Reactions of nitrenes

Nitrenes can also undergo insertion into C—H single bonds, leading to ring closure. Thus, vinyl azidothiophenes 2.91 have been found to be useful precursors for annulation of pyrroles and thiophenes. 

Singlet nitrenes insert into alkyl C-H bonds selectively, with retention of configuration. Triplet nitrenes do not insert into alkyl C-H bonds. 

Hydrogen abstraction from the carbon to the nitrogen leads to the formation of imines. This process is of considerable synthetic importance. When hydrogen abstraction takes place from 4- and 5-position which is followed by ring closure, the products formed are pyrrolidines and piperidines, respectively. 

Aryl nitrene ring expansion and ring contraction 

Nitrenes are also obtained as reaction intermediates in Hofmann, Schmidt and Lossen rearrangements. 

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The reactions of nitrenes are also similar to those of carbenes. As in that case, many reactions in which nitrene intermediates are suspected probably do not involve free nitrenes. It is often very difficult to obtain proof in any given case that a free nitrene is or is not an intermediate. 

This has been mentioned at various points in this paper and may involve either a direct acid-base reaction of nitrene and nucleophile or, in some instances, reaction of the nitrene precursor with the nucleophile (or 1,3-dipolarophile) followed by loss of nitrogen. For example, the reaction of benzenesulphonyl azide with pyridine to give 31 (Ar=Ph) 69> could either involve a free nitrene or a concerted process in which the lone pair on the pyridine nitrogen atom assists the elimination of molecular nitrogen. That some free nitrene can be involved in these reactions is clear from the isolation of some 3-benzenesulphonamido-2,6-lutidine... 

Mechanisms [i] and [II] may occur through the recombination of unpaired electrons which are formed by the hydrogen abstraction of nitrenes, and the insertion reaction of nitrenes to C-H bonds, respectively. However, it has not been revealed in this study which C-H bond is attacked by the nitrene. 

Amines attached to positions 4, 5, 6 or 7 of benzo[6]thiophene behave as normal aromatic amines. The reactions of nitrenes attached to the benzene ring of benzo[6]thiophene are discussed in Section 3.14.3.5 alongside the reactivity of nitrenes attached to the thiophene ring, in order to obtain a unified picture. 

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. 

The rates of reaction of triplet nitrenes and carbenes were compared by DFT and MO calculations.76 Triplet nitrenes are more stable than the analogous carbene and CO bonds stronger than NO bonds the two effects are nearly additive in the reaction of nitrenes and carbenes with molecular oxygen. 

Among these, a variety of oxygen atom transfer reactions have been described [la,b] and highly stereoselective reactions have been reported [2]. Although the formation of aziridines by the reaction of nitrenes with olefins is well known, the efficiency is moderate, because of the competition between hydrogen abstraction and insertion processes [3]. A typical example is shown (Eq. 2) [3d]. 

The typical reactions of nitrenes generated from aliphatic azide precursors are shown below and include isomerization to imines (a), intramolecular insertion into C—H bonds (b) has been questioned (see 1. c. n> p. 57) and intermolecular hydrogen abstraction (c). 

The four ttutin characteristic reactions of nitrenes are summarized in Scheme 2. All of these reactions have parallels in carbene chemistry for example, a full discussion of the C—insertion reaction of car-benes is given in Volume 3, Ch ter 4.2. The first reaction, the addition to an alkene to form an aziridine, is covered in detail in Volume 7, Chapter 3.S. The C—insertion reaction, the subject of this chapter, can, in principle, occur by severid mechanisms. However nx>st of the reactions are believed to involve... 

The C—H insertion reaction of nitrenes is a potentially useful way of functionalizing unactivated C—H bonds, converting hydrocarbons into amine derivatives. In its intermolecular form the synthetic utility of the reaction is highly dependent on the substituents on the nitrene, and on the manner in which it is generated. To exemplify these effects, the results for the functionalization of cyclohexane by insertion of various nitrenes (equation 1) are summarized in Table 1. 

Thus the intramolecular reaction of nitrenes with sp C—H bonds, although it may not involve a genuine C—insertion mechanism, is a useful synthetic method, which extends and complements the nitrene insertions into unaedvated sp C—bonds discussed in earlier sections. 

E. General Considerations on the Reactions of Nitrenes with Thiophene. 179... 

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