Ethylene hydrocyanation

Fig. 9. Mechanism of ethylene hydrocyanation. Dashed arrows imply irreversible reactions.

Ethylene hydrocyanation by [Ni P(0-o-tolyl)3 4] follows the cycle of Eq. 9.20. Oxidative addition of HCN to the metal gives a 16e nickel hydride that undergoes ethylene insertion to give an ethyl complex, followed by reductive elimination to give the EtCN product. The reaction with butadiene is more complex but goes by a closely related route not discussed in detail here. The best ligands are bulky, ir-acceptor P-donors, such as tri-o-tolyl phosphite. 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

Ethylene cyanohydrin can be obtained by the hydrocyanation of ethylene oxide... 

Ethylene cyanohydrin has been prepared by the action of ethylene oxide upon anhydrous hydrocyanic acid 1 but the majority of methods described in the literature have involved the interaction of ethylene chlorohydrin and alkali cyanide. This has been effected in the absence of a solvent by heating to ioo° in a closed vessel,2 by boiling the reagents in 50 per cent aqueous-alcoholic solution,3 by adding a concentrated aqueous solution of potassium or sodium cyanide to a boiling solution of ethylene chlorohydrin in absolute alcohol,4 and in aqueous solution at 45 °.5... 

Acrylonitrile was first produced in Germany and the United States on an industrial scale in the early 1940s. These processes were based on the catalytic dehydration of ethylene cyanohydrin. Ethylene cyanohydrin was produced from ethylene oxide and aqueous hydrocyanic acid at 60°C in the presence of a basic catalyst. The intermediate was then dehydrated in the liquid phase at 200°C in the presence of magnesium carbonate and alkaline or alkaline earth salts of fonnic acid. A second commercial route to acrylonitrile was the catalytic addition of hydrogen cyanide to acetylene. The last commercial plants using these process technologies were shut down in 1970 (Langvardt, 1985 Brazdil, 1991). 

D. Reactions of Epoxides vnth Acids This section is devoted to the addition of acids to ethylene oxides. To facilitate its presentation the material will be divided into two principal categories (1) condensation with mineral adds (3) condensation with organic adds. The first- will include halogen adds and other utxong mineral acids the second will include carboxylic adds, sulfonic acids, and hydrocyanic eoid (hydrogen oyanide). 

The hydrocyanation of ethylene is a model for many monoenes and has been studied in some detail (47, 63). Upon addition of HCN in the presence of excess ethylene at — 40°C, the 31P-NMR resonance of (C2 H4)Ni[P(0-o-tolyl)3]2 at 141.4 ppm is quantitatively replaced by four new singlets at 129.8 (uncoordinated ligand), 118.1, 117.7, and 116.9 ppm with areas in a relative ratio of 1.00 0.14 0.80 0.06 (63). These same signals are produced when HNi[P(0-o-tolyl)3]3CN is treated with excess ethylene at — 50°C (47). These signals persist until HCN and/or ethylene is consumed. In the H-NMR spectrum at — 50°C five Ni—C2H5 protons appear as a single broad resonance at t 9.39, due to accidentally similar chemical shifts the methyl triplet may be observed separately from the methylene near 0°C, but at that temperature the rate of reductive elimination becomes appreciable. 

Fig. 10. Competitive hydrocyanation of ethylene and propylene with 0.17S M Ni[PfO-o-tolyl)3]3 in 75% toluene-d8/25% CH2C12, followed by proton NMR. Used by permission of the New York Academy of Sciences.

The mechanism shown in Fig. 9 for the hydrocyanation of ethylene with (C2H4)Ni[PO-o-tolyl)3]2 is inconsistent with the kinetic data described above for 4PN with Ni[P(0-p-tolyl)3]4 and Lewis acid (A). This is not unreasonable when we remember that the equilibrium constant for binding of ethylene to Ni(0) is 70 times greater than that for binding of 4PN (Table II), whereas P(0-p-tolyl)3 is preferred over P(0-o-tolyl)3 by a factor of 108 (Table I) This leads to the possibility that an intermediate such as 19 is much less important in the 4PN/P(0-o-tolyl)3 system. How the Lewis acid changes the mechanism is also still not clear. 

Hydrocyanation is also catalyzed by [Pd(PPh3)4] (103) and [Pd P(OPh), 4] (132), again in both cases in the presence of excess ligand.604 Complex (132) is an effective catalyst for the addition of hydrogen cyanide to cyclic monoenes and dienes such as norbomene and norbornadiene 605-606 ethylene also reacted readily. The product obtained from norbornene was the exo isomer (equation 165). When norbornadiene was the substrate, some of the endo product was formed.605... 

The DIOP complex of Pd(0) and ethylene, i.e. (DIOP)Pd(C2H4), has been found to induce up to 40% e.e. in asymmetric hydrocyanation of norbomene445. This complex also... 

Catalytic hydrocyanalion of acetylenes. Acetylenes are hydrocyanated to saturated secondary nitriles by 1 with excess cyanide ion and either NaBH, (in ethylene glycol) or Zn (in water) as reducing agent. Terminal acetylenes RC=CH afford RCH(CN)CH3 in high yield, whereas internal acetylenes afford a mixture of regioisomers. 

Hydrocyanic acid adds to bis(dimethylamino)ethylene (516) to give 2,2-bis(dimethylamino)propio-nitrile (517 Scheme 94).28,29,35,977 gy deavage of aminal esters with acyl cyanides or hydrocyanic acid a,a-diaminonitriles (518) are accessible trisaminomethanes have also been cleaved by HCN to give nitriles (518). 

A complete quantitative mechanism cannot be constructed with confidence on the basis of the products since not all of them were determined. A variety of secondary processes is likely. Hydrogen abstraction is expected to involve mainly the N-bound H atom. The activation energy for abstraction by methyl is given as 4.8 kcal/mol (277). Reaction 85 has been found (277) to yield ethylene and nitrogen in the radical-sensitized decomposition of ethyleneimine. Reaction 70 (see above) could explain the production of hydrocyanic acid. 

The mechanism of NiL4-catalyzed hydrocyanation (L=P(0-o-tolyl)3) of ethylene has been studied in detail, offering the advantage that olefin isomerization is avoided (cf. Scheme 1 [10]). Scheme 1 contains the main features of the process, such as oxidative addition, n- and (r-complexes, reductive elimination, and catalyst deactivation by Ni(CN)2 formation. 

Scheme 1. Catalytic loop for the homogeneously catalyzed hydrocyanation of ethylene.

Derivation (1) Condensation of ethylene oxide with hydrocyanic acid followed by reaction with sulfuric acid at 320F (2) acetylene, carbon monoxide, and water, with nickel catalyst (3) propylene is vapor oxidized to acrolein, which is oxidized to acrylic acid at 300C with molybdenum-vanadium catalyst (4) hydrolysis of acrylonitrile. 

According to one process, a mixture of ethylene oxide and hydrocyanic acid is added in batches or continuously at 60-70 C to an aqueous solution of a sterically hindered amine, e.g., diisopropyl amine. The product is fed to a stripping column in which the amine catalyst and water are removed from the higher-boiling cyanohydrin. A preferred procedure in the subsequent dehydration step comprises ad ding the cyanohydrin to a catalyst (e.g., sodium formate) at about 200-240 C, distilling off the acrylonitrile and water, and then fractionating the acrylonitrile. 

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