1.2- Diazetidin-3-ones, dimerization

Cyclohexylamine is reported to react with formaldehyde giving a 60% yield of 1,3-diazetidine. One additional example reports a photochemical closure of iV-cyclohexylben-zaldehydeimine (68JA1666). An excellent review covers much of the earlier work on dimerizations of isocyanates (69ACR186). Aromatic isocyanates readily dimerize to uretidinediones (l,3-diazetidine-2,4-diones) when catalyzed by trialkyl phosphites or pyridine presumably by a dipolar intermediate AH = 2.1 to 4.2 kJ mol-1, AS = -251 to -293 JK-1 mol- (Scheme 86) (66JA3582, 76JGU799). 

Nitrogen substituted carbodiimides are usually not stable in the carbodiimide configuration. Some derivatives can be distilled under vacuum, but on standing they undergo slow dimerization reactions. In one case of N-heterocyclic carbodiimides the 1,3-diazetidine diimine dimers are obtained instead of the monomers. Semiempirical calculations on the formation of 1,3-diazetidine diimine dimers of H2N—N=C=NMe confirm the Z,Z and E,E configurations of the dimers obtained. N-nitrosubstituted carbodiimides are more stable. 

While l,3-diazetidine-2,4-diones are well known and readily prepared by dimerization (vide infra), very few closures from ureas are documented. One simple dialkyl derivative stable enough to be isolated is illustrated in Scheme 60. Smaller alkyl groups like isopropyl are much less stable (71JOC3056). 

The dimerization of aryl isocyanates to l,3-diarylazetidin-2,4-diones is one of the classical methods for the synthesis of 1,3-diazetidinones. In 1993 trimerization of phenyl isocyanate catalyzed by a fluoride ion was also reported, and a small amount of l,3-diazetidin-2,4-dione was obtained at room temperature <1993JOC1932>. 

Diazetidin-3-ones substituted at N-1 dimerized on standing to give materials with the suggested structures (104) (mp 149-151 °C) and (105) (mp 310-315°C) (Equation (24)) <84JOC44l5>. 

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