Properties of Nitrile Ylides

Another mechanism which could also account for the products obtained 

This alternate possibility was discounted by Huisgen, however, who showed that three-membered rings are not primary products in the 1,3-dipolar additions leading to five-membered heterocycles with nitrilium betaines.  

As was mentioned in the previous section, the HOMO and second LUMO of the bent nitrile ylide bear a strong resemblance to the HOMO and LUMO of a singlet carbene. Since carbenes are known to react readily with double bonds, a 1,1-cycloaddition reaction of nitrile ylides can be expected. Padwa and Carlsen uncovered the first example of such a process during an investigation of the photochemistry of a number of 2-ally 1-substituted 2ff-azirines. When a thoroughly deaerated solution of 2-allyl-2-methyl-3-phenyl-2/f-azirine (177) was irradiated in cyclohexane with light of wavelength 280 nm for 15 min, an extremely rapid and clean conversion to 3-methyl-l-phenyl-2-azabicyclo[3.1.0]hex-2-ene (179) was observed. When the irradiation of 177 was carried out to 20% conversion, however, a 1 1 mixture of 179 and 1-methyl-3-phenyl-2-azabicyclo[3.1.0]hex-2-ene (178) was quantitatively isomer-ized to 179. The photochemical behavior of the isomeric 2-allyl-3-methyl-2-phenyl-2if-azirine (180) afforded a quantitative yield of azabicyclohexene (179). A control experiment showed that 177 and 180 were not interconverted by a 

The formation of azabicyclohexenes 178 and 179 from the irradiation of azirine 177 clearly proceeds via a nitrile ylide intermediate since the formation of these compounds is entirely suppressed when the irradiation is carried out in the presence of an added dipolarophile. For example, when dimethyl acetylenedicarboxylate is used, cycloadduct 183 was the only product isolated. The fact that the photolysis of 177 produces a nitrile ylide intermediate which could be trapped by an added dipolarophile (i.e., as 183) eliminates a [2-f2]cycloaddition of the azirine C=N double bond with the olefin and subsequent rearrangement of a hypothetical azatricyclo[2.1.1.0 ]hexane intermediate (184) as the mechanism for the formation of the azabicyclo[3.1.0]hex-2-ene system. 

As unusual aspect of the intramolecular photocyclization of 2-allyl-substituted 2/f-azirines was uncovered during a study of the photochemistry of -2-(2-butenyl)-2-methyl-3-phenyl-2H-azirine (185). Irradiation of 185 in cyclohexane gave rise to one major product ( 95%) which was identified as nrfo-3,6-dimethyl-l-phenyl-2-azabicyclo[3.1.0]hex-2-ene (186). The formation 

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