Carbonylnitrenes

We will follow the lead of Lwowski and classify sulfonyl- and phosphorylnitrenes as a type of carbonylnitrenes. 

Harrison and Shalhoub131 have recently investigated the related carbonylnitrenes HCON, FCON, MeCON, and MeOCON, finding a triplet ground state in all cases. Geometry optimizations were carried out using an STO-3G basis set. 

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 . ... 

It appears that carbonylnitrenes insert at a detectable rate only in their singlet states, while phenylnitrene and cyanonitrene can insert in both their singlet and triplet states. Also, activated C—bonds (such as aromatic C—bonds or those next to ether linkages) may react with triplet carbonylnitrenes . ... 

Intramolecular addition of carbonylnitrenes to double bonds has been used to make a number of highly strained bi- and tricyclic systems . Thus, 4-cycloheptenecarbonyl azide, upon irradiation, gave the tricyclic acylaziridine 44, which is very readily hydrolysed to the hydroxylactam 45. Another example of Edwards intramole-... 

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

Nitriles add carbonylnitrenes to form 1,3,4-oxadiazoles . Alkoxycarbonylnitrenes (made either by azide photolysis, azide thermolysis or by the a-elimination route) give the 2-alkoxy-1,3,4-... 

Photolysis of carbonyl azides gives rise to two types of reactions. The photo-Curtius rearrangement proceeds to form isocyanate. In addition, bimolecidar trapping products, typical of the reactions of singlet carbonylnitrenes, are also observed. 

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