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.
State Key Laboratory of Organometallic Chemistry Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China.
Dongbo
D
Zhao
Jie
J
Sun
Kuiling
K
Ding
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