Diazetidines carbodiimides

The addition of cis-azobenzenes to ketenimines gives 3-imino-l,2-diazetidines which cleave to give carbodiimides and imines upon thermolysis (Scheme 4) (67JHC155, 72JHC555, 74JHC409). l,2-Diphenyl-l,2-diazetidin-3-one has similarly been reported to fragment to isocyanate and imine (56AG71). 

Carbonyl-substituted isocyanates have been reported to add [2 + 2] to carbodiimides, although in one case the reported 4-imino-l,3-diazetidin-2-one (64AP623) was reassigned an oxadiazine structure (72BCJ1534). It is clear that the initially formed [2 + 2] kinetically favored adducts are thermally unstable and ultimately form more stable [4 + 2] products, oxadiazines (Scheme 13) (79BSF(2)499). 

General syntheses of 1,3-diazetidines are [2 + 2] cycloadditions of C = N-containing substrates (Scheme 26). Dimerization of imines and isocyanates furnishes substituted 1,3-diazetidines. Cycloaddition of isocyanates to carbodiimides and isocyanates with imines are alternative syntheses,... 

Fused 2,4-diimino-l,3-diazetidines 26 obtained by aza-Wittig [2+2] cycloaddition of carbodiimide generated in turn from bisiminophosphoranes were confirmed by X-ray diffraction studies <1992CC424>. 

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>. 

The diazetidine 30 was isolated in modest yield (20-40%) by the reaction of a carbodiimide with 1 equiv of triphenylphosphine followed by treatment with an aryl isocyanate 29. 

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... 

The dimerization of diphenylcarbodiimide is catalyzed by tributylphosphine. In this manner l,3-diphenyl-2,4-diphenylimino-l,3-diazetidine 191, mp 162-163 °C, is obtained in 71 % yield on heating of the carbodiimide at 90 °C in the presence of tributylphosphine for 22 hrs. " Heating of diphenylcarbodiimide at 165-170 °C for 16 hrs. in the absence of the catalyst afforded 43 % of the dimer, indicating that the dimerization is a thermal process. 

The reaction of carbodiimides with alkyl- or arylisocyanates proceeds exclusively across the C=N bond of the isocyanates to give 2-imino-l,3-diazetidine-4-ones 209 as evidenced by degradation studies In the case of N-alkyl-N -arylcarbodiimides the reaction proceeds... 

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 Wittig-type reactions of iminophosphoranes with isocyanates and related compounds have also been extensively used in heterocyclic synthesis. Examples include the preparations of the mesoionic [l,3,4]thiadiazolo[2,3-c][l,2,4]triazines (210) from (209), 0 bicyclic guanidines, e.g. (212), from (211), naphthypyridines (215), (216), and (217) from (213) and (214), 2 pyrido[l,2-f]pyrimido-[4,5-d)pyrimidines (218), 7H-pyrido-[4,3-c]- (219) and 10H-pyrido[3,4-b]- (220) carbazoles, tricyclic fused 2,4-diimino-l,3-diazetidines (222) from the bisiminophosphorane (221), benzotriazepines (225) from (223) and (224), 6 and mesoionic thiazolo-[2,3-b]-1,3,4-thiadiazoles (227) and N,N-bisheteroarylamines from the iminophosphorane (226), derived from 3-amino-4-phenylthiazole-2(3H)-thione. The carbodiimides (229), prepared from the iminophosphorane (228), can be converted into quinolines or a-carboline derivatives depending on the nature of the isocyanate used in the reaction with (228) and the reactions of iminophosphoranes (230) and (231) with aryl and styryl isocyanates provide one-pot syntheses of quinoline, a-carboline, and quinindoline derivatives. 9... 

The preparation and intramolecular cyclization of other bis(carbodiimide)s has been described. Treatment of 2,2 -bis[(triphenylphosphoranylidene)aminoj-biphenyl 1726 with carbon dioxide provides, through an intermolecular aza-Wittig reaction, the bis(carbodiimide) 1728 (6% yield), which reacts further in an intramolecular cydization to form diazetidine 1729 (28% yield) [1267]. 

A bis(iminophosphorane) derived from 2,2 -diazidobiphenyl [or a 2,2 -bis(isothio-cyanato)biphenyl reacted with aryliminophosphoranes] serves as the entry point for bis(carbodiimides) and their intramolecular cycUzations to l,3-diazetidine-2,4-diimines. A mechanistic scheme for the aza-Wittig reactions is proposed. 

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. 

Fluorohoric acid Dimerization of carbodiimides 2,4-Diimino-l,3-diazetidines... 

---