Thioketene

In organic chemistry, thioketenes are organosulfur compounds analogous to ketenes with the general formula R₂C=C=S, where R is alkyl or aryl. The parent thioketene (ethenthione) has the formula CH₂=C=S. It is the simplest thioketene.^[1] Ethenthione is stable as a gas, but like most thioketenes, it polymerizes upon condensation.

The general structure of thioketenes

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

Isolable thioketenes

Thioketenes can be stabilized by either steric protection or by electronic effects. Thus, di-tert-butylthioketene is easily isolated and air-stable.^[4] 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.^[5] 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=C=S) is an example of an electronically stabilized thioketene.^[6]

Reactions

Thioketenes are electrophilic. They add amines to give thioamides:^[4]

R₂C=C=S + HNR'₂ → R₂CH−C(S)−NR'₂

With peroxyacids, they produce thioketene-S-oxides:^[5]

R₂C=C=S + R'CO₃H → R₂C=C=S=O + R'CO₂H

Thioketenes bind to metal carbonyls giving adducts.^[7]

Related compounds

• carbon subsulfide (S=C=C=C=S).