Co-dimerization

An example of a biphasic, Ni-catalyzed co-dimerization in ionic liquids with weakly coordinating anions has been described by the author s group in collaboration with Leitner et al. [12]. The hydrovinylation of styrene in the biphasic ionic liq-uid/compressed CO2 system with a chiral Ni-catalyst was investigated. Since it was found that this reaction benefits particularly from this unusual biphasic solvent system, more details about this specific application are given in Section 5.4. 

Effect of dimer formation on deactivation. Another possible mode of deactivation is formation of inactive Co dimers or oligomers. To test for these species, we examined the ESI-mass spectram of fresh and deactivated Co-salen catalysts in dichloromethane solvent (22). The major peak in the mass spectram occurred at m/z of 603.5 for both Jacobsen s Co(II) and Co(III)-OAc salen catalysts, whereas much smaller peaks were observed in the m/z range of 1207 to 1251. The major feature at 603.5 corresponds to the parent peak of Jacobsen s Co(II) salen catalyst (formula weight = 603.76) and the minor peaks (1207 to 1251) are attributed to dimers in the solution or formed in the ESI-MS. The ESI-MS spectrum of the deactivated Co-salen catalyst, which was recovered after 12 h HKR reaction with epichlorohydrin, was similar to that of Co(II) and Co(III)-OAc salen. Evidently, only a small amount of dimer species was formed during the HKR reaction. However, the mass spectram of a fresh Co(III)-OAc salen catalyst diluted in dichloromethane for 24 h showed substantial formation of dimer. The activity and selectivity of HKR of epichlorohydrin with the dimerized catalyst recovered after 24 h exposure to dichloromethane were similar to those observed with a fresh Co-OAc salen catalyst. Therefore, we concluded that catalyst dimerization cannot account for the observed deactivation. 

Lundell, J., 1995, A MPPT2 Investigation of die H20-CO Dimer. A Test of Geometries, Energetics, and Vibrational Spectra , J. Phys. Chem., 99, 14290. 

The standard Sonogashira reaction conditions were not successful for the coupling reaction of 3-chloropyrazine 1-oxide (40) and 1-hexyne. In contrast, treatment of 40 and 1-hexyne with Pd(Ph3P)4 and KOAc produced 3-(l-hexynyl)pyrazine 1-oxide (41), together with the co-dimeric product, (E)-enyne 42 [34]. Presumably, the co-dimerization product 42 resulted from the cis addition of 1-hexyne to adduct 41. 

Remarkably, however, the logarithm of the rate constant varies linearly with the dissociation energy of the allylic C—H bond, which indicates that the rupture of the C—H bond is included in the rate-determining reaction step. Mixed olefin feeds (propene and butene) were also used. It appears that co-dimerization can occur yielding C2 -dimers. 

Bestrahlung von Mischungen aus Butadien und seinen Homologen in Gegenwart von Sensibilisatoren treten auBer homo-dimeren auch co-dimere Cycloaddukte auf (232). 

In order to evaluate the probable existence of the related neutral molecular species, theoretical calculations were carried out on two molecular CO dimer (see Figure 9.14a) and trimer (see Figure 9.14b) structures, applying the CNDO/SM calculation method [123],... 

The relation between the supermolecule coupled cluster approach and the perturbation theory of intermolecular forces in even less obvious than the case of the Mpller-Plesset theory, and no formal analysis has been reported in the literature thus far. Rode et al.68 analyzed the long-range behavior of the CCSD(T) method65, and showed that this method, although very popular and in principle accurate, may lead to wrong results for systems with the electrostatic term strongly depending on the electronic correlation, e.g. the CO dimer. 

It should be stressed that the analysis presented above is general, and applies to any system. However, for the majority of Van der Waals complexes the electrostatic term E will not be as important as it is for the CO dimer. On the other hand, this analysis shows that any supermolecule method should be applied with great care, and an understanding of the supermolecule results in terms of contributions as defined by the symmetry-adapted perturbation theory is necessary. 

A related series of mixed-metal face to face porphyrin dimers (192) has been studied by Collman et al.506 A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 02 to H20 via H202. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co11 /Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemic-ally.507 However for most of the mixed-metal dimers, including a Con/Mnn species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced. 

Normal Butene Reactions. Under alkylation conditions, all four butene isomers are believed to undergo isomerization, dimerization, and co-dimerization when first coming in contact with HF catalyst, i.e., immediately following protonation. These are very rapid, Ionic reactions and take place competitively along with isobutane alkylation. Alkylate compositions from the four butenes are basically similar (see Table VII). However, l-butene produces a C3 fraction containing nearly two times... 

Lundell,., AMPPT2 investigation of the H O—CO dimer. A test of geometries, energetics, and vibrational spectra, J. Phys. Chem. 99, 14290-14300 (1995). 

Irradiation of mixtures of cyclohexene and cycloheptene at 254 nm in the presence of Cu(tfo) (tfo = trifluoromethanesulphonate) promotes a very selective co-dimerization to give (35). A new carbon-carbon bond-forming... 

Taniike et al. used DFT calculations to probe NO-CO reactions on a dimer of Co atoms on a y-alumina support. These dimers had been shown to exist in previous experiments that also suggested an unusual mechanism in which NO is reduced to N2O via interaction with gas phase CO molecules. The calculations indicated that Co dimers allow the formation of an adsorbed cis-(NO)2 species that has an unusually large reactive cross section for reaction with gaseous CO via an Eley-Rideal mechanism. 

In the high-concentration cobalt systems, cobalt ions are reported to be present in several configurations, with the Co" dimer being the most active [71]. Some non-varivalent metal salts, such as ZrO(OAc)2, are reported to promote cobalt catalyzed oxidations. This may be the result of the influence of the promoter on coordination number and monomer-dimer equilibria [72, 73]. 

The hydrovinylation reaction has its origin in the observations made in 1963 that propene dimerizes at a quite remarkable rate in the presence of certain organo-nickel catalysts and that the product distribution can be influenced by introducing auxiliary P-donor ligands [1]. In 1967 it was discovered that in the presence of the chiral ligand P( ranx-myrtanyl)3, 2-butene can be co-dimerized with propene to give 4-methyl-2-hexene in an enantioselective manner and the extension of this co-dimerization reaction to ethylene has become known as hydrovinylation. 

Other Co dimers without carbonyl or nitrosyl ligands include phosphide- ... 

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