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.[1][2][3] The reaction is named after its discoverers, Melvin Spencer Newman[4] and Harold Kwart.[5] 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 | |
|---|---|
| Named after | Melvin Spencer Newman Harold Kwart |
| Reaction type | Rearrangement reaction |
| Identifiers | |
| Organic Chemistry Portal | newman-kwart-rearrangement |
| RSC ontology ID | RXNO:0000412 |
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).[3][6] 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).[7]
A Pd-catalyzed process[2] and conditions under photoredox catalysis[8] (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).[6][9]
