Dimerization reactions 1,3-diazetidines

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. 

The photochemical reaction of 2-phenylbenzoxazole (84CHEC(6)177) forms a head-to-tail dimer, 1,3-diazetidine, (393) (87CC578). Benzoxazole itself, in the presence of oxygen, gives a coupled product, (394) (74TL375). 

The 1,3-diazetidine derivatives are most commonly synthesized by the dimerization of heterocumulenes. At elevated temperature they undergo dissociation and are in equilibrium with their monomers. For example, 1,3-diazetidine 108, formed by the reaction of a-naphthyl isocyanate 106 with dicyclohexylcarbodiimide 107, upon thermal dissociation furnished products of its [2+2] cycloreversion reaction, that is, cyclohexyl isocyanate 110 and a-naphthylcyclohexylcarbodiimide 109 (Scheme 10) <1962AGE621>. 

Risch et al. have reported on the reaction of l,3-diazetidine-2,4-diones, which are used in the preparation of polymers and advanced materials <1999JPR616>. With the help of special catalysts it was possible to dimerize or trimerize diisocyanate to give l,3-diazetidine-2,4-diones 183 or isocyanurates 184 (Scheme 25). 

Carbodiimides undergo [2+2] cycloaddition reactions to furnish l,3-disubstituted-2,4-bisalkyl or arylimino-l,3-diazeti-dine. For example, dibenzyl carbodiimides undergo dimerization to yield l,3-diazetidin-2,4-diimines 296 on heating. The reaction can be catalyzed by the addition of tributylphosphine (1%) (Equation 36) <1940CB1114, 1968CB174>. 

Phenylbenzoxazole (121) forms an amorphous photodimer, either the 1,3-diazetidine (122) and/or the isomer (123). The dimer reverts to the monomer in solution the reaction is very fast under acid catalysis and 116 kJ mol-1 is released. 2-Phenylbenzoxazole is thus an efficient system for the storage and conversion of light energy (82CC380). 

General syntheses of 1,3-diazetidines 200 are [Z + Z] cycloadditions of C=N-containing substrates (Scheme 95) . The dimerization of aryl isocyanates 199 to l,3-diarylazetidin-2,4-diones is one of the classical methods for the synthesis of 1,3-diazetidinones. Carbodiimides 201 undergo [Z + Z] cycloaddition reactions to... 

Diazetidine-2,4-diimines, carbodiimide dimers, formation, reactions, 59, 133, 148 ... 

Carbodiimides undergo cyclooligomerization reactions. In this regard they are similar to isocyanates, the mono imides of carbon dioxide. For example, aliphatic carbodiimides undergo rapid dimerization catalyzed by tetrafluoroboric acid at room temperature to give salts of the cyclodimers 183. Neutralization with dilute sodium hydroxide, or better filtration through basic AI2O3, afford l,3-dialkyl-2,4-bisalkylimino-l,3-diazetidines 184. ... 

Salts of aliphatic carbodiimide dimers are also obtained in the reaction of carbodiimides with dimethyl sulfate. The cyclic dimer of dibenzylcarbodiimide, mp 102-103 °C, was isolated in low yield from the distillation residue of the monomer. The crystal structure of l,3-dicyclohexyl-2,4-bis(cyclohexylimino)-l,3-diazetidine, the cyclodimer of DCC, is... 

Diazetidine iminium salts (142) are formed by the dimerization of aliphatic carbodi-imides that is induced by methylating agents. A similar reaction with tetrafluoroboric acid gave the carbodi-imide dimers (143) via the corresponding iminium salts. 

Azete 6 dimerizes thermally to the 1,3-diazetidine system 7, which can be isomerized to the 1,5-diazocine 7. Azete 6 shows numerous cycloaddition reactions, especially with activated acetylenes and phosphaalkines for its transformations to the pyridine valence tautomers see p. 364. 

Dimerization of Carbodiimides. Treatment of alkyl carbodi-imides with anhydrous HBF4-OEt2 in CH2CI2 results in rapid dimerization to tetrafluoroborate salts in 95% yield (eq 13). Basi-fication converts the salts to diazetidines. In the same work, aryl carbodiimides undergo a similar reaction, but substituted quina-zolines are obtained. 

---