Rhodium complex-catalyzed reaction

Aizenberg and Milstein [78] have found rhodium complex-catalyzed reactions between polyfluorobenzenes and hydrosilanes which resulted in the substitution of fluorine atoms by hydrogen and were both chemoselective and regioselective (Eq. (6) ... 

Mechanism of Asymmetric Hydroformylation 7.2.2.L Rhodium Complex Catalyzed Reactions... 

The rest of the catalyst cycle is identical to that illustrated for the rhodium complex-catalyzed reactions in Scheme 1. It has been proposed that the asymmetric induction occurs during the formation of alkyl-Pt(CO)L2 intermediate through olefin insertion into the Pt-H bond [13]. 

The hydrosilylation of 3-butyn-l-ol, i.e. homopropargyl alcohol, with HSiMe2Ph catalyzed by (Cy3P)Pt(CH2=CH2)2 gives a 4.2 1 mixture of (E)-4-silyl- and (E)-3-silyl-3-buten-l-ols135, whereas the cationic rhodium complex-catalyzed reaction with HSiEt3 affords ( )-4-triethylsilyl-3-buten-l-ol (136) exclusively in 97% yield (equation 55)136. 

Murahashi described a new pyrrole synthesis involving a rhodium complex-catalyzed reaction of isonitriles (e.g., 3) with 13-dicarbonyl compounds 4 to afford the pyrroles 5 <01OL421>. This process is believed to proceed by chemoselective activation of the a-C-H bond of the isonitrile even in the presence of the more acidic dicarbonyl derivative. 

The authors acknowledge the long-term support of the National Science Foundation as well as the National Institute of General Medical Sciences for their research on the discovery and development of a variety of rhodium-complex-catalyzed reactions and processes over the years. Generous support finm the Mitsubishi Chemical Corporation is also gratefully acknowledged. 

The extent of asymmetric hydrosilylation depends strongly upon the structure of hydrosilanes employed in a similar manner to the cases of other chiral rhodium complex-catalyzed reactions with dimethylphenylsilane optical yields are generally more than several times as high as with trimethylsilane. Most remarkable is the fact that the addition of dimethylphenylsilane to pivalophenone gave the silyl ether of (iS)-2,2-dimethyH-phenylpropanol, while that of trimethylsilane led to the (R)-enantiomer. 

The marked effect of hydrosilanes on the stereoselectivity, which is very characteristic of the asymmetric hydrosilylation of ketones as described in the previous Sections, is seen here again. Fairly good optical yields comparable to those obtained in other chiral rhodium complex-catalyzed reactions were attained. For example, the reaction of acetophenone with diphenylsilane catalyzed by (/ )-( S )-MPFA-rhodium complex gave higher optical yield than when (/ )-BMPP or DIOP was used as ligands. 

Rhodium complexes catalyze the oxidative coupling of benzene with ethene to produce styrene directly.45,45a,45b Using Rh(ppy)2(OAc) (ppyH = 2-phenylpyridine), the reaction of benzene with ethene in the presence of 02 and Cu(OAc)2 in benzene and acetic acid at 180 °C gives styrene and vinyl acetate in 77% and 23% selectivities, respectively. 

The enhanced synthetic potential of rhodium-complex-catalyzed enantioselective hydrogenation provided by these advances in ligand design has led to renewed interest in the reaction mechanism, and here we highlight four recent topics (i) the extended base of reactive intermediates (ii) an improved quadrant model for ligand-substrate interactions (iii) computational approaches to mechanism and (iv) (bis)-monophosphine rhodium complexes in enantioselective hydrogenation. These are discussed in turn. 

The rhodium(II)-catalyzed reaction of propargyl compounds 169 and diazo compounds 170 gave corresponding functionalized allenes 171 together with cydopro-penes 172 (Scheme 3.87) [126]. Rh2(pfb)4, where pfb represents perfluorobutyrate, was found to be an excellent catalyst for preparing the allenes 171. An analogous rhodium(II) complex, Rh2(OAc)4, afforded mainly 172 with only a trace amount of 171 (<5%). 

When substituted silanes are used instead of hydrogen, the process is referred to as silylformylation or silylcarbonylation. Only rhodium complexes catalyze the transformation of unsaturated compounds to silylaldehydes via the silylformylation reaction. Iridium complexes also are able to catalyze the simultaneous incorporation of substituted silanes and CO into unsaturated compounds, although during the reaction other types of product are formed. In the presence of [ IrCl(C03) ] and [Ir4(CO)i2]) the alkenes react with trisubstituted silanes and CO to give enol silyl ethers of acyl silanes [58] according to Scheme 14.10. 

