Hydrocyanation

Hydrocyanation is, most fundamentally, the process whereby H⁺ and ^–CN ions are added to a molecular substrate. Usually the substrate is an alkene and the product is a nitrile. When ^–CN is a ligand in a transition metal complex, its basicity makes it difficult to dislodge, so, in this respect, hydrocyanation is remarkable. Since cyanide is both a good σ–donor and π–acceptor its presence accelerates the rate of substitution of ligands "trans" from itself, the "trans" effect.¹ A key step in hydrocyanation is the oxidative addition of hydrogen cyanide to low–valent metal complexes.Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. "Advanced Inorganic Chemistry"; John Wiley & Sons: New York, 1999; pp. 244-6, 440, 1247-9.]

Inorganic Chemistry

Hydrocyanation is performed on alkenes and alkynes with copper, palladium, and most commonly, nickel catalysts. Industrial hydrocyanation utilizes phosphite (P(OR)₃) complexes of nickel. Phosphites give excellent catalysts, whereas the related phosphine (PR₃) ligands, which are more basic, are catalytically inactive. Chiral, chelating aryl diphosphite complexes are commonly employed in asymmetric hydrocyanation. An example of a nickel–phosphite catalyzed hydrocyanation of ethene .

Lewis acids, such as B(C₆H₅)₃, can increase hydrocyanation rates and allow for lower operating temperatures. [Baker, M.J.; Pringle, P.G. "J. Chem. Soc. Chem. Commun." 1991, 1292-3.] Triphenylboron may derive this ability from sterically protecting the ^–CN as it is bound to nitrogen. Rates can also be amplified with electron–withdrawing groups (NO₂, CF₃, CN, C(=O)OR, C(=O)R) on the phosphite ligands, because they stabilize Ni(0). [Goertz, W.; Kramer, P. C. J.; van Leeuwen, P. W. N. M.; Vogt, D. "Chem. Commun." 1997, 1521-2.] A major problem when using nickel catalysts for hydrocyanation is the production of Ni⁰(CN)_x as a result of excess HCN.³ Bulky ligands impede the formation of these unreactive Ni⁰(CN)_x complexes. [Yan, M.; Xu, Q. Y.; Chan, A. S. C. "Tetrahedron":Asymmetry 2000, 11, 845-9.]

Usage

Hydrocyanation is important due to the versatility of alkyl nitriles (RCN), which are important intermediates for the syntheses of amides, amines, carboxylic acids, and ester compounds. [RajanBabu, T. V.; Casalnuovo, A. L. "Pure & Appl. Chem." 1994, 66, 1535-42.] The most popular industrial usage of nickel-catalyzed hydrocyanation is for adiponitrile (NC–(CH₂)₄–CN) synthesis from 1,3–butadiene (CH₂=CH–CH=CH₂). Adiponitrile is a precursor to hexamethylenediamine (H₂N–(CH₂)₆–NH₂), which is used for the production of certain kinds of Nylon. The DuPont ADN process to give adiponitrile is shown below:

This process consists of three steps: hydrocyanation of butadiene to a mixture of 2-methyl-3-butenenitrile (2M3BM) and 3-pentenenitrile (3PN), an isomerization step from 2M3BM (not desired) to 3PN and a second hydrocyanation (aided by a Lewis acid cocatalyst such as aluminium trichloride) to adiponitrile. ["Highly Selective Hydrocyanation of Butadiene toward 3-Pentenenitrile" Bini, L.; Muller, C.; Wilting, J.; von Chrzanowski, L.; Spek, A. L.; Vogt, D. J. Am. Chem. Soc.; (Communication); 2007; 129(42); 12622-12623. DOI|10.1021/ja074922e ]

Naproxen, an anti-inflammatory drug, utilizes an asymmetric enantioselective hydrocyanation of vinylnaphthalene from a phosphinite(OPR₂) ligand, L .The enantioselectivity of this reaction is important because only the S enantiomer is medicinally desirable, whereas the R enantiomer produces harmful health effects. This reaction can produce the S enantiomer with > 90% selectivity. Upon recrystallization of the crude product, the optically pure nitrile can be attained.

=History= Hydrocyanation was first reported by Arthur and Pratt in 1954, when they homogeneously catalyzed the hydrocyanation of linear alkenes. [Arthur Jr., P.; England, D. C.; Pratt, B. C., Whitman, G. M. "J. Am. Chem. Soc." 1954, 76, 5364-7.]

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