Terminal olefins hydrocyanation

The isomerization of the internal olefin 3PN to the terminal olefin 4PN is a critical step in the hydrocyanation of 3PN to ADN [Eqs. (9) and (10)]. Unfortunately, there is a loss in yield because the undesirable conjugated isomer 2PN is also produced. Observations discussed below have led us to the belief that cationic nickel-hydride complexes, HNiL4, may be important in the isomerization process. 

Olefin hydrocyanation using palladium catalysts has been less well studied than with nickel. Nevertheless, zerovalent complexes of palladium, particulrly triarylphosphite complexes, hydrocyanate a wide range of olefins in useful yields (see Table 1). Early work reported the merit of excess phosphorus ligand to promote the reaction, and further paralleling the observations with nickel, Lewis acids have been used to improve catalytic activity. However, addition of ZnClj fails to improve nitrile product yield . Asymmetric induction in hydrocyanation results in optical yields of 30% in the synthesis of exo-2-cyanonorbomane using the chiral ligand DIOP, and studies on the stereochemistry of HCN and DCN addition to terminal alkenes and a substituted cyclohexene with the same catalyst have been reported. ... 

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. 

The stereoselectivity of the reaction was the target of several investigations. The results clearly establish that the addition of hydrogen cyanide to olefins is stereospecifically syn [33, 46 9]. Thus, reaction of terminal, deuterium-substituted olefins yields the corresponding syn addition products. Hydrocyanation of 4-t-butyl cyclohex-1-ene with deuterium cyanide confirmed these results. It is found that the stereospecifity is independent of the catalyst metal employed, since both nickel° and palladium catalysis give the syn addition products [50]. 

The addition of HCN to olefins catalyzed by complexes of transition metals has been studied since about 1950. The first hydrocyanation by a homogeneous catalyst was reported by Arthur with cobalt carbonyl as catalyst. These reactions gave the branched nitrile as the predominant product. Nickel complexes of phosphites are more active catalysts for hydrocyanation, and these catalysts give the anti-Markovnikov product with terminal alkenes. The first nickel-catalyzed hydrocyanations were disclosed by Drinkard and by Brown and Rick. The development of this nickel-catalyzed chemistry into the commercially important addition to butadiene (Equation 16.3) was conducted at DuPont. Taylor and Swift referred to hydrocyanation of butadiene, and Drinkard exploited this chemistry for the synthesis of adiponitrile. The mechanism of ftiis process was pursued in depth by Tolman. As a result of this work, butadiene hydrocyanation was commercialized in 1971. The development of hydrocyanation is one of tfie early success stories in homogeneous catalysis. Significant improvements in catalysts have been made since that time, and many reviews have now been written on this subject. ... 

Nickel-catalyzed hydrocyanation of a-olefins t)q)ically produces the terminal nitrile as the more abundant, but not exclusive isomeric product. In contrast, nickel-catalyzed hydrocyanation of vinylarenes typically generates the branched product. This branched selectivity arises fijom the stability of -phenethyl complexes, as is shown in more detail in Section 16.2.5 on as)unmetric hydrocyanation. The relative rates for hydrocyanation follow the trend ethylene > styrene > propene 1-hexene > disubstituted olefins. Examples of these reactions and selectivities for formation of the linear and branched products are shown in Scheme 16.1. ... 

The enantioselective hydrocyanation of alkenes has the potential to serve as an efficient method to generate optically active nitriles, as well as amides, esters, and amines after functional group interconversions of the nitrile group. As in asymmetric hydroformylation, asymmetric hydrocyanation requires control of both regiochemistry and stereochemistry because simple olefins tend to generate achiral terminal nitrile products. The hydrocyanation of norbomene will give a single constitutional isomer and was studied initially. However, modest enantioselectivities were obtained, and the synthetic value is limited. ... 

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