Fluorous derivatives of [Rh(COD)(dppe)]BX 4 (X=F, Ph): synthesis, physical studies and application in catalytic hydrogenation of 1-alkenes and 4-alkynes

Abstract

Fluorous derivatives of 1,2-bis(diphenylphosphino)ethane (dppe) ( 1 ), containing a para-(1 H,1 H,2 H,2 H-perfluoroalkyl)silyl function, were used in the synthesis of fluorous derivatives of [Rh(COD)(dppe)]BF 4. The single crystal X-ray crystallographic structure of [Rh(COD)( 1a )]BPh 4 ( 7 ) was determined ( 1a =[CH 2P(C 6H 4–SiMe 2CH 2CH 2C 6F 13- p) 2] 2). Large differences in catalytic activity and selectivity (higher hydrogenation activity and lower isomerization activity) relative to [Rh(COD)(dppe)]BF 4 were observed in the hydrogenation of 1-octene using non-fluorous, silyl-substituted [Rh(COD)( 2 )]BF 4 ( 5 ; 2 =[CH 2P(C 6H 4–SiMe 3- p) 2] 2 or [Rh(COD)( 3 )]BF 4 ( 6 ; 3 =[CH 2P(C 6H 4–SiMe 2C 8H 17- p) 2] 2) and even more so with fluorous [Rh(COD)( 1a )]BF 4 ( 4a ). For 4a and 6 , the presence of aggregates was demonstrated by dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryoTEM), which is most probably responsible for these differences. Similar differences between (fluorous) silylated catalysts and non-substituted [Rh(COD)(dppe)]BF 4 were observed in the semi-hydrogenation of 4-octyne. Recycling of highly fluorous catalyst [Rh(COD)( 1c )]BF 4 ( 4c ; 1c =[CH 2P(C 6H 4-Si(CH 2CH 2C 6F 13) 3- p) 2] 2) was investigated in two different fluorous biphasic solvent systems. The catalyst could be recycled with 97.5% (for PFMCH/acetone, 1:1 v/v) and >99.92% (for FC-75/hexanes, 1:3 v/v) retention of rhodium, respectively. The leaching of phosphorus was comparable to the leaching of rhodium (in PFMCH/acetone), showing that dissociation and leaching of free ligand does not take place for these rhodium diphosphine catalysts.