RhCl 3, Wilkinson catalyst

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... 

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). 

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... 

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. 

Buchwald et al. have shown that 5-20 mol % Cp2Ti(CO)2 facilitates the PKR at 18 psi CO and 90 °C, giving yields in between 58 and 95% [38]. Moreover, Mitsudo et al. [39] and Murai et al. [40] reported independently on the employment of Ru3(CO)i2 as active catalyst. Cyclopentenones were isolated in moderate to excellent yields (41-95%). In addition, rhodium catalysts were successfully examined for use in the PKR. Narasaka et al. [41] carried out reactions at atmospheric CO pressure using the dimeric [RhCl(CO)2]2 complex. Also, in the presence of other rhodium complexes like Wilkinson catalyst RhCl(PPh3)3 and [RhCl(CO)(dppp)]2 [42] in combination with silver salts, cyclopentenones were obtained in yields in the range of 20-99%. Some representative examples of the catalytic PKR are shown in Eq. 2. 

For practical hydrogenation of olefins four classes of metal complexes are preferred (a) Rh complexes, the RhCl(PPh3)3, the so-called Wilkinson catalyst and the [Rh(diene)-(PR3)2]+ complexes, (b) a mixture of Pt and Sn chlorides, (c) anionic cyanocobalt complexes and (d) Ziegler catalysts, prepared from a transition metal salt and an alkylaluminum compound. 

Terminal alkyne Id affords 2d, albeit with somewhat lower chemical yield, irrespective of which catalyst was employed. A distinct difference between the two catalysts is observed with the trialkylsilyl-substituted alkyne le. An excellent chemical yield is obtained when [RhCl(CO)2]2 is employed, but the substrate is unreactive with [RhCl(CO)dppp]2. Moreover, Wilkinson catalyst activated by AgOTf is unable to catalyze the PK reaction with this substrate effectively. 

Disappointingly, the trimethylphosphite-modified Wilkinson catalyst, which had proven effective for the allylic substitution reaction [30], furnished only a trace amount of the PK product. By screening various rhodium catalysts for both reactions, it was determined that [RhCl(CO)dppp]2 was the optimum complex for the sequential pro-... 

One of the most carefully studied hydrogenations is the one catalyzed by the Rh(I) complex RhCl(PPh3)3, usually known as the Wilkinson catalyst. It was discovered in 1965 and is easily prepared by the reduction of rhodium trichloride hydrate in the presence of triphenylphosphine. 

Chlorotris(triphenylphosphine)rhodium(I), [RhCl(PPh3)3], was reported independently by three groups in 1965, " and its application to catalytic homogeneous hydrogenation has been studied intensively by Wilkinson s group. The Wilkinson catalyst is now the most widely used for hydrogenation of a variety of unsaturated substrates, and several extensive reviews of this catalyst have been published. ... 

It has been proposed that the decarbonylation of aldehydes by the Wilkinson catalyst [RhCl(PPh3)3] involves a radical pair disproportionation or recombination reaction. A radical pair intermediate in solution is equivalent to a cage reaction (Scheme 6). Table 15 shows the results obtained from the decarbonylation of a series of chiral cyclopropyl aldehydes ... 

In 1965 the compound RhCl(PPh3)3, coined the Wilkinson catalyst , was shown to be an excellent hydrogenation catalyst [17-19]. Although there had been scattered reports on hydrogenation catalysts before [20], it was this discovery which started a worldwide activity in the field of hydrogenation reactions (vide infra). 

Hydrogenations with the Wilkinson catalyst RhCl(PPh3)3 are operationally simple. They are usually carried out at ambient temperatures. In many cases a blanket of hydrogen (1 bar or 0.1 MPa) is sufficient and no hydrogen pressure is necessary. Solvents are usually methanol, ethanol, acetone, THF or benzene [20]. Chloroform and carbon tetrachloride should be avoided because they may undergo H/Cl exchange [40]. 

