Rearrangement of Nitrenes

The preparation and properties of these tertiary aminimides, as weU as suggested uses as adhesives (qv), antistatic agents (qv), photographic products, surface coatings, and pharmaceuticals, have been reviewed (76). Thermolysis of aminimides causes N—N bond mpture foUowed by a Curtius rearrangement of the transient nitrene (17) intermediate to the corresponding isocyanate ... 

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

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. 

When mesitylene-2-sulphonyl azide (3) is heated to 150 °C in n-dodecane, a Curtius-type rearrangement of the nitrene (4) occurs as discussed in Section 2.1 i to give 2,4,6-trimethylaniline and the hexa-methylazobenzene 14>. A similar result has now been observed by a careful analysis of the thermolysis products of durene-3-sulphonyl azide in w-dodecane at 150 °C. The amine is definitely formed but the azo-compound could barely be detected 13>. 

Our calculations showed that the first step, cyclization of the nitrene to an azabicyclo[4.1.0]heptatriene, is rate-determining. Our calculated barriers for cyclization of four fluorinated derivatives of lb are given in Table 6.87 The CASPT2/cc-pVDZ barrier of 13.4 kcal/mol for cyclization of 2,6-difluoro-phenylnitrene (10b) is 4.1 kcal/mol higher than the barrier computed for lb -+ 2b. In contrast, the calculated barriers to rearrangement of 3,5-difluoro-phenylnitrene (lOe) and 4-fluorophenylnitrene (10c) are very similar to that computed for unsubstituted phenylnitrene (lb). These computational results are consistent with the observed reluctance of pentafluorophenylnitrene (10a) and 2,6-difluorophenylnitrene (10b) to rearrange,48 1 81 83 and with the relative ease... 

Another synthetic methodology of growing importance is based on the rearrangement of a transient nitrene, most often generated by thermolysis of an azido group as depicted in Scheme 5. 

Thermal or photochemical rearrangement of acyl azides into amines via isocyanate intermediates. While the thermal rearrangement is a concerted process, the photochemical rearrangement goes through a nitrene intermediate. 

When thermolyzing azido compounds, the question arises as to whether an observed rearrangement is achieved in one step, concerted to the elimination of Nj, or whether there are two steps, elimination and rearrangement, with nitrenes as intermediates. Generally, thermolysis of azidoboranes is more probably a concerted process (22). The decomposition of (tPr2N)2BN3, however, carried out at 450°C, gives a product, besides the main product of Eq. (19), that has to be interpreted in terms of a nitrene stabilized by an intramolecular CH insertion [Eq. (19b)]. 

In contrast to the somewhat limited synthetic utility of nitrenes, there is an important group of reactions in which migration occurs to electron-deficient nitrogen. One of the most useful of these reactions is the Curtius rearrangement 16 This reaction has the same relationship to acylnitrene intermediates that the Wolff rearrangement does to acylcar-benes. The initial product is an isocyanate, which can be isolated or trapped by a nucleophilic solvent. 

Compounds containing neutral, monovalent nitrogen atoms are known as nitrenes. The parent structure, NH, is also called imidogen. Because most stable compounds of neutral nitrogen have a valence of 3, it is no surprise that nitrenes typically are very short lived, reactive intermediates. A short history of nitrenes has been presented by Lwowski who points out that they were first proposed by Tiemann in 1891 as transient intermediates in the Lossen rearrangement. 

In fact, rearrangements of singlet alkylnitrenes generated in frozen matrices cannot even be suppressed by developing strain in imine products such as 1-3, although some triplet nitrene is detected by EPR and UV-vis spectroscopy upon low-temperature photolysis of 1-azidonorbornane. ... 

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 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]... 

Among the more recent approaches to phenoxazines and phenothiazines the reductive cyclization of 2-nitrodiphenyl ethers and sulfides with trialkyl phosphites is the most interesting. Here too a spiro intermediate is involved, produced by attack of an initially formed aryl nitrene on the second aromatic ring. The sulfide (252), for example, reacts with triethyl phosphite to yield 1-methylphenothiazine (253) and it is clear that in this case ring opening of the spiro intermediate also proceeds with a rearrangement of the Smiles type (Scheme 113) (75JCS(P1)2396). 

Thermal decomposition of ethyl azidoformate in the presence of 4-phenyl-l,2-dithiole-3-thione leads to the rare 1,2,3-dithiazine system (254) (76CJC3879). The reaction presumably involves initial attack of the nitrene at S-l, followed by a [1,2] rearrangement of the resulting ylide (Scheme 34). 

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