Carbodiimides insertion reactions

Metathetical reactions between NbCl4(THF)2, NbCls, TaCls, [(Et2N)2TaCl3]2, or (R2N)3Ta(=NBu (R = Me, Et) with various amounts of lithium amidinates have been employed to synthesize the corresponding heteroleptic niobium and tantalum amidinate complexes. The products were investigated as potential precursors to metal nitrides (cf. Section VI) Carbodiimide insertion routes... 

A (pentamethylcyclopentadienyl)iridium chelating guanidinate complex has been conveniently prepared by treatment of [Cp IrCl2]2 with N,N, N"-th-p-tolylguanidine and base in THF at room temperature followed by recrystallization of the green product from toluene and pentane (Scheme 154). Insertion reactions of the product with heterocumulenes (diaryl carbodiimides, aryl isocyanates) have been investigated. It was found that the complex serves as highly active catalyst for the metathesis of diaryl carbodiimides with each other and for the more difficult metathesis of diaryl carbodiimides with aryl isocyanates (cf. Section V.C). ... 

Zirconium and hafnium dialkylamides are highly reactive compounds. They undergo (i) protolytic substitution reactions with reagents such as alcohols, cyclopentadiene and bisftrimethylsilyOamine 63 64 (ii) insertion reactions with C02, CS2, COS, nitriles, phenyl isocyanate, methyl isothiocyanate, carbodiimides and dimethyl acetylenedicarboxylate 69-72 and (iii) addition reactions with metal carbonyls.73 These reactions are summarized with reference to Zr(NMe2)4 in Scheme 1. 

The nitrene 28 is not produced from the azide precursor, but from heterocycles via photolysis and thermolysis as shown in Sch. 11 [20]. Iminoacyl nitrenes react intramolecularly giving benzimidazoles with good yields (Sch. 11), and, dependending on the precursor used and the reaction conditions, varying amounts of carbodiimides are obtained. The reactivity of the acyl nitrenes is influenced by the substituent connected to the acyl group (see Sch. 10), however all acyl nitrenes are quite reactive and therefore rather unselective. Apart from cycloaddition reactions with Tt-bonds, insertion reactions into a-bonds, additions to lone pair electrons of... 

The insertion reaction of heterocumulenes can be used to syuthesize carbodiimides having two different substituents. For example, the carbamate 157 derived from an isocyanate and bis(tributyltin)oxide can be reacted with an isothiocyanate to give a carbodiimide 158 with substituents derived from the isocyanate and the isothiocyanate. ... 

Heterocumulenes undergo insertion reactions with numerous substrates. In general, carbodiimides react faster than isocyanates and isothiocyanates, in that order. Insertions of carbodiimides into metal-hydrogen, metal-halogen, metal-mitrogen, metal-oxygen and metal-sulfur bonds are reported. Also insertions of carbodiimides into carbon-hydrogen bonds are known. 

Examples of insertion reactions include Grignard or alkyl lithium compounds which react with carbodiimides to give formamidines 389 after hydrolysis. ... 

Diarylcarbodiimides also insert into B—Cl, B—OR, B—SR and B—NR2 bonds. Examples include insertions into BCI3, RBCI2 and R2BC1. Sometimes double insertion reactions occur. BCI3 reacts with two equivalents of Carbodiimide to give the double insertion product 394. 

Insertion reactions of carbodiimides are also observed with Sn—OR or Sn—NR2 bonds. For example, reaction of tributyltinmethoxide 405 with diarylcarbodiimides affords the expected insertion product 406. °... 

Insertion reactions involving metal alkoxides are also known. For example, carbon dioxide is known to react with some metal alkoxides as shown in equation (12). The formation of a bidentate ligand is a significant thermodynamic driving force for some of these reactions. The isoelectronic aryl and alkyl isocyanates and carbodiimides can react similarly. Insertion reactions involving alkenes and carbon monoxide are known for platinum alkoxides. 

CO2 analogues such as allenes, ketenes, isocyanates, carbodiimides, and carbonyl sulfide also insert. Studies of insertion reactions of Cp2ZrR2 (R = Me, CH2Ph, Ph) show that the ease of insertion follows the series Me > Py CH2Ph and Ph2C=C=0>PhN=C=0>p-MeC6H4N=C=NC6H4Me-p>C02. The data have been reasonably interpreted as evidence for precoordination before insertion occurs. A four-center transition state similar to that depicted above for the thorium system is favored. 

Insertion Reactions of an Isocyanate, Isothiocyanate, Carbodiimide, Ketene, Diketene, or Ketenimine... 

Insertion Reactions into Element Oxygen and Heavier Group VIB Bonds 11.5.3. Reactions of Isocyanates, Isothiocyanates, and Carbodiimides... 

Titanium and zirconinm tetra-alkoxides undergo insertion reactions of both organic isocyanates and carbodiimides into M-0 bonds, as shown by Eqs (2.290) and (2.291) ... 

The cycloaddition reactions are subdivided into di-, tri- and oligomerization reactions, [2-1-1]-, [2-1-2]-, [3-1-2]- and [4- -2] cycloaddition reactions and other cycloaddition reactions. The insertion reactions into single bonds are also discussed. The cyclodimerization or cyclotrimerization reactions are special examples of the [2-1-2] and the [2-I-2-I-2] cycloaddition reactions, respectively. The cumulenes vary in their tendency to undergo these reactions. The highly reactive species, such as sulfines, sulfenes, thioketenes, carbon suboxide and some ketenes, are not stable in their monomeric form. Other cumulenes have an intermediate reactivity, i.e. they can be obtained in the monomeric state at room temperature and only heat or added catalysts cause di- or trimerization reactions. In this group, with decreasing order of reactivity, are allenes, phosphorus cumulenes, isocyanates, carbodiimides and isothiocyanates. 

I have also included in this book the insertion reactions of carbon cumulenes into polarized metal single bonds, which can be perceived as an initial [2+2] cycloaddition, which subsequently rearranges to give a linear adduct. The reactivity of the metal substituent appears to be NR2 > OR > SR. When the metal compound contains several reactive groups, stepwise insertion occurs. For example, Sn(OR)4 reacts with phenyl isocyanate to give the tetracarbamate Sn[N(Ph)COOR]4. Mixed insertion products are obtained using different isocyanates. In the insertion reactions of carbodiimides sometimes ionic cyclic amidinate complexes are formed. 

Since Alper and coworkers have shown that the dipolar insertion reactions of carbodiimides into three-membered ring aziridines can be extended to four- and five-membered ring nitrogen heterocycles, one wonders if these reactions may also occur with the corresponding oxygen heterocycles. 

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