Phosphine complexes of rhodium

Wilkinson Hyd.rogena.tion, One of the best understood catalytic cycles is that for olefin hydrogenation in the presence of phosphine complexes of rhodium, the Wilkinson hydrogenation (14,15). The reactions of a number of olefins, eg, cyclohexene and styrene, are rapid, taking place even at room temperature and atmospheric pressure but the reaction of ethylene is extremely slow. Complexes of a number of transition metals in addition to rhodium are active for the reaction. 

Scheme 2.12. Enantioselective Reduction of 2-Acetamidoacrylic Acids by Chiral Phosphine Complexes of Rhodium...

An immense number of phosphine complexes of rhodium have been synthesized and detailed compilations of information are available [3d]. In this section, some case studies are presented to illustrate the variations that arise as a result of changes in the steric requirements of the phosphine. 

This index is divided by element into eight parts. Each part is subdivided into sections devoted to each oxidation state, preceded by a general section. Thus if you want to fmd out about phosphine complexes of Rhodium, there is a general entry to phosphine complexes as well as separate references to phosphine complexes trader the headings of Rhodium(0), (I), (II) and (III). 

It is to be mentioned that water-soluble phosphine complexes of rhodium(I), such as [RhCl(TPPMS)3], [RhCl(TPPTS)3], [RhCl(PTA)3], either preformed, or prepared in situ, catalyze the hydrogenation of unsaturated aldehydes at the C=C bond [187, 204, 205]. As an example, at 80 °C and 20 bar H2, in 0.3-3 h cinnamaldehyde and crotonaldehyde were hydrogenated to the corresponding saturated aldehydes with 93 % and 90 % conversion, accompanied with 95.7 % and 95 % selectivity, respectively. Using a water/toluene mixture as reaction medium allowed recycling of the catalyst in the aqueous phase with no loss of activity. 

For this complex, molecular chemistry does not adequately model the surface reactivity and the latter is strongly influenced by the presence of surface hydroxyl groups [22]. The organometallic fragments immobilized on silica have been reacted with trimethylphosphine to afford different silica-supported phosphine complexes of rhodium. The course of the reaction depends strongly on the hydroxyl content of the silica surface [23] (Scheme 7.2). 

The investigation of phosphine complexes of rhodium(I) as catalysts (or catalyst precursors) for the hydroformylation reaction continues both to better elucidate the reaction mechanism and to improve catalyst activity. The presence of dioxygen often decreases the catalytic activity (139), but can also, surprisingly, reactivate hydroformylation catalysts... 

Chemistry of Tertiary Phosphine Complexes of Rhodium, Iridium, and Platinum... 

The products of oxidative addition of acyl chlorides and alkyl halides to various tertiary phosphine complexes of rhodium(I) and iridium(I) are discussed. Features of interest include (1) an equilibrium between a five-coordinate acetylrhodium(III) cation and its six-coordinate methyl(carbonyl) isomer which is established at an intermediate rate on the NMR time scale at room temperature, and (2) a solvent-dependent secondary- to normal-alkyl-group isomerization in octahedral al-kyliridium(III) complexes. The chemistry of monomeric, tertiary phosphine-stabilized hydroxoplatinum(II) complexes is reviewed, with emphasis on their conversion into hydrido -alkyl or -aryl complexes. Evidence for an electronic cis-PtP bond-weakening influence is presented. 

We conclude that in octahedral alkyliridium(III) complexes the presence of tertiary phosphines favors exclusively the n -alkyl over the corresponding secondary alkyl, irrespective of the size or basicity of the phosphine. This preference is probably largely electronic in origin, but steric factors cannot be ruled out. A key step that generates a vacant coordination site for both alkyl-group migration and isomerization in octahedral tertiary phosphine complexes of rhodium(III) and iridium(III) is dissociation of halide ion. 

It should be pointed out that asymmetric reactions other than hydrogenation have been carried out with chiral phosphine complexes of rhodium (and a few other metals). For example, asymmetric hydrosilylations (addition of Si—H across C=C, C=0, and C=N bonds) have been catalyzed by such complexes... 

The tris(tri-n-butyl) phosphine complex of rhodium trichloride has the trans (mer) structure X (23). Two 31P resonances are observed with... 

There are a number of reviews available on homogeneous hydrogenation. Specialized reviews are available on hydrogenation with phosphine complexes of rhodium, " on hydrogenation of arenas, on hydrogenation with water-soluble catalysts, " and on mechanistic aspects. There are general reviews covering the older and more recent literature. 

It is to be mentioned that water-soluble phosphine complexes of rhodium(I), such as [RhCl(TPPMS)3], [RhCl(TPPTS)3], [RhCl(PTA)3],... 

Note added in proof In the preparation of rhodium and iridium complexes from cyclo-octa-1,5-diene, however, all the uncomplexed olefin is converted to the 1,3-diene. The same isomerization is catalyzed by tertiary phosphine complexes of rhodium and iridium, and the 1,4-diene can be detected as an intermediate (117a). 

Enantioselective reduction of 2-acetamidoacrylic acids by chiral phosphine complexes of rhodium... 

Over the last two decades, Wilkinson complex and related phosphine complexes of rhodium(I) have been used in numerous reactions for synthetic purposes, such as in the hydrosilylation of styrene and vinylcyclo-propene to yield ring-opening products of vinylamines. The [ (dippe)Rh 2(/u.-H)2] complex [where dippe = l,2-bis(diisopropylphosphino)ethane] is active in the hydrosilylation of olefins by diphenylsilane (4). Rhodium complexes were extremely favorable catalysts for dehydrogenative silylation of alkenes and divinyldiorganosilanes (4,13). 

The idea of producing lactic acid and alkyl lactates by hydroformylation of vinyl acetate was first patented by Monsanto in 1975, using a phosphine complex of rhodium as a catalyst [36]. Also, Kuraray has shown interest in this route [37], which covers three steps, namely, hydroformylation, subsequent oxidation, and final hydrolysis or alcoholysis. 

Biphenylene underwent dimerization in the presence of phosphine complexes of Group 10 transition metals to afford tetraphenylene (2) [4]. Catalytic hydrogenation of biphenylene with phosphine complexes of rhodium, nickel, palladium, and platinum resulted in the formation of biphenyl (3) [5]. Hydrosilylation and hydroboratlon of biphenylene occurred in the presence of a palladium catalyst to give 2-silylbiphenyls and 2-borylbiphenyl 4, respectively [6]. Disilanes, a... 

The complex [Rh(P-0)2](BPli4) [P-O = (0,P)-chelated Cy2PCFl2CFl20CH3] acted as an efficient catalyst for the hydrogenation of carbon dioxide to produce formic acid. Other chelate phosphine complexes of rhodium also catalyze this reaction. Theoretical studies of this catalytic reaction were performed using Rh(m) and Rh(l)... 

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