Anionic catalyst precursors

Although greater solubility in SCCO2 was observed with the borate (BArF) anion catalyst precursor than the triflate precursor, the more soluble precursor did not always give the greatest enantioselectivity. This article (60) was the first demonstration that asymmetric homogeneous catalysis could be performed in SCFs and the first demonstration that the technique could lead to greater enantioselectivity than that obtained in traditional solvents. 

Anionic Catalyst Precursors. Excellent results in the hydrogenation of olefins are obtained by labilization of the cluster by the formation of anionic species. As discussed in Sect. 2.4.2, this method is often used for the activation of clusters in substitution reactions. 

Togni s [38] approach was therefore to test the ability of sparteine to act as an ancillary ligand in Pd(II)-allyl complexes—susceptible to nucleophilic attack by stabihzed anions such as Na[CH(COOMe)2]—which could be employed as catalyst precursors. In addition he speculated that the rather rigid and bulky sparteine would be able to induce significant differentiation between the two diastereotopic sites of 1,3-disubstituted allyl hgand, thus leading to enantioselection upon nucleophilic attack. 

The catalyst precursor generally used for the reaction is rhodium dicarbonyl acetylacetonate. However, detailed infrared studies under the reaction conditions (ca. 1000 bar CO/H2 and 200°C) have shown both the [Rh(CO)4] and the [Rh12(CO)34 36]2 anions to be present in various concentrations at different stages of the reaction (62, 63). It is suggested that rhodium carbonyl clusters, characterized as having three intense infrared absorptions at 1868 10, 1838 10, and 1785 10 cm-1, are responsible for the catalysis (62), and it is believed that the reaction is dependent upon the existence of the following equilibria ... 

The catalyst precursors (cationic Ir-COD complexes with weakly coordinating anions such as BArF) are air- and moisture-stable, and are therefore easy to handle. It is possible to store them for several months under air. Two of the most versatile catalysts (ThrePHOX catalysts, 12a and 12b) recently became commercially available [33]. 

Some fimctionalized materials have also been prepared by coprecipitation at low supersaturation. A perylene chromophore, for example, has been intercalated into LDH in an attempt to prepare stabihzed pigments [40]. Catalyt-ically active species have also been introduced into the interlayers of LDHs by direct synthesis, e.g. the intercalation of (PWi204o) or (SiWi204o) gives catalysts or catalyst precursors containing interlayer polyoxometalate anions [41]. Vein et al. reported the synthesis of Zr-containing LDH-Hke... 

To examine the effect on the catalysis of the anion of the metal complex, Brandts [96] immobilized the two different catalyst precursor complexes [Rh(COD)2]BE4 and [Rh(COD)Cl]2 on phosphotungstic acid-modified alumina to form 7-AI2O3/... 

The SHB concept was expanded to chiral phosphine catalysts by de Rege et al., who reacted the trifluoromethanesulfonate (triflate) counter anion of the cationic complex [Rh(COD)((R,Rj-MeDuPhos)] with the surface hydroxyl groups of the silaceous mesoporous material MCM-41 [122]. The complex was loaded to a level of 1.03 wt% Rh. A decrease in support surface area and pore volume is consistent with the complex being located within the support pores. The counterion is very important in this process if the anion of the homogeneous catalyst precursor is altered to BArp no adsorption of the catalyst is observed. It is postulated that the mechanism of triflate binding is hydrogen bonding with the support, and that the... 

The effect of cations and anions of several ionic liquids was investigated for the performance of several catalyst precursors, including RuCh and RuCl2(PPh3)3, for aerobic oxidation of alcohols 199).The catalysts in the neutral ionic liquids... 

Under these conditions the catalyst precursor XXb containing the triflate anion could be isolated and evaluated in metathesis. The catalyst XXb underwent efficient RCM reaction of dienes and enynes (Table 8.5) [54]. 

It is noteworthy that the indenyl complex RuCl(ri -C9H7)(PPh3)2l4 provides an efficient catalyst precursor for the anti-Markovnikov hydration of terminal alkynes in aqueous media, especially in micellar solutions with either anionic (sodium dode-cylsulfate (SDS)) or cationic (hexadecyltrimethylammonium bromide (CTAB)) surfactants [38]. This system can be applied to the hydration of propargylic alcohols to selectively produce P-hydroxyaldehydes, whereas RuCl(Cp)(PMe3)2 gives a,P-unsat-urated aldehydes (the Meyer Schuster rearrangement products)(Scheme 10.8) [39]. 

In an earlier related study, Evans and Newell (112) demonstrated the anionic iron carbonyl hydrides [HFe3(CO),][PNP] and [HFe(CO)4][PNP] to be catalytically active for this reaction, generating yields no greater than 5.8 1 moles of formate per mole of catalyst precursor. The low yields were attributed to catalyst degradation caused by oxidation by carbon dioxide as evidenced by the detection of carbonates in the system. 

---