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 ][BF 4 ] 2 ( 1 ), [(η 3 -allyl)Pd(solv) 2 ][SbF 6 ] ( 2; solv = CH 2 Cl 2 ), [(1,10-phenanthroline)Pd(CH 3 )(NC(CH 2 ) 6 CH 3 )][SbF 6 ] ( 6 ), and [(2,9-dimethyl-1,10-phenanthroline)Pd(CH 3 )(NC(CH 2 ) 6 CH 3 )][SbF 6 ] ( 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 Cp 2 ZrCl 2 /MAO (Cp = η 5 -C 5 H 5 ), all of which produced polymers in good yields (73–99%).