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.^[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 heat).^[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]

See also

• Smiles rearrangement
• Chapman rearrangement

References

1. Zonta, C.; De Lucchi, O.; Volpicelli, R.; Cotarca, L. (2007). Thione–Thiol Rearrangement: Miyazaki–Newman–Kwart Rearrangement and Others. Topics in Current Chemistry. Vol. 275. Springer Berlin / Heidelberg. pp. 131–161. doi:10.1007/128_065. ISBN 978-3-540-68099-4. ISSN 0340-1022. PMID 23605512.
2. Harvey, J. N.; Jover, J.; Lloyd-Jones, G. C.; Moseley, J. D.; Murray, P. M.; Renny, J. S. (2009). "The Newman–Kwart Rearrangement of O-Aryl Thiocarbamates: Substantial Reduction in Reaction Temperatures through Palladium Catalysis". Angew. Chem. Int. Ed. 48 (41): 7612–7615. doi:10.1002/anie.200903908. PMID 19746383.
3. Lloyd-Jones, G. C.; Moseley, J. D.; Renny, J. S. (2008). "Mechanism and Application of the Newman-Kwart O→S Rearrangement of O-Aryl Thiocarbamates". Synthesis. 2008 (5): 661–689. doi:10.1055/s-2008-1032179.
4. Newman, M. S.; Karnes, H. A. (1966). "The Conversion of Phenols to Thiophenols via Dialkylthiocarbamates". J. Org. Chem. 31 (12): 3980–3984. doi:10.1021/jo01350a023.
5. Kwart, H.; Evans, E. R. (1966). "The Vapor Phase Rearrangement of Thioncarbonates and Thioncarbamates". J. Org. Chem. 31 (2): 410–413. doi:10.1021/jo01340a015.
6. Burns, Matthew; Lloyd-Jones, G. C.; Moseley, J. D.; Renny, J. S. (2010). "The Molecularity of the Newman−Kwart Rearrangement". J. Org. Chem. 75 (19): 6347–6353. doi:10.1021/jo1014382. PMID 20812755.
7. Jacobsen, Heiko; Donahue, James P. (2011-02-05). "Expanding the scope of the Newman–Kwart rearrangement — A computational assessment". Canadian Journal of Chemistry. 84 (11): 1567–1574. doi:10.1139/v06-161.
8. Perkowski, Andrew J.; Cruz, Cole L.; Nicewicz, David A. (2015-12-23). "Ambient-Temperature Newman–Kwart Rearrangement Mediated by Organic Photoredox Catalysis". Journal of the American Chemical Society. 137 (50): 15684–15687. doi:10.1021/jacs.5b11800. ISSN 0002-7863. PMID 26645387.
9. Melvin S. Newman; Frederick W. Hetzel (1971). "Thiophenols from Phenols: 2-Naphthalenethiol". Org. Synth. 51: 139. doi:10.15227/orgsyn.051.0139.