Palladium(II) Complexes of C2-Bridged Chiral Diphosphines: Application to Enantioselective Carbonyl-Ene Reactions

(11bR,11′bR)-4,4′-(1,2-Phenylene)bis[4,5-dihydro-3H-dinaphtho[2,1-c:1′,2′-e]phosphepin] [abbreviated as (R)-BINAPHANE], (3R,3′R,4S,4′S,11bS,11′bS)-4,4′-bis(1,1-dimethylethyl)-4,4′,5,5′-tetrahydro-3,3′-bi-3H-dinaphtho[2,1-c:1′,2′-e]phosphepin [(S)-BINAPINE], (1S,1′S,2R,2′R)-1,1′-bis(1,1-dimethylethyl)-2,2′-biphospholane [(S,S,R,R)-TANGPHOS] and (2R,2′R,5R,5′R)-1,1′-(1,2-phenylene)bis[2,5-bis(1-methylethyl)phospholane] [(R,R)-i-Pr-DUPHOS] are C2-bridged chiral diphosphines that form stable complexes with palladium(II) and platinum(II) containing a five-membered chelate ring. The Pd(II)-BINAPHANE catalyst displayed good to excellent enantioselectivities with ee values as high as 99.0% albeit in low yields for the carbonyl-ene reaction between phenylglyoxal and alkenes. Its Pt(II) counterpart afforded improved yields while retaining satisfactory enantioselectivity. For the carbonyl-ene reaction between ethyl trifluoropyruvate and alkenes, the Pd(II)-BINAPHANE catalyst afforded both good yields and extremely high enantioselectivities with ees as high as 99.6%. A comparative study on the Pd(II) catalysts of the four C2-bridged chiral diphosphines revealed that Pd(II)-BINAPHANE afforded the best enantioselectivity. The ee values derived from Pd(II)-BINAPHANE are much higher than those derived from the other three Pd(II) catalysts. A comparison of the catalyst structures shows that the Pd(II)-BINAPHANE catalyst is the only one that has two bulky (R)-binaphthyl groups close to the reaction site. Hence it creates a deep chiral space that can efficiently control the reaction behavior in the carbonyl-ene reactions resulting in excellent enantioselectivity.