Polysiloxane derived catalysts

The feasibility of bonding pyridinyl groups to silicon which contains a hydrolytically sensitive functional group has recently been demonstrated 15-71. 2-Fluoro-3-(dimethylchlorosilyl)pyridine and 3-fluoro-4-(dimethylchloiosilyl)pyridine as well as 2-, 3-, and 4-(dimethylchlorosilyl)pyridine were prepared by the reaction of the corresponding lithiopyridines with excess Me2SiCl2- Hydrolysis of the pyridinyl substituted chlorosilanes gave disiloxanes which were insoluble in water. In the present report we will describe extension of this work to include pyridinyl dichlorosilanes which can be hydrolyzed to polysiloxanes. These polymers can be N-oxidized and the resultant derivatives have been shown to be effective hydrophobic transacylation catalysts. 

This initial observation by Inoue et al. triggered intensive research in this area. Most of the efforts were dedicated to structural variation of the catalyst and to elucidation of the catalytic mechanism. With regard to the former, the many structural variations produced mainly confirmed 1 as the optimum catalyst. Variation of the aromatic amino acids involved [25, 26], side-chain methylation and/or modification [27], N-methylation [28], etc., all afforded catalysts of lower selectivity. In contrast, incorporation of a-Me-Phe led to diketopiperazines of activity and selectivity comparable with those derived from non-methylated Phe (for example 1) [29]. Similarly, attachment to Merrifield-resin or polysiloxane polymers proved detrimental to the enantioselectivity of the Inoue-catalyst 1 [30, 31]. Upon incorporation into a silicon based sol-gel glass matrix, however, the excellent enantioselectivity of the cyclic peptide 1 is preserved, and separation of the spent catalyst can easily be achieved by, e.g., filtration, centrifugation or simply decantation [32], Unfortunately, further catalytic cycles afforded much lower ee (ca. 30-35% max.) [32],... 

It is well known that hydrosilylation processes usually catalyzed by Pt and Rh complexes can be efficiently applied in polymer chemistry. Ru3(CO)12 was effectively used for the functionalization of polysiloxanes via hydrosilylation of allyl derivatives with polymethylhydrosiloxanes [175]. On the other hand, polymerization via coupling of activated aromatics with dienes occurs mostly in the presence of ruthenium complexes as catalysts (Eq. 111). For representative references see Ref. [176] and papers cited therein. 

Cinchonidine displays a tertiary amine, an aromatic amine and a free hydroxyl functionality. Direct hydrosilylation with unprotected cinchonidine (derivatized with a double bond for hydrosilylation), led to a cross-linked product. Thus, hydrosilylation was performed on PHMS with the trimethylsilyl derivative 1 (Fig. 10), in the presence of (EtjS)jPtClj as catalyst (0.05%), for 6h at 80°C in toluene. The hydroxyl group was deprotected with methanol at 65°C during 120 h. Size Exclusion Chromatography showed that the polysiloxane backbone was not degraded. A maximal grafting percent of 15% could be obtained, relative to the SiH units. 

The results of these studies and others reported previously demonstrate that the 1-oxypyridinyl group is an effective catalyst for the transacylation reactions of derivatives of carboxylic and phosphoric acids when incorporated in small molecules and polymers. Furthermore, this catalytic site exhibits high selectivity for acid chlorides in the presence of acid anhydrides, amides, and esters. Therefore, catalysts bearing this group as the catalytic site can be used successfully in synthetic applications that require such specificity. The results of this work suggest that functionalized polysiloxanes should be excellent candidates as catalysts for a wide variety of chemical reactions, because they combine the unique collection of chemical, physical, and dynamic-mechanical properties of siloxanes with the chemical properties of the functional group. Finally, functionalized siloxanes appear to mimic effectively enzyme-lipophilic substrate associations that contribute to the widely acknowledged selectivity and efficiency observed in enzymic catalysis. 

Hybrid versions of silicone-thermoplastic semi-IPNs have been developed (19). A hybrid interpenetrating network is one in which the cross-linked network is formed by the reaction of two polymers with structurally distinct backbones. Hydride-functionalized siloxanes can be reacted with organic polymers with pendant unsaturated groups such as polybutadienes (5) in the presence of platinum catalysts. Compared with the polysiloxane semi-IPNs discussed earlier, the hydride IPNs tend to maintain mechanical and morphologically derived properties, whereas properties associated with siloxanes are diminished. The probable importance of this technology is in cost-effective ways to induce thermoset characteristics in thermoplastic elastomers. 

On the other hand, there is a need to use inhibitors of the platinum catalysts temporarily to reduce their catalytic activity in the presence of hydro- and vinyl-polysiloxanes in order to stop the curing process at room temperature, but to allow the platinum catalyst to be activated at elevated temperature. Among the principal types of compounds reported are alkenyl derivatives, esters of unsaturated acids, crown ethers, organic nitrogen compounds, phosphines, linear and cyclic vinyl-siloxanes, and poly(vinyl)siloxanes [2], and recently fumarate [44] and maleinate [33]. New co-activators of the catalysts (precursors) have been revealed in the 1990s to reduce to ppm the levels of platinum required to effect hydrosilylation curing [45, 46]. 

The same authors prepared a sales of allyl-group-containing bifunctional carbohydrate derivatives that were reacted with hydrodimethylsUyl-taminated polysiloxane using Speier s catalyst [132]. 

The application of polymer-supported catalysts has now been extended to the synthesis of complexes between transition metal derivatives and structurally ordered macromolecular ligands to give catalytic systems exhibiting high activity and stereoselectivity. Polystyrene and polymethacrylate resin and polystyrene-divinylbenzene-polystyrene-polybutadiene block copolymers, as well as vinyl-functionalized polysiloxanes grafted onto silica, are very suitable polymers for heterogenization of mostly Pt and Rh complexes. Moreover, polyamides exhibit much higher thermal stability than conventional polystyrene supports (114). 

Examples of sohd-bound Pd-catalyzed carbonylation of aryl and alkenyl halide, allyl alcohol, and derivatives are abundant in the literature. Polyketones have been obtained via carbonylation of ethylene and carbon monoxide catalyzed by palladium complexes of polysiloxane-bound phosphinet t or Pd(dppp) absorbed on alumina.f f Similar processes can also be carried out by catalyst formed simply by absorbing Pd(02CNEt2)2(NHEt2)2 onto silica geL Polyphosphine-bound palladium has been used to prepare ethyl hexanoate from 1-pentene, CO, and ethanol. Similar esterification of styrene has been achieved using a bimetallic system involving palladium and nickel immobilized on poly(Af-vinyl-2-pyrrolidone).f ... 

---