Thioketene



In organic chemistry, thioketenes are organosulfur compounds analogous to ketenes with the general formula R2C\dC\dS, where R is alkyl or aryl. The parent thioketene (ethenthione) has the formula CH2\dC\dS. It is the simplest thioketene. Thioketene is stable as a gas, but like most thioketenes, it polymerizes upon condensation.

Some thioketenes are produced as transient species upon pyrolysis of 1,2,3-thiadiazoles. It has been suggested that thioketene could be involved in cell damage processes.

Isolable thioketenes
Thioketenes can be stabilized by either steric protection or by electronic effects. Thus, di-tert-butylthioketene is easily isolated and air-stable. Several examples have been characterized by X-ray crystallography. The C=S distance is 157 pm and the C=C distance is 124 pm, both bonds being suitable for the C=C=S assignment. The violet color characteristic of thioketenes indicates the small HOMO-LUMO gap. These compound are prepared by treatment of the acid chloride with phosphorus pentasulfide as described by the following idealized equation:
 * RCH2COCl + P4S10  ->  RCH=C=S  +  HCl  +  "P4S9O"

Bis(trifluoromethyl)thioketene ((CF3)2C\dC\dS) is an example of an electronically stabilized thioketene.

Reactions
Thioketenes are electrophilic. They add amines to give thioamides:
 * R2C\dC\dS + HNR'2  ->  R2CH\sC(S)\sNR'2

With peroxyacids, they produce thioketene-S-oxides:
 * R2C\dC\dS + R'CO3H  ->  R2C\dC\dS\dO  +  R'CO2H

Thioketenes bind to metal carbonyls giving adducts.

Related compounds

 * carbon subsulfide (S\dC\dC\dC\dS).