Fluorous phosphine

Fluorous ligands introduce an ease of purification in that the tagged phosphine ligand, the palladium catalyst complexed ligand, and the oxidized ligand can be completely removed by direct fluorous solid-phase separation (F-SPE) prior to product isolation. Similarly, an example of a fluorous palladium-catalyzed microwave-induced synthesis of aryl sulfides has been reported, whereby the product purification was aided by fluorous solid-phase extraction [91]. 

Figure 6.27. Conjugate addition of benzyl alcohol to methyl propiolate catalysed by a fluorous phosphine...

The concept makes use of the complimentary strengths and weaknesses of the two unconventional media. While ionic liquids are known to be excellent solvents for many transition metal catalysts, the solubility of most transition metal complexes in scC02 is poor (if not modified with e. g. phosphine ligands with fluorous "ponytails" [64]). However, product isolation from scC02 is always very simple, while from an ionic catalyst solution it may become more and more complicated depending on the solubility of the product in the ionic liquid and on the product s boiling point. 

The first fluorous phosphine ligands for application in fluorous phase catalysis were fluorous trialkylphosphines 1 [7-9]. 

The fluorous alkylphosphines 2 [8, 12] and 3 [8, 13], containing one or two perfluoroalkyl tails, respectively, have also been prepared. Partition coefficients of the chiral fluorous monophosphines 3a-b were found to be lower than the values measured for the corresponding phosphines 1, which contain one extra perfluoroalkyl tail [13], In addition, fluorous phosphinites, phosphonites and phosphites 4a-e and diphosphonite 5 have become available [8, 14]. 

Some of the improved fluorous arylsilylphosphines in the library 11 (a, b, x, pos) were tested in the context of hydrosilylation catalysis [24]. Rh and phosphine leaching were driven down further to non-detectable levels (<0.1%) and 0.8%, respectively, and it can be expected that even better results are possible for hydrogenation catalysis. 

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. 

Figure 1.6 Like dissolves like perfluoroalkyl ponytails make phosphines more soluble in a fluorous solvent. These phosphines are suitable ligands for metal catalysts, and will therefore aid the solubility of these catalysts in fluorous solvents...

At the time of writing, neither ionic liquids nor fluorous solvents have been used as solvents for commercial processes, although BASF use an ionic liquid as a proton scavenger in phosphine manufacture [36], These are relatively new technologies and time will tell whether or not they will fulfil current high expectations. 

The fluorous solvent alone had a minimal effect on the outcome of the reaction. However, with the fluorinated ligand, the rr.iso ratio was found to increase with increasing phosphine concentration, reaching a value of almost 8 1 at a phosphine to rhodium ratio of 103 1. The beauty of this system was demonstrated by its use in a semicontinuous hydroformylation experiment. After each... 

A fluorous version of the chiral bis-phosphine BINAP has been developed for asymmetric Heck reactions [5], Several fluorous-derivatized binaphthols and BINAP derivatives have been reported, (Scheme 10.4) [6], The silane spacer group present in one of the ligands was used to maximize the percentage fluorine on the molecule. Even so, the partition coefficient1 between FC-72 (see Chapter 3) and benzene was only 2.85, and not surprisingly, the reuse of the catalyst was poor. 

Fluorous-derivatized phosphines have also been used for Heck reactions in scCC>2 together with palladium acetate [7]. The fluorous groups improve the solubility of the catalyst in the SCF compared to nonfluorous ligands. An example of a Heck reaction that uses a fluorous-derivatized phosphine to improve the solubility of a Pd(OAc)2 catalyst is shown in Scheme 10.5. 

The reaction between iodobenzene and methyl acrylate in scCC>2 using the Pd(OAc)2/fluorous-derivatized phosphine catalyst gave a superior yield of methyl cinnamate compared to the same reaction in a conventional organic solvent. The... 

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