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 C2-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)-5 b had a synergistic effect on the enantioselectivity of the reaction compared with its nonsupported precursor (S,S,S,S)-4 c, affording the corresponding allylation products 14 a and 14 b with excellent enantioselectivities (94.6 and 97.2 % ee, respectively). Moreover, the Pd complex of (S,S,S,S)-5 b could easily be recovered and recycled several times without significant loss of enantioselectivity and activity in the allylic substitution reactions.
Zhao
Dongbo
D
State Key Laboratory of Organometallic Chemistry Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China.
Sun
Jie
J
Ding
Kuiling
K
D000498:Allyl Compounds / Q000737:chemistry*; D000925:Anticoagulants / Q000737:chemistry; D002384:Catalysis; D003374:Coumarins / Q000737:chemistry; D003503:Cyclobutanes / Q000737:chemistry*; D019281:Dimerization; D008024:Ligands; D009942:Organometallic Compounds / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry*; D010720:Phosphines / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry*; D012995:Solubility; D013237:Stereoisomerism
D000498:Allyl Compounds; D000925:Anticoagulants; D003374:Coumarins; D003503:Cyclobutanes; D008024:Ligands; D009942:Organometallic Compounds; D010720:Phosphines; D011108:Polymers; D010165:Palladium; C030123:coumarin