Moseley
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Renny
mesh_terms: D006358:Hot Temperature; D007700:Kinetics; D008872:Microwaves; D015394:Molecular Structure; D013237:Stereoisomerism; D013859:Thiocarbamates / Q000737:chemistry*
chemical_list: D013859:Thiocarbamates