Nickel complexes hydrocyanation

Hydrocyanation of aliphatic conjugated dienes in the presence of Ni(0) complexes gives diene rearrangement products and /i.y-unsaUiratcd nitriles in 10-90% yields10. Dienes other than 1,3-butadiene do not produce terminal nitriles, implying that the more highly substituted jr-allyl nickel complex is favored. Thus, reaction of 1-phenylbuta-l,3-diene (1) affords ( )-2-methyl-4-phenylbut-3-enenitrile (2) as the sole product (equation 5). The... 

The nickel-catalyzed hydrocyanation of butadiene is a two-step process (Figure 3.32). In the first step, HCN is added to butadiene in the presence of a nickel-tetrakis(phosphite) complex. This gives the desired linear product, 3-pente-nenitrile (3PN), and an unwanted branched by-product, 2-methyl-3-butenenitrile (2M3BN). The products are separated by distillation, and the 2M3BN is then isomerized to 3PN. In the second step, 3PN is isomerized to 4PN (using the same nickel catalyst), followed by anti-Markovnikov HCN addition to the terminal double bond. The second step is further complicated by the fact that there is another isomerization product, CH3CH2CH=CHCN or 2PN, which is thermodynamically more stable than 4PN. In fact, the equilibrium ratio of 3PN/2PN/4PN is only 20 78 1.6. Fortunately, the reaction kinetics favor the formation of 4PN [95],... 

Successful development of the asymmetric hydrocyanation reaction may provide a versatile route to chiral nitriles, amines, and acids. As we have seen, the mechanistic details of the hydrocyanation reaction of butadiene with zero-valent nickel complexes are well established. By using a nickel complex of a chiral bidentate phosphinite ligand, 9.53, good conversion and enantioselectiv-ity (>85% e.e.) for the hydrocyanation of 6-methoxy 2-vinyl naphthalene have been obtained. 

The formation of complexes with Lewis bases, especially BPh3, that is, MCNBPh3, is of importance in the hydrocyanation of alkenes by nickel complexes (Section 22-4). 

The reaction can be carried out asymmetrically, using nickel complexes of chiral phosphite ligands. Examples are the enantioselective hydrocyanation of norbomene using ligand (22-XVIII),48 and of vinylnaphthalene derivatives with (22-XIX).49 The latter is a precursor for the anti-inflammatory drug naproxen. 

The authors assessed the catalytic activities of these supramolecular nickel complexes for the hydrocyanation of substituted styrenes with... 

Recently, ee s of 85-90% have been obtained for the asymmetric hydrocyanation of 6-methoxy-2-vinyhiaphthalene using nickel complexes of chiral bidentate phosphinites derived from glucose (abbreviated PP, equation 12). This reaction is of great interest to the pharmaceutical industry because the (S) enantiomer of the product nitrile is a useful precursor for the widely marketed antiinflammatory dmg naproxen (equation 13). The same reaction can be applied to a number of other vinyl aromatic compounds, including the precursor for the antiinflammatory drug ibuprofen (6) however, the ee is not as high. 

The biphosphite ligands, (5) and (6), react with [(cod)2Ni] to form nickel complexes of type (7). Nickel phosphite complexes are catalysts in the hydrocyanation of butadiene complex (7) is more robust than the monodentate phosphite analogs. ... 

Hydrocyanation of hexene-1 to a mixture of heptanenitrile and 2-methylhexanenitrile, using Lewis acid promoted zero-valent nickel phosphite complexes, has been studied in some detail (277). The presence of a Lewis acid was advantageous since when the zero-valent nickel complex was used alone the rate of hydrocyanation was very slow. The role of excess ligand and the Lewis acid in the reaction appear to be... 

Steric effects, induced by the substituents of the alkyne derivative, dominate in general the regioselectivity of HCN addition when bulky alkynes are employed [63], The observed selectivity finds its analogy in the hydrocyanation of olefins already discussed. The decisive step involves formation of an -alkenyl-nickel complex. 

The most outstanding example for the applieation of homogeneously catalyzed hydrocyanation is the DuPont adiponitrile process. About 75 % of the world s demand for adiponitrile is covered by hydrocyanation of butadiene in the presence of nickel(O) phosphite species. This process is discussed for the addition of HCN to dienes as an example, because in this case a well-founded set of data is available. Though it was Taylor and Swift who referred to hydrocyanation of butadiene for the first time [45], it was to Drinkard s credit that this principle was fully exploited for the development of the DuPont adiponitrile process [18]. The overall process is described as the addition of two equivalents of HCN to butadiene in the presence of a tetrakisphosphite-nickel(O) catalyst and a Lewis acid promoter. A phosphine-containing ligand system for the catalyst is not suitable, since addition of HCN to the tetrakisphosphine-nickel complex results in the formation of hydrogen and the non-aetive dicyano complex [67], In general the reaction can... 

