Dirhodium diazo compounds

The mechanism through which catalytic metal carbene reactions occur is outlined in Scheme 2. With dirhodium(II) catalysts the open axial coordination site on each rhodium serves as the Lewis acid center that undergoes electrophilic addition to the diazo compound. Lewis bases that can occupy the axial coor-... 

The use of dirhodium(II) catalysts for catalytic reactions with diazo compounds was initiated by Ph. Teyssie [14] in the 1970s and rapidly spread to other laboratories [1]. The first uses were with dirhodium(II) tetraacetate and the more soluble tetraoctanoate, Rh2(oct)4 [15]. Rhodium acetate, revealed to have the paddle wheel structure and exist with a Rh-Rh single bond [16], was conve-... 

Since their first introduction by Brunner and McKervey as chiral catalysts for the asymmetric cyclopropanation of alkenes with diazo compounds, chiral dirhodium tetra(A-arylsulfonylprolinates) complexes have been widely used by Davies,in particular, in the context of these reactions. Therefore, the use of... 

The metal-catalyzed decomposition of diazo compounds has broad applications in organic synthesis [1-8]. Transient metal carbenoids provide important reactive intermediates that are capable of a wide variety of useful transformations, in which the catalyst dramatically influences the product distribution [5]. Indeed, the whole field of diazo compound decomposition was revolutionized in the early 1970s with the discovery that dirhodium tetracarboxylates 1 are effective catalysts for this process [9]. Many of the reactions that were previously low-yielding using conventional copper catalysts were found to proceed with unparalleled efficiency using this particular rhodium catalysis. The field has progressed extensively and there are some excellent reviews describing the breadth of this chemistry [5, 7, 10-17]. 

A vast array of chiral catalysts have been developed for the enantioselective reactions of diazo compounds but the majority has been applied to asymmetric cyclopropanations of alkyl diazoacetates [2]. Prominent catalysts for asymmetric intermolecular C-H insertions are the dirhodium tetraprolinate catalysts, Rh2(S-TBSP)4 (la) and Rh2(S-DOSP)4 (lb), and the bridged analogue Rh2(S-biDOSP)2 (2) [7] (Fig. 1). A related prolinate catalyst is the amide 3 [8]. Another catalyst that has been occasionally used in intermolecular C-H activations is Rh2(S-MEPY)4 (4) [9], The most notable catalysts that have been used in enantioselective ylide transformations are the valine derivative, Rh2(S-BPTV)4 (5) [10], and the binaphthylphosphate catalysts, Rh2(R-BNP)4 (6a) and Rh2(R-DDNP)4 (6b) [11]. All of the catalysts tend to be very active in the decomposition of diazo compounds and generally, carbenoid reactions are conducted with 1 mol % or less of catalyst loading [1-3]. 

Among transition-metal compounds that are effective for metal carbene transformations, those of Cu and Rh have received the most attention [7-10]. Cu catalysis for reactions of diazo compounds with olefins has been known for more than 90 years [11], but the first report of Rh catalysis, in the form of dirhodium(II) tetraacetate, has been recent [12], Although metal carbene intermediates with catalytically active Cu or Rh compounds have not yet been observed, those... 

Rh(II) carboxylates, especially Rh2(OAc)4> have emerged as the most generally effective catalysts for metal carbene transformations [7-10] and thus interest continues in the design and development of dirhodium(II) complexes that possess chiral51igands. They are structurally well-defined, with D2h symmetry [51] and axial coordination sites at which carbene formation occurs in reactions with diazo compounds. With chiral dirhodium(II) carboxylates the asymmetric center is located relatively far from the carbene center in the metal carbene intermediate. The first of these to be reported with applications to cyclopropanation reactions was developed by Brunner [52], who prepared 13 chiral dirhodium(II) tetrakis(car-boxylate) derivatives (16) from enantiomerically pure carboxylic acids RlR2R3CC OOH with substituents that were varied from H, Me, and Ph to OH, NHAc, and CF3. However, reactions performed between ethyl diazoacetate and styrene yielded cyclopropane products whose enantiopurities were less than 12% ee, a situation analogous to that encountered by Nozaki [2] in the first applications of chiral Schiff base-Cu(II) catalysts. 

The use of chiral dirhodium carboxylate, 17 or 18, is preferred over chiral dirhodium carboxamidates for chemical transformations of a-diazo-p-ketocarbonyl compounds primarily because of reactivity considerations, that is, these diazo compounds do not undergo dinitrogen loss with the carboxamidate catalysts even at elevated temperatures. In addition, the orientation of the chiral ligands in 17 and 18 provides closer access to bulky diazo compounds. When the two attachments to the di azomethane unit are vastly unequal in size, high levels of enantiocontrol can result. 

Intermolecular insertion to aryl C—H bonds is possible. The asymmetric intramolecular reaction of the a-diazo compound 354 catalysed by Rh2[(S)-PTTL]4, Rh2[(S)—PTTL]4 = dirhodium tetrakis[N-phthaloyl(S)—t—leucinate], afforded indane... 

In CHEC-II(1996), carbene insertion reactions into the N-H bond to form a fused-ring azetidinone warranted a separate section. In the last decade, the popularity to this approach to bicyclic systems seems to have markedly declined. Nevertheless, dirhodium tetraacetate and rhodium octanoate were used to generate the corresponding bicyclic compounds from the diazo compounds 241 (R2 = H and /3-Me), respectively, via the carbene intermediates. In the latter case, the produced enol was esterified and then the ester group replaced with a hydroxymethyl substituent to give derivatives 242 in a one-pot process <2001JCM166, 1999TL427>. 

