Fluorous rhodium complexes

The ketone hydrosilylation shown in Fig. 7 was used as a test reaction. This can be catalyzed by the fluorous rhodium complexes 16-Rf6 and 16-Rfs under fluorous/organic liquid/liquid biphase conditions [55,56]. These red-orange compounds have very httle or no solubihty in organic solvents at room temperature [57]. However, their solubilities increase markedly with temperature. Several features render this catalyst system a particularly challenging test for recovery via precipitation. First, a variety of rest states are possible (e.g., various Rh(H)(SiR3) or Rh(OR )(SiR3) species), each with unique solubility properties. Second, the first cycle exhibits an induction period, indicating some fundamental alteration of the catalyst precursor. 

Figure 6.2. Fluorous biphasic hydrogenation of methyl t/h-cinnamate catalysed by rhodium complexes.[23]...

Figure 6.7. Hydroformylation of an alkene using a rhodium complex bearing a fluorous ponytail.[l, 22]...

To solve the issue of ligand leaching that was encountered in some of the examples above, fluorous polymeric phosphine ligands 15a-c [28] were developed. The rhodium complexes prepared from 15a-c using a 3 1 ratio of P Rh [28b, 29] displayed good turnover frequencies (TOFs) in the case of 15 a, but reaction rates for 15b,c were lower. The catalyst derived from 15 a was recycled seven times without loss of activity, although leaching was not studied quantitatively. 

Alternatively, an insoluble fluorous support, such as fluorous silica [43], can be used to adsorb the fluorous catalyst. Recently, an eminently simple and effective method has been reported in which common commercial Teflon tape is used for this purpose [44]. This procedure was demonstrated with a rhodium-catalyzed hydrosilylation of a ketone (Fig. 9.27). A strip of Teflon tape was introduced into the reaction vessel and when the temperature was raised the rhodium complex, containing fluorous ponytails, dissolved. When the reaction was complete the temperature was reduced and the catalyst precipitated onto the Teflon tape which could be removed and recycled to the next batch. 

A different extractive work-up is based on fluorous biphasic systems. This concept was first introduced for fhe recovery of rhodium complexes from hydroformylation processes [13] and was soon extended to separation procedures in combinatorial chemistry [14]. It has been fhe subject of several reviews [15-21]. 

Rhodium complexes with the fluorous phosphine P(C6H4-4-OCH2C7F15)3 are active catalysts in the hydroformylation of 1-octene in fluorous biphasic systems [the turnover frequency (TOF) was 380 h 11 (Equation 4.28). Selectivity in aldehydes was as high as 99% and regioselectivity for the linear aldehyde up to an n/iso ratio of 2.8 [49]. 

A new separation procedure for homogeneous catalysts that combines a membrane reactor and SCCO2 was recently developed (Scheme 89 and Figure 4.6). Amixture of a rhodium complex with a fluorous phosphine ligand 49,1 -butene, hydrogen and SCCO2 is introduced into a membrane reactor with a pore size of 0.6 nm. Since... 

Homogeneous hydrogenation in the fluorous phase has been so far reported only for a limited set of simple olefins (Richter et al., 1999, Rutherford et al., 1998), as exemplified with the neutral rhodium phosphine complex 18 as catalyst precursor (eq. 5.7). Isomerization of the substrate 1-dodecene (17a) was observed as a competing side reaction under the reaction conditions. The catalyst formed from 18 could be recycled using a typical FBS protocol, but deactivation under formation of metal deposits limited the catalyst lifetime. 

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