Butadiene catalysts, cobalt complexes

The metal carboxylate insertion mechanism has also been demonstrated in the dicobaltoctacarbonyl-catalyzed carbomethoxylation of butadiene to methyl 3-pentenoate.66,72 The reaction of independently synthesized cobalt-carboxylate complex (19) with butadiene (Scheme 8) produced ii3-cobalt complex (20) via the insertion reaction. Reaction of (20) with cobalt hydride gives the product. The pyridine-CO catalyst promotes the reaction of methanol with dicobalt octacarbonyl to give (19) and HCo(CO)4. 

Donors have been added to the cobalt catalysts used to polymerize butadiene (20). Cobalt chloride-pyridine complexes gave a soluble catalyst with AlEt2Cl which was effective for cis polymerization of butadiene, but at the low concentrations of pyridine employed (Al/Co/py = 1000/1/1 to 4), there was no effect on the polymer structure. However, it was observed that the molecular weight fell as the ratio of pyridine to cobalt was increased. Isopropyl ether in the cobalt octoate-methylaluminum sesquichloride catalyst had a similar effect, although at the highest ether concentrations (typical ratios employed were Al/Co/iPr20 = 160/1/3.5 to 8), a reduction in cis content and polymerization... 

In coordination polymerization it is generally accepted that the monomer forms a 7r-complex with the transition metal prior to insertion into the growing chain. In general these complexes are insufficiently stable to be isolated although complexes of allene [69] and butadiene [70] have been reported. With allene the complex was formed prior to polymerization with soluble nickel catalysts, and cis coordinated butadiene forms part of the cobalt complex, CoCj 2H19, which is a dimerization cateilyst. 

This simple division into polymerization and dimerization catalysts does not apply if the growing chain stabilizes as a jt-allyl—metal complex, as is the case with conjugated diolefins (i ). Certain cobalt complexes, for instance, are perfectly able to polymerize butadiene (19). 

Some of the evidence for such structures comes from the change in product distribution of the butenes as a function of cyanide concentration when butadiene is hydrogenated with pentaeyanocobaltate(II) catalyst or when the a butenyl complex is reduced with the hydride complex [HCo(CN)5] . Thus 1-butene is the major product in the presence of excess CN, and major product in the absence of excess cyanide. The 1-butene presumably arises from the cleavage of a tr complex, and the 2-butene via an intermediate w-allyl complex. The Tr-allyl complexes of cobalt tricarbonyl are well-characterized and can be prepared either from butadiene and HCo(CO)4 or from methallyl halide and NaCo(CO)4 [49). 

Some of these derivatives are useful catalysts for the codimerization of dienes with acrylic esters (82, 138, 140, 143). The reaction between cobalt vapor and butadiene is complex, and the nature of the products remains to be elucidated. However, there is a report of the synthesis of the yellow complex HCo(C4H6)2 from the condensation of a mixture of C4H and Me3CH with cobalt vapor (104, 110). 

Cobalt Ziegler catalysts were also used for the manufacture of 1.2-polybutadiene by Susa (48). He obtained syndiotactic 1.2-poIy-butadiene from catalysts of extremely high ratios of triethylaluminum to cobalt chloride-dipyridine complex. However, as the anionic character of the catalyst system was decreased by the introduction of diethylalumi-num chloride, the 1.2 syndiotactic polymer decreased and increasing... 

Reduction of a cobalt(II) halide in presence of 2,2 -bipyridyl with zinc in THF-ethanol leads to cobalt(I)-bipyridyl complexes which hydrogenate butadiene to cis-2-butene at 25 °C and normal pressure of hydrogen. For different halides the rate decreases in the order I>Br>Cl. 1,10-Phenanthroline complexes were also active.64 Here again, the catalyst does not tolerate an excess of diene. The proposed mechanism for the hydrogenation is given in Scheme 4. 

Butadiene dimerization catalysts have been quite extensively studied, although the major effort has been concentrated on homogeneous catalyst systems using complexes containing nickel (167), iron (168), cobalt (169), and palladium (170). 

