Synthesis of new Biaryl phosphepine ligands for asymmetric catalysis

Ph-BINEPINE is a binaphthyl-based phosphepine ligand. Since its preparation, Ph-BINEPINE has scored a number of excellent performances and now is one of the chiral monodentate phosphorus ligands of choice in asymmetric catalysis.With an increasing interest in the diversity of the supporting scaffold, we explored6-phenyl-6,7-dihydro-5H-dibenzo[c,e]phosphepine, i.e. BIPHENINE (L1); where a biphenyl backbone is in the place of binaphthyl. We targeted synthesis of axially locked and stereocehmically stable1,11-dimethoxy-6-phenyl-6,7-dihydro-5H-dibenzo[c,e]phosphepine, i.e. diMeO-BIPHENINE (L2).We succeeded to synthesize chiral (R)-diMeOBIPHENINE (L2) in fair overall yield following a synthetic sequence initiated by a simple phenolic oxidative coupling in the first step. Resolution of the intermediate 6,6’-dimethylbiphenyl-2,2’-diol with (-)-menthyl chloroformate, established a simple experimental procedure for obtaining (R)-L2 (with >99% e.e.).Several other biaryl phosphepine ligands including heterocyclic phosphepines as well as phosphepines having C1-symmetry; were attempted to be synthesized, that failed to give the desired ligand in the last step.(R)-diMeOBIPHENINE was tested in Pd-promoted alkylation of 1,3-diphenyl-2-propenyl acetate with malonate and Rh-catalyzed hydrogenations of itaconic acid, dimethyl itaconate, and α-amino acids & esters. (R)-diMeOBIPHENINE marginally outperformed Ph-BINEPINE in asymmetric classical hydrogenations achieving upto 96% e.e.