Hydrocyanation, Cyanosilylation, and Other Reactions

There is currently a single example of olefin hydrocyanation under phase transfer catalytic conditions in the literature. Methacrylonitrile in acetonitrile reacts with potassium cyanide and acetone cyanohydrin in the presence of catalytic 18-crown-6 to yield 1,2-dicyanopropane (92%) according to equation 7.12. The cyanohydrin functions as both a proton donor and a source of cyanide ion [6]. 

The protection of carbonyl functions by reaction with trimethylsilyl cyanide to form the cyanohydrin trimethylsilyl ethers is subject to catalysis by potassium cyanide in the presence of 18-crown-6, tetrabutylammonium cyanide or resin bound tetraalkyl-ammonium cyanide [27, 29]. The nucleophilic cyanide ion is believed to attack the carbonyl yielding a cyanohydrin anion. This anion is silylated by trimethylsilyl cyanide with generation of an equivalent of cyanide anion. The sequence is shown in equation 7.13. Accordingly, 3-pentanone and 4- -butylcyclohexanone are cyanosil- 

The mono-cyanosilylation of quinones is recorded in Table 7.7. The aldol condensatior of ethyl a-trimethylsilyldiazoacetate with aldehydes (Eq. 7.15) was found to be catalyzed by the 18-crown-6 complex of potassium cyanide [30]. A mechanism involving the diazoester enolate anion has been proposed. Several examples of this condensation have been recorded in Table 7.8. 

Photochemical cyanation of aromatic hydrocarbons in acetonitrile solution is a higher yield process when the potassium cyanide complex of 18-crown-6 is the cyanide ion source [31] compared to similar reactions in mixed organic aqueous solvent systems [32] (see Eq. 7.16). A ten-fold excess of 18-crown-6/KCN over the aromatic hydrocarbon (present in 10 M) was used. The yield improvements were attributed to increased activity of cyanide due to diminished hydration of the ion. Biphenyl, naphthalene, phenanthrene, and anthracene were photocyanated in 50%, 15%, 25% and 20% yields respectively the latter being an equimolar mixture of mono and dicyanation products [31]. 

Phase transfer agents have been added to organic solutions in order to solubilize salts which comprise supporting electrolytes in electrolytic syntheses [33]. Both naphthalene and anisole were anodically cyanated in dichloromethane according to equation 7.17. The reaction was found to be clean, and high yield, although in both cases, mixtures of positional isomers were isolated. The fact that the reaction was not current efficient was compensated by the inexpensive nature of the supporting electrolyte. 

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