Wilkinson rhodium catalyst

A process for the preparation of fluorobcnzencs comprises the heating of fluorobenzaldehydes in the presence of a catalyst. Suitable catalysts are transition metals from the B groups 1, 11. VI. VII and VIII. The best catalytical properties seem to be held by rhodium and the metals of the platinum group, e.g. formation of 1.3-difluorobenzene (5). The reaction maybe carried out in homogeneous solution with soluble rhodium catalysts (Wilkinson s catalyst) or in heterogeneous phase with the catalyst fixed on a carrier. ... 

As described in this chapter, a number of rhodium-based catalysts have been developed and employed widely for the [2 -I- 2 -I- 2] cycloaddition reactions of alkynes. Among these rhodium catalysts, Wilkinson s complex [RhCl(PPh3)3] and cationic rhodium(I)/biaryl bisphosphine complexes are the most frequently employed. As Wilkinson s complex is a stable single-component catalyst, this complex is the most easily usable catalyst in organic synthesis. On the other hand, cationic rhodium(I)/ (axially chiral) biaryl bisphosphine complexes exhibit excellent catalytic activity and selectivity under mild conditions. These chiral complexes, especially, enable the catalytic enantioselective syntheses of various chiral arenes with high enantioselectivity. 

Similar activation takes place in the carbonylation of dimethyl ether to methyl acetate in superacidic solution. Whereas acetic acid and acetates are made nearly exclusively using Wilkinson s rhodium catalyst, a sensitive system necessitating carefully controlled conditions and use of large amounts of the expensive rhodium triphenylphosphine complex, ready superacidic carbonylation of dimethyl ether has significant advantages. 

Allyl groups attached directly to amine or amide nitrogen can be removed by isomerization and hydrolysis.228 These reactions are analogous to those used to cleave allylic ethers (see p. 266). Catalysts that have been found to be effective include Wilkinson s catalyst,229 other rhodium catalysts,230 and iron pentacarbonyl.45 Treatment of /V-allyl amines with Pd(PPh3)4 and (V,(V -dimethylbarbi Lurie acid also cleaves the allyl group.231... 

In 1968 Wilkinson discovered that phosphine-modified rhodium complexes display a significantly higher activity and chemoselectivity compared to the first generation cobalt catalyst [29]. Since this time ligand modification of the rhodium catalyst system has been the method of choice in order to influence catalyst activity and selectivity [10]. 

The best known rhodium catalyst precursor for hydroformylation is undoubtedly RhH(PPh3)3CO, first reported by Vaska in 1963,167 but its activity for hydroformylation was discovered by Wilkinson and co-workers a few years later.168-171 The chemistry reported in the late 1960s and early 1970s is still... 

Intramolecular process with rhodium catalyst has been described for the syntheses of indane, dihydroindoles, dihydrofurans, tetralins, and other polycyclic compounds. Wilkinson catalyst is efficient for the cyclization of aromatic ketimines and aldimines containing alkenyl groups tethered to the K z-position of the imine-directing group. 

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. 

Z)-Ketene silyl acetals. Hydrosilylation of acrylates with any trialkylsilane catalyzed with Wilkinson s rhodium catalyst results in (Z)-ketene silyl acetals (Z/E 3= 98 2).3 Example ... 

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. 

One last remark concerning the two catalysts we have discussed in more detail, cationic rhodium catalysts and the neutral chloride catalyst of Wilkinson. The difference of the catalytic system discussed above from that of the Wilkinson catalyst lies in the sequence of the oxidative addition and the alkene complexation. The hydrogenation of the cinnamic acid derivative involves a cationic catalyst that first forms the alkene complex the intermediate alkene (enamide) complex can be observed spectroscopically. 

