Newman–Kwart Rearrangement
   HOME

TheInfoList



OR:

The Newman–Kwart rearrangement is a type of
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. :


Mechanism

The Newman–Kwart rearrangement is intramolecular. It is generally believed to be a concerted process, proceeding ''via'' a four-membered cyclic
transition 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). : 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 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). :


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