Dienes homogeneous catalysis

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... 

Nickel is frequently used in industrial homogeneous catalysis. Many carbon-carbon bond-formation reactions can be carried out with high selectivity when catalyzed by organonickel complexes. Such reactions include linear and cyclic oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions [1], Many of the complexes that are active catalysts for oligomerization and isomerization reactions are supposed also to be active as hydrogenation catalysts. 

Homogeneous Catalysis through Heterogeneous Ru Carbenes 471 Tab. 11.1 Ring-closing metathesis of acyclic dienes by Ru complex (10). ... 

Another example is butene dimerization catalyzed by nickel complexes in acidic chloroaluminates 14). This reaction has been performed on a continuous basis on the pilot scale by IFF (Difasol process). Relative to the industrial process involving homogeneous catalysis (Dimersol process), the overall yield in dimers is increased. Similarly, selective hydrogenation of diene can be performed in ionic liquids, because the solubility of dienes is higher than that of monoene, which is higher than that of paraffins. In the case of the Difasol process, a reduction of the volume of the reaction section by a factor of up to 40 can be achieved. This new Difasol technology enables lower dimer (e.g., octenes) production costs 14). 

Such diene complexes can be used to prepare homogeneous hydrogenation catalysts in situ, especially where a variable tertiary phosphine/rhodium ratio is required3 or where an asymmetric tertiary phosphine is employed for asymmetric synthesis.4 The cyclooctadiene complex is also the starting point for the preparation a number of complexes of the type [Rh(l, 5-C8H12)L2]+ (L represents a variety of P— and N— donor ligands) of interest in homogeneous catalysis.s... 

Table IV lists some diene polymers prepared by homogeneous catalysis. The cobalt catalyst for butadiene and the lithium catalyst for butadiene and isoprene are believed to be used commercially in the United States to prepare the so-called stereoelastomers (32, 36).

As mentioned above, many transition metals catalyze the cyclooligomerization of 1,3-dienes. The nickel-catalyzed cyclooligomerization of BD, however, is probably one of the best-understood reactions in the field of homogeneous catalysis. In the 40 years since its discovery a mass of evidence has been collected, indicating that these oligomerizations are the result of a multistep addition-elimination mechanism at a nickel atom template, which constantly flips between two oxidation states. The following strategies played an important role isolation of key intermediates, simulation of the catalytic cycle in a stoichiometric manner, product analysis, and study of model compounds. Detailed analysis of the intellectual development of the mechanism is not included here as this can be followed from excellent reviews [6]. 

The great importance of group VIII complexes in homogeneous catalysis has focused more attention on their insertion chemistry, but the number of authenticated insertions of monoalkenes into M-H bonds is small. Insertions of dienes and allenes are more numerous because of the greater thermodynamic stability of the product xt-allyl complexes. Some examples of insertions of complexes of group VIII are listed in Table... 

Heterogeneous catalysis (Pd) in tritiation of l,4-dien-3-ones leads mainly to l) -tritiation (ca. 76%). Tritium distribution studies indicate that the reaction is probably a 1,4-addition of tritium onto the more exposed p-face of the dienone, which is tilted downwards with respect to ring b (211). Homogeneous catalysis with tris(triphenylphosphine)rhodium chloride, however, is known to favour la-tritiation. Similarly, homogeneous hydrogenation of the less reactive... 

Because of the success experienced in organic synthesis by using phase transfer reaction conditions IS), it was expected that this would be an excellent method for hydrogenating organic soluble dienes by using the water soluble catalyst K3[Co(CN)5H]. Others recently have carried out homogeneous catalysis processes under phase-transfer reaction conditions 14,15,16). 

These unusual properties were the basis of the fluorous biphasic catalysis process (FBC) first published in 1994 by Horvdth and Rdbai and demonstrated using hydroformylation chemistry as a pertinent example (7, 2) in a 1991 Ph.D. thesis, that was unfortunately not readily available to the homogeneous catalysis community nor published in the open literature, M. Vogt, under the guidance of his Ph.D. advisor, W. Keim, of the Rheinisch-WestflUischen Technischen Hochschule in Aachen, Germany, presented the first conceptual aspects of the FBC approach with an emphasis on oligomerization of alkenes, oxidation of alkenes, hydroformylation of olefins, and telomerization of dienes (5, 4). 

Many examples of alkene and alkyne insertion into metal-carbon bonds can also be found in the section on homogeneous catalysis. Other recent examples include the insertion of conjugated dienes into palladium-allyl bonds, olefin arylation in the presence of palladium acetate, and the reaction of ethylene with arylmagnesium halides in the presence of nickel chloride. Reaction of isocyanates with nickel-ethynyl compounds... 

An enhancement of the selectivity in the dicarboxylation of butadiene has been claimed by Tkatchenko and coworkers [83]. When an ether such as tetrahydrofurane or dimethoxyethane is used as solvent and catalytic amounts of an iron carbonyl complex such as Cp2Fe2(C0)4 are added, the electrocarboxylation of butadiene is achieved with an overall current yield of 76 % and a selectivity of the cis- and trans 3-hexenedioic acid up to 75 %. Possibly, this interesting combination of electrochemistry and homogeneous catalysis will enable further enhancements in the C-C linkage of dienes and carbon dioxide. 

---