Butadiene complexes with cobalt

Most organopentacyanides are stable towards [Co(CN)jH], with the exception of allyl complexes which react to liberate propylene derivatives (105). This is one of the steps in the homogeneous hydrogenation of butadienes catalyzed by cobalt cyanide complexes (see Section VII,A). 

Highly c/s-selectivity and low molecular weight distribution polymerization of l -butadiene with cobalt(II) pyridyl bis(imine) complexes in the presence of ethylaluminum sesquischloride effect of methyl position in the ligand... 

The Co2(CO)g/pyridine system can catalyze carbomethoxylation of butadiene to methyl 3-pentenoate (Eq. 6.44) [80]. The reaction mechanism of the cobalt-catalyzed carbalkoxylation of olefins was investigated and the formation of a methoxycar-bonylcobalt species, MeOC(0)Co from a cobalt carbonyl complex with methanol as an intermediate is claimed [81, 82]. 

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). 

The tris-allyl complex, in each case, produced a 1.2 growth step of the butadiene molecule. With the more anionic (or less cationic) cobalt salt, the growth occured to only the dimer before it underwent anionic hydride chain transfer. With less anionic chromium the 1.2 chain growth continued on the produce polymer. 

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. 

Evidence that the butadiene is jr-complexed to cobalt prior to addition to the growing cation was obtained by van de Kamp (365). Aromatic jr-electron donors were found to compete with the butadiene for the cobalt in an equilibrium reaction which decreased polymerization rate. The order of jr-donor strength, K, was determined for a variety of substituted aromatics, taking butadiene as unity. Except for the inversion of mesitylene and durene, these constants are in the same order as the relative basicities determined by equilibrium studies in strong acids (84). 

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. 

An important question in light of the ease of chelation in the synthesis of the carbonyl complexes is whether it is possible to decoordinate the phos-phane arm, possibly to create a vacant coordination site for further chemistry. The question was addressed by treatment of 327 with 1,5-cyclooctadiene under photochemical reaction conditions, using the diene as the solvent, and resulted in a 41% yield of nonchelated cyclooctadiene complex 336 (Scheme 61). Treatment of this complex with diphenylethyne under reaction conditions normally allowing alkyne di- or trimerization reactions gave tetraphenylcyclo-butadiene complex 337 in 64% yield, showing that chemistry at the cobalt atom is possible without inhibition by a chelating phosphane arm. ... 

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. 

Putting aside for the moment the question of the role of a pro-tic substance, such as water, it is evident, according to Sinn et al. (1961), that the important role of cobalt is to orient the monomer in the cis conformation. This conclusion is supported by complexing studies with aromatic ir-electron donors, such as mesit-ylene and durene. These aromatic compounds compete with the butadiene for the cobalt in an equilibrium reaction, thus decreasing the polymerization rate (Van de Kamp, 1962). 

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. ... 

Since the aging reaction of cyanocobaltate(II) results in the formation of hydrido complex, the question arises as to which cobalt species is involved in the absorption of butadiene. If the hydride is the reactive species, absorption would be expected to increase with time. In Figure 3 it may be seen that the absorption of butadiene by cyanocobaltate(II) does increase with time in a manner paralleling the decrease in hydrogen absorption capacity (12). 

Reactions with Hydrido Complex. Upon injection of a prehydrogenated cyanocobaltate(II) solution (0.15M cobalt, CN/Co = 6.0) into an atmosphere of butadiene, the gas was rapidly absorbed, 0.92 mole of butadiene being taken up for each hydrogen atom previously absorbed. Similarly, when the injection was made into a butadiene-saturated cyanocobaltate(II) solution in a butadiene atmosphere, 1.08 moles of butadiene were absorbed. These results provide evidence of the addition of butadiene to the hydrido complex in the following manner ... 

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. 

Cobalt complexes find various applications as additives for polymers. Thus cobalt phthalocyanine acts as a smoke retardant for styrene polymers,31 and the same effect in poly(vinyl chloride) is achieved with Co(acac)2, Co(acac)3, Co203 and CoC03.5 Co(acac)2 in presence of triphenyl phosphite or tri(4-methyl-6- f-butylphenyl) phosphite has been found to act as an antioxidant for polyenes.29 Both cobalt acetate and cobalt naphthenate stabilize polyesters against degradation,73 and the cobalt complex of the benzoic acid derivative (12) (see Section 66.4) acts as an antioxidant for butadiene polymers.46 Stabilization of poly(vinyl chloride)-polybutadiene rubber blends against UV light is provided by cobalt dicyclohexyldithiophosphinate (19).74 Here again, the precise structure does not appear to be known. 

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... 

The amine, by blocking one coordination point, prevents two-point coordination of the butadiene in the cis configuration, and monomer enters with only one double bond coordinated to the cobalt atom. It is presumed that in the active intermediate an organic group is in the bridge of the complex (II) (16), and we would represent the polymerization as indicated by III. 

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). 

The [(T74-butadiene)2Co] X complexes shown in Table VII have been prepared by reacting (CH2 CH2)4CoK with butadiene and the appropriate metal halide (59). The two butadiene molecules are equivalent and symmetrically bonded to the cobalt atom one possible structure is 18. The chemical shift of the central carbon atoms in these complexes falls within a narrow range (77-85 ppm) while that of the terminal carbon atom lies... 

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