In 1971 DuPont started the production of adiponitrile (ADN) as an intermediate in the production of Nylon(6,6). 1 This process is so far the only example of a large-scale industrial application of an alkene hydrocyanation. Originally it was developed using a monodentate phosphite-based zerovalent nickel catalyst. 2 The process consists of three steps. The hydrocyanation of butadiene leads to a mixture of 2-methyl-3-butenenitrile (2M3BN) and 3-pentenenitrile (3PN), obtained in varying ratio (typically 2:3) depending on the ligand employed. In a second step the branched 2M3BN is isomerized to the desired linear 3PN. The last step is the hydrocyanation of 3PN to ADN, requiring a Lewis acid cocatalyst such as AlCl 3 .Many efforts have been made to improve the performance of the nickel catalysts. An important step was the replacement of monodentate ligands by bidentate π-acceptor ligands that lead to higher conversion and selectivity for 3PN up to 70%. 3 A higher selectivity has been claimed only in a patent for bis(diphenylphosphino)ferrocene (DPPF) as ligand used in large excess. 4 Large bite angle ligands, based on a rigid xanthene backbone (e.g., sixantphos or thixantphos) were proven to enhance the Ni(0)-catalyst performance in hydrocyanation. 5 It was proposed that these ligands improve the reductive elimination of the product and stabilize the active Ni-species while suppressing the formation of inactive dicyano Ni(II)-complexes.Triptycene-based bidentate ligands, first described by Hofmann at al., 6 possess both a very rigid backbone and a large bite angle. So far, these systems were described only in the patent literature with examples of Rh-catalyzed hydroformylation reactions. 6,7 In more recent publications different mono-and dinuclear metal complexes with a variety of bite angles and geometries were investigated. 8 As part of our continuing interest in this field, 9 we now describe the use of a triptycene-based diphosphine ligand in the hydrocyanation of butadiene, resulting in unprecedented high selectivities to 3PN of up to 98%. A new route toward the ligand tript-PPh 2 (5) (Scheme 1) has been devised, giving considerably higher yields than the reported 6,7 method; 1,8-dichloroanthraquinone (1) was converted into the difluoro compound (2) and subsequently reduced to 1,8-difluoroanthracene (3) with Zn dust 10,11 (Scheme 1). The triptycene moiety (4) was obtained by reaction of the corresponding anthracene with benzyne, generated in situ from anthranilic acid. Nucleophilic substitution of the fluoro groups with potassium diphenylphosphide gave ligand (5) in 20% overall yield.Reaction of (5) with (cod)PtCl 2 and Ni(cod) 2 , respectively, led to the corresponding complexes (5)PtCl 2 (6) and (5)Ni(cod) (7). The pale-yellow compound 6 was characterized by means of 1 H, 13 C, and 31 P NMR spectroscopy as well as by elemental analysis.The 31 P NMR spectrum of 6 shows a singlet at δ ) 0.42 ppm with platinum satellites ( 1 J Pt-P ) 3761 Hz), suggesting that the ligand is coordinated in a cis fashion. 8c This wa...
Scheme 1. Hydrocyanation of alkenes. Scheme 2. Synthetic versatility of nitriles. Scheme 3. The DuPont Adiponitrile process.
High turnover enantioselective alkene cis-dihydroxylation is achieved with H(2)O(2) catalysed by manganese based complexes containing chiral carboxylato ligands.
The addition of HCN to alkenes is a very useful reaction for the synthesis of functional organic substrates. Industrially the nickel-catalyzed hydrocyanation has gained considerable importance mainly because of the production of adiponitrile in the DuPont process. In this process the hydrocyanation of butadiene is carried out using aryl phosphite-modified nickel catalyst. Since the performance of organo-transition metal complexes is largely determined by the ligand environment of the metal, fundamental understanding and ligand development is of pivotal importance for any progress. This feature article gives an account of the development and application of different mono- and bidentate phosphorus-based ligands in the Ni-catalyzed hydrocyanation reaction of alkenes. Special attention will be paid to the development of insight and understanding of the ligand structural and electronic properties towards the improvement of the catalyst performance in terms of stability, activity, and selectivity.
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