Rhodium -mediated process

Cyclopentane construction by rhodium-mediated intramolecular C-H insertion has been established as a powerful tool for the construction of carbocycHc systems, but there is much yet to be learned about the factors governing selectivity in the C-H insertion process [33]. We look forward to exciting developments in this area in years to come. 

Preliminary efforts to examine the mechanism of C-H amination proved inconclusive with respect to the intermediacy of carbamoyl iminoiodinane 12. Control experiments in which carbamate 11 and PhI(OAc)2 were heated in CD2CI2 at 40°C with and without MgO gave no indication of a reaction between substrate and oxidant by NMR. In Hne with these observations, synthesis of a carbamate-derived iodinane has remained elusive. The inability to prepare iminoiodinane reagents from carbamate esters precluded their evaluation in catalytic nitrene transfer chemistry. By employing the PhI(OAc)2/MgO conditions, however, 1° carbamates can now serve as effective N-atom sources. The synthetic scope of metal-catalyzed C-H amination processes is thus expanded considerably as a result of this invention. Details of the reaction mechanism for this rhodium-mediated intramolecular oxidation are presented in Section 17.8. 

The utilization of rhodium-phosphine complexes in homogeneous catalysis is widespread and encompasses industrially important processes, hydroformylation see Hydroformyla-tion) being one of the most widely known rhodium-mediated conversions see Rhodium Organometallic Chemistry and Carbonylation Processes by Homogeneous Catalysis).It... 

Intermediate enolates derived from Michael-type processes can be isolated. For example, enantiomerically pure enolate (5 )-131 can be isolated, as is the case of the almost enantiomerically pure enolate (5 )-131, prepared by 1,4-addition of the phenyl-substituted borane 130 to enone 129, in the presence of a substoichiometric amount of the chiral rhodium mediator [Rh(OMe)(COD)]2-(5 )-BINAP (COD = 1,5-cyclooctadiene, equation 33). Protonation of (5 )-131 with methanol leads to cyclohexanone (5 )-132 in good yield with no loss of enantiomeric purity (equation 34). The protonation is presumably diastereoselective, taking place on the less hindered face of (5)-131, away from the neighbouring phenyl group, as can be inferred from the stereochemical outcome (133)... 

In recent work, Taber and Hennessy have found that simple a-diazo ketones and esters can, in fact, be. induced to undergo 1,5-insertion in preparatively useful yields. It was already known that, in the rhodium-mediated insertion process, methyl C—is electronically less reactive than methylene C—H or methine C—H. It therefore seemed likely that competing P-hydride elimination would be least likely with a diazoethyl ketone. In fact, on cyclization of (94), only a trace of the enone product from P-hydride elimination is observed (equation 33). The main side reaction competing with 1,5-insertion is dimer formation. 

In the rhodium carboxylate catalyzed process, the transition state leading to C—insertion is highly ordered. Rhodium-mediated C—insertion has further been shown to proceed with retention of absolute configuration. It may likely be possible, therefore, to design enantiomerically pure ligands for rhodium that would direct the absolute course of insertion into a target methylene. 

A rhodium(III)/copper(II)-mediated process was reported to provide tetra-substituted enol esters in a trans-selective fashion. Overall, the reaction consists of a heteroaryl acyloxylation of alkynes. The process was initiated by a rhodium(III)-catalyzed C-2-selective activation of electron-rich het-eroarenes, such as benzo[I>]furan, and furan. Upon addition across an alkyne, a transmetalation to copper(II) enabled reductive C—O bond formation (14AGE14575). 

The direct copper-catalyzed iodosyl-mediated nitrogen transfer to olefins compares with the parent rhodium-catalyzed process that is made possible by the combination of iodosylbenzene diacetate, magnesium oxide, and sulfamates. Other recent promising nitrene transfer methods involve the bromine-catalyzed aziridination of olefins using chloramine-T and the direct electrochemical aziridination with TV aminophthalimide. ... 

Primary amines can be used as substrates for C-C bond activation reactions that consist of four independent transformations [29]. This process is exemplified by reaction of 3-phenylpropan-l-amine (49) with 3,3-dimethylbut-l-ene (47) in the presence of 16 and 21, which produces both the symmetric dialkyl ketone 51 and tmsymmetric ketone 50 (Scheme 10a). The route followed in this reaction (Scheme 10b) begins with rhodium mediated transfer hydrogenation between amine 49 and alkene 47 to generate phenethylimine 52, which then undergoes transimination with 21 to yield the aminopicoline derived imine 53. Chelation-assisted hydroimination of 53 with the olefin then forms ketimine 54, which upon acid promoted hydrolysis produces ketone 50. In a competing pathway, Rh(I)-catalyzed C-C bond activation of ketimine 54, followed by subsequent addition of 47, affords the symmetric dialkyl ketimine 55, which is converted to symmetric dialkyl ketone 51 upon hydrolysis. 

Recently, it was reported that a rhodium-catalyzed reaction of benzyl ketone 1 with thioester 2 resulted in scrambling of the acyl groups to yield 3 and 4 (Eq. (6.4)) [6]. A broad range of benzyl ketones undergo this scrambling reaction with thioesters and esters under these conditions. Although the mechanism for this process remains elusive, rhodium-mediated activation of benzylic C-C=0 bonds... 

The diazo function in compound 4 can be regarded as a latent carbene. Transition metal catalyzed decomposition of a diazo keto ester, such as 4, could conceivably lead to the formation of an electron-deficient carbene (see intermediate 3) which could then insert into the proximal N-H bond. If successful, this attractive transition metal induced ring closure would accomplish the formation of the targeted carbapenem bicyclic nucleus. Support for this idea came from a model study12 in which the Merck group found that rhodi-um(n) acetate is particularly well suited as a catalyst for the carbe-noid-mediated cyclization of a diazo azetidinone closely related to 4. Indeed, when a solution of intermediate 4 in either benzene or toluene is heated to 80 °C in the presence of a catalytic amount of rhodium(n) acetate (substrate catalyst, ca. 1000 1), the processes... 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

Pirrang, Liu, and Morehead [22] have elegandy demonstrated the application of saturation kinetics (Michaehs-Menten) to the rhodium(II)-mediated insertion reactions of a-diazo /9-keto esters and a-diazo /9-diketones. Their method used the Eadie-Hofstee plot of reaction velocity (v) versus v/[S] to give and K, the equilibrium constants for the catalytic process. However, they were unable to measure the Michaelis constant (fC ) for the insertion reactions of a-diazo esters because they proved to be too rapid. 

Vinylidene-mediated reactions involving rhodium and iridium are discussed separately from those involving Groups 10 and 11 transition metals. The reactions of Group 9 metal vinylidenes are more numerous and have more in common with one another. Extensive stoichiometric organometallic literature aids in the understanding of these processes. In contrast, reactions of Groups 10 and 11 metal vinylidenes are more scattered and often controversial. 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

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