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 -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 -carbonyl carbene reaction with alcohols (ethanol or tbutanol) identified by an absorption band at 1694 cm -1 , but find no evidence for a previously proposed ylide pathway. The -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 -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.

The polymerization of photoexcited N-ethylcarbazole (N-EC) in the presence of an electron acceptor begins with an electron transfer (ET) step to generate a radical cation of N-EC (N-EC+.). Here, the...

Ultrafast transient absorption spectra were recorded for solutions of [MnIII(cyclam)(H2O)­(OTf)][OTf]2 (cyclam = 1,4,8,11-tetraazacyclotetradecane and OTf = Trifluoromethanesulfonate) in water to explore the possibility to restrict the equatorial expansion following photoexcitation...

The photoexcitation of α-diazocarbonyl compounds produces singlet carbene intermediates that react with nucleophilic solvent molecules to form ylides. The zwitterionic nature of these newly formed ylides induces rapid changes in their interactions with the surrounding solvent. Here, ultrafast time-resolved infrared absorption spectroscopy is used to study the ylide-forming reactions of singlet carbene intermediates from the 270 nm photoexcitation of ethyl diazoacetate in various solvents and the changes in the subsequent ylide–solvent interactions. The results provide direct spectroscopic observation of the competition between ylide formation and C–H insertion in reactions of the singlet carbene with nucleophilic solvent molecules. We further report the specific solvation dynamics of the tetrahydrofuran (THF)-derived ylide (with a characteristic IR absorption band at 1636 cm –1 ) by various hydrogen-bond donors and the coordination by lithium cations. Hydrogen-bonded ylide bands shift to a lower wavenumber by −19 cm –1 for interactions with ethanol, −14 cm –1 for chloroform, −10 cm –1 for dichloromethane, −9 cm –1 for acetonitrile or cyclohexane, and −16 cm –1 for Li + coordination, allowing the time evolution of the ylide–solvent interactions to be tracked. The hydrogen-bonded ylide bands grow with rate coefficients that are close to the diffusional limit. We further characterize the specific interactions of ethanol with the THF-derived ylide using quantum chemical (MP2) calculations and DFT-based atom-centered density matrix propagation trajectories, which show preferential coordination to the α-carbonyl group. This coordination alters the hybridization character of the ylidic carbon atom, with the greatest change toward sp 2 character found for lithium-ion coordination.

No abstract