The first example involving a rhodium catalyst in an ene reaction was reported by Schmitz in 1976. An intramolecular cyclization of a diene occurred to give a pyrrole when exposed to rhodium trichloride in isobutanol (Eq. 2) [15]. Subsequently to this work, Grigg utilized Wilkinson s catalyst to effect a similar cycloisomerization reaction (Eq. 3) [16]. Opplozer and Eurstner showed that a n -allyl-rhodium species could be formed from an allyl carbonate or acetate and intercepted intramolecularly by an alkene to afford 1,4-dienes (Eq. 4). Hydridotetrakis(triphenylphosphine)rhodium(l) proved to be the most efficient catalyst for this particular transformation. A direct comparison was made between this catalyst and palladium bis(dibenzylidene) acetone, in which it was determined that rhodium might offer an additional stereochemical perspective. In the latter case, this type of reaction is typically referred to as a metallo-ene reaction [17]. 

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-... 

Substitution of the VCP is tolerated both on and adjacent to the cyclopropane ring. Diester-substituted and heteroatom (O, NTs) tethers are well tolerated. Reactions were conducted with 2-10 mol% catalyst at up to 0.20 M, as illustrated. Most importantly, reactions with the naphthalene catalyst were found to be more rapid than those with other catalysts. For example substrate 54 is readily converted in >99% yield to cycloadduct 55 in only 15 min at room temperature (entry 1). Complex 93 efficiently catalyzes the reactions of both alkynes and alkenes with VCPs, offering greater generahty than thus far observed with non-rhodium catalysts. This catalyst is particularly advantageous in the cases of substrates 100 and 102, for which the desired product is not formed cleanly with Wilkinson s catalyst due to product isomerization. 

The ionic liquids acting as solvents in these hydrogenation systems do not show any noticeable difference in the turnover rate with the Wilkinson catalyst. It is important to note that at the end of the hydrogenation reaction the product is removed from the two-phase catalytic system by simple decantation and the rhodium catalysts are almost completely retained in the ionic liquid (Steines et al., 2000). 

To obtain information about the steps in which the asymmetric induction actually takes place, 1-butene, cis-butene, and trans-butene were hydroformylated using asymmetric rhodium catalyst. According to the Wilkinson mechanism, all three olefins yield a common intermediate, the sec-butyl-rhodium complex, which, if the asymmetric ligand contains one asymmetric center, must exist in the two diastereomeric forms, IX(S) and IX(R),... 

Although much effort has been devoted to decarbonylation of cyclopropylcarbonyl metal complexes (vide supra), only (cyclopentadienyl)dicarbonyliron (Fp) derivatives have been successfully decarbonylated either photochemically22,24 or using Wilkinson s rhodium catalyst [(PPh3)2RhCl]2 (equation 35). Further decarbonylation by irradiation led to metallacyclopentane formation, whereas thermal decomposition resulted in the formation of the corresponding Cp(CO)Fe(allyl) complexes. 

A mechanism for this kind of transformation has been established by G. Wilkinson for a rhodium catalyst, RhL3H(CO), in which L is (C6H5)3P, in accord with the cycle of Figure 31-3. This cycle shows that the reaction is closely related to the hydrogenation cycle of Figure 31-2. The new steps are... 

Catalytic hydroboration of perfluoroalkenes 68 with catecholborane provides either terminal 69 or internal alcohols 70 regioselectively <19990L1399>. The regioselectivity is controlled by a judicious choice of catalyst. The anti-Markovnikov alcohol can be obtained with very high selectivity by using cationic rhodium catalysts such as Rh(COD)(DPPB)+BF4, while neutral Rh catalysts such as Wilkinson s catalyst provide the Markovnikov product (COD = cyclooctadiene Equation 3) <19990L1399>. 

The reaction of pinacolborane with styrenes 127 in the presence of bis(chloro-l,5-cyclooctadienylrhodium) at room temperature provides styrenyl pinacol boronate 128 <1999TL2585, 2002BCJ825>. While hydroboration of alkenes is the predominant reaction with phosphine-containing rhodium catalysts such as Wilkinson s catalyst and Rh(PPh3)2COCl, dehydrogenative borylation dominates over hydroboration in the presence of phosphine-free... 

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