Newman–Kwart rearrangement

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). 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). :[[File:NKR-mechanism-2D-skeletal.png|700px]] A Pd-catalyzed process and conditions under photoredox catalysis (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). :[[File:Phenols-to-thiophenols-via-NKR-2D-skeletal.svg|900px]]

References

References

1. (2007). "Thione–Thiol Rearrangement: Miyazaki–Newman–Kwart Rearrangement and Others". Springer Berlin / Heidelberg.
2. (2009). "The Newman–Kwart Rearrangement of O-Aryl Thiocarbamates: Substantial Reduction in Reaction Temperatures through Palladium Catalysis". [[Angewandte Chemie.
3. (2008). "Mechanism and Application of the Newman-Kwart O→S Rearrangement of O-Aryl Thiocarbamates". [[Synthesis (journal).
4. (1966). "The Conversion of Phenols to Thiophenols via Dialkylthiocarbamates". [[Journal of Organic Chemistry.
5. (1966). "The Vapor Phase Rearrangement of Thioncarbonates and Thioncarbamates". [[Journal of Organic Chemistry.
6. (2011-02-05). "Expanding the scope of the Newman–Kwart rearrangement — A computational assessment". Canadian Journal of Chemistry.
7. (2015-12-23). "Ambient-Temperature Newman–Kwart Rearrangement Mediated by Organic Photoredox Catalysis". Journal of the American Chemical Society.
8. (2010). "The Molecularity of the Newman−Kwart Rearrangement". [[Journal of Organic Chemistry.
9. Melvin S. Newman. (1971). "Thiophenols from Phenols: [[2-Naphthalenethiol]]". Org. Synth..