1.3- Butadiene catalysts, nickel complexes

The catalytic cyclo-oligomerization of 1,3-butadiene mediated by transition-metal complexes is one of the key reactions in homogeneous catalysis.1 Several transition metal complexes and Ziegler-Natta catalyst systems have been established that actively catalyze the stereoselective cyclooligomerization of 1,3-dienes.2 Nickel complexes, in particular, have been demonstrated to be the most versatile catalysts.3... 

Scheme 6. Interplay of the C8- and C -production channels for the cyclo-oligomerization of 1,3-butadiene with zero valent PR3/P(OR)3-stabilized nickel complexes as the catalyst. Free energies (AG, AGJ in kcalmol-1) are given relative to the favorable rf-synrfiC A-cis isomer of 2a for catalysts bearing strong a-donor ligands namely I (L = PMe3), III (L = PPrj), VI (L = PBU3), and -acceptor ligands namely V (L = P(OMe)3), IV...

Among transition metal complexes used as catalysts for reactions of the above-mentioned types b and c, the most versatile are nickel complexes. The characteristic reactions of butadiene catalyzed by nickel complexes are cyclizations. Formations of 1,5-cyclooctadiene (COD) (1) and 1,5,9-cyclododecatriene (CDT) (2) are typical reactions (2-9). In addition, other cyclic compounds (3-6) shown below are formed by nickel catalysts. Considerable selectivity to form one of these cyclic oligomers as a main product by modification of the catalytic species with different phosphine or phosphite as ligands has been observed (3, 4). 

These telomerization reactions of butadiene with nucleophiles are also catalyzed by nickel complexes. For example, amines (18-23), active methylene compounds (23, 24), alcohols (25, 26), and phenol (27) react with butadiene. However, the selectivity and catalytic activity of nickel catalysts are lower than those of palladium catalysts. In addition, a mixture of monomeric and dimeric telomers is usually formed with nickel catalysts ... 

Unlike nickel Catalysts, palladium complexes do not catalyze the homo-cyclization reaction to give CDT or COD. The difference seems to be due to a different degree of hydride shift and atomic volume. With palladium catalysts, the hydride shift is easier, and hence linear oligomers are formed. The characteristic reaction catalyzed by palladium is the cocyclization of two moles of butadiene with one-hetero atom double bonds such as C=N and C=0 bonds to give six-membered rings with two vinyl groups (19) ... 

The increased cooling efficiency of thin-walled reactors also has permitted the use of more volatile substrates in near molar quantities. (l-3 6-7 10-12-rj 2,6,10-Dodecatriene-l,12-diyl)nickel has been prepared in multiple gram quantities by cocondensation of nickel vapor and 1,3-butadiene. This method has provided a clean one-step route to this complex, which was first isolated and identified by Wilke et al.1 as an intermediate in the cyclotrimerization of 1,3-butadiene by nickel catalysts. 

Homogeneous nickel complexes proved to be versatile catalysts in dimerization and trimerization of dienes to yield different oligomeric products.46-55 Depending on the actual catalyst structure, nickel catalyzes the dimerization of 1,3-butadiene to yield isomeric octatrienes, and the cyclodimerization and cyclotrimerization to give 1,5-cyclooctadiene and all-trans-l,5,9-cyclododecatriene, respectively46 56 [Eq. (13.13)]. Ziegler-type complexes may be used to form cis,trans,trans-1,5,9-cyclododecatriene37,57 58 [Eq. (13.14)], which is an industrial intermediate ... 

At normal temperatures methyl crotonate does not react with butadiene in the presence of either naked-nickel or the nickel-ligand catalyst. Moreover, since no oligomerization of the butadiene occurs, it is probable that the formation of a stable nickel complex renders the catalyst inactive. 

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. ... 

The synthesis of hexa-1,4-diene has been achieved by the nickel-catalyzed homogeneous addition of ethylene to butadiene. The nickel is introduced in the form of the complex Ni[P(OEt)3]4. The reaction is carried out in acid media, and the active catalyst is the cationic complex NiH[P(OEt)3]3 which is a 16-electron molecule. In Fig. 7 the sequence of reactions that leads to the catalytic formation of isomeric hexadienes is... 

