Isocyanates ligand reactions

It was found that the 15-membered macrocyclic complexes were significantly less acidic. Similar reversible protonations have been shown to occur in related macrocyclic complexes36 37 and this work has been developed by Busch into a major study of ligand reactions,38 which are summarized in equations (16)—(19). It is significant that these reactions include not only typical substitution reactions such as acylation and nitration (equations 16 and 19), but also nucleophilic addition to isocyanates (equation 17) and to a, 3-unsaturated esters (equation 18). 

The first complex containing a bound isocyanate ligand has been prepared. This is ReCl(MeNCO)(dppe)2 (27). The cyanide complex, K4[Re(CN)7],2H20 has been prepared by the reaction of K2Rel6 with hot methanolic KCN solution, or by the reaction of K2ReCl6 with KCN in KCNSe at 523 K. The complex has symmetry as shown by i.r. studies. ... 

Cyclocarbonylation of o-iodophenols 503 with isocyanates or carbodiimides and carbon monoxide in the presence of a catalytic amount of a palladium catalyst (tris(dibenzylideneacetone)dipalladium(O) Pd2(DBA)3) and l,4-bis(di-phenylphosphino)butane (dppb) resulted in formation of l,3-benzoxazine-2,4-diones 504 or 2-imino-l,3-benzoxazin-4-ones 505 (Scheme 94). The product yields were dependent on the nature of the substrate, the catalyst, the solvent, the base, and the phosphine ligand. The reactions of o-iodophenols with unsymmetrical carbodiimides bearing an alkyl and an aryl substituent afforded 2-alkylimino-3-aryl-l,3-benzoxazin-4-ones 505 in a completely regioselective manner <1999JOC9194>. On the palladium-catalyzed cyclocarbonylation of o-iodoanilines with acyl chlorides and carbon monoxide, 2-substituted-4f/-3,l-benzoxazin-4-ones were obtained <19990L1619>. 

The palladium-phosphine-catalyzed cycloaddition reactions of vinyloxetanes 530 with aryl isocyanates or diaryl-carbodiimides led to 4-vinyl-l,3-oxazin-2-ones 531 or l,3-oxazin-2-imines 532, respectively (Scheme 101). In the absence of phosphine ligands (PPhs, bis(diphenylphosphino)ethane (DPPE), l,3-bis(diphenylphosphino)propane (dppp), no conversion of heterocumulenes was observed. Starting from fused-bicyclic vinyloxetanes, both types of cycloadditions proceeded in a highly stereoselective fashion, affording only the r-isomers of alicycle-condensed 1,3-oxazine derivatives <1999JOC4152>. 

Zirconium and hafnium tetraalkoxides are highly reactive compounds. They react with water, alcohols, silanols, hydrogen halides, acetyl halides, certain Lewis bases, aryl isocyanates and other metal alkoxides. With chelating hydroxylic compounds HL, such as j8-diketones, carboxylic acids and Schiff bases, they give complexes of the type ML (OR)4 these reactions are discussed in the sections dealing with the chelating ligand. 

The formation of isocyanates has also been observed in reactions outlined in equations (21)-(23). It has been shown that the reactions of equation (21) most probably proceed via coordination of the amine function followed by rearrangement with elimination of HX. The reaction with PPh3 is interesting because of the initial formation of a nitrene species via N—O bond cleavage. Formation of a nitrene intermediate as a crucial step in the formation of the NCO ligand has also been proposed for the reaction with diazomethane. 

The use of polar solvents or ligands such as 2,2 -bipyridyl decreased the yield. The complex [Pd(diphos)2] (104) was inactive, as were [Ni(PPh3)4] and [Pt(PPh3)4]. Complex (103) was believed to give the carbonyl [Pd(CO)(PPh3)3] prior to commencement of catalytic action. When [Pd(DBA)2](105) was used as catalyst, reaction (139) did not occur at all, the sole product being the vinyl isocyanate formed according to equation (140). A possible mechanism for these reactions is shown in Scheme 51. 

Asymmetric cycloaddition of 2-vinyloxiranes to carbodiimides proceeds in the presence of Pd2(dba)3-CHC13 and TolBINAP as the chiral ligand in THF at room temperature to yield 4-vinyl-l,3-oxazolidin-2-imines with up to 95% ee [74] (Eq. 8A.50). The enantio-determining step is assumed to be the nucleophilic attack of a nitrogen nucleophile on a 7i-allylpalladium intermediate. Reactions of 2-vinyloxiranes with isocyanates using the same catalyst system afford 4-vinyl-l,3-oxazolidin-2-ones with low enantioselectivity. 

Interaction of isocyanates with metal carbonyls [for instance reaction (2.11)] leads to decarbonylation of the ligand and thus, to the formation of a cluster structure 413 [713] ... 

NCS)2] (51), and TBA[Ir(ppy)2(NCO)2] (52). These complexes were conveniently synthesized under inert atmosphere by reaction between the dichloro-bridged Ir dimer [Ir(ppy)2(Cl)]2 in dichloromethane solvent with an excess of a pseudohalogen ligand such as tetrabutylammonium cyanide, tetrabutyl-ammonium thiocyanate, or tetrabutylammonium isocyanate, respectively, with over 70% yields [77]. 

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