Phosphine catalysts hydrogenation

The hydrogenation of C02 in the presence of amines to give dialkylformamides has been carried out directly in an IL/scC02 system. In this case, the ionic liquid was shown to play a dual role [74]. It is an effective solvent for the ruthenium phosphine catalyst and at the same time allows a distinct phase distribution of the polar carbamate intermediates and the less polar products formed during the conversion of C02. As a result, the selectivity of the reaction can be increased over conditions where scC02 is used as the sole reaction medium. 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

Besides the major thrust using chiral phosphine catalysts, other chiral ligands have been used with rhodium and other metals for asymmetric hydrogenation. 

In the hydrogenation of alkenes, rhodium-, ruthenium- and iridium-phosphine catalysts are typically used [2-4]. Rhodium-phosphine complexes, such as Wilkinson s catalyst, are effective for obtaining alkanes under atmospheric pres-... 

Recently, carbene-oxazoline catalysts 33 and carbene-phosphine catalysts 34 (Fig. 29.19) with a chiral paracyclophane backbone have been synthesized and used to hydrogenate a variety of alkenes, with modest selectivity [41]. 

Hydrosilylation of imine compounds was also an efficient method to prepare amines. The hydrosilylation product TV-silylamines can readily be desilylated upon methanol or water treatment, yielding the corresponding amines. The amines can be converted to their corresponding amides by subsequent acyl anhydride treatment. The first attempt to hydrogenate prochiral imines with Rh(I) chiral phosphine catalysts was made by Kagan102 and others. These catalysts exhibited low catalytic activity, and only moderate ee was obtained. 

This type of reverse set-up has been expanded to catalysts with phosphines containing crown ether substituents (Figure 8.1), with the crown ether acting as a built-in phase-transfer function [5], Using a catalyst with this phosphine, the hydrogenation of Li+, Na+, K+ and Cs+ cinnamates in water-benzene solvent mixtures was considerably faster than when the analogous catalyst was used with triphenylphosphine ligands. 

As mentioned earlier, in the Ruhrchemie-Rhone Poulenc process for propene hydroformylation the pH of the aqueous phase is kept between 5 and 6. This seems to be an optimum in order to avoid acid- and base-catalyzed side reactions of aldehydes and degradation of TPPTS. Nevertheless, it has been observed in this [93] and in many other cases [38,94-96,104,128,131] that the [RhH(CO)(P)3] (P = water-soluble phosphine) catalysts work more actively at higher pH. This is unusual for a reaction in which (seemingly) no charged species are involved. For example, in 1-octene hydroformylation with [ RhCl(COD) 2] + TPPTS catalyst in a biphasic medium the rates increased by two- to five-fold when the pH was changed from 7 to 10 [93,96]. In the same detailed kinetic studies [93,96] it was also established that the rate of 1-octene hydroformylation was a significantly different function of reaction parameters such as catalyst concentration, CO and hydrogen pressure at pH 7 than at pH 10. 

The same authors compared catalysts prepared from these precursors and [Ru(BINAP)Cl2]2 adsorbed on MCM-41 (with 26 and 37 A pores) and an amorphous mesoporous silica (with 68 A pores) all treated with combinations of SiPh2Cl2 and Si(CH2)3X (X = NH2, CO2H). Catalysts were also prepared in which the organometallic precursors were immobilized by entrapment into silica (using sol-gel techniques). This is one of the few studies in which the performance of chiral phosphine catalysts immobilized by covalent and noncovalent procedures are compared directly. The materials were examined as catalysts for the hydrogenation of sodium a-acetamidocinnamate and of a-acetamidocinnamic acid under similar conditions to those used for the catalysts on unmodified MCM-41. The catalysts... 

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... 

Other hydrogenation methods are less chemoselective. Use of Raney nickel provides hydroxylamines in low yield °. Hydrogenation of 1-acetonaphthone oxime over rhodium-chiral phosphine catalysts was found to proceed under harsh conditions and provided low... 

An efficient asymmetric hydrogenation of a-acetamidocin-namic acids has been achieved using a rhodium-chiral phosphine catalyst. This paper describes the preparation of the catalyst and the hydrogenation procedure as well as studies on structure vs. activity. 

The catalyst is added to a solution of 4.3 g of avermectin (Bla and Blb mixture) in 25 ml of a mixture of acetone and cyclohexane in a ratio of 2 1. After addition of 51.4 mg of tris-(mexylphenyl)phosphine, the hydrogenation is carried out in a steel autoclave at a hydrogen pressure 5 bar and at 88°C. After a hydrogenation time of 4 hours, 8.9% of starting material, 89.9% of ivermectin (Bla and Blb mixture), tetrahydroavermectin content <0.1% was obtained (according to HPLC analysis). 

Myrcene and a-terpinene contain conjugated double bonds and are not as reactive as limonene and a-pinene. The product mixture is complex besides the hydrogenated products and the alcohols, undefined high boiling point products also occur [42]. The two conjugated terpenes were also studied with rhodium phosphine catalysts. Within 7 h, 96% of a-terpinene reacts to aldehydes with high selectivity for the product shown in Scheme 13 [43]. 

Homogeneous rhodium(I)-chiral tertiary phosphine catalysts have been used to hydrogenate ketones directly and to hydrosilylate ketones and imines thus accomplishing, after hydrolysis, indirect hydrogenation. 

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