Nitrenes laser flash photolysis studies

In the early 1990s, singlet aryl nitrenes had never been detected and the influence of structure on reactivity was only dimly appreciated. Laser flash photolysis studies in combination with theory has provided insight and has led to a comprehensive theory of substituent effects. We have been proud to be part of this process. 

K. A. Schnapp and M. S. Platz, A laser flash photolysis study of di-, tri- and tetrafluo-rinated phenyl nitrenes implications for photoaffinity labeling. Bioconj. Chem. 

Gritsan, N. R, Tigelaar, D., and Platz, M. S., A laser flash photolysis study of some simple para-substituted derivatives of singlet phenyl nitrene, /. Phys. Chem. A, 103, 4465,1999. 

Nitrenium ions (or imidonium ions in the contemporaneous nomenclature) were described in a 1964 review of nitrene chemistry by Abramovitch and Davis. A later review by Lansbury in 1970 focused primarily on vinylidine nitrenium ions. Gassmann s ° 1970 review was particularly influential in that it described the application of detailed mechanistic methods to the question of the formation of nitrenium ions as discrete intermediates. McClelland" reviewed kinetic and lifetime properties of nitrenium ions, with a particular emphasis on those studied by laser flash photolysis (LFP). The role of singlet and triplet states in the reactions of nitrenium ions was reviewed in 1999. Photochemical routes to nitrenium ions were discussed in a 2000 review. Finally, a noteworthy review of arylnitrenium ion chemistry by Novak and Rajagopal " has recently appeared. 

On the basis of product studies, it is clear that irradiation of the naphthyl azides leads to loss of nitrogen with the likely consequent formation of nitrenes. Just as for phenyl azide, the initially formed singlet nitrenes may intersystem cross to the triplet and then dimerize to azo compounds. Clearly in the case of 2-naphthyl azide, but not 1-naphthyl azide, a closed-shell ground-state intermediate that can be trapped with diethylamine can be generated. The intermediate was formulated as the azirine on the basis of product studies [57]. Low temperature absorption spectroscopy and time-resolved laser flash photolysis experiments to be described later support the formation of azirines and provide an explanation for the different reactivity observed between the 1- and 2-substituted azides. 

Kobayashi et al. [86] studied (4-dimethylamino)phenyl azide by means of laser flash photolysis on a picosecond time scale. They found that the triplet nitrene is formed from an unobserved precursor that has a lifetime of approximately 120ps. By consideration of model compounds, these workers suggested that the precursor is the singlet nitrene, but this conclusion must await confirmation by alternative experiments. However, in this work Kobayashi was able to show that triplet (4-dimethylamino)phenyl nitrene reacts to form the (4-dimethylamino)azobenzene by a kinetically second-order process. 

The kinetics of rearrangement of substituted penyl nitrenes have been studied by laser flash photolysis. The temperature-independent observed rate constants are associated with kjsc. Plots of In (koBs-kisc) were linear (Figure 5.23 to Figure 5.25), and these plots were used to deduce the Arrhenius parameters for cyclization of the substituted singlet arylnitrenes (Table 5.7, Table 5.11 to Table 5.13). 

More modern techniques such as laser flash photolysis (LFP) and low temperature matrix studies allow the routine measurement of UV spectra of nitrenes. Triplet phenyinitrene, for example, shows absorptions between 300 and 400 nm at 77 K, while ESR measurements on the same species suggest that there is substantial delocalization of a single electron into the aromatic ring (Figure 6.4). 

