Detection of Ylide Formation between an Alkylidenecarbene and Acetonitrile by Femtosecond Transient Absorption Spectroscopy

Abstract

Femtosecond laser flash photolysis of 3-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-ylidene)tetrahydrofuran produces singlet 3-oxacyclopentylidenecarbene which reacts with acetonitrile solvent to form an ylide. This is the first direct detection of ylide formation by an alkylidenecarbene. This new type of ylide was observed to have a broad absorption band in the visible region with λ max ∼450 nm and a lifetime of ∼13.5 ps. As with other "conventional" carbenes (the divalent carbon atom is separately bound to two substituents), this ylide formation method could be also useful for detecting alkylidenecarbenes, especially those that do not absorb at wavelengths suitable for direct observation. Furthermore, the mechanisms by which 3-oxacyclopentylidenecarbene forms the ylide and the overall favorability of ylide formation, vis-a-vis ring expansion of the carbene to strained 3-oxacyclohexyne, were supported by results from density functional theory calculations.

Direct Observation of the Dynamics of Ylide Solvation by Hydrogen-bond Donors Using Time-Resolved Infrared Spectroscopy

“…In addition, we identified the involvement of the α-carbonyl group in the reactivity of the α-dicarbonyl carbene with alcohols and provided experimental and computational evidence for an enol-forming pathway that bypasses the alcohol ylide . Ylide-forming reaction pathways have also been reported for carbenes that do not bear an α-carbonyl group. , …”

“… 55 Ylide-forming reaction pathways have also been reported for carbenes that do not bear an α-carbonyl group. 56 , 57 …”

“…55 Ylide-forming reaction pathways have also been reported for carbenes that do not bear an α-carbonyl group. 56,57 Solvation of carbenes has been the focus of several prior experimental and theoretical studies, 62−69 but the solvation dynamics of the ylide intermediates of carbene reactions have yet to be examined. Using band shifts observed by TRIR, we present direct spectroscopic evidence for specific solvent interactions between EDA-derived THF ylides (THF-Y) and ethanol (EtOH), chloroform, dichloromethane (DCM), ACN, and cyclohexane HB donors, and we characterize the kinetics of solvation.…”

“… In competition with ylide formation, these carbenes can add to, or insert into, a variety of functional groups, such as CC, C–H, C–X (X = Cl, Br, I), aromatic rings, and O–H bonds, as shown in Scheme , , with the mechanism of insertion typically inferred from the stereochemistry of the products. The direct observation of carbene reaction pathways has been the subject of several previous studies. − By monitoring the CO stretching modes using time-resolved infrared (TRIR) spectroscopy, Xue et al. demonstrated that a variety of ylides could be directly observed from the reaction of the singlet α-carbonyl carbene (2) with nucleophilic solvents such as THF, ACN, and deuterated methanol (MeOD).…”

Polymorphism in 9-[(9H-Fluoren-9-ylidene)methyl]phenanthrene

“…Our laboratory has reported several examples of methylenecyclopropanes ( 1 ) based on the phenanthrene framework as viable photochemical sources of alkylidenecarbenes 2 (Scheme ). − These alkylidenecarbenes typically rearrange to acyclic ,− or strained cyclic ,, alkynes 3 via a 1,2-shift of a substituent attached to the β-carbon.…”