[525-06-4] · C14H10O · Diphenylketene · (MW 194.24)
Physical Data: bp 118-120 °C/1 mmHg.
Solubility: sol benzene, THF.
Form Supplied in: not commercially available.
Preparative Methods: originally prepared by zinc reduction of a-chlorodiphenylacetyl chloride;5 oxidation of benzil monohydrazone with yellow Mercury(II) Oxide followed by thermal rearrangement of the a-diazo ketone;6 dehydrohalogenation of diphenylacetyl chloride with Triethylamine;7 debromination of a-bromodiphenylacetyl bromide by the action of Triphenylphosphine.8
Purification: vacuum distillation at 118-120 °C/1 mmHg.
Handling, Storage, and Precautions: material stored in a tightly stoppered bottle at 0 °C is stable for several weeks. Trace amounts of hydroquinone and storage under nitrogen inhibit polymerization and lead to increased stability. Given the reactivity of diphenylketene with amino acids, care should be taken to avoid skin contact.
Diphenylketene provides a neutral method for the conversion of carboxylic acids into mixed anhydrides.2 Amino acids can be transformed into activated esters through the intermediacy of these mixed anhydrides without racemization (eq 1).9
Diphenylketene undergoes most of the cyclization reactions that are characteristic of ketenes.1 For example, cyclization with hexafluoroacetone leads to a b-lactone (eq 2).10 Reaction with simple alkenes leads to cyclobutanones (eq 3),11 although many of these reactions are prohibitively slow.12 Cyclizations have also been observed with numerous other functionalities, including allenes,13 alkynes,14 azides,15 diimides,16 imines,17 phosphinimides,18 phosphaalkynes,19 and 1,3-dipoles.20
Wittig reaction of diphenylketene leads to the synthesis of allenes (eq 4).4 In fact, tetraphenylallene can be made upon heating diphenylketene in the presence of HMPA.21 Amines also react with diphenylketene; for example, reaction of 2-azabicyclo[2.2.1]hept-5-enes leads to the formation of 2-piperidinones (eq 5).22 Epoxides and oxetanes can be opened with diphenylketene and tetraphenylantimony iodide to generate g- and d-lactones in good yield (eq 6).23
James W. Leahy
University of California, Berkeley, CA, USA