(Cyanovinyl)ketenes From Azafulvenones. An Apparent Retro-Wolff Rearrangement

Flash vacuum pyrolyses or pulsed pyrolysis of azole esters
1b and 20 and their carboxylic acids
1a and
19 or acid chloride 1c give five-membered ring
ketenes (azafulvenones) 2 and 24 which dimerize
to diketopiperazines
3 and 21 but also undergo efficient rearrangement
to (cyanovinyl)- and (o-cyanophenyl)ketene
(6 and 27). The
same products are obtained by elimination of alcohol from esters of
these (cyanovinyl)ketenes. Both types of ketenes
are observed directly by IR spectroscopy and are trapped by alcohols to
give the corresponding azole esters or
(cyanovinyl)acetates. 1-Carbonyl-1H-isoindole
(34) is a highly reactive ketene, dimerizing at 50 K.
All ketene IR
spectra are in good accord with ab initio calculations. The
mechanism of this apparent retro-Wolff rearrangement
is discussed.