Reactions of Cyanates

The carbon atom of cyanates is activated towards nucleophilic attack upon co-ordination of the nitrogen to a metal ion (Fig. 4-32). 

We have already seen an example of this in the nucleophilic attack of water on [Rh(NH3)5(NCO)]2+, which is produced as an intermediate in the hydrolysis of [Rh(NH3)5(H2NCONH2)]3+. The use of the kinetically inert d6 rhodium(m) centre ensures that the Rh-N bond remains intact even during cleavage of the C-N bond (Fig. 4-33). 

In the case of amine nucleophiles, the products from the reaction with co-ordinated cyanates are carbamates or ureas (Fig. 4-34), and this provides a particularly convenient method for the preparation of carbamate complexes. An example of this behaviour is seen in the reaction of 3,5-dimethylpyrazole with cyanate in the presence of copper(n) salts (Fig. 4-35). Similar reactions are observed with co-ordinated thiocyanates and other heterocumulene s. 

4 Other Reactions of Co-ordinated Ligands with Nucleophiles 

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The structures of lithium and sodium cyanates and isocyanates and their related ion-pair 5n2 reactions have been examined by using quantum mechanics at the Hartree-Fock (HF)/6-31G V/HF/6-31G level. (The cyanate ion is NCO the isocyanate ion is CNO. ) The isocyanate ion pairs are the most stable monomeric forms the lowest energy dimers are planar eight-membered rings. For the ionic 5 n2 reaction of cyanate ion with MeF or MeCl, methyl isocyanate is the predicted major product. Predictions about the 5ivr2 reactions of the ion pairs were also made. 

Reaction of cyanates with primary and secondary amines to give 2-substituted pseudoureas [13]. 

Figure 4-35. The copper(n) mediated reaction of cyanate with 3,5-dimethylpyrazole.

The extreme sensitivity of cyanate chemistry to experimental conditions and minor traces of impurities makes the reaction of cyanate not as simple as described by Eq. (2.67). 

A preliminary examination of the reaction of cyanate with chlorine has been published ". ... 

Figure 2. Reaction of cyanate ester with an epoxy to form an oxazoline ring.

Cyanate esters (CEs) are formed in excellent yields by the reaction of the corresponding phenols with cyanogen halide [34]. The reaction scheme is shown in Scheme la. The reaction is usually carried out in solution, in the presence of a tertiary amine as the acid scavenger at very low temperatures. Since the trimer-ization reactions of cyanate esters are highly prone to catalysis by spurious impurities, the most difficult aspect of cyanate ester synthesis is their scrupulous purification. Low molar mass esters are purified by distillation or recrystallization. Polymeric cyanates are purified by repeated precipitation in non-solvents such as water, isopropanol, etc. While distillation and recrystallization lead to pure materials, the precipitation method for polymeric cyanates is not always conducive to obtaining pure materials. A recent patent application claims a purification method for polymeric cyanates based on treatment with cation and anion exchangers [35]. 

Researchers have conducted a number of studies highlighting the synthesis of CEs, reaction conditions including solvents and catalysis [36-38], etc.,but most of the details are covered by patents. The solvents selected include acetone [39], methyl isobutyl ketone [40], dichloromethane [41], etc., and acid scavengers chosen mainly include triethyl amine and pyridine [39, 41]. The use of tri-alkylamines often leads to the formation of dialkyl cyanamides as side products, which are known to catalyze the reactions of cyanate esters, reducing the yield. The formation of these impurities depends on the mode and rate of addition of reagents, reaction temperature, etc. Factors like stirring rate, dropping rate, and... 

Despite their many attractive features, the search for further improvements in performance and reduction in cost of cyanate esters is never ending and a large number of studies have been devoted to matrix modification through blends, and co-curing of these systems. Reactions of cyanate esters with a variety of functional groups like amines [169], hydroxyl [34, 66], epoxy [170-172], phenols, etc., have been reported, among which the most studied are reactions with epoxy [173,174]. Epoxy-cyanate blends are common and found in many commercial and patented resin formulations. 

Figure 2.41 Reaction of cyanate ester resin with water to form carbamate and...

For the reaction at 115°C, the decomposition of excess urea (>2 mol/dm ) generates a large amount of OH ions, which change the medium from acidic to basic (pH 9.7). The reaction of cyanate ions proceeds according to Eq. (2.45). The precipitation of rodlike particles may therefore be represented as ... 

Stark, G. R. Reactions of cyanate with functional groups of proteins. IV. Inertness of aliphatic hydroxyl groups. Formation of carbamyl- and acylhydantoins. Biochemistry 4 2363-2367,1965. 

In Figure 20 the kinetic curves for the formation of pure epoxypolycyanurate (EPCN) and semi-IPNs with 20 and 50 % of polyurethane have been plotted. It can be seen that the reactions of cyanate and epoxy groups are accelerated with increasing LPU content in the system. 

PUMA was synthesized by photo-initiated curing reaction of OUMA in the presence of DCEBA. Finally, the PCN was synthesized by thermal curing reaction of DCEBA. In addition to these reactions (chemical equations can be found in [54]), we have to notice that some chemical grafting of the two networks can also occur by reaction of cyanate groups of growing PCN with urethane groups of PUMA as described above in this Chapter. 

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