Newman–Kwart rearrangement

The Newman–Kwart rearrangement is a type of rearrangement reaction in which the aryl group of an O-aryl thiocarbamate, ArOC(=S)NMe2, migrates from the oxygen atom to the sulfur atom, forming an S-aryl thiocarbamate, ArSC(=O)NMe2. The reaction is named after its discoverers, Melvin Spencer Newman and Harold Kwart. The reaction is a manifestation of the double bond rule. The Newman–Kwart reaction represents a useful synthetic tool for the preparation of thiophenol derivatives.
 * Newman-Kwart-rearrangement-general.png

Mechanism
The Newman–Kwart 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 (ΔH‡ ~ 30 to 40 kcal/mol), necessitating high reaction temperatures (200 to 300 °C, Ph2O as solvent or neat).
 * NKR-mechanism-2D-skeletal.png

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 °C for Pd catalysis, ambient temperature for photoredox).

Use for preparation of thiophenols
The Newman–Kwart 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 °C causes it undergo Newman–Kwart rearrangement to an S-aryl thiocarbamate (4). Alkaline hydrolysis or similar cleavage yields a thiophenol (5).
 * Phenols-to-thiophenols-via-NKR-2D-skeletal.svg