Nitrene rearrangements synthesis

The photoreactions of azides can in most cases be rationalized in terms of the initial formation of nitrenes which then undergo rearrangement, insertion or addition reactions. Nitrene rearrangement is thought to be responsible for the conversion of the a-azido ethers (81) into the imino ethers (82), and the first silanediimine (83) to be described has been prepared in a similar fashion from the diazide (84) as shown in Scheme 4. Intramolecular nitrene addition to suitably aligned nitrogen-nitrogen double bonds has been employed in the synthesis of triaziridines. 

Iminoboianes have been suggested as intermediates in the formation of compounds derived from the pyrolysis of azidoboranes (77). The intermediate is presumed to be a boryl-substituted nitrene, RR BN, which then rearranges to the amino iminoborane, neither of which has been isolated (78). Another approach to the synthesis of amino iminoboranes involves the dehydrohalogenation of mono- and bis(amino)halobotanes as shown in equation 21. Bulky alkah-metal amides, MNR, have been utilized successfully as the strong base,, in such a reaction scheme. Use of hthium-/i /f-butyl(ttimethylsilyl)amide yields an amine, DH, which is relatively volatile (76,79). 

This section deals with transformations of heterocyclic rings and includes not only addition of carbenes and nitrenes, but also rearrangements frequently following these additions. Also included are topics such as the synthesis of Reissert compounds as well as miscellaneous oxidations and reductions. 

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]... 

Abramovitch, Challand, and Yamada322 added thermally DMAD to 2-azido-3,4,5,6-tetrachloropyridine (172) and obtained 85% of the 1,2,3-triazole derivative (173), with no evidence for the generation of the nitrene. A one-step furopyridine synthesis of a mixture of 175 and 176 is achieved by treating the 1-oxide 174 with DMAD in hot toluene, rearrangements having taken place.323... 

Aminooxindols (11/153) are rearranged to 3-cinnolinols 11/154, if treated with equimolar amounts of /-butyl hypochlorite [106] [107] [108]. Compound 11/153 is known as Neber s lactam , and is formed from 11/154 with zinc and sulfuric acid. The mechanism preferred for the 11/153 —> 11/154 transformation involves nitrene formation [106]. - As already mentioned, the Gabriel synthesis (11/155 11/157) is a method for synthesis of primary amines. But the side product is the ring enlarged hydrazide, compare Chapter IV... 

Although this chapter is primarily concerned with the functionalization of unactivated spi C—bonds by nitrene insertion, other relevant aspects of nitrene chemistry are included. Thus the brief section on ni> trene reactivity highlights the rearrangement reactions which often compete with C—H insertion, and the final section covers insertion into sp C—H bonds, since many of these reactions have found wide use in recent years in the synthesis of natural products. 

Even if no nitrenes are produced as intermediates, the isocyanates formed in Curtius-type rearrangements are of value for the synthesis of heterocyclic compounds. For example, transient isocyanates cyclize unto azine rings, giving imidazoazines (Scheme 21).161 N,N-Diphenylcarbamoyl azide rearranges thermally to 1-phenylindazolone (Eq. 35) via the elusive diphenyla-mino isocyanate.162 Further examples have been summarized by Reichen.163... 

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