Hydrocyanation of Non-Activated Monoolefins

A number of olefins are converted in the presence of tetrakis(tri-o-tolyl phos-phite)nickel(O) into the corresponding nitriles. These additions yield the terminal nitriles predominantly [15]. Systematic investigations were performed on the hydrocyanation of olefins containing the norbomene skeleton 9 as a basic structure. Table 1 demonstrates the development of catalysts to gain stereocontrol of product formation. 

Even in the early days of homogeneous hydrocyanation the reaction of norbor-nene with hydrogen cyanide in the presence of tetrakisftriphenyl phosphite)palla-dium(O) 12 indicated the influence of steric factors, since exo-5-cyanobicy-clo[2.2.1]heptane (Structure 10) is obtained stereospecifically. This result was confirmed in similar reactions showing that the entering cyano group is directed into the exo-position of the norbomene system [28]. This is due to the complexa-tion of the palladium(O) center to the exo-face of norbomene. Recent experiments have also utilized the bicyclic system to demonstrate asymmetric hydrocyanation induced by chiral palladium diphosphine complexes. Depending on the applied ligand system 11-17, an enantiomeric excess (ee) up to 40% is obtained [25]. 

If a vinylic double bond is connected to the bicyclic skeleton of norbomene, a competition experiment shows that under the conditions employed hydrogen cyanide addition proceeds only at the endocyclic strained double bond. It is also noted that isomerization of the exocyclic olefinic bond may take place in the course of the reaction [22, 23, 37]. These experiments already reveal the most important features of homogeneously catalyzed hydrocyanation - the influence of the steric structure of the substrate and the fact that the catalyst also promotes isomerizations (cf. Section 2.5.5.1). 

A number of olefins are readily hydrocyanated in the presence of NiL3 or NiL4 [15], but usually catalyst turnover rates demanded (i.e., the number of moles of product formed per mole of catalyst used) and the selectivity tends to be low. It was found that Lewis acids are effective co-catalysts, which enable the reaction pathway and therefore the reaction selectivity to be piloted and accelerate the rate of hydrocyanation [10]. Investigations on the promoting effect of Lewis acids (e. g., AlCL, ZnCL, BPh3 [14, 44]) imply the formation of a 1 1 complex between Lewis acid and NiL4, since at this ratio the reaction rate reaches a maximum [40, 41, 45]. 

The influence of solvent on both selectivity and reaction rate is detectable. Phenolic solvents were found to have a promoting effect on the reaction rate and to increase the rate of straight/branched nitriles. 

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