The use of chiral additives with a rhodium complex also leads to cyclopropanes enantioselectively. An important chiral rhodium species is Rh2(5-DOSP)4, which leads to cyclopropanes with excellent enantioselectivity in carbene cyclopro-panation reactions. Asymmetric, intramolecular cyclopropanation reactions have been reported. The copper catalyzed diazoester cyclopropanation was reported in an ionic liquid. ° It is noted that the reaction of a diazoester with a chiral dirhodium catalyst leads to p-lactones with modest enantioselectivity Phosphonate esters have been incorporated into the diazo compound... 

Examples of more basic, but enantioselective intramolecular carbenoid C-H insertion reactions were displayed in two very similar total syntheses of the phosphodiesterase type IV inhibitor i -(-)-rolipram (179, Scheme 44) [125, 126], In 1999, Hashimoto and coworkers utilized acceptor/acceptor diazo compound 180, with the nitrogen atom protected with a p-nitrophenyl moiety, as the carbenoid precursor. After screening a number of phthalimide-based dirhodium catalysts, Rh2(5 -BPTTL)4 (30) was found to give the optimal results, providing the cyclized product in 74% yield and 88% ee. 

Cyclopropanation of dienes 90 or 94 with 3,3-dichlorodiazopropene (91b) or the parent diazo compound 91 a (X = h) in the presence of dirhodium tetraacetate leads to a mixture of the rearranged fused eyeloheptadienes 93 and 96 and the stable tra .v-l,2-divinylcyclopropanes 92 and 95. The trans- 1,2-divinyl derivatives can be transformed to the seven-meinbered ring by heating to 110 °C854. 

On the other hand, the dirhodium bridge caged within a lantern structure is thought to be essential to the success of dirhodium complexes in which two rhodium atoms are surrounded by four ligands in a nominal symmetry. Both computational studies and characterization of dirhodium car-benoid intermediates suggested that the intermediate adopts a Rh—Rh=C framework. In another word, two rhodium atoms are bound to one carbene center, and the bonding scenario obeys the three-center orbital paradigm. As such, metal carbenoids derived from chiral Rh complexes and donor/ acceptor diazo compounds are routinely utilized. 

Binuclear Rhodium(ll) Catalysts. Soon after the first report of dirhodium(ll) carboxylates 1 (Scheme 1) as effective catalysts for diazo decomposition in 1973 (21), this type of complex was discovered to actively catalyze cyclopropanation (22). Comparison of relative reactivity and stereoselectivity of catalyst 1 (R = CHs) and a stoichiometric carbene complex of (COsWCHPh for cyclopropanation of alkenes with phenyldiazomethane showed rhodium carbene involvement in the rhodium-catalyzed cyclopropanation (23). Catalyst 1 (R = CH3) also demonstrated improvement of cyclopropane production can be achieved by decreasing the available concentration of the diazo compound with a slow addition method (24). 

Diazo Compounds Decomposition with Chiral Rhodium Catalysts. The first chiral rhodium catalyzed asymmetric cyclopropanation was reported in 1989 (75). Structures of the catalysts were based on the framework of dirhodium(II) tetrakis(carboxylate) 1 with the carboxylate ligands replaced with... 

Different from 15, dirhodium(II) carboxamidates 20 (Scheme 15) provide the asymmetric centers of the ligands adjacent to the carbenoid centers. Although catalysts 20 are less reactive toward diazo compounds, they give higher selectiv-... 

Regarding carbon nucleophiles, phosphoric acids have been applied as organocatalysts in multicomponent reactions between diazo compounds, alcohols, or amines, and aldehydes, imines, or Michael acceptors [76]. Diazo compounds can be converted into the respective metallocarbenes in the presence of dirhodium (n) carboxylates complexes [77], Such intermediates can suffer a nucleophilic attack from alcohols or amines generating oxygen or nitrogen ylides that may undergo a proton shift, furnishing the respective O-H or N-H insertion products (insertion pathway. Scheme 26.14). 

A further increase in the d-electron count to 14 brings us to the dirhodium paddle wheels (Rh j) that have found extensive use as catalysts for the decomposition of organic diazo compounds and the subsequent insertion of the carbene group into a C-H bond [7]. The complete occupation of the 8/8 ... 

Cyclopropanation of olefins is currently performed by direct transition metal-catalyzed carbene transfer from a diazo compound to the olefin. Dirhodium(II) carboxylates and carboxamidates have proved to be the catalysts of choice. Other rhodium compounds, such as Rh (CO),6, Rh2(BF4)4, and rhodium(III) porphyrins, have been also investigated, but did not show better reactivity, while rhodium(I) compounds have never been successful [66]. Other complexes containing copper or ruthenium have been tested in cyclopropanation reactions, but have never shown better reactivity or selectivity than rhodinm(II) compounds [67]. 

This overview will concentrate exclusively on the use of dirhodium(II) complexes as these are the most active catalysts for this transformation. The preparation of rhodium(II) carboxylate complexes was first reported in 1960 by the group of Chernyaev following the reaction of rhodium(III) chloride in refluxing formic acid. Their ability to decompose diazo compounds for the formation of a metaUocarbene was then discovered by Teyssie and coworkers a decade later. This seminal study has opened the vast domain of dirhodium(II)-catalyzed carbene additions that has proved highly successful. ° Their use in catalytic nitrene addition, though less extensively investigated, has also led to significant achievements that are summarized below with an emphasis on the latest developments made in the last 5 years. 

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. 

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