Diene complexes cyclooligomerization

The catalytic cyclo-oligomerization of 1,3-butadiene mediated by transition-metal complexes is one of the key reactions in homogeneous catalysis.1 Several transition metal complexes and Ziegler-Natta catalyst systems have been established that actively catalyze the stereoselective cyclooligomerization of 1,3-dienes.2 Nickel complexes, in particular, have been demonstrated to be the most versatile catalysts.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. 

Enantioselective cyclooligomerizations of butadiene can also be achieved when a chiral diaza-diene-iron(II) chloride complex and a fourfold excess of ethylmagnesium iodide are used. Optical yields of 9 to 16% are obtained with two chiral diazadienes of the type R N = CRCR = NR, prepared by formic acid catalyzed condensation of aqueous glyoxal (R1 = H) or 2,3-butadione (R1 = CH3) with (R)-l-phenylethylaminc or (1S,2S,3S,5R)-3-(aminomethyl)pinane. The (di-azadienc)iron(0)-catalyzed dimerization of butadiene also gives 4-vinylcyclohexene with up to 16% ee60. 

The oligomerization and cyclooligomerization of dienes is a synthetically useful and commercially significant reaction catalyzed by transition-metal complexes from different triads. In nearly all examples, -allyl complexes are intermediates, formed by either coupling of dienes about a metal template or by insertion of the diene into a metal-H bond. Allyl complexes are isolated from stoichiometric and catalytic diene oligomeriza-... 

This leads us to propose a theoretically verified, refined catalytic cycle for production of linear and cycHc CiQ-olefin products (cf. Scheme 3). Furthermore, a detailed comparison of crucial mechanistic aspects of the catalytic reaction course for co-oligomerization of butadiene and ethylene and for cyclooligomerization of butadiene promoted by zerovalent bare nickel complexes was undertaken. These contribute (first) to a more detailed understan(fing of mechanistic aspects of the [Ni"]-mediated co-oHgomerization of 1,3-dienes and olefins and (second) to a deeper insight into the catalytic structure reactivity relationships in the transition-metal-assisted co-oHgomerization and oligomerization reactions of 1,3-dienes. 

With their delocalized 7i-electron system, allyl ligands can bond to metals in a manner similar to olefins. Allyl complexes have been detected as intermediates in catalytic processes involving propene or higher olefins and dienes. Examples include the cyclooligomerization of butadiene and the codimerization of butadiene with ethylene. 

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