A cationic rhodium complex-catalyzed codimerization of 1,3-dienes with alkynes gives the corresponding cyclohexadienes in good yields with high regioselectively, as exemplified in the reaction of 2-methyl-l,3-butadiene with phenylacetylene (Eq. 12) [31]. 

N-alkinyldihydropyridone 72 yields bicyclic lactam 73 (92JA7292), and rhodium-complex-catalyzed intramolecular conjugate addition of vinylstan-nanes 74a and b (formed by aza-Diels-Alder reaction) leads to chiral piperidones 75a and b (08T3464). 

One of the success stories of transition metal catalysis is the rhodium-complex-catalyzed hydrogenation reaction. Asymmetric hydrogenation with a rhodium catalyst has been commercialized for the production of L-Dopa, and in 2001 the inventor, Knowles, together with Noyori and Sharpless, was awarded the Nobel Prize in chemistry. After the initial invention, (enantioselective) hydrogenation has been subject to intensive investigations (27). In general, hydrogenation reactions proceed... 

A similar reaction of ylide 200 can also be carried out under thermal conditions or in the presence of catalytic amounts of Cu(acac)2 [143]. The carbenoid reactions of iodonium ylides can also be effectively catalyzed by rhodium(II) complexes [144, 145]. The product composition in the rhodium(II) catalyzed reactions of iodonium ylides was found to be identical to that of the corresponding diazo compounds, which indicates that the mechanism of both processes is similar and involves metallocarbenes as key intermediates as it has been unequivocally established for the diazo decomposition [144]. 

Theoretically, in a simple kinetic resolution the ee value should not exceed 32 % at this specific conversion. In addition to the rhodium complex, this reaction requires acetophenone as stoichiometric hydride acceptor, phenanthroline as coligand and potassium hydroxide as base. An ee value of 98 % at 60 % conversion (theoretical value 67 %)is achieved with [Rh2(OAc)4] without an added base after 3 days. Surprisingly, the enzyme tolerates potassium hydroxide in amounts up to 20 mol% at elevated temperatures however, the enantiomeric excesses are somewhat lower than those obtained in an ordinary kinetic resolution. Unselective, base- or metal-catalyzed acylation might be the reason for the somewhat lower ee value. 

Catalyst decomposition is, overall, receiving little attention in academic work on homogeneous catalysis, and only in recent years has research on decomposition and stabilization of organometallic catalysts started to expand (116), with emphasis on reactions of significant commercial interest such as hydroformylation (117), metathesis 118), crosscoupling, and polymerization 119). Ligand decomposition seems to be a key issue for industrial application, because it affects the total number of turnovers, TON. Phosphine decomposition is an unavoidable side reaction in metal-phosphine complex-catalyzed reactions and the main barrier for commercial application of homogeneous catalysts. There are a few exceptions to this statement for example, the rhodium tppts-catalyzed hydroformylation of propene, a process developed by Ruhrchemie-Rhone Poulenc (now Celanese). 

Expansion of aziridines to -lactams. This rhodium complex catalyzes a regio-specific carbonylation of N-f-butyl-2-arylaziridines (1) to form lactams (2). The reaction fails if the alkyl group on nitrogen contains acidic hydrogens adjacent to nitrogen. 

Copper and rhodium complexes catalyze the reaction of alkenes with diazoacetate to give alkyl cyclopropanecarboxylates [13]. In the presence of Cu(acac)2, the reaction of carbohydrate enol ether 20 with methyl diazoacetate afforded a 1 4 mixture of cis- and frani-cyclopropanes 21 and 22 (c -product 21 was obtained with 95% de). When the reaction was catalyzed by CuOTf in the presence of hgand 23, the tranj -product 22 was obtained with 60% de (Scheme 10.4). The absolute configuration of the major diastereomer was not given [19]. 

Complexes of many transition metals including cobalt, rhodium, iridium, iron, nickel, palladium, and platinum have been found to catalyze double-bond migration in terminal olefins. Evidence for a mechanism of the following type, which is probably also applicable to some of the other catalysts, has been obtained by Cramer 24, 27) for the rhodium chloride-catalyzed reaction (Reaction 37). 

The number of examples of highly selective dehydrogenative silylation is still limited. The most convincing examples are Ru3(CO)i2- and Fe3(CO)i2-cata-lyzed reactions of styrene [106, 114] and vinylsilane [115] with HSiEts, RuH2(H2)2PCy3)2-catalyzed reaction of ethylene with HSiEt3 [116], and cationic rhodium complex-catalyzed dehydrogenative silylation, e.g., [117], as well as the nickel equivalent of the Karstedt catalyst [105]. 

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