4-pentenenitrile, hydrocyanation

Pentenenitnles are produced as intermediates and by-products in DuPont s adiponitrile process. 3-Pentenenitrile [4635-87-4] is the principal product isolated from the isomerisation of 2-methyl-3-butenenitrile (see eq. 4). It is entirely used to make adiponitrile. i7j -2-Pentenenitrile [25899-50-7] is a by-product isolated from the second hydrocyanation step. Some physical properties are Hsted in Table 13. 

There are three commercial routes to ADN in use. The first method, direct hydrocyanation of 1,3-butadiene [106-99-0] has replaced an older process, cyanation via reaction of sodium cyanide with 1,4-dichlorobutane [110-56-5] owing to the lower cost and fewer waste products of the new process. During the initial steps of the direct hydrocyanation process, a mixture of two isomers is generated, but the branched isomer is readily converted to the linear 3-pentenenitrile [4635-87-4]. 

Isomerisation is also an important step in the DuPont process for making adiponitrile (Chapter 11) in which internal pentenenitriles must be converted to the terminal alkene. The catalyst is the same as that used for the hydrocyanation reaction, namely nickel(II) hydrides containing phosphite ligands. 

Hydrocyanation of olefins and dienes is an extremely important reaction [32] (about 75 % of the world s adiponitrile production is based on the hydrocyanation of 1,3-butediene). Not surprisingly, already one of the first Rhone Poluenc patents on the use of water soluble complexes of TPPTS described the Ni-catalyzed hydration of butadiene and 3-pentenenitrile (Scheme 9.10). The aqueous phase with the catalyst could be recycled, however the reaction was found not sufficiently selective. 

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... 

In the absence of Lewis acids, further hydrocyanation of the monoolefin products does not readily occur. However, the addition of a Lewis acid cocatalyst allows pentenenitriles (PN s) to be hydrocyanated to dinitriles. When BD and 4PN are hydrocyanated together with Ni[P(0-p-tolyl)3]4 and ZnCl2 at 80°C, BD hydrocyanates 20 times faster than 4PN. 

Figure 6.24 VIP plot for various descriptors in a PLS model for the hydrocyanation of pentenenitrile in the presence of Ni-biphosphine/biphosphite complexes. Charge descriptors refer to the Mulliken charge calculated at the ligating atoms. A bind is the energy difference between the free ligand and the metal complex, and can be related to the...

In asymmetric hydrocyanation reactions the desired isomers are the chiral branched products only. Good regioselectivity toward the branched product (>98%) is limited to vinylarenes. Hydrocyanation of 1,3-dienes gives a variety of mixtures depending on the catalyst and conditions 1-alkenes give the linear nitrile as major product [34]. Both are seen in the adiponitrile process in which the unwanted branched 2M3BN (hydrocyanation product from 1,3-butadiene) is isomerized to the linear product 3-pentenenitrile, which is then hydrocyanated by in-situ isomerization to 4-pentenenitrile, resulting in the linear adiponitrile. Thus vinylarenes and cyclic alkenes (mainly norbomene) are usually the substrates of choice for the asymmetric hydrocyanation. Hopefully 1,3-dienes will become feasible substrates in the near future. 

The isomerization of 3PN can lead to two possible products, 2-pentenenitrile (2PN), the unwanted isomer, and 4PN, the desired isomer. The former does not undergo hydrocyanation and thermodynamically is the most stable isomer. If the isomerization of 3PN were allowed to reach thermodynamic equilibrium, the concentrations of the three isomers 2PN, 3PN, and 4PN would be approximately 78 20 2. Fortunately the isomerization of 3PN to 4PN is about 70 times as fast as that of 3PN to 2PN. In other words, although 4PN is thermodynamically the less stable isomer, the favorable kinetics allows its preferential formation. 

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). 

Example 8.12. By-product formation in hydrocyanation of 4-pentenenitrile [44]. Hydrocyanation of 4-pentenenitrile (4-PN) to adiponitrile (ADN) was examined in some detail Example 8.7. The du Pont process maximizes the yield of adiponitrile, the desired product, by addition of a Lewis acid such as zinc chloride or tri-alkylborane, but nevertheless some 2-methyl-glutaronitrile (2-MGN) is formed as byproduct ... 

Hydrocyanation of 1,3-butadiene occurs in three stages. Equation 9.39 shows the first stage, which produces a 2 1 mixture of the desired 3-pentenenitrile (68), produced by a 1,4-addition of HCN to 1,3-butadiene, and the branched isomer 2-methyl-3-butenenitrile (69), which results from Markovinikov 1,2-addition. 

The next stage requires equilibration and isomerization of 69 to 68, giving a 9 1 ratio of desired to undesired product. A Ni catalyst is also used for this reaction. Finally, the third stage consists of two transformations, where 68 is first isomer-ized to 4-pentenenitrile (70) under kinetic control, fortunately without producing much of the thermodynamically more stable 2-pentenenitrile (71). Compound 70 then undergoes a second hydrocyanation with anti-Markovnikov orientation (equation 9.40). In this last Ni-catalyzed stage of the overall process, a Lewis acid, such as Ph3B, is added to ensure that linear rather than branched product (72) forms. 

Hydrocyanation of 1,3-butadiene to a mixture of pentenenitriles and 2-methyl-3-butenenitrile (and, in a second step, their reaction with hydrogen cyanide to produce adiponitrile) has received special attention because of its commercial application. ... 

Synthesis of adiponitrile from pentenenitrile has been reported using te-trakis(triarylphosphite)palladium(0), but no report of 1,3-butadiene hydrocyanation by... 

These results coupled with the greater success of nickel and palladium catalysts may explain the long interval preceding the next report on cobalt catalysts . Cobalt(I) complexes (see Table 1) are, however, effective hydrocyanation catalysts, not only in hydro-cyanating olefins such as 4-pentenenitrile, but also are effective for isomerizing 2-methyl-... 

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 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 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. 

The technically relevant hydrocyanation of t3PN (Scheme 8.2, Equation III) represents the most difficult step of the ADN process, and reports in the literature tackling this problem are very scarce. The requirements for the catalyst are quite challenging, as high activity in pentenenitrile isomerization and high substrate selectivity and n-selectivity for the hydrocyanation are essential for the production of the desired adiponitrile (Scheme 8.5). 

Tauchert, M.E. (2009) Nickel-catalyzed hydrocyanation and pentenenitrile isomerization ligand design and mechanistic studies. Dissertation. Universitat Heidelberg. 

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). 

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