Nickel phosphite complexes

Tolman s study of the equilibrium [Eq. (10)] showed that none of the simple fluorinated olefins C2HnF4 were as good as C2H4 in coordinating to the nickel phosphite complex Ni[P(0-o-tolyl)3]3. 

Before discussing hydrocyanation chemistry we will explore the interaction of zero-valent nickel phosphite complexes with various independent components of the catalytic system. Then, in turn, we will examine the catalyzed addition of HCN to butadiene, the isomerization of olefins, and the addition of HCN to monoolefins. Finally, a summary of the mechanism as it is now understood will be presented. 

Various complexes of Cu, Ni, and Pd, especially nickel phosphite complexes, such as Ni[P(0-o-C6H4Me)3]4, are active for the addition of HCN to alkenes and alkynes. Hydrogen cyanide will oxidatively add to low-valent metal complexes,... 

The biphosphite ligands, (5) and (6), react with [(cod)2Ni] to form nickel complexes of type (7). Nickel phosphite complexes are catalysts in the hydrocyanation of butadiene complex (7) is more robust than the monodentate phosphite analogs. ... 

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... 

The nickel phosphite complex, Ni P(OEt)3)4, is important industrially since it will catalyse the coupling of butadiene and ethylene to give hexadiene (12.362). 

The catalyst is similar for all three steps, and consists of a zero valent nickel phosphite complex, promoted with zinc or aluminium chlorides. The direct addition of hydrogen eyanide to butadiene is particularly attractive with the availability of by-product hydrogen cyanide form the manufactnie of acrylonitrile by the ammoxidation of propylene. 

A variety of different routes to cyclobutane systems involving non-photochemical means has also been devised. Acid catalyzed rearrangement of fused cyclopropyl ether (61) by Wenkert et al. (97, 98) afforded the bicyclic dione (62), which through a thioketal-desulfurization process and treatment with hydroxylamine yielded oxime (58) (Scheme 12). The shortest synthesis of grandisol is that reported by Billups et al. (Scheme 13) (99). Dimerization of isoprene in the presence of a zero-valent bis-cyclooctadienyl-nickel-phosphite complex gave the cis-cyclobutane diolefin (65), which could be separated in 12—15% yield from the complex product mixture by low-temperature distillation. Selective hydroboration and oxidation afforded grandisol. 

Substituted Nickel Carbonyl Complexes. The reaction of trimethyl phosphite and nickel carbonyl yields the monosubstituted colorless oil, (CO)2NiP(OCH )2 [17099-58-0] the disubstituted colorless oil, (CO)2Ni[P(OCH )2]2 [16787-28-3] and the trisubstituted white crystalline soHd,... 

The formation of cationic nickel hydride complexes by the oxidative addition of Brdnsted acids (HY) to zero-valent nickel phosphine or phosphite complexes (method C,) has already been discussed in Section II. Interesting in this connection is a recent H NMR study of the reaction of bis[tri(o-tolyl)phosphite]nickelethylene and trifluoroacetic acid which leads to the formation of a square-planar bis[tri(o-tolyl)phosphite] hydridonickel trifluoroacetate (30) (see below) having a cis arrangement of the phosphite ligands (82). 

First we will describe the hydrocyanation of ethene as a model substrate. The catalyst precursor is a nickel(O) tetrakis(phosphite) complex which is protonated to form a nickel(II) hydride. Actually, this is an oxidative addition of HCN to nickel zero. In Figure 11.1 the hydrocyanation mechanism in a simplified form is given the basic steps are the same as for butadiene, the actual substrate, but the complications due to isomer formation are lacking. 

Nickel-catalysed addition of HCN to butadiene was developed by du Pont for adiponitrile production [81]. A Ni(0)-phosphite complex is used as the catalyst in the presence of Lewis acids. Oxidative addition of HCN to Ni(0), followed by insertion of butadiene, generates 7r-allyl intermediate 187. Reductive elimination of 187 yields 188 and 189, and isomerization of the double bond in 189 to the terminal position gives 4-pentenonitrile (190). Then, insertion of 190 to H—Ni—CN affords adiponitrile (191). 

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],... 

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... 

TRIARYL PHOSPHITE COMPLEXES OF COBALT, NICKEL, PLATINUM, AND RHODIUM... 

Industrial uses of HCN are for synthesis of methyl methacrylate and to form adiponitrile (for adipic acid and nylon) by addition to 1,3-butadiene in the presence of nickel(O) phosphite complexes. Waste HCN is also oxidatively hydrolyzed to give oxamide for use as fertilizer. 

Ligand size determines coordination numbers as well as reactivity. Thus phosphine complexes of Pd°, frequently used as precursors in palladium catalyzed reactions, may be 4-, 3-, or 2-coordinate, as in Pd(PMe3)4, Pd(PPr )3, and Pd(PPhBu 2)2, respectively. For nickel phosphine and phosphite complexes, the dissociation constant Kd for the equilibrium... 

The first addition is relatively easy and numerous catalysts have been reported for this reaction 9). In practice, nickel(O) complexes associated with phosphites as ligands, are the most convenient catalysts. 

The DuPont ADN process involves hydrocyana-tion of butadiene (equations 5-8), catalyzed by air-and moisture-sensitive triarylphosphite-nickel(O) complexes [Ni(P(OAr)3)4]. The nickel is zero valent (see Oxidation Number) because it has its full complement of 10 electrons beyond the preceding inert gas (Ar) configuration, and the catalysts can actually be made directly from nickel metal and the phosphite ligands (see P-donor Ligands). The four phosphorus atoms each contribute an electron pair to give a total of 18 electrons, corresponding to the next inert gas (Kr), (see Effective Atomic Number Rule and Electronic Structure of Organometallic Compounds). 

---