Newman–Kwart Rearrangement (NKR) refers to the thermal rearrangement of O-thiocarbamates to their corresponding S-thiocarbamates under high-temperature conditions. The reaction is named after its discoverers, Melvin Spencer Newman and Harold Kwart. The NKR is an important method for the synthesis of thiophenols and their derivatives.

• Reagents: Solvent (e.g., DMA, NMP)

• Reactant: O-Thiocarbamate

• Product: S-Thiocarbamate

• Reaction Type: Rearrangement

Experimental Tips:

• Thiophenol synthesis: Phenol reacts with thiocarbamoyl chloride under basic conditions to form O-aryl thiocarbamates. Upon heating, the O-aryl thiocarbamate undergoes Newman–Kwart rearrangement to form the corresponding S-aryl thiocarbamate, which can then be hydrolyzed to give thiophenol.

• Electron-withdrawing groups at the para or ortho positions significantly enhance reactivity, as they lower the electron density of the aromatic ring and stabilize the resulting negative charge, facilitating nucleophilic attack.

• Photocatalysis [1] or photoredox catalysis [2] can lower the thermal threshold of the reaction, enabling milder reaction conditions.

Mechanism:

The Newman–Kwart rearrangement is an intramolecular process driven by the conversion of a C=S bond into a thermodynamically more stable C=O bond. It is generally believed to proceed via a concerted transition state involving a four-membered cyclic intermediate.

Original Literature:

• Newman, M. S.; Karnes, H. A. J. Org. Chem. 1966, 31(12), 3980–3984.

• Kwart, H.; Evans, E. R. J. Org. Chem. 1966, 31(2), 410–413.

Key References:

• The role of a Lewis acid in the Nenitzescu indole synthesis, Tetrahedron Letters, 2008, 49(50), 7106–7109.

• The Newman–Kwart rearrangement re-evaluated by microwave synthesis, Tetrahedron, 2006, 62(19), 4685–4689.

Further Reading:

• Harvey, J. N.; Jover, J.; Lloyd-Jones, G. C.; Moseley, J. D.; Murray, P. M.; Renny, J. S. Angew. Chem. Int. Ed. 2009, 48(41), 7612–7615.

• Perkowski, A. J.; Cruz, C. L.; Nicewicz, D. A. J. Am. Chem. Soc. 2015, 137(50), 15684–15687.