Ultrafast Photoelimination of Nitrogen from Upper Excited States of Diazoalkanes and the Fate of Carbenes Formed in the Reaction

Abstract

The photochemical reactivity of diphenyldiazomethane 1 and phenyl 1- and 2-adamantyl diazomethanes 2 and 3, respectively, was investigated by transient absorption spectroscopy (TA). Photoelimination of N2 upon UV excitation takes place in the anti-Kasha ultrafast photochemical reaction from the upper excited singlet states to deliver singlet carbenes, which were, in the case of 1 and 2, detected by fs-TA. The reactivity of the carbenes differs with respect to the substituent at the carbene center. The singlet car-1 in a nonpolar solvent delivers the triplet carbene by intersystem crossing (ISC). Singlet car-2 does not undergo ISC but reacts in the intermolecular insertion reactions into C–H bonds. Car-3 has an α-C–H bond next to the carbene center and reacts rapidly in the intramolecular C–H insertion reaction to deliver alkene, precluding its detection by fs-TA. However, the isolation of ketone photoproducts from 3 is highly indicative of triplet car-3’s intermediate formation. The TA spectra from the S1–S3 states of 1–3 were computed using time-dependent density functional theory, while the multiconfigurational perturbation theory to the second order was used for the absorption spectra of the corresponding singlet and triplet carbenes. The modeled and measured spectra are in good agreement, and the computations corroborate the assignments of the key short-lived intermediates.

Two-Dimensional Electronic Spectroscopy Resolves Relative Excited-State Displacements

“…The ability to initiate coherent dynamics and probe the structure of higher-lying excited states is becoming increasingly important in ultrafast photochemistry. Several recent experiments show that the excitation of electronic states higher than S 1 leads to novel photophysical and photochemical processes not accessible by other pathways, encompassing processes such as novel reactive channels in photochromics, charge injection for photovoltaics, and anti-Kasha emission. − The present data suggest that coherence spectroscopies provide new information and potentially suggest routes to control such phenomena.…”

Carbenes from cyclopropanated aromatics

“…Furthermore, there is a legitimate concern that the photochemical generation of carbenes in solution from diazo compounds and diazirines may involve reactions in the excited states (RIES) of the precursors themselves. 33–35 In order to mitigate some of the dangers associated with diazo compounds and diazirines, considerable research has gone into investigating safer, alternative means of carbene generation including the use of N -sulfonylhydrazone family of salts 36,37 and metal-based approaches. 38 In this review, we discuss the emergence of cyclopropanated aromatics—derived from phenanthrene, indane, naphthalene, and 1,4-dydronaphthalene— as safe, viable, and versatile sources of carbenes.…”