The 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 polymers. Out of this work came Nylon-6,6 (9.28), one of the first useful petroleum-based polymers. 

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

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 largely 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. 

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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). 

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. 

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... 

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. 

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

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 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. ... 

A final example of aqueous media used in the hydrocyanation of butadiene is provided by Waddan at ICI [23]. In this chemistry, copper nitrate salts in aqueous media (among many others) are used for the oxidative dihydrocyanation of butadiene to dicyanobutenes (Eq. 9). Good conversions of butadiene are reported in nonaqueous media but no examples are actually provided in which water is added as a solvent. Moreover, because of problems with alkene and HCN dimerization and the risk of explosion hazards, these reactions appear to work best when conducted stepwise (i.e., HCN addition to catalyst followed by oxidation followed, in turn, by butadiene addition), leading one to wonder about the productivity of these systems. 

A final example of aqueous media used in the hydrocyanation of butadiene is provided by Waddan at ICI [20]. In this chemistry, copper nitrate salts in aqueous media (among many others) are used for the oxidative dihydrocyanation of butadiene to dicyanobutenes [Eq. (8)]. 

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 hydrocyanation of butadienes is the basis of DuPont s process for the production of adiponitrile [hexanedinitrile (19), Scheme llj.l l The first step of the process involves hydrocyanation of huta-1,3-diene to produce an isomeric mixture of pentenenitriles. In a second step, nickel-catalyzed double-bond isomerization occurs to produce pent-4-eneni-trile followed by alkene hydrocyanation to produce adiponitrile (19). The details of the al-kene hydrocyanation reaction are discussed in further detail in Section 1.1.4.5. 

The use of a co-catalyst was crucial to the development of practical hydrocyana-tion. The rate and catalyst lifetime for hydrocyanation of simple alkenes increases dramatically by conducting the reactions in the presence of a Lewis acid. As shown in Table 16.1, the reaction of propene occurs much faster in the presence of aluminum and zinc halides. Lewis acid cocatalysts also promote isomerization and selective additions during some steps of the hydrocyanation of butadiene. This effect is presented later in this section. 

While the hydrocyanation of monoolefins remains under development, the hydrocyanation of butadiene shown in Equation 16.3 is one of the largest-scale homogeneous catalytic processes known. This reaction generates the 1,4-dinitrile called adiponitrile, which is the precursor to the diamine monomer in nylon 6,6. The formation of this regioisomer requires a set of reversible hydrocyanations and isomerizations that are discussed in Section 16.2.4 on hydrocyanation of dienes. 

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