Polymer-anchored phosphines

These observations suggested that Pd (II) chloride bound to phosphine-substituted polystyrene might be an eflFective catalyst for the conversion of quadricyclane to norbornadiene. We therefore investigated the preparation of such a polymer-anchored phosphine Pd (II) chloride catalyst. 

Recovery of Pd after reactions is important in commercial processes, but it is not always easy to collect Pd from solutions [31]. Pd can be recovered as insoluble complexes such as the dimethylglyoxime complex or PdCl2(PPh3)2 by treatment with HCl and PPh3. Removal of a very small amount of Pd, remaining in a solution, or purification of reaction products contaminated with a trace of Pd, can be done by treating the solution with active charcoal, polyamines, polymer-anchored phosphines and P(n-Bu)3 [32]. The Pd can be collected in solution by coordination or absorption. 

These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

Figure 7.1. Chiral diphosphine, diphosphite, phosphine-phosphite, phosphinc-phosphinite, aminc-phosphinite, and polymer-anchored ligands used for asymmetric hydroformylation.

The homogeneous chiral phosphine/DPEN-Ru catalyst can be immobilized by use of polymer-bound phosphines such as polystyrene-anchored BINAP (APB-BINAP) [57, 98], Poly-Nap [99], and poly(BINOL-BINAP) [100], poly(BINAP) [101]. These complexes hydrogenate T-acetonaphthone and acetophenone with S/C of 1000-10 000 under 8 10 atm H2 to give the corresponding secondary alcohols in 84-98% e.e. The recovered complexes are repeatedly used without significant loss of reactivity and enantioselectivity. Immobilization allows the easy separation of catalyst from reaction mixture, recovery, and reuse. These advantages attract much attention in combinatorial synthesis. 

Methylmethacrylate has also been used as a reactant for hydroformy-lations with polymer-anchored rhodium catalysts. Low efficiencies were obtained at high temperatures because of competing polymerization and the branched product predominated. The polymer-supported catalysts gave lower activities and a higher proportion of branched product than the homogeneous analogue at comparable phosphine Rh ratios. 

The substituted iron carbonyls Fe(CO)4PPh3 and Fe(CO)3(PPh3)2 have also been examined as photocatalysts for hydrosilation however, the qualitative reactivity features were found to be similar to those of Fe(CO)s [74]. In addition the polymer anchored derivatives Fe(CO)4(PPh2-poly) and Fe(CO)3(PPh2-poly)2 (where PPh2-poly is the polystyrene bound diphenyl phosphine) proved to be effective photocatalysts for lx)th hydrosilation and hydrogenation [74]. Photocatalysis of alkene hydrosilation is also effected by metal carbonyl clusters [61]. 

Hughes, O.R. and Unruh, J.D. (1981) Hydroformylation catalyzed by rhodium complexes with diphosphine ligands. Journal of Molecular Catalysis, 12,71 Sanger, A.R. (1977) Hydroformylation of 1-hexene catalysed by complexes of rhodium(I) with di- or tritertiary phosphines. Journal of Molecular Catalysis, 3,221 Sanger, A.R. and Schallig, L.R. (1977) The structures and hydroformylation catalytic activities of polyphosphine complexes of rhodium(l), and related complexes immobilised on polymer supports. Journal of Molecular Catalysis, 3, 101 Pittman, C.U. and Hirao, A. (1978) Hydroformylation catalyzed by cis-chelated rhodium complexes - extension to polymer-anchored cis-chelated rhodium catalysts. The Journal of Organic Chemistry, 43, 640. 

Hetorogenized Catalysts.—Reaction of [Ru(NH3)60H] + with a Faujasite-type zeolite gives a supported Ru complex, which effects hydroformylation of ethylene the catalytic species may be ruthenium clusters that are trapped in the zeolite cages. The effect of reaction conditions upon the selectivity of the hydroformylation of methyl methacrylate with [RhH(CO)(PPh3)3] or its polymer-anchored analogue has been investigated and hydroformylation of hex-l-ene and cyclo-octa-1,5-diene has been carried out with cobalt, rhodium, and platinum-tin complexes anchored to an ion-exchange resin via quaternary amino-phosphines. ... 

This is an ion-exchanger like the sulfonated polymer. The siUca surface can also be functionalized with phosphine complexes when combined with rhodium, these give anchored complexes that behave like their soluble and polymer-supported analogues as catalysts for olefin hydrogenation and other reactions ... 

An example of such an analysis of rhodium phosphine ligand anchored catalysts is presented (58). A survey scan of a copolymer containing a diphosphine ligand prior to introduction of the metal is shown in Figure 1, spectrum h. The presence of phosphorus in the polymer is evident from this spectrum. 

All the polymers of Table III have been applied for the epoxidation of olefins with alkyl hydroperoxides. For example, the polymers with iminodiacetic acid or diethylene triamine groups were used for the regioselective epoxidation of (E)-geraniol with t-BuOOH to the 2,3-epoxide (225), whereas the Mo anchored to the diphenylphosphinopolystyrene catalyst is used in the epoxidation of cyclohexene with t-BuOOH (228). The polymer-supported molybdenyl thioglycolate has also been used for the catalytic oxidation of thiols and phosphines with air or pyridine N-oxide as the oxidant (234). 

A remarkable example of the cooperation of different active sites in a polyfunctional catalyst is the one-step synthesis of 2-ethylhexanol, including a combined hydroformylation, aldol condensation, and hydrogenation process [17]. The catalyst in this case is a carbonyl-phosphine-rhodium complex immobilized on to polystyrene carrying amino groups close to the metal center. Another multistep catalytic process is the cyclooligomerization of butadiene combined with a subsequent hydroformylation or hydrogenation step [24, 25] using a styrene polymer on to which a rhodium-phosphine and a nickel-phosphine complex are anchored (cf Section 3.1.5). 

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