Direct Observation of Ylide and Enol Intermediates Formed in Competition with Wolff Rearrangement of Photoexcited Ethyl Diazoacetoacetate

The photoexcitation of alpha-diazocarbonyl compounds produces ketenes by both concerted and stepwise Wolff rearrangements. The stepwise mechanism proceeds through singlet carbene intermediates which can also participate in bimolecular reactions such as ylide formation with nucleophiles. Here, ultrafast transient infrared absorption spectroscopy is used to show competitive production of singlet carbene and ketene intermediates from the photoexcitation of ethyl diazoacetoacetate. We provide direct spectroscopic evidence for ylide formation by singlet alpha-carbonyl carbene capture in aprotic nucleophilic solvents (with ylide bands at 1625 cm(-1) in acetonitrile and 1586 and 1635 cm(-1) in tetrahydrofuran) and report an enolmediated pathway for singlet alpha-carbonyl carbene reaction with alcohols (ethanol or tert-butanol) identified by an absorption band at 1694 cm(-1); however, we find no evidence for a previously proposed ylide pathway. The a-carbonyl carbene is monitored by using a band with solvent-dependent wavenumber in the range 1627-1645 cm(-1). A computed two-dimensional cut of the potential energy surface for the reaction of the singlet alpha-carbonyl carbene with methanol shows that the enol forms without a barrier and that this reaction is promoted by an intermolecular hydrogen bond from methanol to the carbonyl oxygen atom. The corresponding ylide structure lies higher in energy, with a barrierless downhill path to isomerization to the enol.