Newman-Kwart rearrangement

03 May 2008 - Named reactions

Name : Newman-Kwart rearrangement
What : rearrangement reaction of O-aryl N,N-dialkyl thiocarbamate to S-aryl N,N-dialkyl thiocarbamate
Year: 1966
Literature: DOI DOI

Why: Convert phenols to thiophenols
Preceded by: Schonberg rearrangement

Kwart and Evans improved yields of the Schonberg reaction by letting the reaction take place in the gas phase or by replacing an alkoxy group in the thiocarbonate by a alkylamino group (thiocarbamate). Hydrolysis by sodium hydroxide yielded the mercaptan. Kwart also speculated on the direct desulfurization (Raney Nickel) of the carbamate to the hydrocarbon effectively opening a method for removing aromatic hydroxyl groups.

A year later Newman pretty much did the same.
The Kwart rearrangement was investigated in 2008 by a group of AstraZeneca researchers (DOI) who gave it a full microwave kinetic treatment. The reaction is promoted by electron-withdrawing groups at the aromatic para position and the entropy of activation is found to be strongly negative. These findings confirm a ipso substitution reaction mechanism with a four-membered transition state as in structure A.

The group also unearthed a competing bimolecular reaction path at higher reactant concentrations (reaction carried out in dichlorobenzene) with a corresponding 8-membered TS.