Cationic Rh complexes

Some chiral mono-, acyl- and di-thioureas have been used as ligand for the Rh-catalysed asymmetric hydroformylation of styrene. Although thiourea ligands form inactive systems with [Rh(COD)Cl]2 as the catalyst precursor, in standard conditions (40 °C, 40 bar CO -l- H2 1/1), the cationic Rh complex [Rh(COD)2]Bp4 combined with monothioureas as the ligand showed moderate to good activity (Scheme 29) [114]. 

This reaction sequence of conjugate reduction followed by aldol reaction is known as the reductive aldol reaction. In certain instances, reductive elimination from the M-TM-enolate species may occur to furnish M-enolate, which itself may participate in the aldol reaction (Scheme 3). This detour may be described as the background path or stepwise path in one-pot. Indeed, it has been reported that certain cationic Rh complexes such as [Rh(COD)(DPPB)] (COD = 1,5-cyclooctadiene, DPPB = diphenylphosphinobutane) catalyze the aldol reactions of silyl enol ethers and carbonyl compounds by serving as Lewis acids [5-8]. 

Under similar conditions, employing a cationic Rh complex (10mol%) and hydrogen (1 atm), the aldehyde-enone 17 was subjected to the cycli-zation to give the cyclic aldol product 18 in 89% with czs-selectivity up to 10 1 (Scheme 19) [31]. Use of (p-CF3Ph)3P as ligand accelerated the reaction... 

The intramolecular reductive aldol reaction of keto-enones was successfully conducted under conditions similar to those described above, employing a cationic Rh complex and PI13P (Scheme 20) [34]. The keto-enone 63 was cyclized in the presence of added K2CO3 to give the ketone-aldol 64 in 72% yield with exclusive ds-selectivity. Dione-enone derivatives, for example 68 and 70, were efficiently cyclized to furnish bicyclic aldol products 69 and 71, respectively, wherein three stereogenic centers of the bicyclic product form stereoselectivity through the intermediacy of a Rh-enolate. 

The coupling of enals and glyoxals was realized by hydrogen-mediated reaction with the cationic Rh complex and PI13P [35]. The intermediate aldehyde enolates derived via Rh-catalyzed hydrogenation were trapped with glyoxals to form (l-hydroxy-y-kclo-aldchydes, which were treated sequentially with hydrazine to give pyridazines in a one-pot transformation to provide, for example, a 62% yield of 72 (Scheme 21). 

The intramolecular cyclization of diketo-enals and keto-enals was accomplished by the combination of a cationic Rh complex and fn(2-furyl)phos-phine (2 - Fur3P). The corresponding bicyclic hydroxy-aldehydes were produced in good to excellent yields, as demonstrated by the formation of 74,76 and 78 (Scheme 22) [36]. 

Isayama described the coupling reaction of N-methylimine 157 and ethyl crotonate catalyzed by Co(acac)2 and mediated by PhSiH3 to produce Mannich product 158 in 82% with syn-selectivity (Scheme 41) [71]. The (i-laclam 159 was readily synthesized by heating 158. In 2002, Matsuda et al. reported cationic Rh complex [Rh(COD) P(OPh)3 2]OTf (1 mol%) as an active catalyst for the reductive Mannich reaction [72]. N-Tosylaldiminc 160 was coupled with methyl acrylate and Et2MeSiH (200 mol%) at 45 °C to give the b-amino ester 161 in 96% with moderate anti-selectivity 68%. 

Matsuda et al. applied aryl isocyanates as acceptors in reductive couplings to methyl acrylate (Scheme 44) [77]. The cationic Rh complex [Rh(COD) P(OPh)3 2]OTf (1 mol%) and Et2MeSiH (200 mol%) catalyze the reaction in refluxing CH2C12 to provide products of hydrocarbamoylation,... 

Rhodium complexes facilitate the reductive cydization of diyne species in good yield, although the product olefin geometry depends on the catalysts used. Moderate yields of -dialkylideneclopentane 169 resulted if a mixture of diyne 146 and trialkylsilane was added to Wilkinson s catalyst ClRh[PPh3]3 (Eq. 33) [101]. If, however, the diyne followed by silane were added to the catalyst, a Diels-Alder derived indane 170 was produced (Eq. 34). Cationic Rh complex, (S-BINAP)Rh(cod) BF4, provides good yields of the Z-dialkylidenecyclopentane derivatives, although in this case, terminal alkynes are not tolerated (Eq. 35) [102]. 

