Cyanates metal complexes

The curing reaction can be carried out thermally or with the addition of a catalyst. The thermal cure is strongly influenced by impurities associated with the synthesis. The greater the degree of monomer purity, the more slowly the thermal cure proceeds. If the monomer is sufficiently purified, the cure rate can be predictably controlled by the addition of catalysts. As with the aromatic cyanate esters, the fluoromethylene cyanate esters can be cured by the addition of active hydrogen compounds and transition metal complexes. Addition of 1.5 wt% of the fluorinated diol precursor serves as a suitable catalyst.9 The acetylacetonate transition metal salts, which work well for the aromatic cyanate esters,1 are also good catalysts. 

The effect of halide, cyanate, cyanide, and thiocyanate ions on the partitioning of Hg in [BMIM][PF6]/aqueous systems (Figure 3.3-2) has been studied [8]. The results indicate that the metal ion transfer to the IL phase depends on the relative hydrophobicity of the metal complex. Hg-I complexes have the highest formation constants, decreasing to those of Hg-F [42]. Results from pseudohalides, however, suggest a more complex partitioning mechanism, since Hg-CN complexes have even higher formation constants [42], but display the lowest distribution ratios. 

Thus transition metal complexes capable of effecting cyanation reactions on aromatic nuclei under mild conditions have been discovered Cassar et al. describe such a catalytic system. The past few years have also seen the discovery of asymmetric catalysis. Asymmetric catalysts contain optically active ligands and, like enzymes, can promote catalytic reactions during which substantial levels of optical activity are introduced into the products. This volume contains examples of asymmetric hydrogenation and asymmetric hydroformylation catalysis in the papers, respectively, by Knowles et al. and Pino et al. 

The Strecker reaction [1] starting from an aldehyde, ammonia, and a cyanide source is an efficient method for the preparation of a-amino acids. A popular version for asymmetric purposes is based on the use of preformed imines 1 and a subsequent nucleophilic addition of HCN or TMSCN in the presence of a chiral catalyst [2], Besides asymmetric cyanations catalyzed by metal-complexes [3], several methods based on the use of organocatalysts have been developed [4-14]. The general organocatalytic asymmetric hydrocyanation reaction for the synthesis of a-amino nitriles 2 is shown in Scheme 5.1. 

Metal complexes can be oxidized or reduced at the inner metal cation and/or at the ligand by other compounds or by electron transfer at an electrode. Conditions can be set as to selectively perform the redox reaction affecting either or both parts of the complex. In the above reaction, [Fe(CN)6]3- oxidizes Co(II) to Co(III) on the other hand, Pb02 is known to selectively oxidize EDTA in this [Co(II)EDTA] complex. Another example is the remediation of effluents containing Cu(CN)2, which has been accomplished electrochemically by removing Cu2+ and CN simultaneously, with reduction of the former to its elemented form and oxidation of the latter to cyanate (see Section 10.1.4). 

Prior chapters have covered the use of transition metals in asymmetric hydrogenations ( 6.2 and 7.1), hydroborations ( 7.3), hydrosilylations and hydro-cyanations ( 6.3, 6.4, 7.4 and 7.5), cyclopropanations ( 7.19), aldol reactions ( 6.11), allylations of carbanions ( 5.3.2), and some sigmatropic rearrangements ( 10.3). This chapter covers other reactions catalyzed by transition metal complexes including coupling of organometallic reagents with vinyl, aryl or allyl derivatives, Heck reactions allylamine isomerizations, some allylation reactions, car-bene insertions into C-H bonds and Pauson-Khand reactions. 

Optically active cyanohydrins can be easily transformed to P-hydroxy amines, a-hydroxy and a-amino carboxylic acids, which represent versatile intermediates for the synthesis of biologically important compounds, including insecticides and medicines [189, 190]. Asymmetric cyanation of carbonyl compounds catalyzed by chiral metal complexes, particularly titanium compounds, has provided one of the most convenient protocols to the access of these type of compounds. The first example of catalytic asymmetric cyanation of aliphatic aldehydes was realized in Reetz s group using BINOL-Ti complex as the catalyst to give the cyanohydrins in up to 82% ee [104] (Scheme 14.85). 

A [3,3]-sigmatropic cyanate-isocyanate rearrangement has been identified as a route to a-isocyanato allylboronic esters that can be trapped with nucleophiles (Scheme 2). A DFT (density functional theory) investigation of [3,3]-sigmatropic rearrangement versus a [2 4-2]-cycloaddition of a formal 5 2 substitution of imido metal complexes with allylic electrophiles has been reported (Scheme 3). ... 

Due to their importance for research but also for industrial chemistry, transition metal based catalysts are intensively investigated. Ananikov et al. [684] reviewed various appUcatimis of hybrid ONIOM methods within this field. This review involves reaction mechanisms and enantioselective reactions of transition metal complexes, e.g. Ti-catalyzed cyanation of benzaldehyde [685], Cu-catalyzed cyclopropanation [686], Mn-porphyrin catalyzed epoxidation of alkene [687], and Mo-catalyzed nitrogen activation [688]. These approaches involve QM/QM as well as QM/MM approaches. 

Scheme 11 General reaction mechanism for poly rare earth metal complex-catalyzed asymmetric cyanation and azidation...

The Strecker reaction is an efQdent method for the preparation of a-amino acids which was first reported by Adolf Strecker over 100 years ago [120]. Now, several methods for asymmetric cyanations catalyzed by metal-complexes or organocata-lysts have been developed [121-124],... 

Vinylic copper reagents react with CICN to give vinyl cyanides, though BrCN and ICN give the vinylic halide instead." Vinylic cyanides have also been prepared by the reaction between vinylic lithium compounds and phenyl cyanate PhOCN." Alkyl cyanides (RCN) have been prepared, in varying yields, by treatment of sodium trialkylcyanoborates with NaCN and lead tetraacetate." Vinyl bromides reacted with KCN, in the presence of a nickel complex and zinc metal to give the vinyl nitrile. Vinyl triflates react with LiCN, in the presence of a palladium catalyst, to give the vinyl nitrile." ... 

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