The NewmanâÂÂKwart rearrangement is a type of rearrangement reaction in which the aryl group of an O-aryl thiocarbamate, ArOC(=S)NMe<sub>2</sub>, migrates from the oxygen atom to the sulfur atom, forming an S-aryl thiocarbamate, ArSC(=O)NMe<sub>2</sub>. The reaction is named after its discoverers, Melvin Spencer Newman and Harold Kwart. The reaction is a manifestation of the double bond rule. The NewmanâÂÂKwart reaction represents a useful synthetic tool for the preparation of thiophenol derivatives.
The NewmanâÂÂKwart rearrangement is intramolecular. It is generally believed to be a concerted process, proceeding via a four-membered cyclic transition state (rather than a two-step process passing through a discrete reactive intermediate). The enthalpy of activation for this transition state is generally quite high for typical substrates (ÃÂH<sup>â¡</sup> ~ 30 to 40 kcal/mol), necessitating high reaction temperatures (200 to 300 ðC, Ph<sub>2</sub>O as solvent or heat).
A Pd-catalyzed process and conditions under photoredox catalysis (both proceeding through complex multistep mechanisms) are known. These catalytic processes allow for much milder reaction conditions to be used (100 ðC for Pd catalysis, ambient temperature for photoredox).
The NewmanâÂÂKwart rearrangement is an important prelude to the synthesis of thiophenols. A phenol (1) is deprotonated with a base followed by treatment with a thiocarbamoyl chloride (2) to form an O-aryl thiocarbamate (3). Heating 3 to around 250 ðC causes it undergo NewmanâÂÂKwart rearrangement to an S-aryl thiocarbamate (4). Alkaline hydrolysis or similar cleavage yields a thiophenol (5).