RhCl 3, Wilkinson

The Wilkinson hydrogenation cycle shown in Figure 3 (16) was worked out in experiments that included isolation and identification of individual rhodium complexes, measurements of equiUbria of individual steps, deterrnination of rates of individual steps under conditions of stoichiometric reaction with certain reactants missing so that the catalytic cycle could not occur, and deterrnination of rates of the overall catalytic reaction. The cycle demonstrates some generally important points about catalysis the predominant species present in the reacting solution and the only ones that are easily observable by spectroscopic methods, eg, RhCl[P(CgH 2]3> 6 5)312 (olefin), and RhCl2[P(CgH )2]4, are outside the cycle, possibly in virtual equiUbrium with... 

Tbe discovery of the catalytic properties of [RhCl(PPh3)3] naturally brought about a widespread search for other rhodium phosphines with catalytic activity. One of those which was found, also in Wilkinson s laboratory, was trans-[Rh(CO)H(PPh3)3 which can conveniently be... 

The paramagnetic impurity which invariably accompanies Wilkinson s catalyst has proved difficult to identify. It is probably the air-stable, green, irans- (RhCl(CO)(PPh3)2j- see K. R. Dunbar and S. C Harfner, Innrg. Chem. 31, 36"76-9 (1992),... 

Complex formation with the Wilkinson catalyst RhCl(PPh3)3 gives 351 [870M(319)311]. The reaction with [Rh(Ti -C2H4)2Cl]2 gives the triple-decker... 

More recently. Baker, Tumas, and co-workers published catalytic hydrogenation reactions in a biphasic reaction mixture consisting of the ionic liquid [BMIM][PFg] and SCCO2 [10]. In the hydrogenation of 1-decene with Wilkinson s catalyst [RhCl(PPh3)3] at 50 °C and 48 bar H2 (total pressure 207 bar), conversion of 98 %... 

Tertiary phosphine complexes [42] are the most important rhodium(I) compounds. RhCl(PPh3)3 ( Wilkinson s compound ), a hydrogenation catalyst, is the most important, but they exist in a range of stoichiometries. Synthesis follows several routes ... 

Aldehydes, both aliphatic and aromatic, can be decarbonylated by heating with chlorotris(triphenylphosphine)rhodium or other catalysts such as palladium. The compound RhCl(Ph3P)3 is often called Wilkinson s catalyst.In an older reaction, aliphatic (but not aromatic) aldehydes are decarbonylated by heating with di-tert-peroxide or other peroxides, usually in a solution containing a hydrogen donor, such as a thiol. The reaction has also been initiated with light, and thermally (without an initiator) by heating at 500°C. 

Complex 5 was more active than the well-known precious-metal catalysts (palladium on activated carbon Pd/C, the Wilkinson catalyst RhCl(PPh3)3, and Crabtree s catalyst [lr(cod)(PCy3)py]PFg) and the analogous Ai-coordinated Fe complexes 6-8 [29] for the hydrogenation of 1-hexene (Table 2). In mechanistic studies, the NMR data revealed that 5 was converted into the dihydrogen complex 9 via the monodinitrogen complex under hydrogen atmosphere (Scheme 4). 

Several techniques have been used to activate the zinc metal and improve yields. For example, pretreatment of zinc dust with a solution of copper acetate gives a more reactive zinc-copper couple.168 Exposure to trimethylsilyl chloride also activates the zinc.169 Wilkinson s catalyst, RhCl(PPh3)3 catalyzes formation of Reformatsky reagents from diethylzinc, and reaction occurs under very mild conditions.170... 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

The groups of Loupy and Jun have presented a chelation-assisted rhodium(I)-cata-lyzed ortho-alkylation of aromatic imines with alkenes (Scheme 6.57) [119]. The use of 2 mol% of Wilkinson s catalyst, RhCl(PPh3)3, and 5 equivalents of the corresponding alkene under solvent-free conditions proved to be optimal, providing the desired ortho-alkylated ketones in high yields after acidic hydrolysis. Somewhat lower yields were obtained when the imine preparation and the ortho-alkylation were realized in a one-pot procedure. 

Keurentjes et al. performed a continuous hydrogenation of 1-butene in supercritical carbon dioxide.[9,10] A fluorous derivative of Wilkinson s catalyst was prepared in situ by mixing the ligand with [(COD)RhCl]2 under hydrogen / carbon dioxide pressure (Figure 4.37). 

PEGs with average molecular weights above 1000 are waxy solids under ambient conditions, but they melt under C02 pressure to become liquids under typical conditions of scC02 catalysis [63], The approach was demonstrated for the rhodium catalysed hydrogenation of styrene as a test reaction using Wilkinson s complex [(PPhs RhCl] as the catalyst (Scheme 8.6) [61],... 

