Fluorous Catalyst Research Articles

Rhodium‐Catalyzed Hydrogenation of Alkenes by Rhodium/Tris(fluoroalkoxy)phosphane Complexes in Fluorous Biphasic System

AbstractThe reaction of [Ir(μ‐Cl)(COD)]2 with various fluorous derivatives of triphenylphosphane containing a para‐, meta‐, or ortho‐(1H,1H‐perfluoroalkoxy)‐substituted fluorous phosphane P(C6H4‐ORf)3 (Rf=CH2C7F15, CH2CH2CH2C8F17) and CO (1 atm) gives the corresponding trans‐[Ir(μ‐Cl)(CO){P(C6H4ORf)3}2]. The IR νCO values of these complexes give some information on the donor/acceptor properties of the phosphanes. These fluorous derivatives of triphenylphosphane, as well as a phosphane bearing two (1H,1H‐perfluoroalkyloxy) chains at the 3,5‐positions, were used in association with [Rh(μ‐Cl)(COD)]2 or [Rh(COD)2]PF6 in the reduction of methyl cinnamate, 2‐cyclohexen‐1‐one, cinnamaldehyde, and methyl α‐acetamidocinnamate in a two‐phase system D‐100/ethanol under 1 bar hydrogen at room temperature. Some differences in catalytic activity were observed in the reduction of methyl cinnamate, the most active catalyst being the rhodium complex containing the phosphane with the p‐fluorous ponytail. Recycling of the fluorous catalyst was possible, particularly using the p‐substituted phosphane, where no significant loss of catalyst or activity was observed, and generally with very low leaching of rhodium or phosphane in the organic phase.

Enhanced Hydrogenation Activity and Recycling of Cationic Rhodium Diphosphine Complexes through the Use of Highly Fluorous and Weakly‐Coordinating Tetraphenylborate Anions

AbstractThe effect of the nature of the anion on the performance of ionic rhodium catalysts has received little attention. Herein it is shown that the use of highly fluorous tetraphenylborate anions can enhance catalyst activity in both conventional and fluorous media. For hydrogenation catalysts of the type [Rh(COD)(dppb)][X] {COD=1,5‐cis,cis‐cyclooctadiene; dppb=1,4‐bis(diphenylphosphino)butane; X=BF4− (1a), [BPh4]− (1b), [B{C6H4(SiMe3)‐4}4]− (1c), [B{C6H3(CF3)2‐3,5}4]− (1d), [B{C6H4(SiMe2CH2CH2C6F13)‐4}4]− (1e), [B{C6H4(C6F13)‐4}4]− (1f) and [B{C6H3(C6F13)2‐3,5}4]− (1 g)} the activity towards the hydrogenation of 1‐octene in acetone increased in the order 1c <1b <1e <1a <1d ~ 1f <1g with 1g being twice as active as the commonly applied 1a. Despite the fluorophilic character introduced by the substituted tetraarylborate anions, the presence of some perfluoroalkyl‐substituents in the cation was still required for achieving high partition coefficients. Therefore, [Rh(COD)(Ar2PCH2CH2PAr2)][X] {Ar=C6H4(SiMe2CH2CH2C6F13)‐4, X=[B{C6H3(C6F13)2‐3,5}4]− (3f); Ar=C6H4(SiMe(CH2CH2C6F13)2)‐4 and X=[B{C6H4(C6F13)‐4}4]− (2g)} were prepared, which were active in the hydrogenation of 1‐octene, 2g even more so than 3f. Both these highly fluorous catalysts could be recycled with 99% efficiency through fluorous biphasic separation, whereas the corresponding BF4− complex of 2g (2a) did not show any affinity for the fluorous phase.

Use and recovery of a homogeneous catalyst with carbon dioxide as a solubility switch.

A method for fluorous biphasic catalysis is described,in which the fluorous liquid is replaced by fluorinated silica, the fluorous catalyst is induced to dissolve in the organic solvent by the presence of CO2, and the recovery of the catalyst after the reaction is achieved by release of the CO2 pressure.

Hydrolytic Kinetic Resolution of Terminal Epoxides Catalyzed by Fluorous Chiral Co(salen) Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

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

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.

