Nitrenes as Intermediates

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)]. 

Azides (24) thermally decompose to tetrazines (Equation (8)) <66TL5369> and 4-aminotriazoles (25) are cleaved by lead tetraacetate oxidation (Equation (9)) <70TL385l>, both presumably via the nitrene as intermediate. 

Nitrenes as intermediates in synthesis of N-heterocycles 82MI14. Nitroacetic acid and its esters in synthesis of N-heterocycles 79S666. Nitro compounds of aliphatic series, synthesis of N-heterocycles from ... 

Nitrenes undergo addition reactions to neutral nucleophiles.60 51 Accordingly, they add to the nitrogen atom of pyridines, forming iV-imines. This method of preparation was described first by Curtius et a/.62-66 They heated aromatic sulfonyl azides in pyridine and obtained various A-sul-fonylimines [Eq. (5) ] the structures of which were not at that time recognized. They formulated sulfonyl nitrenes as intermediates in this reaction. 

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 ... 

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). 

From N-oxides of aromatic bases oxaziridines were obtained only at very low temperatures, but oxaziridines were often postulated as intermediates in the photoconversion of such N-oxides (Section 5.08.3.1.2). Isolation of the more stable photoisomers of nitrones also causes some problems due to their thermal and photochemical instability leading to acid amides, e.g. (69TL2281), or, by fragmentation, to carbonyl compounds and products of stabilization of nitrenes, e.g. from (260) (69ZN(B)477). 

There are four types of organic species in which a carbon atom has a valence of only 2 or 3/ They are usually very short lived, and most exist only as intermediates that are quickly converted to more stable molecules. However, some are more stable than others and fairly stable examples have been prepared of three of the four types. The four types of species are carhocations (A), free radicals (B), carbanions (C), and carbenes (D). Of the four, only carbanions have a complete octet around the carbon. There are many other organic ions and radicals with charges and unpaired electrons on atoms other than carbon, but we will discuss only nitrenes (E), the nitrogen analogs of carbenes. 

The hydrogenation of nitroacetophenones has been studied and considerable kinetic and mechanistic information obtained. Differences in reaction rate, bonding and selectivity have been observed. The formation of 1-indolinone from 2-NAP was unexpected and revealed the presence of a surface nitrene. This intermediate has not been postulated in nitroaromatic hydrogenation previously. Hydrogenation in the presence of deuterium revealed, as well as a kinetic isotope effect, that it is likely that... 

The great majority of matrix isolation studies of carbenes and nitrenes have employed their formal adducts with molecular nitrogen, that is, diazo compounds or diazirines in the case of carbenes, azides in the case of nitrenes, as precursors for their in situ generation. Usually, these compounds will readily release N2 on irradiation with a low-pressure mercury lamp (254 nm), and this fragment has the advantage that it will usually not react with or perturb the targeted reactive intermediate (see Scheme 17.2). 

Nitrenes, as is the case with other electron-deficient high energy species, are sufficiently short-lived that special experimental techniques are required for positive verification. Bonding patterns, such as those above, can just as readily involve nitrenoids which are electron-deficient in character but not truly monovalent. Here we will not attempt to make a careful distinction between these intermediates, so long as cyclization is the synthetic outcome. 

Singlet nitrenes such as intermediates 3-7 (Sch. 1) are known to undergo intra- and intermolecular insertion into C-H- and O-H-bonds as well as cycloaddition towards 7r-bonds. 

Other classes of heterocycles undergo thermolytic fragmentation to give imido-nitrenes, as typified by the thermolysis of 1,5-diphenyltetrazole 41, the intermediates 42 can either cyclize on to an aromatic ring to form benzimidazoles 43 or undergo a Wolff-type rearrangement to carbodiimides 44 (Scheme 7). Thermal decomposition of 2-acyltetrazoles... 

A variety of pyrrole ring-closure reactions are conveniently formulated as proceeding via nitrene-type intermediates. Pyrolysis of tf-nitrobiphenyls with iron(II) oxalate <1961T80>, or reduction under milder conditions with triethyl phosphite <1965JCS4831> or tris(trimethylsilyl) phosphite <1979TL375>, leads to the carbazole, as does the pyrolysis or photolysis of 2-azidobiphenyls (Scheme 13) <1975JA6193>. 

The existence of ground-state triplet carbenes, nitrenes, and their derivatives as intermediates (291) in... 

At least two types of nitrenium ions the nitrogen analogs of carbocations, can exist as intermediates, although much less work has been done in this area than on carbocations. In one type (65), the nitrogen is bonded to two atoms (R or R can be and in the other (66) to only one atom. " When R = H in 65 the species is a protonated nitrene. Like carbenes and nitrenes, nitrenium ions can exist in singlet or triplet states." ... 

Clearly, in many cases where the formation of N-nitrenes has been postulated as intermediates, the process really proceeds by an alternate non-nitrene pathway. For example, the mechanism of such an apparently simple reaction as the formation of )V,N -diazoles may be presented as (1) the result of dimerization of two nitrene particles (2) interaction of the (V-nitrene with the starting amine affording a tetrazane followed by its... 

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