Butadiene, coupling reactions

A very special complication can arise from the breakdown of the single configuration approximation. This is most likely to be a problem in systems where the d-orbital energy ordering alters drastically during a reaction an example may be the butadiene coupling reaction discussed below. 

Recent discoveries have expanded the utility of nickel-catalyzed coupling reactions. Inclusion of butadiene greatly improves the efficiency of the reactions.268... 

It should be noted here again that the catalytic reaction does not involve a change of valence of the metal. In general, catalytic olefin addition reactions that involve a hydride transfer do not require change of valence in the metal catalyst. On the other hand, carbon-carbon bond formation by coupling reactions which involve electron shifts, such as in Wilke s Ni°-catalyzed butadiene oligomerization reaction [Eq. (1)], requires a valence change on the metal. 

Corriu et al. have reported that the coupling reaction of 2-(iV,iV-dimethylaminomethyl)phenyllithium with (McvSi)vSiCI 53 affords 2-(iV,iV-dimethylaminomethyl)-l-[tris(trimethylsilyl)silyl]benzene 894. No evidence has been found that the intramolecular iV-ligand coordinates to the silicon atom of 894. Upon UV irradiation, the trisilane forms a transient silyene 895, which has been trapped with 2,3-dimethyl-2,3-butadiene and triethylsilane to give the oligosilanes 896 and 897 as well as 898-900, (Scheme 126).859 Apparently, the bulk on the two ligands is insufficient to provide kinetic stabilization of the silylene intermediate 895. 

The application of in situ-generated (alkoxy)palladium(II) species (Scheme 14.23) can be extended to reactions of a-carbonates with organoboron compounds. Crosscouplings of allenes 108 with aryl (or alkenyl) boron acids or their esters catalyzed by a palladium(O) complex afforded the 2-aryl(alkenyl)-l,3-butadienes 109 in excellent yields (Scheme 14.24) [53], The coupling reactions of 9-BBN-derived intermediates such as ester 111 can be accelerated by applying K3P04 as additive (Eq. 14.15). 

Figure 4 in Scheme 2.3-4 demonstrates that when using a triphenylphosphane-modified Ni-catalyst, butadiene reacts with 2-butyne to form a 2 1-adduct whereas with methyl 2-butynoate, a 1 2 co-oligomer is obtained. Butadiene and phenyl-acetylene also form 1 2 products As we may have shown, a change from X- to C- or Z-type substituents in the co-substrates alters the ratio from 2 1 to 1 2 in a synthon coupling reaction. 

As discussed in connection with olefin-coupling reactions and shown in Fig. 4, the coupling of vinyl Grignard reagents is stereospecific and dependent upon the transition metal catalyst used (32, 33). The dimerization of ethylene, shown in Fig. 6, was also shown to produce primarily the terminal olefin 1-butene (35). The size of the metal has also been shown to influence the course of the catalyzed oligomerization reactions of butadiene. When bis-(ir-allyl) metal complexes are used as... 

A gel permeation chromatogram obtained by Heller (13) indicated that Kraton 101 contained 1% polystyrene, 22% polystyrene/1,4-poly-butadiene, and 77% polystyrene/1,4-polybutadiene/polystyrene, apparently because the material is manufactured by coupling of the diblock. The diblock would be present if the coupling reaction is not 100% efficient. Kraton 101 would thus be essentially a blend of ABBA and AB block copolymers. Assuming that Kraton 102 is manufactured in the same way, the molecular weight of the ABBA block copolymer results as about 72,000 from the above data, the polystyrene blocks having a molecular weight of about 12,000, and the polybutadiene center block one of about 48,000. 

Hegedus and Varaprath studied the reactions of various bromodienes with Ni(CO)4 and with bis(cyclooctadiene)nickel. l-Bromo-2,5-hexadiene and 2-bromomethyl-1,3-butadiene give the stable products 62 and 63, respectively, which resemble allyl nickel halides in their properties (217). Similar compounds had been prepared several years previously from geranyl halides (218). l-Bromo-2,4-pentadiene and l-bromo-2,4-hexadiene, however, formed intractable materials which could not be isolated and purified. In these cases the red color of the solution which was first produced faded and NiBr2 was deposited. The desired compounds, however, could be generated in situ at — 30° C and used in coupling reactions with aryl, alkenyl, and allyl halides (217). 

Thiele and Beckhaus reacted zirconocene dichloride with two molar equivalents of vinyl lithium. The expected divinylzirconocene product proved not to be stable under the applied reaction conditions. It was initially not even observed as an intermediate but reacted further by cr-ligand coupling to give a good yield of the stable (butadiene)zirconocene (see Scheme 23).72 This coupling reaction was actually used synthetically to... 

The (butadiene)zirconocene template coupling reaction has found application in organic synthesis. Coupling of [(butadiene)ZrCp2] with [W(CO)6] followed by the reaction of the steroid ketone 3-methoxy-l,3,5(10)-trien-17-one (108) gave the corresponding metallacyclic... 

CO, alternating copolymerization, 273-274 Cp2ZrCl2/LiAlH4, 32 Cross-coupling reactions, 266-270 Cyclopentadienyl-free (butadiene)M(IV) complexes, 118-120... 

The same catalysts were also claimed to catalyze the coupling reaction between butadiene and acetylene to produce 1,4-cyclohexadiene, i.e.,... 

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