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The formation of the diazetidines 15 can be explained by an intramolecular [2+2] cycloaddition reaction of the bis(carbodiimide)14. This hypothesis is availed by two facts:

When the reaction was monitored by IR spectroscopy, we could follow it as the strong band corresponding to carbodiimide, clearly apparent at the beginning of the reaction, decreased in intensity when the mixture was refluxed in benzene 12 h. This likely indicates that at this moment the band correspond only to the carbodiimide fragment of the N,N'-diarylcarbodiimide present in the final reaction mixture.

In an attempt to isolate the 2,2'-biphenylene bis(carbodiimide) 14 we chromatographed the reaction mixture (5', room temp., dry dichloromethane) from which we isolated an inseparable mixture of the diazetidine 15 and another compound, (presumably 14, IR = 2142 cm^-1) which we studied by VT-NMR: on heating the compound 14 gradually disappeared while the proportion of 15 increased notably in the mixture.

Another attempt to synthesize 14 from the dehydrosulfurization of the bis(thiourea) 16 failed, and we isolated the guanidine shown below:

A tentative mechanism to explain this result:

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If in these reactions the intermediate 17 is involved likely it would lead primarily to the corresponding P⁺-N^- bines shown in the next scheme. These bines could react with arylisothiocyanates or arylisocyanates in several ways. To clarify these points we are studying the reactivity of N-[2'-(azidobiphenylene-2-yl)],N'-arylcarbodiimide with phosphines.

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