Nitrenes alkyl, rearrangement

The N-diazeniumdiolates are quite photosensitive. Studies of various 02-substi-tuted compounds, both alkyl and aryl, revealed a primary photochemical reaction involving cleavage of the N=N bond to yield a nitrosoamine and an O-substituted nitrene which rearranges to a C-nitroso compound (Scheme 3.31), the latter is often isolated as the oxime [224]. 

Adamantanyl nitrene (102) is generated by photolysis of 1-adamanlane azide (101). Nitrene 102 rearranges by an alkyl shift to the unstable imine 103 (Scheme 10.25). The C=N double bond is quite reactive because it is located at a bridge-head. Therefore, 103 dimerizes to azetidine 105 in a head-to-tail fashion in an alkane solution or in the solid state. 

Rearrangements. Alkyl nitrenes do not generally give either of the two preceding reactions because rearrangement is more rapid, for example,... 

Intramolecular insertion and addition reactions are very rare for alkyl nitrenes. In fact, it is not clear that the nitrenes are formed as discrete species. The migration may be concerted with elimination, as is often the case in the Wolff rearrangement.251... 

N-benzylaniline with phosgene, and then with sodium azide to produce carbonyl azide 52. On heating, nitrogen is evolved and a separable mixture of nitrene insertion product 53 and the desired ketoindazole 54 results. The latter reaction appears to be a Curtius-type rearrangement to produce an N-isocyanate (54a), which then cyclizes. Alkylation of the enol of 54... 

Aryldioxophosphoranes such as aryl- and alkyl-iminooxophosphoranes number among the short-lived metaphosphonates. The former are best obtained by fragmentation of cyclic phosphinic esters, and the latter by rearrangement of aryl- and alkyl-substituted phosphoryl azides and nitrenes, respectively. This reaction is reminiscent of the phosphorylcarbene/methyleneoxophosphorane rearrangement discussed in Section 2. 

The rearrangements that we have considered to date all have one feature in common the migration of an alkyl or aryl group, with its electron pair, to a carbon atom which, whether it be a carbocation or not, is electron-deficient. Another atom that can similarly become electron-deficient is nitrogen in, for example, R2N or RN (a nitrene, cf. carbenes above), and it might be expected that alkyl or aryl migration to such centres would take place, just as it did to R3C and R2C this is indeed found to be the case. 

Polyfluorination does seem to suppress rearrangements of alkyl azides and to extend the lifetimes of the corresponding singlet nitrenes. Photolysis of 5 in cyclohexane produces insertion adduct 6." ... 

The intramolecular reaction of alkyl nitrenes with a 1,3-diene is unsuitable for the preparation of fused bicyclic dihydropyrroles142. However, the intramolecular 1,3-dipolar cycloaddition reactions of the same substrates to give vinylaziridines, followed by rearrangement, can be utilized to achieve the desired target. 

Alkyl-2-phenylquinazolines 9 are readily available by reaction of 5,S -dimethyl-A-(A-aryl-benzimidoyl)sulfimides 7 with enamines 8 in refluxing Tetralin (1,2,3,4-tetrahydronaphthalene). The mechanism of quinazoline ring formation probably involves a thermal cleavage of the imidoylsulfimides into imidoyl nitrenes, nitrene addition to the enamine double bond and subsequent rearrangement of the aziridine intermediate thus formed to the final product 9. Small amounts (10-15%) of 4-unsubstituted quinazolines 10 are obtained as byproducts. The formation of these byproducts involves a known intramolecular rearrangement of the benz-imidoylsulfimides employed. ... 

Eor a treatise on azides, which includes discussion of rearrangement reactions, see Scriven, E.F.V. Azides and Nitrenes, Academic Press, NY, 1984. For a review of rearrangements of alkyl and aryl azides, see Stevens, T.S. Watts, W.E. Selected Molecular Rearrangements, Van Nostrand-Reinhold, Princeton, NJ, 1973, pp. 45-52. For reviews of the formation of nitrenes from alkyl and aryl azides, see, in Lwowski, W. Nitrenes, Wiley, NY, 1970, the chapters by Lewis, E.D. Saunders, Jr., W.H. pp. 47-97, 47-78 and by Smith, P.A.S. pp. 99-162. 

With the exception of the above report, there exists very little evidence for the intermediacy of discrete nitrene intermediates in alkyl azide thermolyses. A unimolecular process may arise either from a rearrangement concerted with elimination of nitrogen, or from a ratedetermining elimination of nitrogen followed by a fast rearrangement (equation 48). Several tertiary aliphatic azides were synthesized to determine whether processes other than rearrangement could be... 

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