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The Newman–Kwart rearrangement is a type of 
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).
rearrangement reaction
In organic chemistry, a rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another at ...
in which the aryl group of an ''O''-aryl thiocarbamate, ArOC(=S)NMe2, migrates from the oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
atom to the sulfur
Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
atom, forming an ''S''-aryl thiocarbamate, ArSC(=O)NMe2. The reaction is named after its discoverers, Melvin Spencer Newman and Harold Kwart. The reaction is a manifestation of the double bond rule
In chemistry, the double bond rule states that elements with a principal quantum number (''n'') greater than 2 for their valence electrons ( period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds). Do ...
. The Newman–Kwart reaction represents a useful synthetic tool for the preparation of thiophenol
Thiophenol is an organosulfur compound with the formula C6H5SH, sometimes abbreviated as PhSH. This foul-smelling colorless liquid is the simplest aromatic thiol. The chemical structures of thiophenol and its derivatives are analogous to phen ...
derivatives.
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Mechanism
The Newman–Kwart rearrangement is intramolecular. It is generally believed to be a concerted process, proceeding ''via'' a four-membered cyclictransition state
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked w ...
(rather than a two-step process passing through a discrete reactive intermediate
In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these comp ...
). The enthalpy of activation for this transition state is generally quite high for typical substrates (Δ''H''‡ ~ 30 to 40 kcal/mol), necessitating high reaction temperatures (200 to 300 °C, Ph2O as solvent or heat).
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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. Aphenol
Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire.
The molecule consists of a phenyl group () ...
(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).
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See also
* Smiles rearrangement *Chapman rearrangement
Carboximidates (or more general imidates) are organic compounds, which can be thought of as esters formed between an imidic acid () and an alcohol, with the general formula .
They are also known as imino ethers, since they resemble imines () w ...
References
{{DEFAULTSORT:Newman-Kwart rearrangement Rearrangement reactions Name reactions