2-methyl-3-butenenitrile isomerization

Ligand Design and Mechanistic Studies for Ni-Catalyzed Hydrocyanation and 2-Methyl-3-Butenenitrile Isomerization Based upon Rh-Hydroformylation Research... 

The mechanism and the scope of the hydrocyanation and 2-methyl-3-butenenitrile isomerization reaction has been studied at DuPont in great detail for nickel catalysts featuring monodentate phosphite ligands, such as P(Otolyl)3. The results of these studies have been published in a review by Tolman and coworkers [9]. Later advances in the field have been summarized in recent reviews [10, 11], and book chapters [12, 13]. The aim of this chapter is to give an account of those developments that have not been covered in these reviews yet or are directly relevant to our own SFB-based research conducted in the field. 

Scheme 11 DPT calculated pathways for 2-methyl-3-butenenitrile isomerizations with [Ni(dmpe)]...

The current hydrocyanation process can be broken down into two major steps. In the first, HCN is added to butadiene in the presence of an NiL4 catalyst to give 3-pentenenitrile (3PN) and 2-methyl-3-butenenitrile (2M3BN) [Eq. (7)]. Fortunately the branched 2M3BN may be isomerized to the linear 3PN isomer [Eq. (8)]. In the second step, a Lewis acid promoter is added to the NiL4 (L = a phosphorus ligand) catalyst to effect the double bond isomerization of 3PN to 4-pentenenitrile (4PN) concurrently with the... 

The first HCN addition (eq. 3) occurs at practical rates above 70°C under sufficient pressure to keep butadiene condensed in solution and produces the 1,4- and 1,2-addition products (3-pentenenitrile [4635-87-4 ], 3PN, and 2-methyl-3-butenenitrile [16529-56-9 ], 2M3BN) in a 2 to 1 ratio. Fortunately, thermodynamics favors 3PN (about 20 1) and 2M3BN may be isomerized to 3PN (eq. 4) in the presence of a nickel catalyst. 

Pentenenitriles are produced as intermediates and by-products in DuPont s adiponitrile process. 3-Pentenenitrfl.e [4635-87 4]y is the principal product isolated from the isomerization of 2-methyl-3-butenenitrile (see eq. 4). It is entirely used to make adiponitrile. af-2-Pen tenenitrile [25899-50-7]> is a by-product isolated from the second hydrocyanation step. Some physical properties are listed in Table 13. 

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],... 

Equation 24 describes the Ni(0)-catalyzed addition of HCN to butadiene, which leads to 3-pentenenitrile together with its allylic isomer 2-methyl-3-butenenitrile (1.5 1 molar ratio). The branched allylic isomer, however, progressively isomerizes to the linear 3-pentenenitrile. 

Double-bond isomerization of 3-pentenenitrile (Equation 25) gives an equilibrium mixture containing only ca.1% of 4-pentenenitrile with respect to the 3-isomer. This isomerization occurs much faster than that leading to the thermodynamically favoured 2-pentenenitrile. 4-Pentenenitrile is preferentially hydrocyanated to adiponitrile because it reacts faster than the 3-isomer for steric reasons (Equation 26). In addition to adiponitrile, the final reaction mixture contains 2-methylglutaronitrile (from hydrocyanation of 2-methyl-3-butenenitrile), ethylsuccinonitrile (from cyanation of 3-pentenenitrile before isomerization) and 2-pentenenitrile (from isomerization of 3-pentenenitrile, which does not undergo further hydrocyanation). 

An attempt to polymerize 2-methyl-2-butenenitrile, MBN, by first isomerizing to methylenebuteronitrile (eq 46) led to isomerization instead (eq 47). 

Huser and Perron have extended this work to the isomerization of 2-methyl-3-butenenitrile (2M3 BN) to 3-PN (isomerization step Eq. (6) 92% yield) [17]. This patent mentions the use of iron and palladium catalysts but does not provide examples beyond nickel. In other work these same inventors discuss the use of other water-soluble ligands such as those containing carboxylate, phosphate, and alkyl-sulfonate substituents [18], while also exploring a wide range of Lewis acid co-catalysts for the addition of HCN to 3-pentenenitrile (Eq. 7) [19]. In general, the addi-... 

Huser et al. have extended Kuntz s work to the isomerization of 2-methyl-3-butenenitrile (2M3BN) to 3-PN (isomerization step Eq. (6) 92% yield) [17-19]. 

Isomerization processes involving homogeneous catalysts are mostly intermediate steps in industrial processes. For example, in the Shell oxo process, inner olefins are converted to primary alcohols. The isomerization occurs prior to CO insertion. The key step in the above mentioned DuPont process is the isomerization of 2-methyl-3-butenenitrile to a linear nitrile. A further example is the CU2CI2 catalyzed isomerization of dichlorobutenes [10]. 

