Hydrocyanation of butadiene

The existence of proteins (9.27) suggested to Carothers at du Pont that the peptide link, -NHCO-, might be useful for making artificial polymns. Out of this work came nylon-6,6 (9.28), one of die first useful petroleum-based polymers. 

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

4-pentene nitrile, once formed, is rapidly hydrocyanated selectively to the linear adiponitrile product all the other possible dinitriles are formed at a much slower rate. 

An important step at several points in the catalytic cycle is loss of L to open up a vacant site at the metal. The rate and equilibrium constant for these dissociative steps are controlled lai ely by the bulk of the ligand. Electron-withdrawing ligands are required to facilitate the other steps in the cycle, so that one of the best is o-tolyl phosphite, which combines steric bulk with a strongly electron-withdrawing character. 

The Lewis acid BPhj is a useful co-catalyst for the reaction. Such additives are often termed promoters. In this case the promoter improves the selectivity of the system for linear product (it is not clear exactly why) and improves the life of the catalyst. A catalyst deactivates when it loses. some or all of its activity by going down an irreversible path that leads to an inactive form of the metal complex. In this case, the formation of the inactive Ni(CN)2 is the principal deactivation step. This can happen in several ways an example is shown here  

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Until the 1960s, adipic acid [124-04-9] was virtually the sole intermediate for nylon-6,6. However, much hexamethylenediamine is now made by hydrodimerization of acrylonitrile (qv) or via hydrocyanation of butadiene (qv). Cyclohexane remains the basis for practically the entire world output of adipic acid. The U.S. capacity for adipic acid for 1993 was 0.97 X 10 t/yr (233). 

Another example is the du Pont process for the production of adiponitrile. Tetrakisarylphosphitenickel(0) compounds are used to affect the hydrocyanation of butadiene. A multistage reaction results in the synthesis of dinitrile, which is ultimately used in the commercial manufacture of nylon-6,6 (144-149). 

Some companies are successfully integrating chemo- and biocatalytic transformations in multi-step syntheses. An elegant example is the Lonza nicotinamide process mentioned earlier (.see Fig. 2.34). The raw material, 2-methylpentane-1,5-diamine, is produced by hydrogenation of 2-methylglutaronitrile, a byproduct of the manufacture of nylon-6,6 intermediates by hydrocyanation of butadiene. The process involves a zeolite-catalysed cyciization in the vapour phase, followed by palladium-catalysed dehydrogenation, vapour-pha.se ammoxidation with NH3/O2 over an oxide catalyst, and, finally, enzymatic hydrolysis of a nitrile to an amide. 

Hexamethylenediamine (HMDA), a monomer for the synthesis of polyamide-6,6, is produced by catalytic hydrogenation of adiponitrile. Three processes, each based on a different reactant, produce the latter coimnercially. The original Du Pont process, still used in a few plants, starts with adipic acid made from cyclohexane adipic acid then reacts with ammonia to yield the dinitrile. This process has been replaced in many plants by the catalytic hydrocyanation of butadiene. A third route to adiponitrile is the electrolytic dimerization of acrylonitrile, the latter produced by the ammoxidation of propene. 

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 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... 

The indirect hydrocyanation of butadiene as practiced by du Pont (/) involved the electrolysis of sodium chloride, formation of sodium cyanide from HCN using the NaOH, chlorination of butadiene to give 1,4-dichloro-but-2-ene, chloride displacement with sodium cyanide, and subsequent hydrogenation, as indicated in Eqs. (l)-(5), with the net result of Eq. 6. 

The hydrocyanation of butadiene is an important industrial route to adiponitrile (equation 163).602 Again, complex (131) is used as the catalyst for the reaction. The hydrocyanation of dienes proceeds mainly by 1,4-addition and r/ -allyl complexes are believed to be intermediates (Scheme 59).603 The l-cyano-2-butene is then isomerized to l-cyano-3-butene which undergoes further hydrocyanation to give adiponitrile.601"603... 

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],... 

It has a global capacity of 1.3 million tonnes per year. In the United States, production of ADN is based on the hydrocyanation of butadiene or electrochemical conversion from acrylonitrile. In Western Europe, companies produce ADN from adipic acid, butadiene and acrylonitrile. In Japan, the sole producer makes ADN from the electrodimerization of acrylonitrile. Demand for ADN is expected to be around 2% per year through at least 201 (f A comparison of the costs associated with two of the ADN processes are shown in Table 22.1275. 

Metric Unit3 Hydrocyanation of Butadiene Electrohydrodimerization of Acrylonitrile... 

DuPont manufactures adiponitrile (ADN), a raw material for nylon 6,6, by the hydrocyanation of butadiene using homogeneous nickel catalysts. As shown... 

Interestingly, the by-product in the above-described hydrocyanation of butadiene, 2-methylglutaronitrile, forms the raw material for the Lonza process for nicotinamide (see earlier) [123]. Four heterogeneous catalytic steps (hydrogenation, cyclisation, dehydrogenation and ammoxidation) are followed by an enzymatic hydration of a nitrile to an amide (Fig. 1.50). 

The most important use is the hydrocyanation of butadiene to adiponitrile, NC—(CH2)4—CN, a precursor to hexamethylenediamine for the synthesis of nylon. The process goes stepwise. The first addition of HCN involves nickel allyl intermediates and gives a mixture of linear and branched products in a ratio of —70 30. 

The active species in the methoxycarbonylation is presumably CoH(CO)4 (in equilibrium with Co2(CO)g and Co(CO)4 ) which adds Co-H 1,4- to the diene this is followed by carbonylation of the Co-C bond, methanolysis of the RCO-Co bond by MeOH or OMe , and regeneration of the hydride. The methoxycarbonylation route to adipic acid is an alternative both to the du Pont (Ni(II)/Lewis acid (BPhs)) catalyzed double hydrocyanation of butadiene (Section 5.4.4) and to the process based on the oxidation of cyclohexane (Section 2.2). 

Figure 13 Schematic representation of the steps presumably involved in the Ni-catalyzed hydrocyanation of butadiene to adiponitrile.

Improvements in existing processes accompagnied by new techniques. The first edition of this book presented 70 processes. It now discusses 140. Admittedly these are not all innovations. Many of them are different versions of the same chemical reaction or of an already existing separation method. Others, more innovative, only made headway slowly their industrial penetration was hindered by the slowdown in economic expansion new solvents in extractive distillation for benzene production, metathesis of olefins (Shell), olefins for oxo synthesis (Dimersol, Instituc Franfais du Pitrole), adiponitrile by direct hydrocyanation of butadiene (Dm Pont de Nemours), or by the conversion of 1,6-hexanediol (Celanese), laur IIactam from cyclododecane [ATO, Huls). 

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. ... 

Some of the earlier reviews summarizing this extensive chemistry are those of Brown [8, 16], Hubert and Puentes [17], James [18], and Tolman [15]. Low-valent organonickel chemistry was reviewed by Jolly and Wilke [19]. Newer developments, especially the employment of bidentate ligands for the generation of more active catalysts as well as the induction of asymmetry in the product nitriles, are generally reviewed by Casalnuovo and RajanBabu [20] the exploration of water-soluble catalysts for hydrocyanation of butadiene is summarized by Bryndza and Harrelson [21],... 

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... 

Metric Unit" Hydrocyanation of butadiene Electrohydrodimerization of acrylonitrile... 

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