The intercalated catalysts can often be regarded as biomimetic oxidation catalysts. The intercalation of cationic metal complexes in the interlamellar space of clays often leads to increased catalytic activity and selectivity, due to the limited orientations by which the molecules are forced to accommodate themselves between sheets. The clays have electrostatic fields in their interlayer therefore, the intercalated metal complexes are more positively charged. Such complexes may show different behavior. For example, cationic Rh complexes catalyze the regioselective hydrogenation of carbonyl groups, whereas neutral complexes are not active.149 Cis-Alkenes are hydrogenated preferentially on bipyridyl-Pd(II) acetate intercalated in montmorillonite.150 The same catalyst was also used for the reduction of nitrobenzene.151... 

An extensive array of chiral phosphine ligands has been tested for the asymmetric rhodium-catalyzed hydroboration of aryl-substituted alkenes. It is well known that cationic Rh complexes bearing chelating phosphine ligands (e.g., dppf) result in Markovnikoff addition of HBcat to vinylarenes to afford branched boryl compounds. These can then be oxidized through to the corresponding chiral alcohol (11) (Equation (5)) ... 

The result of the described methodical solution to monitor gas-consuming reactions at reduced partial pressure under isobaric conditions is shown in Figure 10.8 for the catalytic hydrogenation of COD with a cationic Rh-complex. The slope of the measured straight lines corresponds to the maximally obtainable rate (Vsat = k2 [E]0 = k 2 [H2] [E]0) [42 b], which is directly proportional to the hydrogen concentration in solution and at validity of Henry s law to the hydrogen partial pressure above the reaction solution. The experiments prove that the dilution factor of the gas phase can adequately be found in the rate constant (Further examples can be found in [47].)... 

Scheme 12.3 Formation of dihydride intermediates of a cationic Rh complex via displacement of the NMD ligand in the DIPHOS-derived catalyst (S = solvent).

Table 22.1 Partitioning of aldolization and 1,4-reduction pathways depends critically on the use of cationic Rh-complexes and mildly basic additives.a)...

Furthermore the complexes 4 were reacted towards cationic Rh complexes (generated from [Rh(CO)2Cl]2 and T1PF6) to form twofold trans substituted products 14a,b (Eq. 13) [16]. Both the reaction with [M(CO)5(thf)] (M=Cr, W) as well as with [Rh(CO)2L2] reveal the tendency of the [(N3N)M=P]... 

High catalytic activities have been achieved by the PYRPHOS- [18], PPCP [20], BICHEP- [21], Et-DuPHOS-Rh [19] complexes among others, allowing the reaction with a substrate-to-catalyst molar ratios (S/C) as high as 50,000. With a [2.2]PHANEPHOS-Rh complex, the reaction proceeds even at -45°C [27], Supercritical carbon dioxide, a unique reaction medium, can be used in the DuPHOS and BPE-Rh-catalyzed hydrogenation [43], A highly lipophilic counteranion such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) or trifluoromethanesulfonate is used to enhance the solubility of the cationic Rh complexes. Under the most suitable reaction conditions of 102 atm of carbon dioxide, 1 atm of hydrogen, and 22°C, a-amino acid derivatives are produced with up to 99.7% ee. 

A DuPHOS-Rh catalyst reduces both E and Z enamides with a high enantioselectivity even in an alcoholic solvent without E/Z isomerization [57], Notably, (3,(3-disubstituted oc-enamides are also smoothly hydrogenated to (3-branched a-amino acids [58], Sterically less-hindered Me-DuPHOS- and Me-BPE-Rh catalysts provide a high enantioselectivity. The cationic Rh complexes of TRAP [24], BisP [30], and [2.2]PHANEPHOS [27] are also active for hydro-... 

Only limited successful examples of asymmetric hydrogenation of acrylic acids derivatives have included the use of chiral Rh complexes (Scheme 1.17). The diamino phosphine (28) utilizes selective ligation of the amino unit to a Rh center and also exerts electrostatic interaction with a substrate. Its Rh complex catalyzes enantioselective hydrogenation of 2-methylcinnamic acid in 92% optical yield [116], Certain cationic Rh complexes can attain highly enantioselective hydrogenation of trisubstituted acrylic acids [ 1171. 2-(6 -Methoxynaphth-2 -yl)acrylic acid is hydrogenated by an (.S ..S )-BIPNOR- Rh complex in methanol at 4 atm to give (.S)-naproxen with 98% ee but only in 30% yield [26]. 

