Nitrenes dimerization

An intensely colored by-product of the photolysis reaction of methyl-2-azidobenzoate has been identified as the first known derivative of 3,3 -diazaheptafulvalene 70 (94LA1165). Its molecular mass was established by elemental analysis and mass spectroscopy as that of a formal nitrene dimer, whereas and NMR studies demonstrated the twofold symmetry as well as the existence of a cross-conjugated 14 7r-electron system in 70. Involving l-azido-2,3-dimethoxy-5,6-dimethoxycarbonylbenzene in thermal decomposition reactions, the azaheptafulvalene 71 could be isolated and characterized spectroscopically and by means of X-ray diffraction. Tliis unusual fulvalene can be regarded as a vinylogous derivative of azafulvalenes (96JHC1333) (Scheme 28). 

To add to the confusion, various groups reported that gas-phase photolysis of phenyl azide produced the absorption and emission spectra of triplet phenylni-trene. " These observations were reconciled by the work of Leyva et al. who discovered that the photochemistry of phenyl azide in the presence of diethylamine was very sensitive to temperature. Above 200 K, azepine 30 is formed, but <160 K, azobenzene, the product of triplet nitrene dimerization, is produced. The ketenimine can react with itself or with phenyl azide to produce a polymer, which can be converted into an electrically conducting material. Gritsan and Pritchina pointed out that at high-dilution ketenimine 30 can interconvert with singlet phenylnitrene which eventually relaxes to the lower energy triplet that subsequently dimerizes to form azobenzene. 

Nitrene can be stored for some time below 40 K. At room temperature, nitrene dimerizes very quickly to diimtne according to (77). 

Comparable results are obtained from the nanosecond time scale flash photolysis study of (4-nitro)phenyl azide reported by Liang and Schuster [45]. Irradiation of this azide leads to rapid formation of the triplet nitrene. This nitrene was identified by comparison of the observed spectrum with that recorded at low temperature, and by its observed dimerization to form the (4-nitro)azobenzene. The rate constant for the triplet nitrene dimerization is 1.0 x 109 M 1 s 1 a value approximately 10 times below the diffusion limit. This relatively small rate constant is consistent with the demands of spin statistics. Combination of two triplets to form a singlet product can be successful only one ninth of the time [64],... 

From the effect of solvent (Table 15) it is evident that the reactions discussed are nitrene reactions hydrogen-rich solvents suppress ring contraction and give rise to solvent dimer (bibenzyl) and/or a yellow nitrene dimer. The structure of the dimer is not known, but one possibility is shown in 144. A similar (colorless) dimer was obtained from 9-phenanthridylnitrene at 500 ° 7). Xhe two dimers formed from 137 and 141 in cyclohexane have nearly identical IR spectra. How could a hydrogen-rich solvent promote dimeriztion There is evidence from aryl azide decomposition in solution that amino radicals are formed first, and these dimerize and dehydrogenate as shown for 1-naphthylnitrene in [Eq. (48)] 82). 

Nitrenes for the most part being electron deficient are highly electrophilic intermediates and therefore react with nucleophiles of all types. Tertiary amines, phosphines, sulfides, and sulfoxides all react with nitrenes to give ylides, in a reaction that is the reverse of their formation. In practice, dimethyl sulfoxide (DMSO) is often the most convenient nucleophilic trap since it can be used as the reaction solvent, and gives relatively stable sulfoximides (Scheme 6.40). Azo compounds, which are formally nitrene dimers, are common by-products in many nitrene reactions. However, the dimerization of two highly reactive species in solution is extremely unlikely on statistical grounds, and therefore the mechanism of azo compound formation probably involves the reaction of a nitrene, as an electrophile, with its precursor. 

