Pd²⁺-Catalyzed Cyclocopolymerization of 1,5-Hexadiene and CO:  Regioselectivity of Olefin Insertion

The cyclocopolymerization of 1,5-hexadiene and CO with Shell-type catalyst systems comprising palladium(II) complexes in the presence of chelating phosphines yields soluble cyclocopolymers containing 5- and 6-membered cyclic ketones. Cyclocopolymerization of 1,5-hexadiene and CO in the presence of Pd(OAc)2, 1,3-bis(diphenylphosphino)propane (Dppp), 1,4-naphthaquinone, and Ni(ClO4)2·6H2O in chloroform/methanol (100−20/1) afforded a soluble cyclocopolymer 2 containing both 5- and 6-membered ring ketones in the polymer backbone. Cyclocopolymerization under similar conditions in the presence of 1,3-bis(diisopropylphosphino)propane (Dipp) gave a soluble cyclocopolymer 3 containing only 6-membered rings. Epimerization of the 6-membered ring cyclocopolymer 3 with 4-(dimethylamino)pyridine provided evidence for an initial prevailingly cis microstructure (ca. 3/1 cis/trans) for the 2,5-disubstituted cyclohexanone repeating units of the cyclocopolymer. The regioselectivity for insertion of 1,5-hexadiene into the Pd−acyl bonds was inferred from the nature of the ring formed in the cyclocopolymerization. The formation of 5-membered cyclopentanone repeating units was interpreted as a signature for an initial 2,1-insertion of 1,5-hexadiene; the formation of 6-membered cyclohexananone rings was interpreted in terms of an initial 1,2-insertion of the 1,5-hexadiene.