Zero-valent nickel complexes are known to reduce 1,2-dihalides to olefins and to mediate C,C-coupling reactions of vinyl halides. Based on these facts, lyoda and coworkers developed a two-step, one-pot synthesis of alkyl-substituted [4]radialenes which starts from 2,3-dihalo-l,3-butadienes and 1,4-dichloro-2-butyne derivatives and circumvents the isolation of the butadiene intermediates. Furthermore, the synthesis can be made catalytic in nickel when the Ni(0) complex is generated from NiBr2(PPh3)2 with a more than stoichiometric quantity (based on the dihalide) of zinc. Again, the formation of radialene 94 must compete with that of 95 and 96. With preformed Ni(PPh3)4 and Ni(PBu3)4, the [4]radialene is normally favored in benzene solution, but formation of 95 and/or 96 becomes important in the more polar solvents THF and DMF. With a catalyst... 

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... 

Use of nickel carbonyl to add 1 mol of HCN to 1,3-butadiene may be the first example of hydrocyanation by a homogeneous nickel catalyst. That work also recorded the important observation that substantial improvement in nitrile product yield results from conducting the reaction in the presence of ( 115)3 or (C H5)3As. This work led to extensive studies to develop effective nickel hydrocyanation catdysts. Virtually all subsequent developments have focused on finding the most effective nickel complex and the identification and application of promoters to improve catalyst efficiency and life. ... 

Scheme 3 Condensed Gibbs free-energy profile (kcal mol ) of the complete catalytic cycle of the co-oligomerization of 1,3-butadiene and ethylene catalyzed by zerovalent bare nickel complexes affording linear and cyclic Cio-olefins, focused on viable routes for individual elementary steps. The favorable [Ni (ri -frans-butadiene)2(ethylene)] isomer of the active catalyst species lb was chosen as reference. Activation barriers for individual steps are given relative to the favorable stereoisomer of the respective precursor (given in italics)...

Scheme 4 Interplay of the alternative reaction channels for production of CiQ-olefins via butadiene/ethylene co-oligomerization and of Ci2-olefins via butadiene cyclooligomerization with zerovalent bare nickel complexes as the catalyst. Gibbs free energies (kcal mol ) are reported relative to the favorable [Ni (ip -syM,ip (C ))A-c s,-octadienediyl)(ethylene)] isomer of 2, while activation barriers are given relative to the respective precursor...

Oligomerisation.—Much detailed discussion, based to a large extent on product characterisation and distribution observations, has appeared on catalysis of olefin oligomerisation by nickel complexes. Thus for the first example 7r-2-butenylnickel chloride, in contrast to the analogous 7r-allyl compound, is not a good catalyst for polymerisation of butadiene. But in the presence of trichloroacetate the 7r-2-butenyl complex does exhibit catalytic activity. This enhancement of activity is ascribed to the intermediacy of charge-transfer complexes. The mechanism of cyclotrimer-isation of butadiene has been considerably clarified by the characterisation of an intermediate (21) and determination of the distribution of isomers... 

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. ... 

Although the linear dimerization of butadiene has been conducted with many catalyst systems, one of the best understood and most specific systems is the nickel(0)-tri-ethylphosphite-morpholine catalyst developed by Heimbach (Scheme 22.19). Coupling of butadiene by Ni(0) produces a bis-(-r -allyl) nickel complex. Protonation of an internal position of one of the T) -allyl groups by morpholine, followed by proton abstraction a to the other Tfj -allyl group, produces the observed octatrienes and regenerates the catalyst. 

Numerous complexes of nickel(II) and nickel(O) catalyze the addition of the Si—H bond to olefins. Among such catalysts are nickel-phosphine complexes, such as [Ni(PR3)2X2] (where X = Cl, I, and NO3 R = alkyl and aryl), [Ni(PPh3)4], and [Ni(CO)2(PPh3)2], as well as bidentate complexes [NiCl2-(chelate)], [Ni(acac)2L] (L = phosphine), and [Ni(cod)2(PR3)2l (3,6,10,64). Ni(0)-phosphine complexes were used for the hydrosilylation of alkenes and butadienes (65,66). Cationic nickel complexes, such as [(indenyl)Ni(PPh3)]+ (67) and [Ni( r-allyl)PR2(CH2CH=CH2)]+ (68), were reported as novel effective catalysts of regioselective hydrosilylation of styrene with PhSiHa. 

In the hydrocyanation of butadiene, 2 mol of HCN are added to butadiene with a nickel complex as catalyst to obtain adiponitrile directly. 

---