Catalysts for Hydrocyanation

All the reactions of the hydrocyanation process are catalyzed by zero-valent nickel phosphine or phosphite complexes. These are used in combination with Lewis acid promoters such as zinc chloride, trialkyl boron compounds, or trialkyl borate ester. The ability of the precatalyst to undergo ligand dissociation 

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FIGURE 22 Preparation of supramolecular catalysts for hydrocyanation reactions (82) (A) assembly of heterodimeric chelating ligands (B) structure of the optimal nickel-diphosphine complex for hydrocyanation (other ligands of the metal center are omitted for clarity) and (C) hydrocyanation of functionalized styrenes. (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

Some of the earlier reviews summarizing this extensive chemistry are those of Brown [8, 16], Hubert and Puentes [17], James [18], and Tolman [15]. Low-valent organonickel chemistry was reviewed by Jolly and Wilke [19]. Newer developments, especially the employment of bidentate ligands for the generation of more active catalysts as well as the induction of asymmetry in the product nitriles, are generally reviewed by Casalnuovo and RajanBabu [20] the exploration of water-soluble catalysts for hydrocyanation of butadiene is summarized by Bryndza and Harrelson [21],... 

The addition of HCN to olefins catalyzed by complexes of transition metals has been studied since about 1950. The first hydrocyanation by a homogeneous catalyst was reported by Arthur with cobalt carbonyl as catalyst. These reactions gave the branched nitrile as the predominant product. Nickel complexes of phosphites are more active catalysts for hydrocyanation, and these catalysts give the anti-Markovnikov product with terminal alkenes. The first nickel-catalyzed hydrocyanations were disclosed by Drinkard and by Brown and Rick. The development of this nickel-catalyzed chemistry into the commercially important addition to butadiene (Equation 16.3) was conducted at DuPont. Taylor and Swift referred to hydrocyanation of butadiene, and Drinkard exploited this chemistry for the synthesis of adiponitrile. The mechanism of ftiis process was pursued in depth by Tolman. As a result of this work, butadiene hydrocyanation was commercialized in 1971. The development of hydrocyanation is one of tfie early success stories in homogeneous catalysis. Significant improvements in catalysts have been made since that time, and many reviews have now been written on this subject. ... 

Nickel catalysts for hydrocyanation are poisoned by the formation of dicyanide complexes, LjNifCN). The formation of this material is second order in HCN. Thus, hydrocyanation reactions are t57pically nm under conditions in which HCN is dilute. The presence of added phosphite also helps to minirnize deactivation of the catalyst. 

Kee and coworkers reported that Al(salen) (73a) and Al(salan) (67b) complexes catalyze hydrophosphonylation of benzaldehyde derivatives [70]. Enantioselectivi-ties were modest in the reactions catalyzed by each catalyst. Interestingly, compared to Al(salen) complex (73a), Al(salan) complex (67) results in better enantioselectiv-ity (Scheme 6.55). The structure of (67b) in solution was identified as a dimeric hydroxyl-bridged structure with the twisted ligand geometry. In addition, Jacobsen s Al(salen)Cl (67a), which is well known as an excellent asymmetric catalyst for hydrocyanation of aromatic imines, was not an effective catalyst for this reaction [62]. 

Nickel plays a role in the Reppe polymeriza tion of acetylene where nickel salts act as catalysts to form cyclooctatetraene (62) the reduction of nickel haUdes by sodium cyclopentadienide to form nickelocene [1271 -28-9] (63) the synthesis of cyclododecatrienenickel [39330-67-1] (64) and formation from elemental nickel powder and other reagents of nickel(0) complexes that serve as catalysts for oligomerization and hydrocyanation reactions (65). 

Starting from enantiomerically pure 4-methylsulfanyl-mandelonitrile, thiamphenicol and florfenicol have been enantioselectively synthesized (Figure 5.14). The enantiomerically pure 4-methylsulfanyl-mandelonitrile was obtained by hydrocyanation reaction of 4-methy lsulfany 1-benzaldehyde catalyzed by (M)-hydroxynitrile lyase of Badamu (almond from Xinjiang, China) (Prunus communis L. var. dulcis Borkh), which, after an extensive screening, was found to be a highly effective bio-catalyst for this reaction [85]. 

