Dirhodium tetracarboxylate

Deuterium isotope effects in CDCI3 have been applied in studies of proton transfer equilibrium for optically active Schiff bases [24], [25] as well as their dirhodium tetracarboxylate adducts.11 The AC-2(D) values were in the range from +541 to —546 ppb for imines and from +524 to 255 ppb for their adducts. Exceptionally low values of deuterium isotope effects of - 20 —80 ppb were observed for the Schiff bases being derivatives of 3,5-dinitrosalicylaldehyde which exist in pure NH-form. This behaviour was explained by different electronic structures of the NH... 

Asymmetric synthesis of the rocaglamides was accomplished by employing [3+2] photo-cycloaddition mediated by functionalized TADDOL based chiral Brpnsted acids. The synthesis consisted of a [3+2] dipolar cycloaddition, a base-mediated a-ketol rearrangement and a hydroxyl-directed reaction <06JA7754>. Asymmetric synthesis of 1,2-dihydrobenzo /j]furans was achieved by adamantylglycine derived dirhodium tetracarboxylate catalyzed C-H insertion <06OL3437>. 

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]. 

An important paper describes the formation of the metal-metal bonded dirhodium tetracarboxylate trifluorophosphine complex [Rh2(OCOCH3)4(PF3)2] made directly from the dirhodium tetraacetate complex by direct addition of PF3 to the formally metal-metal triple bond. The structure was determined by a single-crystal X-ray study, and has been compared with other [Rh2(OAc)4X2] systems (X = py, Et2NH, CO, and P(OR)3). 

Dirhodium tetracarboxylate complexes are among the most successful and well-studied catalysts for C-H amination. Early work by Miiller provided support for a concerted asynchronous reaction mechanism for intermolecular amination reactions using Rh2(OAc)4 and NsN=IPh [22-24]. Du Bois and coworkers have shown that carbamate and sulfamate esters can engage in oxidative cyclization reactions promoted by these same types of Rh complexes using PhI(OAc)2 as the terminal oxidant [93-96]. Mechanistic studies, which include Hammett analysis (p = 0.55... 

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. 

Smith RC, Tennyson AG, Lippard SJ (2006) Polymer-bound dirhodium tetracarboxylate films for fluorescent detection of nitric oxide. Inorg Chem 45 6222-6226... 

Hilderbrand SA, Lim MH, Lippard SJ (2004) Dirhodium tetracarboxylate scaffolds as reversible fiuorescence-based nitric oxide sensors. J Am Chem Soc 126 4972—4978... 

High-resolution NMR spectroscopic studies of dirhodium tetracarboxylates in their crystal and liquid-crystalline phases have provided a direct probe of the local symmetry and dynamics of the columnar mesophase of these complexes. In particular, it has shown that the dimetallic... 

Quite recently, Davies and co-workers developed a new class of sterically demanding dirhodium tetracarboxylate catalysts, especially Rh2(R-BPCR)4, that changed the site selectivity of the C(sp )—H bond insertion reaction. In the presence of catalytic amount of Rh2(R-BPCR)4, the primary C—H bond is the preferred reaction site of various substrates containing primary benzylic C—H bonds, allylic C—H bonds, or C—H bonds a to oxygen, which is complementary to Rh2(i -DOSP)4 which favors secondary C—H bonds (Scheme 1.17a-c). Moreover, the use of this methodology was further proved by the selective C—H bond functionalization of complex molecules such as (-)-a-cedrene (Scheme 1.17d). 

In 2006, Davies and co-workers demonstrated dirhodium tetracarboxylate, Rh2(S-TCPTAD)4 as an efficient catalyst to enable the intermolecular and intramolecular (not shown here) C—H bond amination reactions, respectively. Utilizing this catalyst, the aminated product 98 was obtained in excellent... 

SO2 and CO, CO2, N2O, NO2, and H2O. Lippard and coworkers also utilized the Lewis acid-base interaction for fluorescent detection of nitric oxide (NO). Fluorophores (dansylimidazole or dansylpiperazine) bind to the axial site of the metal center in a dirhodium tetracarboxylate complex. Displacement of fluorophores by NO causes a fluorescence change. 

The dirhodium tetracarboxylate, Rh2(S-PTAD)4, derived from adamantylglydne, is a very effective chiral catalyst for carbenoic reactions and high asymmetric induction was obtained for intermolecular cyclopropanation (Scheme 80). ... 

The X-ray crystal structure of Rh2(02CCH3)4(IMes) (64) (Figure 9.23 IMes = l,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) was determined by Chang et al. [150]. Catalyst 64 was employed for the arylation at position 8 of quinoline (Scheme 9.21), which is a prominent structural entity in many natural products. The dirhodium tetracarboxylate itself failed to catalyze the reactions, but C-C bonded quinoline products were obtained in high yield with the dirhodium-NHC catalyst when quinoline and aryl bromide were reacted. 

In a computational study on the competing intramolecular amidation and aziridina-tion reactions, catalysed by dirhodium tetracarboxylate, four pathways were examined... 

With the aim to better understand the exceptional reactivity of the Rh2(esp)2 catalyst, Du Bois has demonstrated that dirhodium(II) complexes undergo an one-electron oxidation to give a mixed-valent Rh /Rh species. While classical dirhodium tetracarboxylate complexes decompose... 

Reddy RP, Davies HML. Dirhodium tetracarboxylates derived from adamantylglycine as chiral catalysts for enantioselective C-H aminations. Org Lett. 2006 8 5013-5016. 

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. 

Mattiza JT, Harada N, Kuwahara S, Hassan Z, Duddeck H. Comparing various chiral dirhodium tetracarboxylates in the dirhodium method. Chirality 2009 21 843-849. 

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