Isoxazole-oxazole photoisomerization was studied by irradiation of matrix-isolated 3,5-dimethylisoxazole (18) at 222 nm. 2-Acetyl-3-methyl-2H-azirine (20) was obtained, likely through an acetyl vinyl nitrene intermediate 19 as primary photoproduct, while upon longer time UV irradiation, two additional photoproducts were identified as acetyl nitrile ylide 21 and 2,5-dimethyloxazole (22) (13JOC10657). Analogously, 3,5-diphenylisoxazole and 2-benzoyl-3-phenyl-2f/-azirine behaved as precursors to triplet vinyl nitrene (of type 19) through laser flash photolysis (13JOC11349). Reductive heterocycle—heterocycle transformations of (2-nitrophenyl)isoxazole precursors, such as 23 and 26, afforded 4-amino quinolines of type 24, quinolin-4(lfJ)-ones 25, and 3-acylindoles 27. Che-moselective heterocyclizations were observed from 3,4-,4,5-, and 3,4-bis(2-nitrophenyl)isoxazoles (13OL2062). 

According to the (4,4) CASSCF/6-31G calculations, the nitrene 29a in the A" state can close to the azirine without any barrier and this state was found to be the transition state for interchange of the enantiotopic pair of hydrogens in 2H-azirine (27a). ° Therefore, if a barrier does exist, it is probably very small. This conclusion, based on the results of calculations, is wholly consistent with the fact noted above, that the triplet and singlet vinyl nitrenes have escaped detection. However, further experimental studies, using very fast laser flash photolysis techniques, along with higher level ab initio calculations are certainly warranted. 

Recently, the photochemistry of azide 42 was studied by laser flash photolysis (Aex = 266nm) techniques in Fieon-113 (CF2CICFCI2) at room temperature. The formation of at least two intermediates, viz., triplet nitrene M3 (Aa,ax = 4(X)nm, lifetime 1.5 fis) and ethoxycarbonyl radical 44 (2 = 333 nm, lifetime 0.4/is), was observed (Scheme 11.22). The singlet nitrene M3 was deduced to have a lifetime between 2 and 10 ns in Freon-113 at ambient temperature. The kinetics of reactions of M3 with tetramethyleth-ylene and triethylsilane were also measured. ... 

The effect of substituents placed para to the nitrene center was investigated in a series of matrix isolation, laser flash photolysis, and product studies. Table 44.1 Ksts a number of rate constants (k,sc for intersystem crossing to the triplet ground-state and for rearrangement to the didehydroazepine). For comparison, the kinetic parameters of several other singlet aryl nitrenes are also given. 

Aryl substituents also activate benzyHc C-H bonds adjacent to a nitrene center. An interesting study on this topic has dealt with the atropoisomeric 3,5-dimethyl-2-(9-fluorenyl)phenylazides 49 and 52. ° The low-temperature photochemistry (77 K) of these rotameric azides proved to be fundamentally different Photolysis of 49 resulted in the formation of nitrene 50 in addition to azanorcaradiene 51, while photolysis of 52 gave nitrene 53 in addition to iminoquinone methide 54. This subsequently underwent intramolecular cycloaddition to yield dihydroindenocridine 55. Laser flash photolysis showed that the formation of iminoquinone methide 54 from azide 52 occurred in less than 10 ns. Deuteration of the 9-position of the fluorene chromophore resulted in a significantly diminished yield of 55. LFP of the rotameric azide 49 yielded the rate constant for the formation of azanorcaradiene 51 which was determined as k = 7.1 x 10 ... 

Chemiluminescence has been used to measure the relative yields of excited ketones formed from self reaction of alkoxyl and alkylperoxyl radical pairs . In the photochemistry of aryl azides a dehydroazepine is detected by time resolved infra red spectroscopy and flash photolysis at room temperature . Singlet and triplet nitrenes and dehydroazepenes have also been detected in the photochemistry of 3- and 4-nitrophenyl azides . Picosecond and nanosecond laser photolysis of p-nitrophenyl acetate in aqueous media produces a triplet state of the -nitrobenzylanion and CO2 after cleavage of the rnr triplet. Absorption, emission, and reaction kinetics of dimethylsilylene produced by flash photolyses of dodecamethylcycloherasilane is another interesting study 2,... 

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