Article pubs.acs.org/JPCA
A Laser Flash Photolysis Study of Azo-Compound Formation from Aryl Nitrenes at Room Temperature Alec Q. Ribblett and James S. Poole* Department of Chemistry, Ball State University, Muncie, Indiana 47306, United States S Supporting Information *
ABSTRACT: The species 4-nitrenopyridine 1-oxide is known to exhibit triplet nitrene dominated chemistry to yield azodimer products exclusively, even at room temperature. As such, this species, and its analogue 4-nitrenoquinoline 1-oxide, are useful models to probe the mechanism of formation of azodimers, which is postulated to proceed by self-reaction of the nitrene or reaction of nitrene with the parent azide. A laser flash photolysis study is described where the kinetics of formation of azo-dimer were found to be most adequately modeled by competition between both mechanisms, and rate coefficients for the competing reactions were determined.
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(3.2 ± 0.3) × 106 s−1 at room temperature. Platz et al. have demonstrated that strongly electron donating substituents such as −NR2 or −OR at the para-position on the ring will enhance the rate of intersystem crossing by 2 or more orders of magnitude, to the point where it may successfully compete with rearrangement reactions.12 (2) Retard the rate of rearrangement: for example, the rate of rearrangement can be retarded somewhat by substitution at the position ortho to the nitrene, which results in a steric or electronic barrier to the initial cyclization step leading to rearrangement. Alkyl substitutents have been used to demonstrate the former effect,13 whereas fluoro substituents provide examples of the latter.14,15 In previous contributions,16,17 we reported that the chemistry of 4-nitrenopyridine 1-oxide (1) was dominated by triplet chemistry, even at room temperature (Scheme 1). Calculations show that the S1 state of the nitrene is selectively stabilized by an iminyl−aminoxyl biradical resonance contributor that decreases the T0−S1 energy gap relative to say, phenyl nitrene. This stabilization both enhances the rate of intersystem crossing from S1 to T0 and makes the energy barriers for cyclization of the singlet nitrene prohibitively high. The pyridine 1-oxide group has been demonstrated by Creary to be an extremely effective biradical stabilizing group.18 On the basis of Creary’s approach, Wenthold et al.19,20 have demonstrated computationally that a similar degree of stabilization could be expected in certain furanyl systems, although this has yet to be confirmed experimentally. Given this behavior, it would appear that the quinoline analogue, 4-azidoquinoline 1-oxide (2), provides an additional system for studythe photochemistry of this azide mirrors that
INTRODUCTION Aryl nitrenes have been the subject of considerable experimental and theoretical study, and their chemistry has been described in a number of recent reviews.1−3 To briefly summarize, aryl nitrenes, often generated by photolysis or thermolysis of their parent azides, exhibit room temperature chemistry consistent with an open-shell singlet state. Rearrangement typically yields didehydroazepine (ketenimine) products, which may be trapped with strong nucleophiles4,5 or, in the absence of such nucleophiles, will polymerize. At low temperatures (
