Nitrenes triplet nitrene trap

Matrix isolated NH reacts with NO, an excellent nitrene trap, to form trans-HNNO, which was characterized by infrared (IR) spectroscopy. Triplet imidogen also reacts with oxygen in matrices. 

More recently, the e.s.r. spectra of triplet species trapped in rigid glasses have been examined. Absorption is observed when a molecular axis is approximately parallel to the magnetic field (Yager et al., 1962). The triplets examined include methylenes such as Ph2C (Murray et al., 1962) and PhCH (Trozzolo et al., 1962) and nitrenes (Smolinsky et al., 1962). 

Alkenyl and allyl azides give nitrenes, which undergo intramolecular addition to give highly strained compounds (Scheme 6.20). Conversely, addition of triplet (radical) traps such as dienes or a-methylstyrene increases the stereospecificity by selective removal of the triplet. Photolysis is less stereospedfic since a higher percentage of the nitrenes is generated directly as triplets. 

Similar products are obtained from the photosensitized decomposition of the tertiary azides, suggesting that decomposition may result from the triplet azides under both direct and sensitized photolysis/461 Additional evidence for a discrete nitrene intermediate comes from the observation that this intermediate can be trapped by decomposition of the azides in the presence of good hydrogen donors such as tri- -butyItin hydride and jec-butyl mercaptan. Triarylamines result ... 

The chemical reactions of sulphonyl nitrenes include hydrogen abstraction, insertion into aliphatic C—H bonds, aromatic substitution , addition to olefinic double bonds, trapping reactions with suitable nucleophiles, and Wolff-type rearrangement. Hydrogen-abstraction from saturated carbon atoms is usually considered to be a reaction typical of triplet... 

Nitrenes, the nitrogen analogs of carbenes, are too reactive for isolation under ordinary conditions. The ground state of NH, and of most nitrenes, is a triplet. Aryl nitrenes have been trapped at 77 K,186 the more reactive alkyl nitrenes in matrices at 4 K.187... 

Nitrenes,239 R—N, are the nitrogen analogs of carbenes, and most of what we have said about carbenes also applies to them. Nitrenes are too reactive for isolation under ordinary conditions. Alkyl nitrenes have been isolated by trapping in matrices at 4 K,24n while aryl nitrenes, which are less reactive, can be trapped at 77 K.241 The ground state of NH, and probably of most nitrenes,242 is a triplet, though nitrenes can be generated in both triplet and singlet states. In additions of EtOOC—N to C=C double bonds two species are involved,... 

Thermal decomposition of aryl azides under a nitrogen atmosphere leads to nitrenes which are able to substitute fluorine in polyfluorinated naphthalenes, albeit with a poor yield (4%). Since no substitution product was isolated when oxygen was present during this reaction, the intermediacy of a triplet nitrene was proposed, which is completely trapped by oxygen.207... 

From the ratios of cis- and trans-N-carbethoxy-2-methyl-3-isopropylaziri-dines (38) and (39) it was concluded that 30% of the photochemically generated nitrene was formed in the triplet state 86>. With a-methylstyrene (40), which was used as triplet trap, carbethoxynitrene does not lead to aziridines. As 1 1 adduct 3-carbethoxyamino-2-phenyl-propene-l (42) was isolated 83>, which was pre-... 

Of particular mechanistic interest is the photochemistry of ethyl azidoformate which does not normally undergo the Curtius rearrangement (the migratory aptitude of the . oxy group is low), and where the nitrene can be efficiently trap. d, e.g. by acetylenes and nitriles . Lwowski was able to establish the spin state of carbethoxynitrene (ethoxycarbonyl nitrene ) by an elegant method adapted from the work of Skcll on carbenes " - It is based on the assumption, fully justified by the results, that a singlet species deficient in two electrons will add stereospecifically to a double bond. A triplet reactant can accept only one electron at a time and will close the bond with the second electron only after one of the spins has... 

However, Sundberg et al.304-305 demonstrated that two intermediates must be formed from the photolytically generated nitrene 240. These were supposed to be the immediate carbazole precursor 241, and the azirine 243 (Scheme 47) because part of the nitrenes could be trapped with diethylamine, giving 245. In conformity with the discussion in Sections VIII,A-C we have added the azacycloheptatetraene 244 in Scheme 47. Since the trapping reaction (to 245) was found to be a factor 102 faster than carbazole formation, but the formation of 242 could not be completely suppressed, 241 was assumed to be the nontrappable intermediate. It was also found304 that the deoxygenation of 2-nitrosobiphenyl proceeded in a similar manner, and that a similar product ratio (242 245) resulted. Therefore, a free nitrene appears to be involved in this reaction also. A more detailed flash-photolytic study305 excluded the triplet nitrene 240 as a principal carbazole precursor. The nature of the observable transient is still unclear, however. 

The research results described in this introductory section present a somewhat confusing picture of aryl azide chemistry. In particular, it seems that different analytical methods lead to contradictory conclusions concerning the identity of reactive intermediates and their proper role in the chemical transformations of azides. Analysis at low temperature by EPR spectroscopy reveals a triplet nitrene, but IR spectroscopy requires a dehydroazepine. Irradiation at room temperature gives triplet nitrene-derived products unless a trap for a closed-shell intermediate (either a benzazirine or a dehydroazepine) is present in solution. The last ten years have witnessed remarkable progress in resolving these contradictions and questions. The remainder of this chapter is devoted to the presentation and analysis of this more recent work. 

Azobenzene formation signals reaction of the triplet nitrene, substituted 3H-azepines come from the trapping of dehydroazepines. Clearly substituents on aryl azides affect the formation and reactivity of these intermediates. The precise nature of the substituent effects was revealed by application of time-resolved absorption experiments that will be described later. However, from the perspective of product yields and synthetic applications, two noteworthy trends should be mentioned here. 

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. 

The conclusion seems inescapable that lowering the temperature changes the identity of the reactive intermediate formed on photolysis of phenyl azide from an electrophilic species that is trapped by amines, to triplet phenyl nitrene. This result requires that there is a branching point in the photochemistry of phenyl azide and that the rate constants of these two branches have very different Arrhenius parameters. Because triplet phenyl nitrene is... 

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