Formation of complexes of the type [Rh(NH3)6(NH2X)] + by trapping the nitrene [Rh(NH3>6N] +, generated by the thermal or photochemical dissociation of the [Rh(NH3)6N3] + ion, with an anion X - (e.g. Cl or SOa ") was discussed in Section 4. Evidence for the formation of a nitrene dimer was also presented. Replacement of both chloride ions in a-cir-[Rh(trienX3J by other anions proceeds in two stages. The rates of these reactions are independent of the concentrations of the added anions, and so a mechanism is proposed in which the ratedetermining step for each stage is aquation, followed by rapid anation of the aqua-intermediates, e.g. 

The solution phase photochemistry of phenyl azide 47 is temperature dependent/ Photolysis of 47 in the presence of diethylamine at ambient temperature yields azepine 48b. Lowering the temperature suppresses the yield of 48b and below 160K, azobenzene, the product of triplet nitrene dimerization, is produced. Thus, high temperature favors reactions of singlet state intermediates whilst low temperatures favor reactions associated with triplet phenylnitrene. 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

Dimerization. One of the principal reactions of NH is dimerization to diimide (N2H2). Azobenzenes are often obtained in reactions where aryl nitrenes are... 

It would thus seem that dimerization is more important for nitrenes than it is for carbenes, but again it has not been proved that free nitrenes are actually involved. 

As shown in the previous two sections, rhodium(n) dimers are superior catalysts for metal carbene C-H insertion reactions. For nitrene C-H insertion reactions, many catalysts found to be effective for carbene transfer are also effective for these reactions. Particularly, Rh2(OAc)4 has demonstrated great effectiveness in the inter- and intramolecular nitrene C-H insertions. The exploration of enantioselective C-H amination using chiral rhodium catalysts has been reported by several groups.225,244,253-255 Hashimoto s dirhodium tetrakis[A-tetrachlorophthaloyl-(A)-/ r/-leuci-nate], Rh2(derived rhodium complex, Rh2(i -BNP)4 48,244 afforded moderate enantiomeric excess for amidation of benzylic C-H bonds with NsN=IPh. 

From these results, the crystaline compound was determined to be a dimerized 3-methyl-2-butylideneaniline[6]. The formation of such a dimer from the residue suggests that the residual matter is 3-methyl-2-butylideneaniline. This presumption is supported by the fact that a similar product can be obtained from the reaction of aliphatic nitrene with unsaturated olefines. 

Flash vacuum thermolysis of 6-chloro-3-diazoindazole (2b) resulted, upon loss of nitrogen, in the formation of carbene 102, which could intra-molecularly rearrange to the nitrene 103, or to the azabenzocyclopropene 104, or to azacycloheptatrienylidene 105 (Scheme 29). The only isolable product was 106, formed by dimerization of the nitrene 103 (78CB2258). 

Intermolecular amination experiments described by Muller using 02NC,5H4S02N=IPh (NsN=IPh) as the nitrene source underscore the value of certain rhodium(II) catalysts for C-H insertion (Scheme 17.5) [12, 34—36]. In accord with Breslow s finding, dirhodium carboxylates were demonstrated to catalyze the amination of allylic, benzylic, and adamantyl substrates. Notably, structurally related tetracarboxamide dimers fail to give... 

Several lines of inquiry have been explored to address key mechanistic issues in the rhodium-catalyzed C-H insertion of carbamates and sulfamates (Scheme 17.32) [99]. A pathway involving initial condensation between substrate 96 and PhI(OAc)2 to form iminoiodinane 97 was envisioned in the original design of this chemistry. Coordination of 97 to an axial site on the rhodium dimer would promote nitrene formation and the ensuing C-H insertion event Surprisingly, control experiments with PhI(OAc)2 and sulfamate 96 (or analogous carbamates) give no indication for a reaction between these two components. 

The simple photochemical transformation of o-azidobiphenyls into carbazoles is preparatively useful. Considerable effort has been expended on study " of the mechanism of this process. Both carbazole and dimerized nitrene, for example 257, can be produced in the process in... 

In the absence of nucleophiles the cyclic ketenimine polymerizes. At high dilution it can slowly revert to benzazirine BZ, and to the singlet nitrene. Eventually the singlet nitrene relaxes to the lower-energy triplet nitrene (at high dilution), which subsequently dimerizes. ... 

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