It is now well established that many cobalt compounds activated with an alkylaluminum halide may induce polymerization of butadiene to a polymer of over 96% cis-1,4 content (17, 22, 25, 28, 29, 36, 37). To obtain a catalyst of high catalytic activity, it is desirable to use cobalt compounds that are soluble in the polymerization solvent such as cobalt naphthenate or a complex of C0CI2 with A1C13 (37) or pyridine (28). An effective catalyst is also formed by dissolving C0CI2 in ethanol (solubility, 54 grams in 100 ml.) and dispersing this solution in the polymerization solvent (6). 

Catalysts from Group VIII metals have given unsatisfactory results. In the polymerization of butadiene with soluble cobalt catalysts tritium is not incorporated when dry active methanol is employed [115], although it is combined when it has not been specially dried [117, 118]. Alkoxyl groups have been found when using dry alcohol [115, 119] but the reaction is apparently slow and not suited to quantitative work [119]. Side reactions result in the incorporation of tritium into the polymer other than by termination of active chains [118], probably from the addition of hydrogen chloride produced by reaction of the alcohol with the aluminium diethyl chloride [108], Complexes of nickel, rhodium and ruthenium will polymerize butadiene in alcohol solution [7, 120], and with these it has not been possible to determine active site concentrations directly. 

Active catalysts for butadiene polymerization are obtained from aluminium alkyl halides and soluble Co and Co salts and complexes. The structure of the organic grouping attached to the cobalt is not important, but compounds most widely employed are acetylacetonates and carboxylic acid salts such as the octoate and naphthenate. The activity of the catalyst and structure of the polymer are affected by the groupings in the complex. Catalysts from aluminium trialkyls and cobalt salts other than halides are relatively unstable and give syndiotactic 1,2-polybutadiene. If halogens are present, e.g., from CoClj or CoBrj,... 

Allyl complexes of cobalt in the oxidation states III and I have been identified as stmcturally defined catalysts for stereospecific butadiene polymerization. 

On the other hand, the well-known 5-methylheptatrienyl-butadiene-cobalt(I) complex [Co( /, 7 -CH3C7Hio)( 7 -C4H6)] has been proved recently to be a very highly active catalyst for the formation of syndiotactic 1,2-polybutadiene [51]. The activity increases strongly with the acceptor properties of the solvent in the order heptane < toluene < dichloromethane < carbon disulfide, and can be extremely enhanced by the addition of alumoxane. 

Acylmetal complexes generally react with conjugated dienes to produce useful organic compounds. For example, acylcobalt carbonyls add to butadiene, producing 1 -acylmethyl-7r-allylcobalt derivatives, which then undergo elimination of the elements of HCo(CO)3 on treatment with base to give 1-acyl-diene u6). This reaction can be made catalytic with respect to the cobalt catalyst under proper reaction conditions. 

Co2(CO)8 and HCo(CO)4 using HP-IR. A cobalt carbonyl-butadiene complex and an allylcobalt carbonyl species were formed from the reaction of butadiene with Co2(CO)8. Reaction with HCo(CO)4 gradually afforded an alkenyl complex which reacted to form further byproducts. The reaction of cobalt carbonyls with hydrogen or deuterium, and the stoichiometric interaction of the resultant hydride with alkenes has also been studied with HP-IR. Ojima reported HP-IR studies into Co-Rh bimetallic carbonyl catalysts, and observed a CoRh(CO)7 species which was active for hydroformylation-amidocarbonylation. 

The mechanism of trimerisation of butadiene by a mixed cobalt(ii) chloride-aluminium triethyl catalyst has been inferred from the natures of the three products characterised. The determination of the enthalpy of dimerisation of aluminium triethyl provides a useful piece of thermochemical data for quantitative discussion of the role and energetics of aluminium triethyl in this type of reaction. Polymerisation of isoprene in the presence of Fe(acac)3-aluminium triethyl-pyridine derivatives mixtures has a negative apparent activation enthalpy, which can be attributed to the instability of the catalytic complex at elevated temperatures. Bis-cyclo-octatetraeneiron(o) is an effective oligomerisation catalyst. The composition of products accessible only by hydrogen migration indicates an oxidative addition-reductive elimination mechanism rather than insertion. 

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. ... 

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