A Laser Flash Photolysis Study of a Carbene−Ether Ylide

Abstract

Laser flash photolysis (308 nm) of carbomethoxy-2-naphththyldiazomethane in deoxygenated Freon-113 (CF2ClCFCl2) solution containing tetrahydrofuran (THF) produces a transient spectrum with a sharp absorption maximum at 330 nm and a shoulder at 375 nm. The observed rate constant of formation of transient absorption is the same at 330 and 400 nm. The transient spectrum is the same 100 ns, 1 μs, and 10 μs after the laser pulse at ambient temperature and is attributable to a single carrier. The transient shows little decay over 10 μs in the presence and absence of oxygen. Similar results were obtained at −60° C. This pattern of observations is not consistent with assigning the transient to triplet carbomethoxy-2-naphthylcarbene or its related radical. The rate constant of formation of the transient, k THF, is first order in the concentration of THF; thus, the transient is attributed to a carbene−ether ylide. To our knowledge, this is the first direct observation of a carbene-ether ylide. An Arrhenius plot of k THF is nonlinear due to a change in mechanism of the reaction of the carbene with THF or to reversion of the ylide to carbene and THF at elevated temperatures.

Photochemistry of an Azido-Functionalized Cryptand: Controlling the Reactivity of an Extremely Long-Lived Singlet Aryl Nitrene by Complexation to Alkali Cations

“…In a related system, we were recently able to obtain indirect evidence for the formation of a nitrene-amine ylide . Very little evidence for nitrene-ether ylides has been reported so far, while more data have been presented for carbene-ether ylides . The formation of these two species from the S 1 state of the nitrene might involve a conical intersection connecting the open-shell nitrene and closed-shell ylide surfaces .…”

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

“…“Saturated” electrophilic singlet carbenes ( 1 ), endowed with an empty p-orbital on the formally sp 2 -hybridized divalent and neutral carbon atom, are known to accept an electron pair from heteroatom-containing moieties (e.g., 2 ) to form the corresponding ylides ( 3 ) (Scheme ). − These carbene-derived ylides have proven to be of considerable synthesis value, − and in many instances, they have been directly observed and spectroscopically characterized. − The structurally different “unsaturated” alkylidenecarbenes ( 4 ), which feature an sp-hybridized carbon atom attached to only one substituent by a double bond, prefer singlet ground states and are electrophilic in nature (Scheme ). − As expected, carbenes such as 4 also form ylides 5 by accepting an electron pair in their vacant p-orbital from appropriate nucleophilic donors ( 2 ) (Scheme ).…”

Carbene Complexes of Nonmetals

“…[52] LFP of the diazo precursor 17 in the presence of THF generated a long-lived ylide 19e whose rate of formation was proportional to the concentration of THF. [53] However, the intermolecular reactivity of 19e was not explored. More evidence was adduced from LFP studies of 20.…”