Matrix isolation and computational study of the photochemistry of p-azidoaniline

Abstract

The photochemistry of p-azidoaniline was studied in argon matrices in the absence and presence of oxygen. With the help of quantum chemical calculations we were able to characterize the triplet p-aminophenylnitrene as well as the cis-and trans-p-aminophenylnitroso oxides. It was found that the latter two isomers can be interconverted by selective irradiation and that they are ultimately converted into p-nitroaniline. Although restricted wavefunctions of the nitroso oxides are unstable, CASSCF calculations turned up no evidence for the claimed diradical character of these compounds. Also we found no evidence for dioxaziridines as intermediates of the conversion of the nitroso oxides to p-nitroaniline. IntroductionGreat progress has been made in understanding the photochemistry of aryl azides over the past few years. 1,2 Singlet phenylnitrene and a series of its simple derivatives have been detected directly and the effects of substituents on the spectra and the decay kinetics of singlet aryl nitrenes have been examined systematically. 2 Thereby it was found that groups which act as strong p donors dramatically accelerate the rate of intersystem crossing. This could be the reason why photolysis of p-azidoaniline (1) and p-azidodimethylaniline, unlike that of phenyl azide and most of its derivatives, gives only products of triplet nitrene reactions. 3,4 For instance, in the presence of oxygen at ambient temperature p-nitroaniline (2) and p-nitrosoaniline (3) are formed in quantitative yield upon photolysis of 1 (Scheme 1). 3,5 Since the early seventies it has been known that triplet arylnitrenes react with oxygen by formation of adducts. [5][6][7][8][9] In an early study on 1,4-diazidobenzene, two types of adducts (diamagnetic and paramagnetic) were detected in glassy matrices. 6 It was proposed that the diamagnetic adduct is an arylnitroso oxide, R-NQO 1 -O À , and the paramagnetic species is a ''spin isomer'' thereof, the triplet aryliminodioxy diradical, R-N-O -O . 6,10 Although subsequent studies did not confirm the formation of triplet adducts of arylnitrenes with oxygen, 3,5,7,8 speculations about the occurrence of aryliminodioxy diradicals in the photooxidation of aryl azides continue to surface occasionally in the literature. 11 This paper is devoted to a comprehensive experimental and computational study of the reactions of triplet arylnitrenes with oxygen in order to unambiguously identify the resulting adducts by their experimentally observed spectra, and to understand their properties. It should be noted that the earlier assignments of a strong near-UV absorption to arylnitroso oxides 3,5,7-9 were never supported by calculations. p-Azidoaniline was used in our matrix isolation study because, as noted above, it gives exclusively products of triplet nitrene reactions in solution, and we thought that this would also simplify the photochemistry in Ar matrices. In addition, the reaction of triplet p-aminophenylnitrene (4) with oxygen in solution and in glassy matrices has previously been studied. 5 2 Experim...

Computational Rationalization for the Observed Ground-State Multiplicities of Fluorinated Acylnitrenes

“…Regardless of whether 1 HCON or 3 HCON is lower in energy, it is clear that ΔE ST > 0 for FCON. ΔΔE ST for the pair is in the range 10 kcal/mol (7−8 kcal/mol according to three of the methods and 13 by MP2), which is by far the largest effect of any of the comparisons due to fluorination in this paper.…”

“…It is well understood that the high polarity of C−F bonds results in opening up of the bond angles between the other substituents (e.g., the H−C−H angles in FCH 3 ). This is most often attributed to the electronegativity of fluorine causing a rehybridization of the carbon favoring greater p-orbital participation in the C−F bond.…”

“…For example, alkoxycarbonylnitrenes 2 have been shown to have triplet states because conjugation of the oxygen into the carbonyl (available in the uncyclized triplet nitrene) is more favorable than into the oxazirene by just enough that 3 2 is more stable than 1 …”

Thermally Persistent Carbonyl Nitrene: FC(O)N

“…10,20,21 In contrast, formation of triplet carbonyl nitrenes was experimentally confirmed by EPR spectroscopy during the photolysis of alkoxycarbonyl azides (ROC(O)N 3 ), such as (4-acetylphenoxyl)carbonyl azide 11 and carbethoxy azide. 16 Quite recently, the ground-state multiplicities of fluorinated acylnitrenes, including FC(O)N, were computationally rationalized.…”

“…11,15,16 Benzoyl nitrene (PhC(O)N), generated by photolysis of benzoyl azide 2,7,9,10 or sulfilimine-based nitrene precursors, 17 has been detected by ultrafast IR spectroscopy and matrixisolation IR and UV/vis spectroscopy. The absorption data and the absence of EPR signal suggested a singlet ground state for this nitrene, 10,18 although the sign of the calculated relative energy between the singlet and triplet (ΔE ST ) was found to depend on the applied methods.…”