Nitrenes carbonylnitrenes

If this mechanism is really operative, the "abstraction product" formed in benzene is not necessarily due to a triplet nitrene precursor. Recently a careful study of the thermolysis of methylazide in substituted benzenes demonstrated that the unsubstituted primary sulfonamide is a product of hydrogen abstraction by the nitrene 72). On the other hand there are remarkable differences in isomer ratios (o m p) of the ring-substituted anilides formed depending on the spin state of the reacting nitrene. The triplet was shown to attach the aromatic nucleus mainly in the o-position, as is expected from a highly electrophilic diradical. Dehydrogenations by carbonylnitrenes have been reported by several authors for a variety of systems. In the direct photolysis of ethylazidoformate 29 in cyclohexene, the amide 30 and the bicyclohexenyl 31 were isolated 35b Both products result from an abstraction reaction. 

Reactions of azides (of all types) that involve nitrogen evolution have often been assumed to proceed by a two-step nitrene mechanism , in which nitrogen is lost in the first step R—N3—>N2 -f R— N. The nitrene R—N then leads to the observed reaction products. For azides in general, nitrene reactions are treated in Chapter 5, but the formation and the reactions of carbonylnitrenes, CO—N, will be dealt w ith here. 

Carbonylnitrene intermediates have been postulated since 1891 , but a problem recurs for every reaction which can be formulated as a nitrene reaction one can always write a reasonable azide mechanism that leads to the same products. One such alternative is a two-step process in which the azide reacts with the substrate in a first step, to give an adduct or an intermediate radical or ion pair, which loses nitrogen in a second step. Another alternative is a concerted process, in which loss of nitrogen and product formation are simultaneous. An example of the former azide mechanism is the aziridine formation... 

Dissociation of acyinitrenes occurs, presumably from the triplet state, when carbonylnitrenes are produced in very unreactive media, or in a vacuum. Ethoxycarbonylnitrene, produced by flash photolysis in vacuo, dissociates to "NCO and EtO, the former being identified by its detailed and intense u.v. absorption spectrum. The NCO radical is not produced, however, when cyclohexene vapour is present. In this case, the normal double bond adduct, 7-ethoxycarbonyl-7-azabi-cyclo[4.1.0]heptane is formed, as identified by its i.r. spectrum . Pivaloylnitrene, i-BuCO—N, when produced in unreactive solvents, such as dichloromethane or neopentane, apparendy dissociates in a similar manner. A polymer suggested as being the product from <-Bu and NCO is formed, and isobutene can be obtained in 22% yield (based on nitrene formed) when a stream of nitrogen is passed through the reaction mixture . ... 

Alkynes add carbonylnitrenes to give l,3-oxazoles ° °, which may add another molecule of the nitrene to give the di-adduci 48 ... 

The proposed triplet mnltiplicity of the gronnd state of nitrene 21 was proven by ESR spectroscopyWasserman detected the ESR spectrnm of 21 at low temperature in a rigid matrix with a Z) value of 1.603 cm. Some interaction of one of the spins with the carbonyl group is evident in the nonzero E value of 0.0215 cm In subsequent studies, Autrey and Schuster reproduced the ESR spectrum of 21 and, along with Sigman et al., recorded a very similar ESR spectrum ( D = 1.65 cm E = 0.024 cm ) upon irradiation of a suspension of (4-acetylphenoxy)carbonyl azide (22) in Fluorolube at 77 K. This spectrum was assigned to triplet (4-acetylphenoxy)carbonylnitrene (23). 

Until recently singlet carbonylnitrenes, in a manner reminiscent of alkyl and vinyl nitrenes, had escaped direct spectroscopic detection. Only the ESR spectra of alkyloxy and aroyloxycabonyl nitrenes [RO-CO-N ( 21, R=Et and 23, R=Ar)] have been detected.eSR spectra of carbonyl nitrenes that do not have an oxygen atom attached to the carbonyl group [R-CO-N (R=alkyl, aiyl)] have never been observed. ° ... 

Therefore, the experimental data are most consistent with a singlet ground state of carbonyl nitrenes. Triplet carbonylnitrenes were not detected in either chemical trapping or spectroscopic experiments. The explanation of these resnlts was available only recently. 

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