Nickel hydrocyanations

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. ... 

Although application of nickel hydrocyanation catalysts to the production of adiponitrile has been extensively studied, much remains to be learned about other applications of these catalysts. 

Nickel plays a role in the Reppe polymeriza tion of acetylene where nickel salts act as catalysts to form cyclooctatetraene (62) the reduction of nickel haUdes by sodium cyclopentadienide to form nickelocene [1271 -28-9] (63) the synthesis of cyclododecatrienenickel [39330-67-1] (64) and formation from elemental nickel powder and other reagents of nickel(0) complexes that serve as catalysts for oligomerization and hydrocyanation reactions (65). 

Nickel is frequently used in industrial homogeneous catalysis. Many carbon-carbon bond-formation reactions can be carried out with high selectivity when catalyzed by organonickel complexes. Such reactions include linear and cyclic oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions [1], Many of the complexes that are active catalysts for oligomerization and isomerization reactions are supposed also to be active as hydrogenation catalysts. 

In an extension of an early work on the nickel-catalyzed addition of hydrogen cyanide to unsaturated compounds, a basic reaction in various large-scale processes in the polymer industry, the hydrocyanation of butadiene (equation 15) and the efficiency of catalysis of this reaction by low-cost copper salts has been studied extensively by Belgium researchers47,48. 

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... 

Isomerisation is also an important step in the DuPont process for making adiponitrile (Chapter 11) in which internal pentenenitriles must be converted to the terminal alkene. The catalyst is the same as that used for the hydrocyanation reaction, namely nickel(II) hydrides containing phosphite ligands. 

First we will describe the hydrocyanation of ethene as a model substrate. The catalyst precursor is a nickel(O) tetrakis(phosphite) complex which is protonated to form a nickel(II) hydride. Actually, this is an oxidative addition of HCN to nickel zero. In Figure 11.1 the hydrocyanation mechanism in a simplified form is given the basic steps are the same as for butadiene, the actual substrate, but the complications due to isomer formation are lacking. 

Hydrocyanation of butadiene is more complicated than that of ethene it requires two hydrocyanation steps and several isomers can be observed. The isomers obtained in the first step of the HCN addition to butadiene are shown in Figure 11.3. The addition first leads to compounds 1 and 2, in a 1 2 ratio, but they equilibrate to a favourable 1 9 ratio via the retro-reaction. The retro reaction involves a C-C bond breaking reaction, which is rare, but in this case the intermediate is a Tt-allyl species and a stable, anionic cyanide group. Electron-rich nickel species (Ni-dippe) can cleave aromatic nitrile C-C bonds... 

Table 10.1. Nickel-catalysed hydrocyanation of styrene, using diphosphine ligands. ...

In the presence of a large excess of cyanide, the catalyst prepared from [Ni(COD)2] and TPPTS was also active in the hydrocyanation of allylbenzene however, at low cyanide/nickel ratios isomerization to propenylbenzene became the main reaction path (Scheme 9.9) [5]. 

Leeuwen, P.W.N.M., Vogt D. and Keim, W. (1995) Effect of the bite angle of diphosphine ligands on activity and selectivity in the nickel-catalysed hydrocyanation of styrene. /. Chem. Soc. Chem. Commun., 2177-1778. 

Because of its low acidity, hydrogen cyanide seldom adds to nonactivated multiple bonds. Catalytic processes, however, may be applied to achieve such additions. Metal catalysts, mainly nickel and palladium complexes, and [Co(CO)4]2 are used to catalyze the addition of HCN to alkenes known as hydrocyanation.l67 l74 Most studies usually apply nickel triarylphosphites with a Lewis acid promoter. The mechanism involves the insertion of the alkene into the Ni—H bond of a hydrido nickel cyanide complex to form a cr-alkylnickel complex173-176 (Scheme 6.3). The addition of DCN to deuterium-labeled compound 17 was shown to take place... 

Hydrocyanation of alkenes usually gives anti-Markovnikov products. Interestingly, however, addition of HCN to styrene yields mostly the branched (Marko-vnikov) adduct. This was suggested to result from stabilization of the branched alkylnickel cyanide intermediate by interaction of nickel with the aromatic ring.176... 

Hydrocyanations -nickel catalysts [NICKEL COMPOUNDS] (Vol 17) -use of dimethylacetamide [ACETIC ACID AND DERIVATIVES - DIMETHYLACETAMIDE] (Voll)... 

Whereas many nickel-catalyzed olefin hydrocyanation reactions may be run in the batch mode (i.e., all reagents charged to the vessel at the beginning of reaction), it is often preferable to feed one or more components in a... 

In this article, we will discuss the chemistry behind the du Pont adiponitrile process from a mechanistic viewpoint (10). It is not intended to be a comprehensive review of the hydrocyanation literature. We will restrict ourselves rather to homogeneous nickel-catalyzed hydrocyanation of olefins and will depend primarily on du Pont studies. Reviews which explore hydrocyanation in a more general way include those of Brown (77), Hubert and Puentes (72), and James (73). A general review of low-valent organo-nickel chemistry has been published by Jolly and Wilke (14). 

Before discussing hydrocyanation chemistry we will explore the interaction of zero-valent nickel phosphite complexes with various independent components of the catalytic system. Then, in turn, we will examine the catalyzed addition of HCN to butadiene, the isomerization of olefins, and the addition of HCN to monoolefins. Finally, a summary of the mechanism as it is now understood will be presented. 

A variety of ligands other than phosphites are of course also important in hydrocyanation. The isolation of the 16-electron complex Ni[P(0-o-tolyl)3]3 (17) provided a remarkable opportunity for the study of how various components of the catalytic system interact with nickel. The addition of nitriles to this complex led to the observation of the first nitrile complex of zero-valent nickel (25). When the phosphite ligands are very bulky, as in the case of P(0-o-tolyl)3, a nitrile complex of formula (RCN)NiL3 is formed nearly quantitatively even in the presence of excess L, and is identified easily... 

Hydrogen cyanide smoothly adds to butadiene (BD) in the presence of zero-valent nickel catalysts to give (3PN) and (2M3BN) [1,4- and 1,2-addition products, respectively, Eq. (7)]. A variety of Ni[P(OR)3]4 (R = alkyl or aryl) complexes are suitable as catalysts. The reaction may be carried out neat or in a variety of aromatic or nitrile solvents at temperatures from 50-120°C. Whereas in many olefin hydrocyanations it is desirable to keep the HCN concentration very low to protect the nickel from degradation, with butadiene HCN may be added batchwise as long as the HCN concentration is kept near the butadiene concentration. In the case of batch reactions one must be cautious because of possible temperature rises of 50°C or more over a period of a few minutes. Under typical batch conditions, when Ni[P(OEt)3]4, butadiene, and HCN are allowed to react in a ratio of 0.03 1.0 1.0 at 100°C for 8 hr, a 65% conversion to 3PN and 2M3BN (1.5 1) is observed (7). 

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