New types of soluble polymer-supported bisphosphine ligands with a cyclobutane backbone for Pd-catalyzed enantioselective allylic substitution reactions

A highly efficient and practical optical resolution of anti head-to-head racemic coumarin dimer 7 has been achieved by molecular complexation with TADDOL, (-)-8, through a hydrogen bonding interaction to afford the corresponding two enantiomers, (-)- and (+)-7, in 70 and 75% yields, respectively, with >99% ee. Starting from enantiopure (-)-7, a new type of C-2-symmetric bisphosphine ligand (S,S,S,S)-3 with a cyclobutane backbone has been synthesized in good yield by facile transformations. The asymmetric induction efficiency of these chiral bisphosphine ligands in Pd-catalyzed asymmetric allylic substitution reactions was evaluated. Under the experimental conditions, the allylic substitution products could be obtained in excellent yields (up to 99%) and enantioselectivities (up to 98.9% ee). By taking advantage of the high enantioselectivity of this catalytic reaction and the easily derivable carboxylate groups on the cyclobutane backbone of ligand (S,S,S,S)-3, a new type of analogous ligand (S,S,S,S)-4 as well as the MeO-PEG-supported soluble ligand (S,S,S,S)-5 (PEG = polyethylene glycol) have also been synthesized and utilized in asymmetric allylic substitution reactions. In particular, the MeO-PEG supported (S,S,S,S)-5b had a synergistic effect on the enantioselectivity of the reaction compared with its nonsupported precursor (S,S,S,S)-4c, affording the corresponding allylation products 14a and 14b with excellent enantioselectivities (94.6 and 97.2 % ee, respectively). Moreover, the Pd complex of (S,S,S,S)-5b could easily be recovered and recycled several times without significant loss of enantioselectivity and activity in the allylic substitution reactions.