Hydrolytic kinetic resolution of terminal epoxides catalyzed by fluorous chiral Co(salen) complexes

Cobalt complexes of fluorous chiral salen ligands have been synthesized and tested as catalysts in the hydrolytic kinetic resolution of terminal epoxides. Whereas the activity of heavily fluorinated complexes was found to be rather low, a ‘light fluorous’ complex was shown to be an efficient and highly selective catalyst for this asymmetric ring-opening reaction. Several strategies for the isolation of reaction products and the recovery of the fluorous catalyst are also discussed.

A Fluorous Super Brønsted Acid Catalyst: Application to Fluorous Catalysis without Fluorous Solvents

The preparation of a fluorous super Bronsted acid catalyst, 4-(1H,1H-perfluorotetradecanoxy)-2,3,5,6-tetrafluorophenylbis(trifluoromethanesulfonyl)methane (3d), is described. The fluorous catalyst 3d can be recycled by using liquid/solid phase separation without fluorous solvents. A perfluorocarbon solvent is not essential for fluorous biphasic catalysis.

Chiral fluorous catalysts: synthesis and purposes

Despite the growing number of applications of fluorous biphasic catalysis and related techniques, only a few examples of asymmetric reactions carried out under these conditions have been reported up to now. Scope and limitation of this approach are thus far to be assessed. Our efforts toward this goal will be presented here, with emphasis on the design of efficient chiral fluorous catalysts.

Asymmetric Epoxidation of Alkenes in Fluorinated Media, Catalyzed by Second-Generation Fluorous Chiral (Salen)manganese Complexes

The synthesis of sterically hindered chiral (salen)manganese complexes bearing perfluoroalkyl ponytails and their use in asymmetric epoxidation reactions are described. For better understanding of the relative influences of steric and electronic effects on the enantioselectivity of the fluorous catalysts, the epoxidation of 1,2-dihydronaphthalene and benzosuberene was first studied under homogeneous conditions. It was shown that the presence of sterically demanding tert-butyl groups and, to a lesser degree, the displacement of the electron-withdrawing perfluoroalkyl substituents from the ligand core provide ees higher than those attainable with first generation fluorous chiral (salen)manganese complexes featuring perfluoroalkyl substituents in the key positions (3,3′ and 5,5′) in the ligand. Second generation catalysts (Mn-6)C7F15COO and (Mn-7)C7F15COO were successfully employed in the fluorous biphase epoxidation of alkenes with PhIO as the oxidant and pyridine N-oxide as an additive. Epoxide yields (68−98%) and ees (50−92%) were similar to those obtained with the same oxidizing system and standard (salen)manganese complexes under homogeneous conditions. When the reaction was complete, the fluorous layer in which the catalyst was immobilized was easily recoverable by simple phase separation at room temperature and could be used up to three times before significant decline in yield and enantioselectivity was observed.

Fluorous Versions of Wilkinson's Catalyst. Activity in Fluorous Hydrogenation of 1-Alkenes and Recycling by Fluorous Biphasic Separation

Two novel fluorous tris(arylphosphine)rhodium(I) chloride complexes RhCl(P{C6H4-p-SiMe2- (CH2)2CnF2n+1}3)3 (3a: n ) 6, 3b: n ) 8) being fluorous analogues of Wilkinson's catalyst show high activity in hydrogenation of 1-alkenes under single phase fluorous conditions and can be effectively recycled using the new concept of fluorous biphasic separation. For comparison the p-(trimethylsilyl)-substituted derivative RhCl(P(C6H4-p-SiMe3)3)3 (3c) has also been synthesized and the X-ray structure of dimeric (Rh(I-Cl){P(C6H4- p-SiMe3)3}2)2 (4c) was determined. A comparison of the catalytic activity of the fluorous catalysts and the nonfluorous derivatives under identical conditions revealed a decreasing activity in the order 3c > 3a > RhCl- (PPh3)3 > 3b. The recycling efficiency of the new catalysts (> 98% for 3b) is much better than expected on the basis of the fluorous phase affinity of the free phosphine ligands themselves and is in fact the result of the much higher fluorous phase affinity of the phosphine-containing rhodium species that are present during and after catalysis. Hence it is demonstrated that assembling multiple fluorous ligands in a specific configuration at one metal center can drastically improve the fluorous phase affinity of the resulting complexes.