How to turn the catalytic asymmetric hydroboration reaction of vinylarenes into a recyclable process

Abstract

Optically pure rhodium(I) complexes [Rh(cod)(Lbond;L)]X (cod=cyclooctadiene; L-L= (R)-2,2'-bis(diphenylphosphino)1-1'-binaphthyl ((R)-BINAP), (S,S)-2,4-bis(diphenylphosphino)pentane ((S,S)-BDPP), 2-diphenylphosphino-1-(1'-isoquinolyl)naphthalene ((S)-QUINAP); X=BF(4), PF(6), SO(3)CF(3), BPh(4)) were immobilised onto smectite clays such as montmorillonite K-10 (MK-10) and bentonite (Na(+)-M). (19)F, (31)P and (11)B NMR experiments recorded in CDCl(3) during the impregnation process provided evidence that montmorillonite K-10 may immobilise ionic metal complexes throughout the cationic and anionic counterparts. However, when bentonite was used as the solid, only the cationic metal complex was immobilised through cationic exchange while the counteranion remained in solution. When we used these preformed catalytic systems in the hydroboration of prochiral vinylarenes, we obtained high activities and enantiomeric excess with (S)-1-(2-diphenylphosphino-1-naphthyl)isoquinoline-modified rhodium complexes. These activities and selectivities are competitive with the homogeneous counterparts. The significant features of this method are the simple separation and good retention of the active metal in the solid, which allows efficient recycling even on exposure to air.