Butadiene coordination

The trans/cis ratio of the product must, therefore, be determined at an earlier reaction stage and most probably by the ratio of species 27a and 27b. Steric or electronic factors affecting this ratio will influence the trans/cis ratio of the resulting 1,4-hexadiene. The phosphine and the cocatalyst effect on the stereoselectivity can thus be interpreted in terms of their influence on the mode of butadiene coordination. Some earlier work on the stereospecific synthesis of polybutadiene by Ni catalyst can be adopted to explain the effect observed here, because the intermediates that control the stereospecificity of the polymerization should be essen-... 

Avery active form of Ni(0) is generated from bis-7r-allylnickel (12) and Ni(cod)2 (13) in the absence of phosphine ligand, and is often called naked Ni(0). Three butadienes coordinate to naked Ni(0) and the 18-electron trimeric complex 10 is... 

The reaction is catalyzed by Pd(0) complexes, but the pre-catalyst can be either a Pd(0) or a Pd(II) compound that is to be reduced in situ. Oxidative coupling of two butadienes coordinated to a Pd(0) species of the phosphine ligand (species A, Scheme 4) results in the formation of intermediate [Pd(l,2,3,8-r 4-octa-2,6-diene-l,8-diyl)(L)] (B). This oxidative coupling is rather facile and reversible the latter illustrated by the fact that B is only stable at high butadiene concentration. Subsequent protonation of B by NuH at the 6-position of the ri r -octadienyl ligand leads to [Pd(l,2,3,7,8-ri5-octa-2,7-dien-l-yl)(L)]Nu (C). The complex of type C is a pivotal intermediate as several reaction pathways are available from here. 

The catalytic cycle proposed for the dimerization of butadiene is shown in Fig. 7.8. As shown by 7.24, two molecules of butadiene coordinate to NiL. A formal oxidative addition, as shown by Eq. 7.8, produces two nickel-carbon bonds and the carbon-carbon bond required for ring formation. The structure of 7.25 with two nickel-carbon bonds (see Fig. 7.8), is a hypothetical one that helps us to understand the carbon-carbon bond formation process. The actual catalytic intermediates that have been observed by spectroscopy have an rf-allyl type of bonding. As shown by reaction 7.9, species 7.25 can reductively eliminate 1,5-cyclooctadiene and the zerovalent nickel complex Ni-L. 

The Mode of Butadiene Coordination and the Structure of the Butenyl Anion at the Metal... 

Depending on the butadiene coordination in the single cis or the single trans form, by anti or syn insertion a cis- or a trans-C4 unit can be generated. Furthermore, a certain probability exists for a reaction between the C(3) atom of the butenyl group and uncoordinated butadiene, which might be the reaction channel for the low 1,2-selectivity, which is also observed in hydrocarbon solution. 

If the 1,4-polymerization is realized by the (T-allyl insertion mechanism, then the butenyl group in the anti and in the syn structure should be practically equally reactive and the cis-trans selectivity can be determined by the different mode of the butadiene coordination in the butenyl-lanthanide complex. 

Both modes of butadiene coordination are exemplified by appropriate model complexes [NiCp(CH3)( 7 -C4H6)] [65] and [Ni(l,3-(CH3)2C3H3)( 7 -1,4-... 

Therefore the cis-lrans selectivity can be determined neither solely by the mode of butadiene coordination nor by the rate of anii-syn isomerization as has been suggested in the literature [50, 68-70] rather, it is regulated kinetically by the different reactivity of the anti- and the iyn-butenylnickel(II) complex, depending on the mode of butadiene coordination, and thermodynamically by the concentration of the stmcturally different butadiene complexes. 

Fig. 7. The site of protonation of ri butadiene coordinated to Mo(cp) (CO)2 predicted by simple Extended Hiickel MO calculations and observed experimentally (Ref. 112)...

At a more qualitative level, the EHMO method continues to provide important insights into the mechanism of organometallic reactions. For example, the sites of protonation of q3-butadiene coordinated to [Mo(CO)2L] species can be successfully predicted based on a charge controlled mechanism [112] (Fig. 7). Tsipis [73] gives numerous other examples of the use of this method for studying qualitatively a whole range of organometallic reactions. 

This reversible olefin coordination can also be observed based on spectral changes in the UV absorption spectra [21]. A strong and broad absorption band at 220-230 nm for the propylene coordinated with the AgBp4-PVMK complex ([Ag ] [C=0] = 1 2) disappeared and a new band appeared at around 258 nm for the coordinated 1,3-butadiene when the propylene coordinated film was exposed to a 1,3-butadiene atmosphere followed by a nitrogen gas purge. The absorption band of the coordinated propylene at 220-230 nm reappeared when propylene was introduced into the cells containing the 1,3-butadiene coordinated film. 

In cyclization of conjugated dienes, typically butadiene, coordination of two molecules of butadiene gives rise to the bis-jr-allyl complexe 12. The distance between the terminals of two molecules of butadiene becomes closer by 7r-coordination to Pd(0), and the oxidative cyclization is thought to generate either the l-pallada-2,5-divinylcyclopentane 13 or l-pallada-3,7-cyclononadiene 14. 

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