Diphenyl ketene was first isolated by Hermann Staudinger in 1905 and identified as the first example of the exceptionally reactive class of ketenes with the general formula R1R2C=C=O.
Preparation
The first synthesis by H. Staudinger was based on 2-chlorodiphenylacetyl chloride from which two chlorine atoms are cleaved with zinc in a dehalogenation reaction: An early synthesis uses benzilmonohydrazone, which is oxidized with mercuryoxide and calcium sulfate to form mono-diazoketone, and is then converted into the diphenylketene at 100 °C under nitrogen elimination in 58% yield: A further early diphenylketene synthesis originates from Eduard Wedekind, who had already obtained diphenyl ketene in 1901 by the dehydrohalogenation of diphenylacetyl chloride with triethylamine, without isolation and characterization though. This variant was also described in 1911 by H. Staudinger. A standard laboratory protocol is based on the Staudinger method and yields diphenyl ketene as an orange oil in yields of 53 to 57%. In a more recent process, 2-bromo-2,2-diphenylacetyl bromide is reacted with triphenylphosphine to give diphenyl ketene in yields up to 81%. Recently, a synthesis of diphenyl ketene from diphenylacetic acid and the Hendrickson reagent with water elimination in 72% yield has been reported.
Diphenylketene can undergo attack from a host of nucleophiles, including alcohols, amines, and enolates with fairly slow rates. These rates can be increased in the presence of catalysts. At present the mechanism of attack is unknown, but work is underway to determine the exact mechanism. The high reactivity of the diphenyl ketene is also evident in the formation of three dimers:
the cyclic diketone 2,2,4,4-tetraphenylcyclobutane-1,3-dione by heating with quinoline
the β-lactone 4--3,3-diphenyloxetan-2-one by heating with sodium methoxide and
the tetraline derivative 2,2,4-triphenylnaphthalene-1,3--dione by heating with benzoyl chloride
and oligomers produced therefrom.
Application
Ketenes have many parallels to isocyanates in their constitution as well as in their reactivity. Diphenyl ketene reacts with water in an addition reaction to form diphenylacetic acid, with ethanol to diphenyl acetic ethyl ester or with ammonia to the corresponding amide. Carboxylic acidsproduce mixed anhydrides of diphenylacetic acid, which can be used to activate protected amino acids for peptide linkage. The protected dipeptide Z-Leu-Phe-OEt is thus obtained in 59% yield via the activation of Z-leucine with diphenyl ketene and subsequent reaction with phenylalanine ethyl ester. Diphenyl ketene is prone to autoxidation, in which the corresponding polyester is formed at temperatures above 60 °C via an intermediate diphenyl acetolactone. In a Wittig reaction, allenes can be prepared from diphenyl ketene. With triphenylphosphine diphenylmethylene and diphenyl ketene, at e. g. 140 °C and under pressure tetraphenyl allenes are formed in 70% yield. The synthetically most interesting reactions of diphenyl ketene are cycloadditions, e.g. the reaction with cyclopentadiene yielding a Diels-Alder adduct. Imines such as benzalaniline form β-lactams with diphenyl ketene. With carbonyl compounds β-lactones are formed analogously. The cycloaddition of diphenyl ketene with phenylacetylene leads first to a cyclobutenone which thermally aromatizes to a phenyl vinyl ketene and cyclizes in a cycloaddition to 3,4-diphenyl-1-naphthol in 81% yield. From this so-called Smith-Hoehn reaction a general synthesis method for substituted phenols and quinones has been developed.
