Carbodiimide, synthesis

Isocyanate formation through multiple bond metathesis of C02 with carbodiimide has been also demonstrated [112]. This transformation can be promoted by titanium isopropoxide, at 383 K, in THF as solvent. It is worth noting that the reverse process, which opens an entry into carbodiimide synthesis, is a well-known process that is catalyzed by several other systems, including trialkylphosphine oxides [113] or vanadium-oxo or -imido complexes [114]. 

Carbodiimide synthesis.6 1,3-Disubstituted ureas or thioureas react with triphenylphosphine, carbon tetrachloride, and triethylamine in methylene chloride at 40° to give carbodiimides in yields of 85-92 %. 

The addition occurs in the presence of polymerization inhibitors such as copper stearate, aromatic polynitro compounds, or sterically hindered phenols and then sometimes requires days or weeks. In spite of the often tedious procedure the method is of great interest since, from cheap materials, it often affords good yields of compounds containing the phospholene ring system which is otherwise difficultly accessible. Compounds of type (1) and (2) have also considerable interest as efficient catalysts for the carbodiimide synthesis (cf. page 504). 

Reviews on the synthesis and chemistry of carbodiimides are given in [1248-1250]. Carbodiimides are mainly synthesized in one of three ways from ureas or thioureas, from isocyanates, or from isocyanides. Several reagents have been employed in carbodiimide synthesis phosgene [1252, 1253], dimethylphosgenimi-nium chloride [1254], tiiphosgene [561, 562], phosphorus pentoxide [1255], phos-phoryl chloride [1256], triphenylphosphine dibromide [758, 1257-1261], triphenylphosphine/tetrahalomethanes [1262, 1263], iminophosphoranes [1264-1277], Mitsunobu reagent [1278, 1279], p-tosyl chloride [1280, 1281], and CDC [1137] oxidative additions have also been used [1282-1284]. 

Saponification next frees the carboxy group for condensation with -butyl 7-aminocephalosporinate mediated by dicyclohexyl-carbodiimide and 1-hydroxybenzotriazole. The synthesis is... 

This method may be applied to the synthesis of a variety of aryl and alkyl carbodiimides. Other catalysts may also be used,10 but the especially active one described here is the one most easily obtained. The method is superior to other methods... 

Proline analogs, biaryl-substituted, fluorous synthesis 112 PS-anthracene, dienophiles 151 PS-carbodiimide 136 Pimine C-2, nucleophilic displacement 119... 

The synthesis, structures, and reactivity of neutral and cationic mono- and bis(guanidinato)zirconium(rV) complexes have been studied in detail. Either salt-metathesis using preformed lithium guanidinates or carbodiimide insertion of zirconium amides can be employed. Typical examples for these two main synthetic routes are illustrated in Schemes 73 and 74. Various cr-alkyl complexes and cationic species derived from these precursors have been prepared and structurally characterized. 

The reaction between acyl halides and diazomethane is of wide scope and is the best way to prepare diazo ketones. Diazomethane must be present in excess or the HX produced will react with the diazo ketone (10-74). This reaction is the first step of the Amdt-Eistert synthesis (18-8). Diazo ketones can also be prepared directly from a carboxylic acid and diazomethane or diazoethane in the presence of dicyclohexyl-carbodiimide. ... 

The treatment of isocyanates with 3-methyl-l-ethyl-3-phosphoIene-l-oxide (59) is a useful method for the synthesis of carbodiimides in good yields. The mechanism does not simply involve the addition of one molecule of isocyanate to another, since the kinetics are first order in isocyanate and first order in catalyst. The following mechanism has been proposed (the catalyst is here represented as... 

An effective synthesis of the pyrido[T,2 l,2]imidazoI5,4-c]isoquinoline system 41 uses the 3-aceta (or benza)mido-2-phenylimidazo[l,2-a]pyridine 40 and phosphorus oxychloride. An alternative approach using the carbodiimide 42, obtained from the corresponding iminophosph(Mane and an isocyanate, is also successful <95H(41)2019>. 

Organic synthesis [OS 15] Dipeptide synthesis by carbodiimide coupling using... 

Smith, M., Moffatt, J.G., Khorana, H.G. (1958) Carbodumides. VIII. Observations on the Reactions of Carbodiimides with Acids and Some New Applications in the Synthesis of Phosphoric Acid Esters. Journal of the American Chemical Society, 80, 6204-6212. 

A first terminal imido complex of nickel (121) was prepared according to Equation (3).468 The synthesis goes via the Ni11 amido compound (122) and uses the steric bulk of the arylimido group for stabilization. The Ni11 center in (121) is planar and three-coordinate. Reaction of (121) with CO or benzyl isocyanide leads to formal nitrene transfer with formation of (124) and (125), respectively. Further reaction with CO liberates the isocyanate and carbodiimide (Equation (4)). 69... 

Let us return to the hexanucleotide synthesis described above the yields of new matrix were not as high as had been hoped. There are various reasons for this for example, the condensation agent was inactivated by hydrolysis, and higher amounts of carbodiimide had to be used. In order to obtain higher yields of C, it was necessary to modify the building block B (for example by an amino modification). These changes led to an increase of the reaction rate by a factor of 104. 

An aza-Wittig reaction-cycloaddition reaction sequence was utilized for the synthesis of pyridotriazines 94. Treatment of iminophosphorane 92 with phenylisocyanate leads to the formation of the corresponding carbodiimide intermediate, which with another molecule of isocyanate affords 94 in [4+2] heterocycloaddition reactions (Equation 13) <1997T16061>. 

An alternative strategy for the synthesis of these tricyclic compounds involves the reaction of the azidoalkenyl-functionalized furo[3,2-A]pyrrole 30, which reacts with triphenylphosphine to give the corresponding iminophos-phoranes 31 these upon reaction with aryl isocyanates give the pyrrolo[2, 3 4,5]furo[3,2-r-]pyridines 32, via the corresponding carbodiimides which are not isolated < 1994H(37) 1695, 1992M807> (Scheme 9). 

Synthesis of the annulated thicno[3, 2 5,6]pyndo[4,3-z/]pynmidinoncs 74 and 75 involves the formation of the outer six-membered (pyrimidine) ring via a carbodiimide (Scheme 23) <2006HCA1337>. 

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