Nickel is frequently used in industrial homogeneous catalysis. Many carbon-carbon bond-formation reactions can be carried out with high selectivity when catalyzed by organonickel complexes. Such reactions include linear and cyclic oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions [1], Many of the complexes that are active catalysts for oligomerization and isomerization reactions are supposed also to be active as hydrogenation catalysts. 

New catalyst design further highlights the utility of the scaffold and functional moieties of the Cinchona alkaloids. his-Cinchona alkaloid derivative 43 was developed by Corey [49] for enantioselective dihydroxylation of olefins with OsO. The catalyst was later employed in the Strecker hydrocyanation of iV-allyl aldimines. The mechanistic logic behind the catalyst for the Strecker reaction presents a chiral ammonium salt of the catalyst 43 (in the presence of a conjugate acid) that would stabilize the aldimine already activated via hydrogen-bonding to the protonated quinuclidine moiety. Nucleophilic attack by cyanide ion to the imine would give an a-amino nitrile product (Scheme 10). 

The hydrocyanation of butadiene is an important industrial route to adiponitrile (equation 163).602 Again, complex (131) is used as the catalyst for the reaction. The hydrocyanation of dienes proceeds mainly by 1,4-addition and r/ -allyl complexes are believed to be intermediates (Scheme 59).603 The l-cyano-2-butene is then isomerized to l-cyano-3-butene which undergoes further hydrocyanation to give adiponitrile.601"603... 

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... 

Hoveyda, Snapper, and co-workers identified a series of Ti(IV)-based catalysts 10 through systematic screening [53] of modular Schiff base ligands [54, 55], Their methodology proved to be very general for hydrocyanation of ald-... 

The search for other amino acid-based catalysts for asymmetric hydrocyanation identified the imidazolidinedione (hydantoin) 3 [49] and the e-caprolactam 4 [21]. Ten different substituents on the imide nitrogen atom of 3 were examined in the preparation, from 3-phenoxybenzaldehyde, of (S)-2-hydroxy-2-(3-phenoxy-phenyl)acetonitrile, an important building block for optically active pyrethroid insecticides. The N-benzyl imide 3 finally proved best, affording the desired cyanohydrin almost quantitatively, albeit with only 37% enantiomeric excess [49]. Interestingly, the catalyst 3 is active only when dissolved homogeneously in the reaction medium (as opposed to the heterogeneous catalyst 1) [49]. With the lysine derivative 4 the cyanohydrin of cyclohexane carbaldehyde was obtained with an enantiomeric excess of 65% by use of acetone cyanohydrin as the cyanide source [21]. 

For hydrocyanation of MVN with bidentate phosphinites zero-order in substrate and in HCN was again observed. Yet first-order in catalyst was obtained because of the bidentate ligand. The hydrocyanation followed in time should, in theory, fit a simple zero-order model (d[RCN]/dt= ki[Ni]). The catalyst is, however,... 

Over the past half-dozen years, many laboratories have focused their efforts on the development of chiral hydrogen bond donors that function as catalysts for enantioselective organic reactions. One of the earliest successes in this area came from Jacobsen and co-workers, who reported the use of peptide-like chiral urea-based catalysts for the hydrocyanation of aldimines and ketoimines [40, 41]. Several other laboratories have also reported highly enantioselective transformations catalyzed by a chiral hydrogen bond donor. The following sections provide a summary of the many developments in hydrogen bond-catalyzed enantioselective reactions, along with a discussion of mechanisms and selectivity models. 

The Strecker reaction is defined as the addition of HCN to the condensation product of a carbonyl and amine component to give a-amino nitriles. Lipton and coworkers reported the first highly effective catalytic asymmetric Strecker reaction, using synthetic peptide 43, a modification of Inoue s catalyst (38), which was determined to be inactive for the Strecker reactions of aldimines (see Scheme 6.5) [62], Catalyst 43 provided chiral a-amino nitrile products for a number of N-benzhydryl imines (42) derived from substituted aromatic (71-97% yield 64->99% ee) and aliphatic (80-81% yield <10-17% ee) aldehydes, presumably through a similar mode of activation to that for hydrocyanations of aldehydes (Table 6.14). Electron-deficient aromatic imines were not suitable substrates for this catalyst, giving products in low optical purities (<10-32% ee). The a-amino nitrile product of benzaldehyde was converted to the corresponding a-amino acid in high yield (92%) and ee (>99%) via a one-step acid hydrolysis. 

