Cyclic carbodiimides

Chemistry and Technology of Carbodiimides Henri Ulrich 2007 John Wiley Sons, Ltd. ISBN 978-0-470-06510-5 

In contrast, the seven membered ring tricyclic carbodiimide 9, obtained in the pyrolysis of tetrazolophenanthridene, is stable to -40 °C and it dimerizes to give the [2+2] cyclodimer 10 in high yield.  

Generation of 10 from the corresponding bis(iminophosphorane) using B0C2O in the presence of DMAP, resulted in the formation of the corresponding cyclic urea derivative. However, low yields of 10 are also obtained from bis(iminophhosphoranes) and isothiocyanates. 

The bicyclic nine membered ring carbodiimide 11 is stable at room temperature. Also 12 is obtainable in the dehydrosulfurization of the corresponding cyclic thiourea, and 13 is a stable solid, but the tricyclic carbodiimide 14 has only been postulated as an intermediate.  

Stable macrocycles with one or two carbodiimide groups in the ring have been synthesized by Molina and his coworkers (see Section 11.2.3). 

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Extension of these studies to benzologues of tetrazolo[l,5- ]pyridine, that is, for tetrazolo[l,5- ]quinoline 21 and tetrazolo[5,l- ]isoquinoline 22, led to interesting results <2003JOC1470> as shown in Scheme 7. Both of these fused tetrazoles resulted in formation of a nitrene 23 and 24, respectively, which could be interconverted via formation of the fused cyclic carbodiimide derivative 25. Isoquinolylnitrene 24, furthermore, was found to undergo subsequent reactions ring opening afforded the vinylnitrene 26, which was transformed to o-cyanophe-nylacetonitrile 27 by a 1,2-H shift and to 4-cyanoindole 28 by an intramolecular cyclization in 40% and 25% yields, respectively. 

A series of transformations via nitrene formation similar to the previously discussed case was also found under flash vacuum thermolytic (FVT) conditions by the same team as shown in Scheme 8 <2003JOC1470>. 9-Phenyltetrazolo[l,5- ]quinoline 29 underwent nitrene 30 and cyclic carbodiimide 31 formation, and this intermediate - similar to the previous case - could open up to the isoquinoline nitrene 32 in which, however, proximity of the nitrene to the phenyl substituents allowed the ring closure to the stable tetracyclic ring system 33 which was obtained in 73% yield. 

Azido-4,6-dichloro-133-triazine 35, matrix isolated in argon at 10 K, yields triplet nitrene 36 and the strained cyclic carbodiimide 37 upon photolysis <99CC2113>. 

Cyclic carbodiimides have been prepared where n = 5 or greater [61] (see structures IV and V). 

Intramolecular cyclization of the cyclic carbodiimide 159 provides an interesting route to the imidazo[l,2-a]benzimidazole (160 R1 = R2 =... 

Behringer and Meier found that treatment of thiourea 105 with yellow mercury(Il) oxide gave traces of N, AT-pentamethylenecarbodiimide (107), the smallest known cyclic carbodiimide. Compound 107 was reported as unstable and able to polymerize readily (larger ring homologs were, ex-... 

The flash-vacuum pyrolysis of tetrazolo[l,5-z/] pyridine 38, and its benzologues, gives nitrenes that rapidly undergo subsequent ring insertion, affording cyclic carbodiimides, e.g., 39 40. 

Cyclic carbodiimide 48 theoretically exists as two conformational isomers. Comparison of the coupling constant values, calculated using AMI Hamiltonian and Karplus relationship, with the experimental vicinal coupling constants of 8.33 and 1.05 Hz, undoubtedly prove its methyl-out structure 48 <1996JOC4289>. 

Bis(iminophosphorane) 225 was reacted with carbon dioxide in dry benzene at 70°C in a sealed tube to afford the nine-membered cyclic carbodiimide 48 in 98% yield (Equation 24) < 1996JOC4289>. 

Cyclic carbodiimides react with HFA to form six- and four-membered rings 123 and 124, which have been investigated spectroscopically. Isolation was successful only in the case of the 10-membered ring 123b the other two compounds are in equilibrium with their precursors (225). 

Carbodiimides have chiral structures similar to allenes, i.e., they can exist in optically active forms. Schloegl and Mechtler were the first to report a partial optical separation of N,N"-diferrocenylcarbodiimide into enantiomers by chromatography on acetylated cellulose, but other authors doubt the validity of these results. According to theoretical calculations a separation of carbodiimide enantiomers is not possible. N,N -diferrocenylcarbodiimide was also obtained in optically active form by kinetic resolution in the reaction with (-)-S-6,6"-dinitrodiphenic acid. Cervinka and coworkers isolated both enantiomers of (R,S)-N,N"-bis(o -phenylethyl)carbodiimide, and they found that they undergo racemization at room temperature. A recent study on the racemization mechanism of macrocyclic carbodiimides indicates that the open chain as well as the large ring carbodiimides racemize by nitrogen inversion or tra 5-rotation, while medium size cyclic carbodiimides racemize by cw-rotation. ... 

From cyclic carbodiimides and carbon disulfide the corresponding diisothiocyanates are obtained (see Section 11.3.3). 

The cyclic thiourea 19, upon reaction with HgO affords the cyclic carbodiimide 20, but the compound has not been isolated in pure form. ... 

Thermolysis of 2-azidopyridine 25 at 480 °C generates the nitrene, which rearranges at low temperature to the cyclic carbodiimide 26. At -70 °C the carbodiimide band in the infrared spectrum disappears and the isolated product is 2-cyanopyrrol 27 ... 

The reaction of bisaryliminophosphoranes with di-t-butylcarbonate(Boc)20 in the presence of 4-dimethylaminopyridine, or carbon dioxide results in the formation of an intermediate isocyanate, which reacts with the second -N=PPh3 group to form a cyclic carbodiimide or a cyclic dicarbodiimide. Reaction of the bisaryliminophosphorane with carbon disulfide generates a bis-isothiocyanate which undergoes reaction with the bisaryliminophosphorane to give the corresponding cyclic dicarbodiimide in high yield. Examples of these reactions are listed in Tables 11.1 and 11.2. 

Seven membered ring cyclic carbodiimides, such as 1,3-diazahepta-l,2-diene 37 and tetramethyl-l,3-diazahepta-l,2-diene are not stable in solution, but rapidly form dimers and oligomers. For example, from 37 the cyclodimer 38 formed by a [2+2] cycloaddition reaction and the cyclic trimer 39 are formed. ... 

Cyclic carbodiimides also undergo a facile [2+2] cycloaddition reaction with alkyl- and aryl isocyanates to give the cycloadducts 44. With aryl isocyanates the reaction is exothermic and is completed within several minutes. 

The reaction of cyclic carbodiimides 47 (n = 4,5), generated in situ, with carbon disulfide, results in the formation of the diisothiocyanates 48 in 23-28 % yield. ... 

Cyclic carbodiimides 49 (n = 5,6,11) react with phenyl-, phenethyl- and diphenylketene to give the [2-1-2] cycloadducts 50 in yields of > 90... 

Cyclic carbodiimides also undergo a rapid reaction with phosgene in methylenechloride to form the expected adducts 55, which on hydrolysis give cyclic allophanoyl chlorides 56.22... 

The reaction of cyclic carbodiimides with oxalyl chloride affords bicyclic para-banic acid derivatives 57 (when n = 11) or spiro(oxazolisine) derivatives 58 (when... 

Methyloxalyl chloride reacts with cyclic carbodiimides (n = 6) to form the spirobi(oxazoli-dine) derivative 59. ... 

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