Preparation and chemistry of an unexpectedly stable alpha-oxoketene-pyridine zwitterion, 2,2-bis(tert-butylcarbonyl)-1-[4-(dimethylamino)pyridinio]ethen-1-olate

Treatment of dipivaloylketene 1 and its dimers 2 and 3 with 4-(dimethylamin o)pyridine (DMAP) in acetonitrile affords the alpha -oxoketene-pyridine zwi tterion 4 as a pale yellow solid. This is the first example of a stable zwi tterion obtained from a true alpha -oxoketene. In anhydrous solution at roo m temp., compound 4 largely cleaves into the reactants 1 and DMAP. At -60 d egreesC the equilibrium is shifted entirely towards the zwitterion 4, as sh own by C-13 NMR measurements. Calculations using density functional theory (B3LYP/6-311+G**) are in excellent agreement with the formation and relativ e stability of 4. N-15 labelling experiments demonstrate that the ring nitr ogen atom of DMAP is involved in generating the new zwitterionic C-N bond. Reactions of 4 with NH and OH nucleophiles in solution afforded the corresp onding dipivaloyl acetic acid derivatives 6 and 7, whereas acetone or benzy lidene aniline undergo cycloaddition reactions of the hetero-Diels-Alder ty pe.