Dirhodium tetracarboxylate catalyst

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). 

On the other hand, other chiral dirhodium(II) tetracarboxylate catalysts based on azetidine- and aziridine-2-carboxylic acids have been prepared by Zwanenburg et al. and submitted to the cyclopropanation of styrene with... 

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]. 

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. 

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... 

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). ... 

A strategy directed at exploiting another bimetallic framework with tethered carboxylate groups led to the isolation of an unusual dirhodium(ll) tetracarboxylate catalyst with calbc[4]arene (CLX), which was synthesized (Scheme 9.17) and used in C-H amination reactions [143] the structures of... 

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. 

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. 

Extensive studies involving the use of dirhodium catalysts for cyclopropanation and cyclopropenation reactions have been reported by Doyle [80], Davies [81], and Fox etal. [82] with chiral dirhodium(II) tetracarboxylates and their derivatives being the most common [83]. A large number of chiral dirhodium(II) carboxamidate complexes have been developed, primarily by Doyle et al, [60a] to perform asymmetric cyclopropanation [84] and cydopropenation [85] reactions. 

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