cis-Oxypalladation Complexes Derived from (1R,5R)-2(10),3-Pinadiene and Their Utilization in Pd(II)-catalyzed Enantioselective Cyclization of 2-(trans-2-Butenyl)phenols

Article abstract of DOI:10.1246/bcsj.58.3282

(1R,5R)-2(10),3-Pinadiene, when treated with either Na2PdCl4 in MeOH or Pd(OAc)2 in AcOH and NaCl, gives di-μ-chloro-bis<(3,2,10-η-cis-4-methoxy or acetoxypinene)palladium(II)> (4a) or (4b), respectively.These complexes represent the firstly isolated cis-oxypalladation adduct.The ligand exchange of 4b with AgOAc affords di-μ-acetato-bis<(3,2,10-η-cis-4-acetoxypinene)palladium(II)> (5b) which serves as the catalyst for the asymmetric cyclization of 2-(trans-2-butenyl)phenols leading to 2-vinyl-2,3-dihydrobenzofurans (13).Although the enantioselectivities induced in this asymmetric cyclization are not high (1-29percent ee), noteworthy is that the cis-complex 5b affords (R)-(-)-enantiomer of 13 while the parent di-μ-acetato-bis<(3,2,10-η-pinene)palladium(II)> give the (S)-isomer.As an application of the present asymmetric cyclization, attempts to synthesize (S)-(+)-tremetone have been made.