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Rhodium complexes carbene insertion reactions

Transition-metal carbene complexes undergo insertion reactions with C-H compounds. For example, under the action of PF or HCl, the rhodium cyclopen-tadienyl carbene complex gives the products of an migratory insertion of the CPh unit into an Ar/j -C-Hbond of the cyclopentadienyl ring [64] ... [Pg.180]

A similar type of chiral rhodium porphyrin was found to be effective for the carbene-insertion reaction to olefins, where formation of the carbene complex takes place. Chiral rhodium complexes for catalytic stereoselective-carbene addition to olefins were prepared by condensation of a chiral aldehyde and pyrrole. Formation of the metal-carbene complex and substrate access to the catalytic center are crucial to the production of optically active cyclopropane derivatives. Optically active a-methoxy-a-(trifluoro-methyOphenylacetyl groups are linked witfi the amino groups of a,p,0L,p isomers of tetrakis-(2-aminophenyI)por-phyrin through amide bonds. Oxidation reactions of the... [Pg.285]

The reactions of organorhodium compounds or the catalytic reaction of a rhodium complex are insertions, formation reactions of carbene, hydrogenations, hydrometalations, decarbonylations, carbonylations, hydroformylations, cycliza-tions and cyclometalations, etc. [Pg.397]

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. [Pg.238]

As shown in the previous two sections, rhodium(n) dimers are superior catalysts for metal carbene C-H insertion reactions. For nitrene C-H insertion reactions, many catalysts found to be effective for carbene transfer are also effective for these reactions. Particularly, Rh2(OAc)4 has demonstrated great effectiveness in the inter- and intramolecular nitrene C-H insertions. The exploration of enantioselective C-H amination using chiral rhodium catalysts has been reported by several groups.225,244,253-255 Hashimoto s dirhodium tetrakis[A-tetrachlorophthaloyl-(A)-/ r/-leuci-nate], Rh2(derived rhodium complex, Rh2(i -BNP)4 48,244 afforded moderate enantiomeric excess for amidation of benzylic C-H bonds with NsN=IPh. [Pg.196]

The catalytic activity of rhodium diacetate compounds in the decomposition of diazo compounds was discovered by Teyssie in 1973 [12] for a reaction of ethyl diazoacetate with water, alcohols, and weak acids to give the carbene inserted alcohol, ether, or ester product. This was soon followed by cyclopropanation. Rhodium(II) acetates form stable dimeric complexes containing four bridging carboxylates and a rhodium-rhodium bond (Figure 17.8). [Pg.364]

Electrophilic carbene complexes generated from diazoalkanes and rhodium or copper salts can undergo 0-H insertion reactions and S-alkylations. These highly electrophilic carbene complexes can, moreover, also undergo intramolecular rearrangements. These reactions are characteristic of acceptor-substituted carbene complexes and will be treated in Section 4.2. [Pg.169]

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. [Pg.797]

Although the first catalysts were copper-based, the insertion of metal-associated carbenes into carbon-hydrogen bonds has undergone a renaissance with the advent of rhodium(II) carboxylate catalysts [56]. Metal-catalyzed enan-tioselective C-H insertions of carbenes have not been studied in great detail. Most of the efficient enantioselective versions of this reaction involve chiral rhodium complexes and until recently, the use of chiral catalysts derived from metals other than copper and rhodium for the asymmetric C-H insertion of metal-associated carbenes are still unexplored. [Pg.575]

A dirhodium tetracarboxylate complex coordinated by two bromocalix[4]arene macrocycles exhibited two toluene molecules coordinated to the rhodium centers and inserted in the clefts, which are formed by the vicinal -bromophenyl rings of the two calixarene units (Figure 29). This complex has been found to be an efficient catalyst for two carbene transfer reactions, alkene cyclopropanation, and intramolecular C-H insertion, in terms of stereo- and regioselectivity. [Pg.797]

An important factor in the efficiency of this reaction may well be complexation between the rhodium carbene and the bond into which carbene insertion is to take place certainly rhodium mediated C—H bond activation is known to take place. The addition to alkenes can also be catalysed " rearrangement of the product provides a useful route to cyclopentane rings. [Pg.560]

