Frédéric Robin

FROM 2H-PHOSPHOLES TO BIPNOR, A NEW EFFICIENT BIPHOSPHINE FOR ASYMMETRIC CATALYSIS

Abstract For many years now, we have studied the 1H-/2H-phosphole equilibrium and its synthetic applications. On reaction with alkynes, 2H-phospholes yield the corresponding 1-phosphanorbornadienes. As ligands of rhodium(I), these phosphines show some potential in catalytic hydrogenation and hydroformylation of alkenes. Starting from 3,3′,4,4′-tetramethyl-1,1′-biphospholyl and tolan, we have similarly obtained the corresponding 2,2′-bis-(1-phosphanorbornadienyl) (BIPNOR) with two chiral, non-racemisable, phosphorus atoms at the bridgeheads. The pure enantiomers of BIPNOR appear to be efficient ligands in asymmetric hydrogenation of CC and CO double bonds.

γ-Functionalisation of the 1-Phosphanorbornadiene Structure

3-Methyl-substituted 1-phosphanorbornadiene oxides or sulfides such as 1a,b are either metalated by a strong base (n-BuLi or LDA) or brominated by NBS at the methyl substituent. The reaction of the resulting delocalised carbanions 2a,b takes place either at C-2 (H+, MeI) or at the methylene group (Me3SiCl, ClPPh2, I2). In the last case, γ-functional (5a,b or 6a,b) or bridged (7) phosphanorbornadienes are obtained. Similarly, the reaction of 3-bromomethyl-1-phosphanorbornadiene oxides such as 8 with nucleophiles [NaCH(CO2Me)2, PhO–, Me2NH, PhCH2NH2] yields the corresponding γ-functional phosphanorbornadienes 9–12. This chemistry has also been applied to the BIPNOR dioxide 13.