An ultrafast time‐resolved infrared and UV–vis spectroscopic and computational study of the photochemistry of acyl azides

Abstract

The photochemistry of pivaloyl, benzoyl, 4‐phenylbenzoyl, and 2‐anthroyl azides has been studied using femtosecond (fs) time‐resolved infrared (TRIR) and UV–vis spectroscopy and interpreted with the aid of computational chemistry. Density functional theory calculations revealed a significant difference in the nature of the lowest singlet excited state for these carbonyl azides. The lowest singlet excited states (S1) of p‐phenylbenzoyl and 2‐anthroyl azides are (π,π*) in nature, while the pivaloyl and benzoyl azides S1 states involve (n,π*) excitations. Nevertheless, for all acyl azides studied here, a similar, and intense, IR band at about 2100 cm−1 has been detected in the ultrafast TRIR experiments following 270 nm excitation. These bands were shifted to lower energy by about 100 cm−1 relative to the N3 stretching mode for the ground states of these azides. These 2100 cm−1 vibrational bands were assigned to the S1 states of acyl azides in agreement with density functional theory calculations. The decay of the acyl azide S1 states was described by bi‐exponential functions. The fast component was attributed to the decay of the hot S1 state and the longer component to the decay of the thermally relaxed S1 state. A strong and broad transient absorption in the 350–650 nm spectral range was observed in the fs UV–vis experiments for p‐phenylbenzoyl and 2‐anthroyl azides. The carrier of this absorption also decayed bi‐exponentially, and the time constants were in excellent agreement with those found in the fs TRIR experiments. The slow component of the S1 state decay was found to be dependent on the solvent polarity. When the lifetime of the acyl azide S1 state is substantially longer than the time constant for vibrational cooling of nascent (hot) isocyanate, the correlation between the S1 decay and isocyanate formation was clear. The 270 nm excitation populates the Sn (n ≥ 2) states of these acyl azides. It was established that a hot nitrene is produced more efficiently from both the Sn and hot S1 states than from the relaxed S1 state of these acyl azides. Thus, time‐resolved study provides direct experimental evidence that the S1 state is the precursor of nitrene only when the S1 state is pumped directly and when the S1 state lifetime is longer than the time constant of vibrational cooling of the newborn nitrene. All of these results are consistent with the data obtained recently for 2‐napththoyl azide. Copyright © 2012 John Wiley & Sons, Ltd.

Formyl Azide: Properties and Solid‐State Structure

“…Similar results were also found for other singlet nitrenes, and attributed to an intramolecular O→N interaction 15a. 19 Thus, nucleophilic attack of CO to the nitrene center is expected to occur from the opposite side of this supposed NO interaction, that is, the anti position with respect to the carbonyl oxygen.…”

Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling

“…Note that lower excited states of many organic azides, which correspond to the states populated after the nonadiabatic dynamics observed here, generally have lifetimes of a few hundred femtoseconds (62)(63)(64), while the lowest excited state has a lifetime of a few to several hundred picoseconds (65)(66)(67). The asymptotic (longtime) products formed from VUV excitation of methyl azide, which occurred after the short-timescale nonadiabatic dynamics discussed here, have been explored in detail by Wodtke and coworkers (68)(69)(70)(71), including minor pathways for ion-pair formation (71) and cyclic N 3 formation (70).…”

The Curtius rearrangement: mechanistic insight and recent applications in natural product syntheses

“…There were virtually no insertion or addition products observed as the analytical method of detection would have identified 1% yield of any of these products. 29 These experiments, along with a large number of other experimental, 31 theoretical, 32,33 and computational studies, 34,35 concluded that the thermal Curtius rearrangement is a concerted reaction as shown in Scheme 2. 31,32,36…”