Rhodium(II)-MEPY and rhodium(II)-MACIM (methyl 1-acetylimidazolidin-2-one-4-carboxylate) complexes are efficient chiral catalysts for intramolecular carbon-hydrogen insertion reactions of diazoacetates (224) and metal carbene transformations (225). Dirhodium(II) carboxylates of similar structure (eg, piperidinonate complexes of the Rh2(ligand)4 type) have been found efficient catalysts for asymmetric cyclopropanation of olefins (226). [Pg.701]

By analogy with the rhodium carbene intermediate proposed in the C H insertion reaction with diazo compounds, C— H amination is believed to proceed via a rhodium nitrene species, although such an intermediate has never been characterized. However, as chiral dimeric rhodium complexes lead to the formation of enantioen-riched amination products, it suggests that the metal center is closely associated with the reactive nitrogen during the C—H insertion step. Both a rhodium nitrene or rhodium phenyliminoiodinane species may be involved (Figure 5.2). [Pg.147]

It is well established that the rhodium-carbene species generated upon activation of diazo compounds by rhodium complexes can undergo insertion into a X—H bond (X= C, Si, O, N) to form a new C—X bond under mild conditions (Scheme 3.58). This reaction involves formation of an onium yUde intermediate and subseqnent proton transfer [156]. The high reactivity of these onium ylide species, which can be trapped by an electrophile prior to the proton transfer leading to a second bond formation. [Pg.101]

Diazo compounds, with or without metal catalysis, are well-known sources of carbenes. For synthetic purposes a metal catalyst is used. The diazo compounds employed are usually a- to an electron-withdrawing group, such as an ester or a ketone, for stability. In the early days, copper powder was the catalyst of choice, but now salts of rhodium are favoured. The chemistry that results looks very like the chemistry of free carbenes, involving cyclopropanation of alkenes, cyclopropenation of alkynes, C-H insertion reactions and nucleophilic trapping. As with other reactions in this chapter, free carbenes are not involved. Rhodium-carbene complexes are responsible for the chemistry. This has enormous consequences for the synthetic applications of the carbenes - not only does the metal tame the ferocity of the carbene, but it also allows control of the chemo-, regio- and stereoselectivity of the reaction by the choice of ligands. [Pg.312]

Carbene complexes, generated by the reaction between metal salts and diazo compounds can insert into C-H bonds in a form of CH activation (see Chapter 3 for other CH activation reactions). While early reactions involved the use of copper salts as catalysts (Schemes 8.143 and 8.144), rhodium complexes are now more widely used. In molecules such as cyclohexane, there is no issue of regioselectivity, but this issue is critical for the use of the reaction in synthesis. Both steric and electronic factors influence selectivity. Carbon atoms where a build up of some positive charge can be stabilized are favoured. Hence, allylic positions and positions a- to a heteroatom such as oxygen or nitrogen, are favoured. The reaction at tertiary C-H bonds, rather than primary C-H bonds is also favoured for the same reason, but, in this case, are also disfavoured by steric effects. Reactivity and selectivity are also influenced by both the structure of the catalyst, and the... [Pg.315]

Tryptophan offers an indole side chain that can be used for ligation chemistry. A water-compatible rhodium carbene can be added to the indole ring (19) [105,139]. The reactive species is generated in situ by a conjugated diazo compound by a rhodium catalyst like rhodium(II) acetate [63,139,149]. The reaction takes place in the two- and three-position of indole. Thus, a mixture of N-alkylated and C-alkylated product is obtained. It is necessary to add hydroxylamine hydrochloride as an additive to bind to the distal rhodium carbenoid complex. The usage of this salt lowers the pH value below 3.5 and therefore limits the scope of this methodology. As a side reaction, the carbene inserts into the O-H bond of water (Table 6). [Pg.57]

