Electrophilic reactions metal cyanides

Although not discussed in this chapter, the Tsuji-Trost reaction159 is undoubtedly the most extensively investigated Pd-catalyzed allylation with allyl electrophiles. There have also been some uncatalyzed and Cu-catalyzed reactions of allyl electrophiles with alkyl metals and metal cyanides. On the other hand, the Pd- or Ni-catalyzed reactions of allyl electrophiles with organometals containing allyl-, benzyl-, propargyl- and other alkylmetals do not appear to have been extensively investigated. 

Pyrimidine-5-carbonitriles are best available by primary syntheses. Cyano groups in electrophilic positions have been introduced by nucleophilic substitution reactions with a metal cyanide. 

A Pd-catalyzed cyanation procedure using aryl halides and potassium cyanide was reported in 1973 (Scheme 3.79) [258, 259]. Many studies investigated effective Pd catalysts, metal cyanides, solvents, or additives, intending to provide the efficient catalytic reactions of a variety of carbon electrophiles including aryl halides under rrald conditions. In addition to Zn(CN)2, KCN, and NaCN, several other cyanide sources including NaCN, K4[Fe(CN),5], cyanohydrines, and trimethylsilyl cyanide (TMSCN) have been shown to participate in Pd-catalyzed cyanations [258b]. 

An approach to isobacteriochlorins1 ln-e makes use of Pd(II) or metal-free bilatrienes 1 as starting materials. Cyclization of the corresponding bilatriene derivatives is induced by base in the presence of palladium(II) or zinc(II) which exercise a template effect. Zinc can be readily removed from the cyclized macrotetracycles by acid whereas palladium forms very stable complexes which cannot be demetalated. Prior to the cyclization reaction, an enamine is formed by elimination of hydrogen cyanide from the 1-position. The nucleophilic enamine then attacks the electrophilic 19-position with loss of the leaving group present at the terminal pyrrole ring. 

New anions of the general formula [Ta(CO)sL]z prepared by this method included [Ta(CO)5(AsPh3)]" and [Ta(CO)5(CN)]2 (54). The cyanide derivative is of particular interest because its synthesis suggests that other more interesting dianionic species, such as [Ta(CO)5(C=CR)]2 and related [Ta(CO)5(f/1-hydrocarbyl)]2 species, might be accessible materials. Clearly, much chemistry of [Nb(CO)5]3 and [Ta(CO)5]3 remains to be explored. Of particular interest will be a systematic examination of the reactions of these materials with transition metal and organic electrophiles. 

Four oxidation states of palladium are encountered in organometallic chemistry see Palladium Inorganic Coordination Chemistry) In order of importance, they are Pd , Pd , Pd, and Pd . With the reduction of palladium from Pd to Pd , the metal changes its reactivity from electrophile to nucleophile. However, unlike main group nucleophiles such as thiolates or cyanide, Pd complexes react with both alkyl halides and aryl or vinyl halides. Reactions of Pd complexes with these latter sp halides generate new Pd aryl or vinyl bonds through the process of oxidative addition. 

Whereas, there is an almost complete analogy between metallocenes and phos-phametallocenes in terms of electrophilic substitutions, sharp differences appear with nucleophilic reagents. It has never been possible to observe C-metalations with phos-phametallocenes. In all cases, nucleophilic attack takes place at phosphorus. This is quite logical since it must be remembered that the LUMO is mainly localized on phosphorus, at least in the case of phosphacymantrenes. Phosphacymantrenes are so sensitive to nucleophilic media that they are destroyed even by sodium cyanide in ethanol and reaction of butyllithium directly results in a high yield of 1-butylphospholes ... 

Electrophilic catalysis of the departure of halogens in the century-old Koenigs-Knorr reaction is implicit in the use of heavy metal bases such as silver oxide and mercuric cyanide, but the first demonstration of electrophilic catalysis in water (in the hydrolysis of the p-glucoside of 8-hydroxyquinoline by first-row transition metals (Cu Np > C")) was by Clark and Hay in 1973. The observations were expanded to the more conveniently followed (because more labile) benzaldehyde methylacetals or tetrahydropyranyl derivatives of 8-hydroxyquinoline, whose hydrolysis is now known to give solvent-equilibrated oxocarbenium ions (Figure 3.19). Surprisingly, however, the observation of electrophilic catalysis of glycoside hydrolysis itself was not picked up by paper... 

A close analogy also exists between the behaviour of dinitrogen, cyanides and isocyanides in their attachment to electron-rich d metal centres, e.g. Mo(0), W(0) and Re(I). This attachment activates the ligand for electrophilic attack. This activation plays a role in the reduction by nitrogenases. For cyanides the following type of reaction has been observed... 

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