Hydrocyanations Lewis acids

Because of its low acidity, hydrogen cyanide seldom adds to nonactivated multiple bonds. Catalytic processes, however, may be applied to achieve such additions. Metal catalysts, mainly nickel and palladium complexes, and [Co(CO)4]2 are used to catalyze the addition of HCN to alkenes known as hydrocyanation.l67 l74 Most studies usually apply nickel triarylphosphites with a Lewis acid promoter. The mechanism involves the insertion of the alkene into the Ni—H bond of a hydrido nickel cyanide complex to form a cr-alkylnickel complex173-176 (Scheme 6.3). The addition of DCN to deuterium-labeled compound 17 was shown to take place... 

Among other nonaddition processes, adiponitrile may be manufactured by the direct hydrocyanation of 1,3-butadiene (DuPont process).169 172,187 196 A homogeneous Ni(0) complex catalyzes both steps of addition of HCN to the olefinic bonds (Scheme 6.4). The isomeric monocyano butenes (20 and 21) are first formed in a ratio of approximately 1 2. All subsequent steps, the isomerization of 20 to the desired 1,4-addition product (21), a further isomerization step (double-bond migration), and the addition of the second molecule of HCN, are promoted by Lewis acids (ZnCl2 or SnCl2). Without Lewis acids the last step is much slower then the addition of the first molecule of HCN. Reaction temperatures below 150°C are employed. 

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. 

When HCN is added to 3PN in the presence of NiL4 and a Lewis acid, the onset of isomerization to 4PN is observed before any significant hydrocyanation occurs. Observation of this isomerization reaction over the first few... 

The unpromoted hydrocyanations of monoolefins discussed so far generally involved only a few catalytic cycles on nickel. The development of a practical commercial process depended on getting many cycles. Certain Lewis acids are quite remarkable in increasing (1) catalyst cycles, (2) the linearity of products obtained, and (3) the rates of reaction. The effects depend on the Lewis acid, the phosphorus ligand used, and the olefin substrate (72). 

The mechanism shown in Fig. 9 for the hydrocyanation of ethylene with (C2H4)Ni[PO-o-tolyl)3]2 is inconsistent with the kinetic data described above for 4PN with Ni[P(0-p-tolyl)3]4 and Lewis acid (A). This is not unreasonable when we remember that the equilibrium constant for binding of ethylene to Ni(0) is 70 times greater than that for binding of 4PN (Table II), whereas P(0-p-tolyl)3 is preferred over P(0-o-tolyl)3 by a factor of 108 (Table I) This leads to the possibility that an intermediate such as 19 is much less important in the 4PN/P(0-o-tolyl)3 system. How the Lewis acid changes the mechanism is also still not clear. 

Fig. 16. The mechanism of PN isomerization/hydrocyanation in the absence of a Lewis acid. R"Ni = NCCH2(CH3CH2)CHNi, R" Ni = NC(CH3CH2CH2)CHNi.

The hydrocyanation of 1-hexene catalyzed by [Ni P(OPh)3 4] has been studied.600 Isomerization of the alkene accompanies the reaction. Best rates were obtained in the presence of excess P(OPh)3 and a Lewis acid (ZnCl2). The excess P(OPh)3 was believed to suppress the formation of the cyano complex [Ni(CN)2 P(OPh)3 2], which does not catalyze hydrocyanation. ZnCl2 increases... 

An enantioselective Strecker reaction involving Brpnsted acid catalysis uses a BINOL-phosphoric acid, which affords ees up to 93% in hydrocyanations of aromatic aldimines in toluene at -40 °C.67 The asymmetric induction processes in the stereoselective synthesis of both optically active cis- and trans-l-amino-2-hydroxycyclohexane-l -carboxylic acids via a Strecker reaction have been investigated.68 A 2-pyridylsulfonyl group has been used as a novel stereocontroller in a Strecker-type process ees up to 94% are suggested to arise from the ability of a chiral Lewis acid to coordinate to one of the sulfonyl (g)... 

Figure 3.34 a Formation of the various products in the second hydrocyanation step, co-catalyzed by Lewis acids (LA) b bulky Lewis acids such as BPh3 shift the linear/branched product equilibrium toward the desired linear product. 

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... 

Figure 7.13 First stage of hydrocyanation. Conversion of butadiene to 3PN. Under the reaction conditions 2M3BN is isomerized to 3PN. Interaction of Lewis acid with coordinated nitrile is not shown for clarity. The left and right side involve CN addition in an anti-Markovnikov and Markovnikov manner. L = P(OEt)3 or P(0-o-tolyl)3.

Figure 7.15 Second stage of hydrocyanation. Conversion of 4PN to adiponitrile (ADN) and MGN. A is the Lewis acid. The left loop dominates, giving ADN as the main product. Analogues of 7.45 and 7.46 with Lewis acid are not shown for clarity but are definitely involved.