Many rhodium(II) complexes are excellent catalysts for metal-carbenoid-mediated enantioselective C-H insertion reactions [101]. In 2002, computational studies by Nakamura and co-workers suggested the dirhodium tetracarboxylate catalyzed diazo compounds insertion reaction to alkanes C-H bonds proceed through a three-centered hydride-transfer-like transition state (Fig. 25) [102]. Only one rhodium atom of the catalyst is involved in the formation of rhodium carbene intermediate, while the other rhodium atom served as a mobile ligand, which enhanced the electrophilicity of the first one and facilitate the cleavage of rhodium-carbon bond. In this case, the metal-metal bond constitutes a special example of Lewis acid activation of Lewis acidic transition-metal catalyst. [Pg.179]

While major advances in the area of C-H functionalization have been made with catalysts based on rare and expensive transition metals such as rhodium, palladium, ruthenium, and iridium [7], increasing interest in the sustainability aspect of catalysis has stimulated researchers toward the development of alternative catalysts based on naturally abundant first-row transition metals including cobalt [8]. As such, a growing number of cobalt-catalyzed C-H functionalization reactions, including those for heterocycle synthesis, have been reported over the last several years to date (early 2015) [9]. The purpose of this chapter is to provide an overview of such recent advancements with classification according to the nature of the catalytically active cobalt species involved in the C-H activation event. Besides inner-sphere C-H activation reactions catalyzed by low-valent and high-valent cobalt complexes, nitrene and carbene C-H insertion reactions promoted by cobalt(II)-porphyrin metalloradical catalysts are also discussed. [Pg.319]

The different synthetic applications of acceptor-substituted carbene complexes will be discussed in the following sections. The reactions have been ordered according to their mechanism. Because electrophilic carbene complexes can undergo several different types of reaction, elaborate substrates might be transformed with little chemoselectivity. For instance, the phenylalanine-derived diazoamide shown in Figure 4.5 undergoes simultaneous intramolecular C-H insertion into both benzylic positions, intramolecular cyclopropanation of one phenyl group, and hydride abstraction when treated with rhodium(II) acetate. [Pg.178]

Few examples of preparatively useful intermolecular C-H insertions of electrophilic carbene complexes have been reported. Because of the high reactivity of complexes capable of inserting into C-H bonds, the intermolecular reaction is limited to simple substrates (Table 4.9). From the results reported to date it seems that cycloalkanes and electron-rich heteroaromatics are suitable substrates for intermolecular alkylation by carbene complexes [1165]. The examples in Table 4.9 show that intermolecular C-H insertion enables highly convergent syntheses. Elaborate structures can be constructed in a single step from readily available starting materials. Enantioselective, intermolecular C-H insertions with simple cycloalkenes can be realized with up to 93% ee by use of enantiomerically pure rhodium(II) carboxylates [1093]. [Pg.189]

Silanes can react with acceptor-substituted carbene complexes to yield products resulting from Si-H bond insertion [695,1168-1171]. This reaction has not, however, been extensively used in organic synthesis. Transition metal-catalyzed decomposition of the 2-diazo-2-phenylacetic ester of pantolactone (3-hydroxy-4,4-dimethyltetrahydro-2-furanone) in the presence of dimethyl(phenyl)silane leads to the a-silylester with 80% de (67% yield [991]). Similarly, vinyldiazoacetic esters of pantolactone react with silanes in the presence of rhodium(II) acetate to yield a-silylesters with up to 70% de [956]. [Pg.192]

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. [Pg.294]

Rhodium Catalysts. The catalytical behavior of Rh complexes in oligomerizations has been extensively studied " . A wide variety of olefins such as a-olefins, chlorinated olefins, styrene, and acrylates (but not acrylonitrile) are dimerized by RhClj-SHjO. The reactions are carried out at T from 20 to 200°C and are generally not very dependent on the solvent, although small amounts of alcohols lead to increased activity. A detailed insertion mechanism based on an anionic Rh(III)-H-intermediate has been suggested but a carbene mechanism and a metallacycle pathway have also been discussed. [Pg.402]


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See also in sourсe #XX -- [ Pg.575 , Pg.576 ]




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