Thioketene
Organic compounds with the structure >C=C=S
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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 (ethenethione) has the formula CH2=C=S. It is the simplest thioketene.[1] Ethenethione is stable as a gas, but like most thioketenes, it polymerizes upon condensation.
Synthesis
Thioketones may be prepared by treatment of acid chlorides with phosphorus pentasulfide as described by the following idealized equation:
- RR'CHC(O)Cl + P4S10 → RR'C=C=S + HCl + "P4S9O"
Some thioketenes are produced as transient species upon pyrolysis of 1,2,3-thiadiazoles.[2] Elimination from α-chloroalkenyl thiolates RR'C=C(Cl)S− also yields thioketenes.[3] These intermediates are believed to be responsible for the cytotoxicity and mutagenicity of trichloroethylene, as well as certain other polyhalogenated alkenes, with toxication occurring via conjugation with glutathione.[4][5]
Isolable thioketenes
Thioketenes can be stabilized by either steric protection or by electronic effects. Thus, di-tert-butyl thioketene is easily isolated and air-stable.[6] 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.[7]
Bis(trifluoromethyl)thioketene ((CF3)2C=C=S) is an example of an electronically stabilized thioketene.[8]
Reactions
Thioketenes are electrophilic. They add amines to give thioamides:[6]
- R2C=C=S + HNR'2 → R2CH−C(S)−NR'2
With peroxyacids, they produce thioketene-S-oxides:[7]
- R2C=C=S + R'CO3H → R2C=C=S=O + R'CO2H
Thioketenes bind to metal carbonyls giving adducts.[9]
Related compounds
- carbon subsulfide (S=C=C=C=S).