Information published from several sources about 1970 presented details on both the halide-containing RhCl(CO)(PPh3)2- and the hydride-containing HRh(CO)(PPh3)3-catalyzed reactions. Brown and Wilkinson (25) reported the relative rates of gas uptake for a number of different olefinic substrates, including both a- and internal olefins. These relative rates are listed in Table XV. 1-Alkenes and nonconjugated dienes such as 1,5-hexadiene reacted rapidly, whereas internal olefins such as 2-pentene or 2-heptene reacted more slowly by a factor of about 25. It should also be noted that substitution on the 2 carbon of 1-alkene (2-methyl-l-pentene) drastically lowered the rate of reaction. Steric considerations are very important in phosphine-modified rhodium catalysis. 

Mechanistic details for hydrogenation of ethylene and cyclohexene catalyzed by the well-known Wilkinson catalyst, RhCl(PPh3)3 (7, p. 204) have been further elucidated (69-74) (Fig. 1). Studies on the analogous... 

The review of Morrison et al. (10) traces the development of the use of rhodium-chiral phosphine catalysts to about the end of 1974. This field was initiated by the suggested incorporation (216) of chiral phosphines, instead of triphenylphosphine, into the so-called Wilkinson catalyst, RhCl(PPh3)3 (Section II,A), or into closely related systems. Horner s group (217, 218) used such catalysts, formed in situ in benzene... 

Witulski introduced a novel protocol for crossed alkyne cyclotrimerizations of systems such as 87 mediated by Grubb s catalyst to produce 4,6-disubstituted indolines 88 <00CC1965>. Interestingly, use of Wilkinson s catalyst [RhCl(PPhj)3] allows for the regioselective synthesis of the corresponding 4,5-substituted isomers. 

In the presence of nickel(0), tethered diene-VCPs react to produce eight- and five-membered ring products (Scheme 2). Palladium(O) and cobalt(m) were also tried but produced only decomposition products. However, in the presence of Wilkinson s catalyst (RhCl(PPh3)3), tethered diene-VCP 1 was cleanly converted to triene 4 in 91% yield. Although the desired cycloaddition reaction was not obtained, the cleavage of the cyclopropane ring was encouraging.22... 

Among recent examples that highlight the synthetic utility of transition metal-catalyzed hydroborations are its direction toward a formal syntheses of the non-steroidal anti-inflammatory agents Ibuprofen 131 and Naproxen 13214 15 139 as well as the anti-depressant Sertraline 133 (Figure 14).140 In the majority of cases, rhodium-catalyzed hydroboration is utilized and the rhodium(i) source generally is Wilkinson s catalyst RhCl(PPh3)3. 

The hydride route involves the initial reaction with hydrogen followed by coordination of the substrate the well-known Wilkinson catalyst [RhCl(PPh3)3] is a representative example. A second possible route is the alkene (or unsaturated) route which involves an initial coordination of the substrate followed by reaction with hydrogen. The cationic catalyst derived from [Rh(NBD)(DIPHOS)]+ (NBD = 2,5-norbornadiene DIPHOS = l,2-bis(diphenyl)phosphinoethane) is a well-known example. The above-mentioned rhodium catalysts will be discussed, in the detail, in the following sections. 

The widely studied [RhCl(PPh3)3] complex, usually known as Wilkinson s catalyst, was discovered independently in 1965 by Wilkinson (a recipient of the Nobel Prize in 1973) and other groups [14]. This compound catalyzes the chemo-specific hydrogenation of alkenes in the presence of other easily reduced groups such as N02 or CHO, and terminal alkenes in the presence of internal alkenes [16]. The rate of hydrogenation parallels their coordination ability (Scheme 1.4), but tetrasubstituted alkenes are not reduced. 

The water-soluble analogue of Wilkinson s catalyst, [RhCl(TPPMS)3] [TPPMS = PPh2(C6H4S03Na)], prepared in situ from Rh(/<-Cl)(diene)]2 and TPPMS, reacts with hydrogen in aqueous solution to yield [RhH(TPPMS)3], instead of [RhH2(TPPMS)3], according to Eq. (6) ... 

Following Wilkinson s discovery of [RhCl(PPh3)3] as an homogeneous hydrogenation catalyst for unhindered alkenes [14b, 35], and the development of methods to prepare chiral phosphines by Mislow [36] and Horner [37], Knowles [38] and Horner [15, 39] each showed that, with the use of optically active tertiary phosphines as ligands in complexes of rhodium, the enantioselective asymmetric hydrogenation of prochiral C=C double bonds is possible (Scheme 1.8). 

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