Rhodium catalysis substitution

Information published from several sources about 1970 presented details on both the halide-containing RhCl(CO)(PPh3)2- and the hydride-containing HRh(CO)(PPh3)3-catalyzed reactions. Brown and Wilkinson (25) reported the relative rates of gas uptake for a number of different olefinic substrates, including both a- and internal olefins. These relative rates are listed in Table XV. 1-Alkenes and nonconjugated dienes such as 1,5-hexadiene reacted rapidly, whereas internal olefins such as 2-pentene or 2-heptene reacted more slowly by a factor of about 25. It should also be noted that substitution on the 2 carbon of 1-alkene (2-methyl-l-pentene) drastically lowered the rate of reaction. Steric considerations are very important in phosphine-modified rhodium catalysis. 

Because these insertion reactions create new bonds at completely unfunctionalized centres, they can be very useful in synthesis. This next carbene is created between two carbonyl groups from a diazocompound with rhodium catalysis and selectively inserts into a C-H bond five atoms away to form a substituted cyclopentanonc. 

Allyl methyl ether (ethyl diazoacetate, rhodium catalysis) and allyl terf-butyl ether (dimethyl diazomalonate, copper catalysis) yield cyclopropanes exclusively. With y-substituted allyl methyl ethers, C-0 insertion is generally strongly favored over cyclopropanation, even with tetraacetatodirhodium as catalyst.In view of these findings, the cyclopropanation of ( )- ,4-dibenzyloxybut-2-ene in moderate yield, only, to give (la,2a,3j5)-31 is notable. 

Evans and coworkers developed a rhodium-catalyzed allylic substitution of chiral acyclic tertiary allylic carbonates with acyl anion equivalents. This substitution pathway provides a route to undergo a stereo- and regioselective substitution to provide the quaternary carbon 73. The enantioenriched product is isolated in greater than 95 5 selectivity and 87% yield. This pathway provides a product that could be obtained through enolate chemistry, only lacking the same atom economy obtained with rhodium-catalysis. 

Catalytic Reactions under Rhodium Catalysis. The /3-C-H bond of 1,1-dimethylsilacyclobutane undergoes insertion of an ethoxycarbonyl-substituted rhodium carbenoid in a highly regio-selective fashion (eq 10). The reaction provides a facile access to functionalized 1,1-dimethylsilacyclobutanes. 

When 6-diazopenicillanates are irradiated in the presence of sulfur nucleophiles, predominantly 6/3-substitution products are obtained (77JOC2224). When BFs-EtiO is used to catalyze the reaction with nucleophiles, however, the products are primarily the 6a-isomers (78TL995). The use of rhodium or copper catalysis led primarily to ring-opened thiazepine products, presumably by way of the intermediate (56 Scheme 39) (80CC798). 

Rhodium(II) acetate was found to be much more superior to copper catalysts in catalyzing reactions between thiophenes and diazoesters or diazoketones 246 K The outcome of the reaction depends on the particular diazo compound 246> With /-butyl diazoacetate, high-yield cydopropanation takes place, yielding 6-eco-substituted thiabicyclohexene 262. Dimethyl or diethyl diazomalonate, upon Rh2(OAc)4-catalysis at room temperature, furnish stable thiophenium bis(alkoxycarbonyl)methanides 263, but exclusively the corresponding carbene dimer upon heating. In contrast, only 2-thienylmalonate (36 %) and carbene dimer were obtained upon heating the reactants for 8 days in the presence of Cul P(OEt)3. The Rh(II)-promoted ylide formation... 

The interesting complex chemistry of rhodium has been rather neglected this is probably because most of the synthetic methods for obtaining complexes have been tedious. In general, substitutions of chlorine atoms bonded to rhodium by other ligands are slow, and products have usually been mixtures. The situation is now changing, since novel catalytic approaches to rhodium complexes have been developed.1 The catalysis in the present synthesis involves the rapid further reaction of the hydrido complex formed from l,2,6-trichIorotri(pyridine)rho-dium(III) in the presence of hypophosphite ion. 

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