When certain cyclodipeptides are used as catalysts for the enantioselective formation of cyanohydrins, an autocatalytic improvement of selectivity is observed in the presence of chiral hydrocyanation products [80]. A commercial process for the manufacture of a pyrethroid insecticide involving asymmetric addition of HCN to an aromatic aldehyde in the presence of a cyclic dipeptide has been described [80]. Besides HCN itself, acetone cyanohydrin is also used (usually in the literature referred to as the Nazarov method), which can be activated cata-lytically by certain lanthanide complexes [81]. Acetylcyanation of aldehydes is described with samarium-based catalysts in the presence of isopropenyl acetate cyclohexanone oxime acetate is hydrocyanated with acetone cyanohydrin as the HCN source in the presence of these catalytic systems [82]. 

Ditertiary phosphines such as (86), (92), and (98) (100) (Scheme 6) have found important uses as ligands for metal-catalyzed transformations, including e.g., palladium-catalyzed Grignard cross couplings,194,205 rhodium-catalyzed Michael additions,2 hydrocyanations,206 copolymerizations,20 and palladium-catalyzed animations.208 Rhodium complexes of (86) are catalysts for the carbonylation of methanol.188 More recently the ligand bite angle of ditertiary phosphines such as (100) has been shown to influence catalytic activity/selectivity in several important catalytic processes.209-213... 

Homochiral 3,3 -dimethyl-2,2 -bisquinoline N,N -dioxide serves as a Lewis base catalyst for asymmetric hydrocyanation of aromatic N-(diphenylmethyl)imines, although the enantioselectivity is still modest (37-77% ee) [659]. 

Asymmetric hydrocyanation of ketimines with TMSCN, a more challenging subject, has been reported by Vallee et al. They investigated the utility of chiral Ti-BI-NOL complexes for hydrocyanation of the N-benzylketimine derived from acetophenone [663]. The best result (80% conversion, 56% ee) was obtained by catalytic use of Ti(Oi-Pr)2(BINOL) (10 mol%) in the presence of TMEDA (20 mol%). More recently they have found that Sc(BINOL)2Li works as an efficient chiral catalyst for the same hydrocyanation (10 mol% of the catalyst >95% conversion, 88% ee) [664]. 

Cyclic dipeptides, especially cyclo[(S)-phenylalanyl-(S)-histidyl], are efficient and selective catalysts for the hydrocyanation of aromatic aldehydes (Fig. 5) [41]. The catalysts are not available commercially but can be synthesized by conventional methods and their structure can be varied easily (Fig. 5) [41,42,43]. The catalysts are only selective in a particular heterogeneous state, described as a clear gel [41,43]. It seems that their method of precipitation is crucial [41,44] and that reproducing literature results is not always easy [42]. A recent study confirmed the importance of the aggregate formation and reported a second order rate dependence on the concentration of the cyclic dipeptide [45]. These findings indicate that the enantioselective catalytic species is not monomeric but either a dimer or polymer. 

Nonactivated olefins fail to react even under strenuous conditions with cyanide anion catalysis. Due to this lack of reactivity coupled with the inherent desirability of the products, much research has focused on developing catalysts for the hydrocyanation of these nonactivated olefins. This has led to nickel, palladium, copper, and cobalt-based catalysts effective at 25-125°C with or without a solvent. Most were developed for the hydrocyanation of unactivated olefins, but many are equally applicable for oAer olefins. For example, much work has been reported on butadiene hydrocyanation employing all of the catalysts mentioned above except palladium. 

These catalyst systems are effective for hydrocyanation of only 1,3-butadiene (see Table 1). Substituted butadienes give lower yields, and most other activated or nonac-tivated olefins do not react, although low yields of hydrocyanation products are obtained from vinyl ethers . An extension of this chemistry is the conversion of butadiene to trans-... 

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