(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.