The Nickel Catalyzed Cyclooligomerization of Butadiene

The nickel-catalyzed cyclooligomerization of butadiene (BD) is one of the most thoroughly studied homogeneous catalytic reactions. Extensive NMR studies of intermediates and model compounds have recently been reported. It provides one of the best examples of the ability of phosphorus ligands to control both rates and product distributions in homogeneous catalysis and shows just how complex catalytic systems can become. 

In the absence of added phosphorus ligands, the principal product (80-90%) is trans, trans, rran5-l,5,9-cyclododecatriene (ttt-CDT 49), with smaller amounts of trans, trans, cw-l,5,9-cyclododecatriene ttc-CDT 50), trans, cis, c/5-l,5,9-cyclododecatriene (tcc-CDT 51), 1,5-cyclooctadiene (COD 52), 4-vinylcyclohexene (VCH 53), and c/5-l,2-divinylcyclobutane (DVCB 54). With the addition of phosphorus ligands, especially bulky phosphites, COD can become 96% of the product, and the rate of butadiene consumption can be increased by a factor of 10 (at 80°).  

Ethylene can be readily cooligomerized with BD to produce cis, trans-1,5-cyclododecadiene (55) in good yield, but propylene reacts only poorly in this way and 2-butene not at all. The strained olefin norbor-nene, however, gives 56 in high yield, and dimethyl acetylene can be used to prepare 57.  

A variety of nickel catalysts may be used, including Ni(II) with reducing agents, (7r-C3H5)2Ni, Ni(COD)2, Ni(CH2=CHCN)2, Ni(CO)4, and even atomic Ni (by metal atom evaporation). Phosphorus ligands may be added separately or coordinated in an added nickel complex such as (CDT)NiL, (COD)NiL2, or NiL4- 

Rates and product distributions depend on L Ni ratio, as well as ligand type, temperature, and BD conversion. Table 6 shows the effect of varying the ligand Ni ratio for PPhs. The rate of CDT formation (sum of all isomers 49-51) is reduced from 25 cycles/hri with no added PPhs, to 4 at 1 1, and to less than 0.4 with PPhsiNi of 2 1 or greater. The maximum rate of Cg products (COD + VCH) is realized at a 3 1 ratio (not shown) but decreases from 105 cycles/hr to 45 as the L Ni ratio is increased to 8 1. 

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Comparing [L]-control maps for up to now fifteen different ligands of the nickel-catalyzed cyclooligomerization of butadiene we found eight independent productdetermining l and-association processes (I — VIII). To find out the respective changes in the [L]-control maps, it is extremely helpful to construct product stream diagrams as described in Scheme 3.2-3. 

It is appropriate to include here two bis(T)3-allyl)nickel complexes that are formed by reacting zerovalent nickel species with 1,3-dienes and that have been shown to be involved as intermediates in the nickel-catalyzed cyclooligomerization of butadiene, viz., (T)3,i73-C12H18)Ni and (rj3-QH NiPRa. 

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