Catalytic dimerization of conjugated dienes

In a method of producing a dimer with a six-membered ring by contacting a catalyst containing iron compounds and organoaluminum compounds with a conjugated diene which is butadiene, isoprene, 1,3-pentadiene, cyclopentadiene or cyclohexadiene, the improvement comprising using a catalyst consisting essentially of the reaction product of:

• A. an iron compound selected from at least one of the group consisting of:
• iron chloride, iron bromide, iron iodide, iron sulfate, iron carbonate, iron hydroxide, iron acetate, iron oxalate, iron acetylacetonate, and ferrocene, present in from about 0.00001 to about 0.05 moles per mole of the conjugated diene;
• B. an organoaluminum compound selected from at least one of the group consisting of compounds having the formula
• R.sub.2 AlR'
• wherein each R is one of the group consisting of hydrogen, alkyl having 1 to 8 carbon atoms and phenyl, and R' is the same as R or is a halogen, with the proviso that when R' is a halogen each R is an alkyl having 1 to 8 carbon atoms, present in from about 0.1 to about 1,000 moles per mole of iron compound;
• and
• C. nitric oxide, present in from about 0.1 to about 5 moles per mole of organoaluminum compound.