Nickel, 1,3-butadiene trimerization with structures

Some insight into the nature of the coordination and addition steps can be deduced from the cyclo-oligomerization work reported by Wilke (316). Butadiene can be converted into several isomers of cyclododeca-triene using (C2H5)aAlCl/TiCl4, AlEta/Cr02Cl2 and AlEta/nickel acetyl-acetonate catalysts. With the nickel catalysts, open chain, jr-complexed intermediates have been isolated for both the butadiene trimer and the dimer. The open chain dimer structure is shown below, where Do = a bulky Lewis base and the nickel is apparently zero valent. 

Diene Cyclization. In 1952 Reed (157) discovered the catalytic dimerization of butadiene with Reppe catalyst in the presence of acetylene. Important results were obtained by Wilke (200) in the cyclization of butadiene with a nickel(0) catalyst. With bis-7r-allylnickel, biscyclo-i,5-octadienenickel, or cyclododecatrienenickel, he obtained the trimerization of butadiene to cyclododecatetraene while, with a catalyst of the type Ni(PR3)4, in which perhaps one coordination site cannot be replaced, he obtained the dimerization to cycloocta-l,5-diene. The mechanism of these reactions, in which 7r-allyl systems can be in equilibrium with o--7r-allyl systems (Figure 7), have been proved by Wilke and co-workers who isolated the intermediate compounds. It is worth noting that all these catalysts have ligands of weak -acceptor character which are labile and do not prevent butadiene from coordinating. The presence of weak t acceptors on the nickel tends to favor the structure of the diene, as was emphasized by Mason (112). 

When butadiene is treated with RuCls in 2-methoxyethanol at 100°C, yellow-brown crystals of composition (C4H8)3RuCl2 are obtained 381). Crystal structural analysis has identified 381, 382] the complex as (2,6,10-dodecatriene-l,12-diyl)dichlororuthenium (146), analogous to the nickel complex (194) suggested 608) as the catalytic intermediate in the nickel-catalyzed cyclic trimerization of butadiene. In this complex, the ruthenium atom has a trigonal-bipyramidal configuration with the... 

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