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]
^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.
ISBN978-3-540-68099-4.
ISSN0340-1022.
PMID23605512.
^
abHarvey, 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.
PMID19746383.
^
abLloyd-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.
^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.
^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.
^
abBurns, 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.
PMID20812755.