Nitrenes with aromatic compounds

In the present experiment, we have studied the mechanism of photocrosslinking of 1,2-polybutadiene by aromatic azide, based on the reaction of aromatic nitrene with unsaturated hydrocarbon monomeric compounds. 

Organic azides (RNS) are isoelectronic with diazo-compounds, and like them are yellow in colour. On irradiation azides lose nitrogen to produce monovalent nitrogen species, nitrenes lRN . In the absence of an addend, aliphatic nitrenes generally undergoa shift of hydrogen to give an imine (5.34), whereas aromatic nitrenes can dimerize to yield an azo-compound (S.3SI. 

Coordination chemistry reveals how two ArN species can be coupled into one azobenzene molecule. In the case of the reaction of Fe3(CO)12 with aromatic nitro compounds in benzene1161, formation of derivatives such as in structure 111 has been proven by X-ray diffraction. Azoxybenzene can be formed by reaction of nitrene with nitrosobenzene, formed by reduction of nitrobenzene. 

By reaction in benzene of P 3aromatic nitro compounds, derivatives such as <1) and <3) reacts with Ru3ruthenium nitrene complex <5) reaction with excess nitrobenzene in boiling benzene [653. On the other hand even the thermal disproportionation of <5) (M=sRu R=Ph) under an inert atmosphere and in solution leads to the formation of <1), and other as yet uncharacterised products[663. Complexes following sequence of reactions]603s... 

Phosphinazine derivatives (6.352) and amidophosphoric compounds (6.353,6.354) can be obtained from phosphites. With aromatic nitrites, nitrenes can be formed (6.355). 

Trifluoroethanol has been shown to promote the addition of nitrenes, generated by the reaction of nitroso-compounds with phosphites, to aromatic hydrocarbons, e.g. (57), (58), and (59) are formed from the reaction... 

The Sundberg indole synthesis using aromatic azides as precursors of nitrenes has been used in synthesis of various indoles. Some kinds of aryl azides are readily prepared by SNAr reaction of aromatic nitro compounds with an azide ion. For example, 2,4,6-trinitrotoluene (TNT) can be converted into 2-aryl-4,6-dinitroindole, as shown in Eq. 10.60.83... 

In contrast to considerations of 50 years ago, today carbene and nitrene chemistries are integral to synthetic design and applications. Always a unique methodology for the synthesis of cyclopropane and cyclopropene compounds, applications of carbene chemistry have been extended with notable success to insertion reactions, aromatic cycloaddition and substitution, and ylide generation and reactions. And metathesis is in the lexicon of everyone planning the synthesis of an organic compound. Intramolecular reactions now extend to ring sizes well beyond 20, and insertion reactions can be effectively and selectively implemented even for intermolecular processes. 

Iwayanagi (40) recently extended this system into the mid-UV range by changing the sensitizer to an aromatic monoazide compound (4-azidochalcone). Insolubilization of this mid-UV resist does not result in a cross-linked matrix as occurs with the bisazide sensitized MRS. The primary reaction appears to be insertion of the reactive nitrene into a C-H bond on the ring, forming a secondary amine. 

The reduction of aromatic nitro compounds to the corresponding amines was catalyzed by [Ru3(CO)i2] in combination with aliphatic amine cocatalysts (95). A mixture of diglyme and water was used as a solvent, turnover frequencies were about 5000 h-1, and a CO partial pressure of 20-50 atm was necessary. The reaction is highly selective for aromatic amines. It was speculated that the reaction proceeds via an intramolecular hydrogen transfer in a hydrido-metal-nitrene intermediate without prior formation of H2 in the water gas shift reaction. 

V-Aminopyrroles, easily prepared from the reaction of azoalkenes with enamines and /3-dicarbonyl compounds, have been shown to react with electron deficient alkynes to afford substituted benzenes (79TL2969). While the N-methoxycarbonylaminopyrrole (208) reacted with DM AD under rather vigorous conditions to afford (211) in only 13% yield, the N-unsubstituted aminopyrrole (209) prepared from (208) by NaCN treatment reacted with DMAD in CHC13 solution at room temperature to give (211) in 50% isolated yield. The formation of the aromatic system probably occurs by extrusion of the heteroatom bridge from (210) to afford a relatively stable nitrene (212 Scheme 45). 

