Palladium(II)-Catalyzed Rearrangement and Oligomerization Reactions of cis-Bicyclo[4.2.0]oct-7-ene

In the presence of several Pd(II) catalysts, cis-bicyclo[4.2.0]oct-7-ene (4a) was found to undergo olefin isomerization ("ring walking") and oligomerization, resulting in the formation of cis-bicyclo[4.2.0]oct-2-ene (4d) and the low-molecular-mass cycloaliphatic oligomers 5a–d, respectively. The catalysts studied are [Pd(NCEt)4][BF4]2 (1), [(η3-allyl)Pd(solv)2][SbF6] (2; solv = CH2Cl2), [(1,10-phenanthroline)Pd(CH3)(NC(CH2)6CH3)][SbF6] (6), and [(2,9-dimethyl-1,10-phenanthroline)Pd(CH3)(NC(CH2)6CH3)][SbF6] (7). Isomerization included the formation of both 4d and the olefinic end groups of 5a–d and ranged from 94% using catalyst 7 to 29% employing catalyst 2. Ab initio and DFT calculations at the LMP2/6-31G** and B3LYP/6-31G** levels show that the thermodynamic stabilities of the bicyclo[4.2.0]octene isomers increase in the order 7-ene 4a < 1(8)-ene 4b < 1-ene 4c ≈ 1(6)-ene 4e ≈ 3-ene 4f < 2-ene 4d. A mechanism of isomerization via subsequent β-hydride eliminations and olefin reinsertions is proposed. These results are in contrast to the reactions of bicyclo[3.2.0]hept-6-ene (3a) catalyzed by 1, 2, and 7 and the reaction of 4a catalyzed by Cp2ZrCl2/MAO (Cp = η5-C5H5), all of which produced polymers in good yields (73–99%).