The first rapid and practical system for the homogenous reduction of alkenes, alkynes and other unsaturated substances at 25° C and 1 atm pressure used the complex RhCl(PPh3)3 known as Wilkinson catalyst. It dissociates to only a small extent at 25° C. 

The hydroboration of alkenes is known to be activated either by pressure or catalysis. Consequently, the combination of these techniques might open the way for the hydroboration of particularly unreactive substrates. Maddaluno el al. recently investigated the hydroboration of some functionalized alkenes, comparing different reagents (catecholborane (CBH) versus pinacolborane (PBH)), and activation by Wilkinson catalyst (RhCl[PPh3]3) and pressure [24]. While bromoalkenes and al-lylamines were found to give the best results with CBH at ambient pressure, 2,3-dihydrofuran (52a, Scheme 7.17) was hydroborated most effectively by PBH in the... 

Wilkinson catalyst, RhCl(PPh3)38 are prototypes of the OA reaction critically important to hydrogenation and homogeneous catalysis in general. It was now possible to directly observe the electronic and stereochemical properties of binding of a gaseous diatomic molecule on a metal complex and understand the factors that determine its activation toward cleavage. 

Qualitative ideas of catalytic H2 activation on metals were devised in the late 1950s in connection with the formation of transition states or intermediates prior to OA of H2 to hydride complexes (see Chapter 2). Surprisingly, there was no molecular orbital analysis of this intriguing theoretical problem until Dedieu carried one out in 1979.17 Both extended Huckel and ab initio Hartree-Fock calculations were carried out on H2 addition to square-planar d8 RhCl(PH3)3, a model for the well-known Wilkinson catalyst in which the phosphine is PPh3. In this 16e complex, the H2 approaches the filled dzi metal orbital. Calculations indicate that at the beginning of the reaction, end-on (i/1) approach of H-H is preferred over side-on (tf2) approach (Eq. 4.1). 

Thus, detailed experimental and theoretical studies are highly desirable on the mechanism of the transition-metal-catalyzed olefin hydroboration reactions, as well as on the role of the transition-metal center, substrates, and electronic and steric factors in the mechanism. MMM [67] have presented the first detailed ab initio molecular orbital (MO) study of possible reaction pathways illustrated in Fig. 22 for the reaction of C2H4 with the boranes HB(0H)2 and HB02(CH2)3 catalyzed by the model Wilkinson catalyst RhCl(PH3)2. The reaction of BH3 with C2H4 catalyzed by the Rh(PH3)2Cl have been studied by MMM [68] and DS [69]. 

Impressive calculations for several sequences of reaction steps of the catalytic hydroboration of ethylene by HB(OH)2 using a model Wilkinson catalyst, RhCl(Ph3)2, have been done by Musaev et al. [64]. Different mechanisms were compared involving more than 30 intermediates and transition states. Full-geometry optimizations were performed at the MP2 level. This study shows that it is now possible to understand catalytic reactions at a very detailed level. The hydroboration reaction is just one example, although a particularly spectacular one, of the type of detailed studies that have been performed by Morokuma et al. For earlier work along similar lines, see Ref. 5. 

In order to eliminate the possibility for in situ carbene formation Raubenheimer et al. synthesized l-alkyl-2,3-dimethylimidazolium triflate ionic liquids and applied these as solvents in the rhodium catalyzed hydroformylation of l-hejEne and 1-dodecene [178]. Both, the classical Wilkinson type complex [RhCl(TPP)3] and the chiral, stereochemically pure complex (—)-(j7 -cycloocta-l,5-diene)-(2-menthyl-4,7-dimethylindenyl)rhodium(i) were applied. The Wilkinson catalyst showed low selectivity towards n-aldehydes whereas the chiral catalyst formed branched aldehydes predominantly. Hydrogenation was significant with up to 44% alkanes being formed and also a significant activity for olefin isomerization was observed. Additionally, hydroformylation was found to be slower in the ionic liquid than in toluene. Some of the findings were attributed by the authors to the lower gas solubility in the ionic liquid and the slower diffusion of the reactive gases H2 and CO into the ionic medium. 

---