Nitrenes dimerization reactions

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). 

It is clear that the ortho and para substituted diarylamines 16 and 17 are derived from capture of the singlet nitrene and the product of benzylic CH insertion 15 can be formed from either the triplet or singlet state of the nitrene. Decafluoroazobenzene is derived from a dimerization reaction of the triplet nitrene and pentafluoroaniline is formed by hydrogen atom abstraction reactions of triplet pentafluorophenyl nitrene, and possibly by some photoreduction of an excited state of the azide. It is clear from this data that the singlet and triplet nitrene are not rapidly interconverting, and there is no evidence for uphill intersystem crossing from the triplet to the singlet nitrene. 

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). 

We conclude this section by stating that both mechanisms, i.e., radical cation-radical cation and nitrene-anilinc coupling reactions, may be operative for the aniline dimerization reaction. As pointed out by Genies et al. [49], the nilrene mechanism may be... 

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. 

Copper-catalyzed decomposition of benzenesulfonyl azide in the presence of cyclohexene was the first reported evidence of a metal-catalyzed nitrene insertion reaction [25]. This seminal discovery was then followed by the pioneering work of Breslow and Gellman who introduced the use of iminoiodinanes as metal nitrene precursors as well as rhodium dimer complexes as catalysts [26,27]. They showed the formation of the corresponding benzosultam in 86% yield in the presence of rhodium (II) acetate dimer (Rh2(OAc)4) via an intramolecular metal nitrene C—H bond insertion reaction (Eq. (5.1)). 

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. 

Upon the photolysis of 9-azidoacridine hydrochloride, the resulting acridyl nitrene was protonated at the endocyclic nitrogen atom. Evidently, the positive charge of the acridine nucleus prevents the dimerization reaction to the azoacridine dication. Therefore, the consecutive abstraction of two hydrogen atoms from solvent moleeules with formation of protonated 9-aminoacridine (Scheme 3), whieh gives the free amine upon neutralization, is the main reaction route of the aeridyl nitrene eation. 

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... 

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. 

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. 

Nitrenes have a short lifetime (only several microseconds)86- 8 and undergo stabilization by the following reactions isomerization to imines, dimerization to azo compounds, hydrogen abstraction followed by ring closure to heterocyclic compounds, bimolecular insertion into C-H bonds to secondary amines, addition to solvent yielding ylids, and addition to unsaturated systems yielding heterocyclic compounds. Table 117-106 includes the reaction products and references for the different classes of nitrenes. 

The photochemistry of aryl azides is quite complex, suggesting that the nitrene 14 may not be the only reactive intermediate and that insertion reactions may not be the only route to form photoconjugates.Although aryl nitrenes are much less susceptible to rearrangements than acyl nitrenes, they may still occur and lead to the formation of reactive intermediates such as azepines, which may go on to react with nucleophiles.[911 141 Addition of nitrenes to double bonds will generate azirines, while dimerization will produce azobenzenesJ11 Aryl azides are stable to most of the procedures used in the course of peptide synthesis except for reduction reactions. Non-photochemical reduction of aryl azides to the primary amines by thiols has been reported by Staros et al.[15]... 

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