Rhodium catalysts, Wilkinson catalyst

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. 

A process for the preparation of fluorobenzenes comprises the heating of fhiorobenzaldehydes in the presence of a catalyst.99100 Suitable catalysts are transition metals from Ihe B groups 1, 11. VI. VII and VIII. The best catalytieal 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.100... 

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. 

The avermectins also possess a number of aUyflc positions that are susceptible to oxidative modification. In particular the 8a-methylene group, which is both aUyflc and alpha to an ether oxygen, is susceptible to radical oxidation. The primary product is the 8a-hydroperoxide, which has been isolated occasionally as an impurity of an avermectin B reaction (such as the catalytic hydrogenation of avermectin B with Wilkinson s rhodium chloride-triphenylphosphine catalyst to obtain ivermectin). An 8a-hydroxy derivative can also be detected occasionally as a metaboUte (42) or as an impurity arising presumably by air oxidation. An 8a-oxo-derivative can be obtained by oxidizing 5-0-protected avermectins with pyridinium dichromate (43). This also can arise by treating the 8a-hydroperoxide with base. 

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. 

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... 

Wilkinson s catalyst has also been utilized for the hydroboration of other alkenes. Sulfone derivatives of allyl alcohol can be hydroborated with HBcat and subsequently oxidized to give the secondary rather than primary alcohol. This reactivity proves to be independent of substituents on the sulfur atom.36 Similarly, thioalkenes undergo anti-Markovnikoff addition to afford a-thioboronate esters.37 The benefits of metal-catalyzed reactions come to the fore in the hydroboration of bromoalkenes (higher yields, shorter reaction times), although the benefits were less clear for the corresponding chloroalkenes (Table 3).38,39 Dienes can be hydroborated using both rhodium and palladium catalysts [Pd(PPh3)4] reacts readily with 1,3-dienes, but cyclic dienes are more active towards [Rh4(CO)i2].40... 

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... 

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 ... 

A key feature of the mechanism of Wilkinson s catalyst is that catalysis begins with reaction of the solvated catalyst, RhCl(PPh3)2S (S=solvent), and H2 to form a solvated dihydride Rh(H)2Cl(PPh3)2S [1], In a subsequent step the alkene binds to the catalyst and then is transformed into product via migratory insertion and reductive elimination steps. Schrock and Osborn investigated solvated cationic complexes [M(PR3)2S2]+ (M=Rh, Ir and S= solvent) that are closely related to Wilkinson s catalyst. Similarly to Wilkinson s catalyst, the mechanistic sequence proposed by Schrock and Osborn features initial reaction of the catalyst with H2 followed by reaction of the dihydride with alkene for the case of monophosphine-ligated rhodium and iridium catalysts [12-17]. Such mechanisms commonly are characterized... 

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. 

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