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　photolysis　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.　©　2014　Published　by　NRC　Research　Press.