I.4.I.2. Amino, Hydroxy, and Phenylthio Ketones Asymmetric hydrogenation of amino ketones, in either a neutral or hydrochloride form, has extensively been studied. Both Rh(I) and Ru(II) complexes with an appropriate chiral diphosphine give a high enantioselectivity. As described in Scheme 1.42, a-aminoacetophenone hydrochloride is hydrogenated using a cationic Rh complex with (R)-MOC-BIMOP, an unsymmetricaJ biaryl diphosphine, to give the... 

Intramolecular hydrosilylation of siloxy acetone 55 catalyzed by a cationic Rh complex with DuPHOS-i-Pr (56), [Rh(COD)(DuPHOS-i-Pr)]OTf, to give the corresponding cyclic silyl ether with 93% ee (5) [42]. The product was converted to 1,2-diol 57, which can also be prepared by asymmetric dihydroxylation of propene. In the same reaction, the use of BINAP 58 gave only... 

Asymmetric [4+1] cycloaddition of vinylallenes and carbon monoxide is promoted by a cationic Rh complex formed in situ from [Rh(cod)2]PF6 and chiral diphosphine ligand, (R.R)-Me-DuPHOS to afford 2-alkylidene-3-cyclopentenones with high asymmetric induction [72] (Eq. 8A.48). 

However, the Rh-catalysed hydrosilylation of terminal alkynes affords the thermodynamically unfavorable anti product 583 as the main product [224], The symanti ratio changes depending on the catalysts and solvents. The syn adduct 584 is obtained by a cationic Rh complex in MeCN [225]. The Ru catalyst gives the anti adduct. Formation of the anti adducts is explained by the following mechanism [224, 226]. Insertion of alkyne to the R3S-RI1 bond generates 585 which, due to steric repulsion, isomerizes to 588 via the carbene species 586, or the metallacyclopropene 587, and gives the anti adduct 583. 

Enamines RR CHCR CR NR4 (R-R3 = H, alkyl, aryl R4 = H, alkyl, cycloalkyl R5 = alkyl, cycloalkyl NR4R5 = heterocyclic amine) were prepared by isomerization of allylamines RR1C=CR2CHR3NR4R5 in the presence of a cationic Rh complex (RhLL1)+C104 [L = norbornadiene, L1 = R-( + )-BINAP]. 

Cationic Rh complexes [RhL(C0)(PPh3)2]C104 (L = unsaturated nitrile) show catalytic activities for the hydrogenation of a,3-unsaturated nitriles under mild conditions (30 °C). Selective reduction of the carbon-carbon double bonds of a,3-unsaturated carbonyls also occurs in high yield with [ Rh( 1,5-hexa-diene)Cl 2] and a phase transfer catalyst in aqueous media. ... 

A landmark process using a cationic Rh complex with a BINAP ligand is working at Takasago International Corporation, Japan, on up to a nine-ton scale. 

The cationic Rh complex effectively catalyzes the hydrogenation of imines under mild conditions using a nonionic surfactant, Triton X-100, as the PT agent [58]. 

Although cationic Rh complexes have found some use in reducing alkenes to alkanes, converting alkynes to m-alkcnes, and transforming ketones to alcohols, the most important function of these catalysts has been to promote asymmetric hydrogenation of a C=C bond. Equation 9.31 shows a key step that Knowles... 

In 1995, Burk and Tumas reported for the first time the asymmetric hydrogenation of several a-enamides in SCCO2 using a cationic Rh complex with Et-DuPHOS [5]. The reaction proceeded homogeneously under 5000 psi of the supercritical phase (H2 partial pressure 200psi) at 40°C. The enantioselectivities obtained in SCCO2 were comparable to or higher than those obtained in either methanol or hexane (Scheme 7.34). 

Development of a more C02-philic cationic Rh complex was reported by Lange et al. [Eq. (17), with ligand in Scheme 1] (100) ... 

In the initial screening, use of cationic Rh-complexes [Rh(COD)2]BF4 and diphosphines, with a substrate/catalyst ratio of between 20 and 50, led to a facile hydrogenation. Excellent conversions to 20 could be obtained at industrially attractive conditions (room temperature, <10 bar, <12 hours) using a variety of chiral diphosphines. A selection of the results is outlined in Tab. 10. 

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