Photoaffinity Labeling via Nitrenium Ion Chemistry: Protonation of the Nitrene Derived from 4-Amino-3-nitrophenyl Azide to Afford Reactive Nitrenium Ion Pairs

Abstract

Phenyl azides with powerful electron-donating substituents are known to deviate from the usual photochemical behavior of other phenyl azides. They do not undergo ring expansion, but form basic nitrenes that protonate to form nitrenium ions. The photochemistry of the widely used photoaffinity labeling system 4-amino-3-nitrophenyl azide, 5, has been studied by transient absorption spectroscopy from femtosecond to microsecond time domains and from a theoretical perspective. The nitrene generation from azide 5 occurs on the S 2 surface, in violation of Kasha's rule. The resulting nitrene is a powerful base and abstracts protons extremely rapidly from a variety of sources to form a nitrenium ion. In methanol, this protonation occurs in about 5 ps, which is the fastest intermolecular protonation observed to date. Suitable proton sources include alcohols, amine salts, and even acidic C-H bonds such as acetonitrile. The resulting nitrenium ion is stabilized by the electron-donating 4-amino group to afford a diiminoquinone-like species that collapses relatively slowly to form the ultimate cross-linked product. In some cases in which the anion is a good hydride donor, cross-linking is replaced by reduction of the nitrenium ion to the corresponding amine.

Electronic Properties of Nitrenium Ions

“…24 In relation to characterizing intermediates involved in photoaffinity labeling, Wilson et al generated and characterized the 4-(diethylamino)-3-nitrophenyl nitrenium 38 ion using picosecond LFP. 29 This species, which lives for several hundred nanoseconds in isopropyl alcohol, absorbs at 486 nm, in reasonable agreement with a value of 441 nm predicted by TD-DFT.…”

Primary reactions in the photochemistry of hexahalide complexes of platinum group metals: A minireview

“…The fitting programme corrected for the group velocity dispersion and calculated the response time of the instrument. Experiments on ultrafast kinetic spectroscopy of the Pt IV Br 6 2À , Ir IV Br 6 2À and Os IV Br 6 2À complexes were performed using the setup described in [56][57][58]. Two TOPAS-C optical parametric amplifiers generated 420 or 530 nm pump pulses of 2 mJ and probe pulses tunable between 270 and 400 nm.…”

Ultrafast Spectroscopy and Computational Study of the Photochemistry of Diphenylphosphoryl Azide: Direct Spectroscopic Observation of a Singlet Phosphorylnitrene

“…Both theoretical methods provided remarkably similar predictions in the past that were comparable to the experimental observations. 21, 22,26,27 However, the preceding computations were performed on aryl azides, for which the lowest excited singlet state always had (π, π* in-plane ) characteristics of the azide moiety, thereby leading to singlet nitrene formation. Based on these computational results, we predict that photolysis of DPP-N 3 at about 250 nm will result in pumping the precursor molecule to either a singlet azide excited state or a singlet (π, π*) excited state of the phenyl ring, which will then transfer its energy to the azide unit.…”