Reversible Interconversion between Singlet and Triplet 2-Naphthyl(carbomethoxy)carbene

Abstract

Photolysis of methyl α-diazo-(2-naphthyl)acetate at 450 nm yields primarily the triplet ground state of 2-naphthyl(carbomethoxy)carbene (3NCC), which was characterized by its UV/visible, IR, and ESR spectra as well as by trapping with O2 and CO. Bleaching of the weak visible bands of 3NCC (λ > 515 nm) leads to the disappearance of the triplet ESR spectrum and to a new optical spectrum with broad bands in the near-UV region. The spectra of 3NCC can be almost fully recovered in the dark at 12 K. The same process also occurs upon irradiation at 450 nm, with the exception that the recovery of 3NCC is accompanied by the formation of small amounts of 2-naphthyl(methoxy)ketene and 2-(2-naphthyl)propiolactone. The observed optical and IR spectra of the intermediate that is formed reversibly from 3NCC are in full accord with quantum chemical model calculations for the singlet state of the same carbene 1NCC, but not with the tricyclic cyclopropene derivative observed previously for the parent 2-naphthylcarbene or the 1-(2-naphthyl)-2-methoxyoxirene. Exploration of the potential energy surfaces for singlet and triplet NCC show that a barrier between the two states is created by a pronounced change in conformation of the carbomethoxy group, which may furthermore be hindered in a solid matrix environment.

Ultrafast time‐resolved studies of the photochemistry of diazo carbonyl compounds

“…Photoexcitation of diazo ester 1 (with λ exc = 266 nm) results in a stepwise decomposition mechanism, as the singlet carbene (1650 cm −1 ) is observed and it decays with the same time‐constant as the ketene (2096 cm −1 ) band rises 10. This result follows Kaplan's rule as calculations indicate that ester 1 exists almost entirely (99%) in the anti conformation 12…”

100 Years of the Wolff Rearrangement

“…[150] The reversible interconversion between singlet and triplet 2-naphthyl-(methoxycarbonyl)carbene (162) has recently been monitored. [151] Photolysis (450 nm) of matrix-isolated diazo ester 159 produced 3 162 which was identified by its UV/Visible, IR, and ESR spectra. Irradiation of 3 162 (Ͼ 515 nm) generated a new species whose optical and IR spectra were in agreement with DFT calculations for 1 162.…”

Carbene Complexes of Nonmetals

“…Oxonium ylides studied by time-resolved spectroscopy In contrast, 2-naphthyl(methoxycarbonyl)carbene (18e) is a spin-equilibrated carbene with a small singlet/triplet energy gap (∆G ഠ 0.8 kJ/mol). [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.…”