The active species in the methoxycarbonylation is presumably CoH(CO)4 (in equilibrium with Co2(CO)g and Co(CO)4 ) which adds Co-H 1,4- to the diene this is followed by carbonylation of the Co-C bond, methanolysis of the RCO-Co bond by MeOH or OMe , and regeneration of the hydride. The methoxycarbonylation route to adipic acid is an alternative both to the du Pont (Ni(II)/Lewis acid (BPhs)) catalyzed double hydrocyanation of butadiene (Section 5.4.4) and to the process based on the oxidation of cyclohexane (Section 2.2). 

The mechanism of hydrocyanation by nickel catalysts should proceed through a nickel hydride addition on the double bonds. The nickel hydrides should result from the oxidative HCN addition to the metal, or from the above Lewis acid-assisted dissociation of HCN. The oxidative HCN addition to low-valent metal complexes has been demonstrated, particularly by NMK spectroscopy with Ni(0)(P(OF.t)3 4. 

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 hexene-1 to a mixture of heptanenitrile and 2-methylhexanenitrile, using Lewis acid promoted zero-valent nickel phosphite complexes, has been studied in some detail (277). The presence of a Lewis acid was advantageous since when the zero-valent nickel complex was used alone the rate of hydrocyanation was very slow. The role of excess ligand and the Lewis acid in the reaction appear to be... 

Orthoesters react with hydrocyanic acid, catalyzed by ZnCh, to give 2,2-dialkoxycarbonitriles (424 equation 201)." These nitriles can also be obtained by treatment of orthoesters with acyl cyanides" or trimethylsilyl cyanide in the presence of Lewis acids (BF3, SnCh)." Cyanoselenation of ketene 0,0-acetals affords the nitriles (425 Scheme 77)," " from which other compounds of this type can be prepared, e.g. (426) and (427), by splitting off the phenylselenyl group. The acetonitrile derivative (428 equation 202) is a byproduct (23%) in the photochemical cycloaddition of ketene diethylacetal to 6-cyanouracil." ... 

A number of olefins are readily hydrocyanated in the presence of NiL3 or NiL4 [15], but usually catalyst turnover rates demanded (i.e., the number of moles of product formed per mole of catalyst used) and the selectivity tends to be low. It was found that Lewis acids are effective co-catalysts, which enable the reaction pathway and therefore the reaction selectivity to be piloted and accelerate the rate of hydrocyanation [10]. Investigations on the promoting effect of Lewis acids (e. g., AlCL, ZnCL, BPh3 [14, 44]) imply the formation of a 1 1 complex between Lewis acid and NiL4, since at this ratio the reaction rate reaches a maximum [40, 41, 45]. 

The most outstanding example for the applieation of homogeneously catalyzed hydrocyanation is the DuPont adiponitrile process. About 75 % of the world s demand for adiponitrile is covered by hydrocyanation of butadiene in the presence of nickel(O) phosphite species. This process is discussed for the addition of HCN to dienes as an example, because in this case a well-founded set of data is available. Though it was Taylor and Swift who referred to hydrocyanation of butadiene for the first time [45], it was to Drinkard s credit that this principle was fully exploited for the development of the DuPont adiponitrile process [18]. The overall process is described as the addition of two equivalents of HCN to butadiene in the presence of a tetrakisphosphite-nickel(O) catalyst and a Lewis acid promoter. A phosphine-containing ligand system for the catalyst is not suitable, since addition of HCN to the tetrakisphosphine-nickel complex results in the formation of hydrogen and the non-aetive dicyano complex [67], In general the reaction can... 

In the first stage Lewis acids are absent and further hydroeyanation of the monoolefm products 3-PN 40 and 2M3BN 41 does not readily oeeur. The monoeyanation of butadiene is similar to HCN addition to olefins. An individual feature of hydrocyanation of conjugated dienes is the intermediate appearance of TT-allylic complexes 43, which participate in the successive carbon-carbon coupling. Equations (12) and (13) demonstrate the reaction of butadiene with the hydrido-nickel complex 42 leading to formation of nitrile 40 (a) and explain the generation of byproducts, i.e., the branched nitrile 41 via an alternative pathway (b) [68-70]. 

The hydrocyanation of 1,3-cyclohexadiene does not require the presence of Lewis acids, in contrast to the results found in the case of HCN addition to mono-olefins [11, 74, 75]. 

The assymetric Strecker reaction of diverse imines, including aldimines as well as ketoimines, with HCN or TMSCN provides a direct access to various unnatural and natural amino acids in high enantiomeric excesses, using soluble or resin-linked non-metal Schiff bases the corresponding chiral catalysts are obtained and optimized by parallel combinatorial library synthesis [93]. A rather general asymmetric Strecker-type synthesis of various imines and a, 9-unsaturated derivatives is catalyzed by chiral bifunctional Lewis acid-Lewis base aluminum-containing complexes [94]. When chiral (salen)Al(III) complexes are employed for the hydrocyanation of aromatic substituted imines, excellent yields and enatio-selectivities are obtained [94]. 

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