The hydrocyanation of alkenes and dienes has similarly provided an exceptionally useful process for the conversion of simple feedstocks into more complex structures. [Caution Hydrogen cyanide is a highly toxic gas.] The process is best known as a key step in the DuPont adiponitrile process, which involves the dihydrocyanation of 1,3-butadiene (Scheme 3-95). The overall sequence first involves butadiene hydrocyanation to afford a mixture of 3-pentenenitrile and 2-methyl-3-butenenitrile. The unwanted branched isomer 2-methyl-3-butenenitrile is isomerized to 3-pentenenitrile under different conditions, and then 3-pentenenitrile is isomerized to 4-pentenenitrile in a subsequent nickel-catalyzed process in the presence of Lewis acidic additives. Finally, hydrocyanation of the remaining alkene generates the desired product adiponitrile, which serves as a precursor for nylon. A vast number of studies describing the optimization and mechanistic study of this process has appeared, and the interested reader is referred to the many excellent studies describing the details of this process. " ... 

The isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile is thought to occur by elimination and re-addition of HCN. A number of labeling experiments have been conducted to reveal the order and reversibility of the steps of hydrocyanation an experiment that addresses the mechanism of isomerization of the branched nitrile is depicted in Equation 16.8. As shown on the left of this equation, allylic transposition of the nitrile group without elimination would lead to 5-deuterio-3-pentenenitrile as the only isotopomer of 3-PN-iij. However, elimination to form an H-Ni-CN complex and free 1-deuteriobutadi-ene would lead to a mixture of two labeled 3-PN-iij isotopomers after re-insertion of the labeled butadiene and reductive elimination of the free labeled 3-PN. A mixture of the two isotopomers was formed, and this result indicates that isomerization occurs by elimination and re-addition of HCN. 

Miscellaneous Reactions. Hydwcyanation of olefins was among the very first reactions investigated in aqueous organic biphasic systems. [Ni(TPPTS)4] prepared separately or in situ from Ni(II) salts and TPPTS catalyze efficiently the anti-Markovnikov addition of HCN to butadiene and 3-pentenenitrile and also the isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile (Scheme 40). Lewis acid cocatalysts such as ZnCl2 facilitate the reaction. 

When the first HCN adds to butadiene, some undesired branched 2-methyl-3-butenenitrile, 9.29 in Figure 9.6, is formed along with the desired linear 3-butenenitrile. Interestingly, the first HCN addition to butadiene is reversible, because the branched nitrile can be isomerized to the linear form with NiL . This means that 9.29, which is an activated allylic nitrile, can oxidatively add to the nickel to give back the V-allyl nickel cyanide. Labelling studies suggest that this intermediate goes back to HCN and butadiene, before readdition... 

In the pastfewyears, application of electron-rich phosphine [23-27] and carbene [28] ligands has attracted some attention for the Ni-catalyzed 2M3BN isomerization reaction. These catalyst systems commonly yield 2-pentenenitriles (2PN) and branched 2-methyl-2-butenenitriles (2M2BN). Although the formation of (X,P-unsaturated nitriles 2PN and 2M2BN is of no technical relevance, it has been studied in quite some detail because the transformation is an interesting model reaction to study Ni-catalyzed C-H bond activation (Scheme 8.6). 

Structural characterization of these species has revealed that complex 69 adopts a trigonal-bipyramidal geometry in which the methylallyl moiety occupies the apical position (Ni-GN 189 pm), whereas complex 71 adopts a square-pyramidal structure with the cyanide ligand at the apical position at a relatively long distance from the Ni center (Ni-CN 199 pm). The latter structure is similar to the bromo analog NiBr(allyl)(dippe), prepared from the reaction of the nickel(i) hydrido dimer [NiH(dippe)]2 and allyl bromide. The involvement of allyl cyano species such as 69 and 71 in the catalytic hydrocyanation of butadiene is supported by the following observations (i) complex 69 catalyzes the isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile (ca. 100 turnovers at 100 °G), (ii) complex 71 decomposes slowly to give Ni(0) complexes of cis- and // 77i--crotonitrile (Scheme 20). 

Scheme 8 Nickel catalyzed isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile via 1,3-allyl shift...

It is well know that nickel(O) complexes play a crucial role in the commercial synthesis of adiponitrile (AdN), the major nylon-6,6 precursor. In the global process, the isomerization of the branched 2-methyl-3-butenenitrile (2M3BN) to the linear 3-pentenenitrile (3PN) is a key step. " Such isomerization is obtained through the C-CN bond activation involving a Ni° intermediate. 

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