Rhodium catalyst RhCl

The oxidative C-arylation of five- to eight-membered (NH)-heterocycles such as pyrrolidine, piperidine, morpholine, etc., is observed in the reaction with iodoarenes in the presence of a rhodium catalyst, RhCl(CO)[P(Fur)3]2.95... 

Since the PK reaction with electron-defident alkynes was also problematic, even when stoichiometric Co2(CO)g was employed, promoters such as trialkylamine N-oxide were required for the reaction to proceed [14]. Alternatively, W(CO)5-THF may be employed semi-catalytically for this class of substrates [9cj. The rhodium catalyst [RhCl(CO)2]2, has shown great versatility for electron-deficient alkynes (Scheme 11.4) the reaction times are much shorter (1-3 h) than those of the usual examples (Tab. 11.3). This rhodium-catalyzed PK reaction may be extended to the synthesis of 6,5-fused ring analogs, as exemplified in the synthesis of bicyclo[4.3.0]nonenone 2o from the 1,7-enyne lo (Eq.4) [13 bj. 

Ethylene hydrosilylation with triethoxysilane has not yet been described in the literature. There are data concerning ethylene and propylene hydrosilylation in the presence of rhodium catalyst [RhCl(CO)2]2- However, it is known that a side reaction, dehydrogenating silylation, is typical for rhodium catalysts (Scheme 1). 

A rhodium catalyst, RhCl(CO)PMe3, was also used in the photochemical coupling of benzene (2a) with methyl acrylate (4g). In this case, the product alkene 5g served as a hydrogen scavenger giving rise to the formation of the saturated product (11, Figure 4.10) [20]. 

Terminal alkynes substituted with chiral substituents have been polymerized by using a rhodium catalyst, [RhCl(NBD)]2 (NBD = norbomadiene) [6]. As shown in Scheme 3, polymerization of a chiral (carbamoyloxy)phenylacetylene 4 forms a cis-substituted polyacetylene 5. Due to the bulkiness of the substituents, these polymers show a helical conformation with no extended conjugation in the polymer chain. These materials are potentially useful as enantioselective permeable membranes to separate racemic amino acids and alcohols in water or in methanol. They can be also used as chiral stationary phase for enantioselective high-performance liquid chromatography (HPLC) analysis. 

Other irmovative approaches have been used to promote aromatization, once the RCM reaction involving the two alkenes has been accomplished. These two approaches are shown in Scheme 17.4. In the first one, triene 24 underwent an RCM reaction to afford the exocycUc double bond-containing compound 25 [18]. Use of the rhodium catalyst [RhCl(cod)]j then facihtated formation of the desired phenol 26. Another neat way to accomplish formation of the aromatic benzene ring was to allow the intermediate cyclic Michael acceptor 28 (formed by RCM of 27) to undergo a Heck-Mizoroki reaction with p-methoxybenzenediazonium tetrafluoroborate 30 to afford the second phenol 29 [18]. 

Ruthenium, iridium, and rhodium are the most frequently used metals. The best known is the rhodium catalyst RhCl(PPh3)3 first discovered by Wilkinson and bearing his name. 

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

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. 

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... 

Electron spin resonance (ESR) signals, detected from phosphinated polystyrene-supported cationic rhodium catalysts both before and after use (for olefinic and ketonic substrates), have been attributed to the presence of rhodium(II) species (348). The extent of catalysis by such species generally is uncertain, although the activity of one system involving RhCls /phosphinated polystyrene has been attributed to rho-dium(II) (349). Rhodium(II) phosphine complexes have been stabilized by steric effects (350), which could pertain to the polymer alternatively (351), disproportionation of rhodium(I) could lead to rhodium(II) [Eq. (61)]. The accompanying isolated metal atoms in this case offer a potential source of ESR signals as well as the catalysis. 

The method of competitive reactions was used (38) to measure the relative rates of addition of Et3 ClnSiH (n = 0-3) to 1-heptene with Co2(CO)8 and RhCl(PPh3)3 as catalysts. With the rhodium catalyst at 80°C no hydrosilation took place at 120°C only that silane with a greater number of Si—Cl bonds yielded products. 

The sp2 nitrogen in a pyridine ring can activate a C-H bond of 2-arylpyridines (Equation (13)). The alkylation of the 2-arylpyridine is satisfactorily catalyzed by rhodium catalysts such as [RhCl(coe)2]2/PCy3 and [RhCl(coe)2]2/PPh3.13 13a... 

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. 

Finally, other rhodium catalysts for the selective diene hydrogenation worth mentioning include [RhCl(PPh3)3] (64), [RhCl(nbd)]2 (65), and the catalytic sys-... 

Water-soluble complexes constitute an important class of rhodium catalysts as they permit hydrogenation using either molecular hydrogen or transfer hydrogenation with formic acid or propan-2-ol. The advantages of these catalysts are that they combine high reactivity and selectivity with an ability to perform the reactions in a biphasic system. This allows the product to be kept separate from the catalyst and allows for an ease of work-up and cost-effective catalyst recycling. The water-soluble Rh-TPPTS catalysts can easily be prepared in situ from the reaction of [RhCl(COD)]2 with the sulfonated phosphine (Fig. 15.4) in water [17]. 

Phosphonium salts can be synthesized by the transition-metal-catalyzed addition reaction of triaryphosphines and acids to unsaturated compounds. The reaction of PPh3, CH3SO3H, and alkynes in the presence of a palladium or rhodium catalyst gave alkenylphosphonium salts. Although Pd(PPh3)4 directed the C-P bond formation at the internal carbon atom of aliphatic 1-alkynes (Markovnikov mode), [RhCl(cod)]2... 

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. 

Oligomerization and polymerization of terminal alkynes may provide materials with interesting conductivity and (nonlinear) optical properties. Phenylacetylene and 4-ethynyltoluene were polymerized in water/methanol homogeneous solutions and in water/chloroform biphasic systems using [RhCl(CO)(TPPTS)2] and [IrCl(CO)(TPPTS)2] as catalysts [37], The complexes themselves were rather inefficient, however, the catalytic activity could be substantially increased by addition of MesNO in order to remove the carbonyl ligand from the coordination sphere of the metals. The polymers obtained had an average molecular mass of = 3150-16300. The rhodium catalyst worked at room temperature providing polymers with cis-transoid structure, while [IrCl(CO)(TPPTS)2] required 80 °C and led to the formation of frani -polymers. 

Wilkinson s catalyst [RhCl(PPh3)3], a standard catalyst for this reaction, is reported to give lower yields with less regioselectivity in these reactions. Conjugated dienes gave mixtures of 1,4- and 1,2-addition products in the presence of rhodium-NHC systems, whereas [RhCl(PPh3)3] leads to selective 1,4-addition. 

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