In the first stage Lewis acids are absent and further hydroeyanation of the monoolefm products 3-PN 40 and 2M3BN 41 does not readily oeeur. The monoeyanation of butadiene is similar to HCN addition to olefins. An individual feature of hydrocyanation of conjugated dienes is the intermediate appearance of TT-allylic complexes 43, which participate in the successive carbon-carbon coupling. Equations (12) and (13) demonstrate the reaction of butadiene with the hydrido-nickel complex 42 leading to formation of nitrile 40 (a) and explain the generation of byproducts, i.e., the branched nitrile 41 via an alternative pathway (b) [68-70]. 

Based on the extensive studies reported for the triaryl phosphite-nickel complex catalyzed hydrocyanation of alkenes [1] and our own findings, we propose a general catalytic cycle shown in Scheme 5 for the Ni(COD)2/L/MVN system. This simplified catalytic cycle is meant to encompass the various diastereomeric pathways generated by the ligand Cj symmetry. 

Use of nickel carbonyl to add 1 mol of HCN to 1,3-butadiene may be the first example of hydrocyanation by a homogeneous nickel catalyst. That work also recorded the important observation that substantial improvement in nitrile product yield results from conducting the reaction in the presence of ( 115)3 or (C H5)3As. This work led to extensive studies to develop effective nickel hydrocyanation catdysts. Virtually all subsequent developments have focused on finding the most effective nickel complex and the identification and application of promoters to improve catalyst efficiency and life. ... 

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. ... 

Mechanistic studies on the reductive elimination of square-planar type aryl(j7 -allyl)palladium complexes demonstrated occurrence of bond formation between the aryl carbon and one of the allyl termini that are located cis to each other (Scheme 8.53) [91]. The allyl ligand remained 17 -coordinated during the coupling. Similar reductive elimination between 17 -allyl and cyano ligands may be a key step in the industrially important nickel catalyzed hydrocyanation of dienes (Scheme 8.54) [92]. 

The hydrocyanation of butadiene with two moles of HCN in the presence of nickel complexes to give adiponilrile with high regioselectivity has been developed to industrial scale by DuPont. 

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 mechanism of hydrocyanation of alkenes catalyzed by soluble complexes is closely related to the mechanism of hydrogenation and hydrosilation. Hydrocyanation occurs by a sequence consisting of oxidative addition of HCN, olefin insertion into the M-H bond, and reductive elimination to form the new C-C bond. The mechanism of the original hydrocyanation catalyzed by cobalt carbonyl has not been studied in depth, but the mechanism of the reactions catalyzed by nickel complexes has been studied in depth and is better defined. 

Certain reaction conditions and properties of the nickel complexes promote hydrocyanation. The oxidative addition of HCN requires an open coordination site. Catalysts containing P(0-o-Tol)3 as ligand are particularly reactive because the imsatu-rated L Ni complex is the most stable form of the Ni(0) complex, whereas the saturated 18-electron L Ni complex is the most stable form of the catalysts containing smaller phosphite ligands. The need to imderstand the steric and electronic properties of the ligand on the dissociation of phosphine led to the classic work of Tolman on cone angles and electronic parameters. ... 

In the hydrocyanation of butadiene, 2 mol of HCN are added to butadiene with a nickel complex as catalyst to obtain adiponitrile directly. 

Allyl complexes have contributed significantly to the development of the organometallic chemistry of nickel and the applications of nickel complexes in organic synthesis, for example, nucleophilic attack on coordinated allyl ligands. In addition, allylnickel complexes have been identified as key intermediates in the oligomerization and cyclization of olefins and dienes. For example, the Ni(0)-catalyzed hydrocyanation of butadiene to adiponitrile, the main component of a major commercial process for the production of nylon, involves Ni (7r-allyl) intermediates. Moreover, the 77-rearrangements of allylnickel species have helped explain the facile isomerization of olefins in the presence of nickel complexes. The Ni-catalyzed homoallylation of carbonyl compounds with 1,3-dienes also involves Ni(7r-allyl) complexes this subject has been reviewed recently. New applications include the cleavage of G-G bonds in the deallylation of malonates, the preparation of cyclopentenones by carbonylative cycloaddi-... 

The commercial introduction of hydrocyanation has been pioneered by Du Pont. The nickel complex catalyzed addition of two molecules of HCN to butadiene yielding in high regioselectivity adiponitrile is practized in two plants. 

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