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

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Abstract
(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 (1–29% 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.