Direct Observation of Ylide and Enol Intermediates Formed in Competition with Wolff Rearrangement of Photo-Excited Ethyl Diazoacetoacetate

The photoexcitation of α-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 infra-red 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 α-carbonyl carbene capture in aprotic nucleophilic solvents (with ylide bands at 1625 cm^-1 in acetonitrile, and 1586 cm^-1 and 1635 cm^-1 in tetrahydrofuran) and report an enol mediated pathway for singlet α-carbonyl carbene reaction with alcohols (ethanol or t-butanol) identified by an absorption band at 1694 cm^-1, but find no evidence for a previously proposed ylide pathway. The α-carbonyl carbene is monitored 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 α-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.