Polymer-supported oxidation catalysts

Scheme 10.19 Non-chiral polymer-supported oxidation catalysts and the use of one them in the direct oxidation of alcohols to a-tosyloxyketones (115).

Several examples are known of the enantioselective conversion of alkenes into epoxides with the use of polymer-supported oxidation catalysts. This can be traced to the pioneering work by Julia and Colonna in 1980. They demonstrated that highly enantioselective epoxidations of chalcones and related a, 3-unsaturated ketones can be achieved with the use of insoluble poly(a-amino acids) (116, Scheme 10.20) as catalysts [298-301]. The so-called Julia-Colonna epoxidation has been the object of several excellent reviews [302-306]. The terminal oxidant is H202 in aq. NaOH. With lipophilic amino acids as the components, such as (SJ-valine or (SJ-leucine, enantioselectivities as high as 96-97% ee were obtained. The enan-tioselectivity depends of several factors, including the side-chain of the amino acid, the nature of the end groups and the degree of polymerization. Thus, for instance,... 

Oxidative cleavage of alkenes using sodium periodate proceeds effectively in a monophasic solution of acetic acid, water, and THF with very low osmium content or osmium-free. The orders of reactivity of alkenes are as follows monosubstituted trisubstituted >1,2 disub-stituted > 1,1-disubstituted > tetrasubstituted alkynes.100 Cleavage with polymer-supported OSO4 catalyst combined with NaI04 allows the reuse of the catalyst.101... 

Shibasaki et al. developed a polymer-supported bifunctional catalyst (33) in which aluminum was complexed to a chiral binaphtyl derivative containing also two Lewis basic phosphine oxide-functionahties. The binaphtyl unit was attached via a non-coordinating alkenyl Hnker to the Janda Jel-polymer, a polystyrene resin containing flexible tetrahydrofuran-derived cross-Hnkers and showing better swelling properties than Merifield resins (Scheme 4.19) [105]. Catalyst (33) was employed in the enantioselective Strecker-type synthesis of imines with TMSCN. 

Table 15) highlights the stability of this system compared to the PS/MTO system (entry 6, Table 15), which shows a decrease in activity during recycling. This difference in behaviour may be due to the weaker interaction between MTO and the PS polymer, which is only accomplished by the physical envelopment of the benzene ring. The PVP/MTO combination was successfully used for other compounds of biological interest, such as ter-penes. Even highly sensitive terpenic epoxides, hke a-pinene oxide, can be obtained in excellent yields using polymer-supported MTO catalysts [73] (Scheme 20, Table 16). 

Miyamura et al. [170] and Kanaoka et al. [171] have succeeded in stabilizing Au clusters on polymer supports for aerobic oxidation at room temperature in the mixed solvent of water-benzotrifluoride and in water, respectively. Polymer supports could also offer new functions, such as a recycling system by using a thermoresponsive polymer-supported Au catalyst [171]. 

Photocatalytic Polymers. The electrochemical experiments cited above were chosen from a vast body of recent polymer coated electrode work. Likewise the field of polymer supported photoredox catalysts is also broad and has a more extended history. Possibly a common linkage between electrochemical and photochemical catalyses, assisted by polymers, can be traced to oxidation of ascorbic acid (AH2). In 1966 Davidov (20a) found that light exposed, polyacrylonitrile (PAN) containing solutions of AH2 consumed oxygen in a measurably different manner than similar solutions without AH2. (Simple photoabsorption of 02 also occurs). 

Although the oxidation of tertiary phosphines by these catalytic processes has minimal useful application, it needs to be considered as a problematic side reaction in homogeneous catalysis. Much effort is being currently expended to immobilize platinum metal phosphine complexes on heterogenized tertiary phosphine supports, and irreversible oxidation at phosphorus on these supports effectively destroys the supported catalyst. Recent observations that the compound Rh6(CO)i6 catalyzes the oxidation of tertiary phosphines correlate with the report that phosphine oxidation occurs with molecular oxygen on Rh6(CO)i6 bound to diphenylphosphino-functionalized poly(styrenedivinylbenzene). Thus, in order to use these phosphinated polymer-supported rhodium catalysts, one needs either to rigorously exclude oxygen, or to find a way to inhibit the simultaneous catalyzed phosphine oxidation. 

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