The most reliable method for generating nitrenes is the thermal or photochemical elimination of nitrogen from azides. An alternative method which is useful for indole and carbazole synthesis is the deoxygenation of aromatic nitro compounds with trivalent phosphorus compounds. Triethyl phosphite is the most commonly used reagent, though more reactive compounds may be useful in special cases (B-79MI30600). 

More importantly, this silver system catalyzes the intermolecular amination of hydrocarbons, as shown in Table 6.3. In addition to animating weaker benzylic C-H bonds, stronger aliphatic C-H bonds such as those in cyclohexane were also reactive. Although yields with more inert hydrocarbons were modest with the bathophenan-throline system, the discovery of the first silver-catalyzed intermolecular amination opens opportunities for further developments. This reaction favored tertiary cyclic sp3 C-H bonds over secondary cyclic sp3 C-H bonds, and showed limited success with simple linear alkanes. No conversion was observed with any aromatic C-H bonds. The compound NsNH2 was tested as the nitrene precursor with different oxidants. The use of PhI(OAc)2 as oxidant gave the expected amination product with a lower yield, while persulfate and peroxides showed no reactivity. 

Abramovitch et al. [97] demonstrated that the pentafluorophenyl nitrene intermediate generated from treatment of nitroso compound 13 with triethyl-phosphite reacted with electron rich aromatics. Adducts were formed with benzene, toluene, anisole, and mesitylene in modest yield. 

There are many other kinds of reactive intermediates, which do not fit into the previous classifications. Some are simply compounds that are unstable for various possible reasons, such as structural strain or an unusual oxidation state, and are discussed in Chapter 7. This book is concerned with the chemistry of carbocations, carbanions, radicals, carbenes, nitrenes (the nitrogen analogs of carbenes), and miscellaneous intermediates such as arynes, ortho-quinone methides, zwitterions and dipoles, anti-aromatic systems, and tetrahedral intermediates. This is not the place to describe in detail the experimental basis on which the involvement of reactive intermediates in specific reactions has been estabhshed but it is appropriate to mention briefly the sort of evidence that has been found useful in this respect. Transition states have no real hfetime, and there are no physical techniques by which they can be directly characterized. Probably one of the most direct ways in which reactive intermediates can be inferred in a particular reaction is by a kinetic study. Trapping the intermediate with an appropriate reagent can also be very valuable, particularly if it can be shown that the same products are produced in the same ratios when the same postulated intermediate is formed from different precursors. 

These species are isoelectronic with carbenes. The parent compound is nitrene NH (also known as imidogen, azene, or imene), which is formed when hydrazoic acid is irradiated with UV Hght in an aromatic solvent, which produces a small amount of primary aromatic amine. In the presence of ethylene, nitrene is trapped to form aziridine (Scheme 6.1). Nitrenes are also referred to as derivatives of imidogens as aminyls, azene, azylene, azacarbene, imene, or imines. 

Nitrobiphenyl gave 2-aminobiphenyl, but no carbazole therefore a nitrene intermediate appeared unlikely in this reduction. The reaction between Kj[HFe(CO)43 and aromatic nitro compounds proceeds vigorously and exothermally in alcohol at room temperature to give the corresponding aromatic amines in excellent yields[76j. The reaction is not catalytic and is not affected by reaction atmospherearomatic nitro compounds with Fe3methanol-benzene is catalysed by the crown ether, 18 crown-6, with aqueous K0H[77j, or by PhCH2N3+Cl " with aqueous NaOH[64,78l. 

ArNHC02Me) as the major product, with ureas as the main by-products. Without the metal carbonyls, the aromatic nitro compounds are recovered unchanged from the reaction. A nitrene-alkoxy-carbonyl complex was suggested as the intermediate of the reactions... 

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