Pyrazinyl- and pyrimidinylketenes, bisketenes, and ylides: direct observation and nucleophilic

Pyrazinylketene (9) and 4-pyrimidinylketene (11), identified by their IR absorption at 2128 and 2130 cm−1, respectively, are formed in CH3CN as transient intermediates by photolysis of the corresponding diazoketones. The corresponding ylides 10 and 12 are formed concurrently as longer-lived intermediates identified by their distinctive IR and UV absorption. Reactions of 2-pyridylketene and of 9 and 11 with diethylamine form initial amide enol intermediates leading to dihydroheteroarene intermediates and then to amides, whereas amide enols are not observed in reactions with n-butylamine. Reactions with water give observable dihydroheteroarenes. 2,5-Bis(ketenyl)pyrazine (33) is formed by pho- tolysis of 2,5-bis(diazoacetyl)pyrazine (32) together with the corresponding bis(ylide) 35. The latter is calculated to have two geometrically isomeric structures (bond-stretch isomers) with similar energies, one with a central six-membered aromatic ring and another with a 10-pi electron aromatic system.