Dirhodium carboxylate catalyst

The aziridination of olefins, which forms a three-membered nitrogen heterocycle, is one important nitrene transfer reaction. Aziridination shows an advantage over the more classic olefin hydroamination reaction in some syntheses because the three-membered ring that is formed can be further modified. More recently, intramolecular amidation and intermolecular amination of C-H bonds into new C-N bonds has been developed with various metal catalysts. When compared with conventional substitution or nucleophilic addition routes, the direct formation of C-N bonds from C-H bonds reduces the number of synthetic steps and improves overall efficiency.2 After early work on iron, manganese, and copper,6 Muller, Dauban, Dodd, Du Bois, and others developed different dirhodium carboxylate catalyst systems that catalyze C-N bond formation starting from nitrene precursors,7 while Che studied a ruthenium porphyrin catalyst system extensively.8 The rhodium and ruthenium systems are... 

Although dirhodium(II) carboxamidates are less reactive toward diazo decomposition than are dirhodium carboxylates, and this has limited their uses with diazomalonates and phenyldiazoacetates, the azetidinone-ligated catalysts 11 cause rapid diazo decomposition, and this methodology has been used for the synthesis of the cyclopropane-NMDA receptor antagonist milnacipran (17) and its analogs (Eq. 2) [10,58]. In the case of R=Me the turnover number with Rh2(45-MEAZ)4 was 10,000 with a stereochemical outcome of 95% ee. 

In 2005, Doyle et al. reported an original sequence of two successive intramolecular cyclopropanations involving a bis(diazoacetates), using a sterically encumbered oxaimidazolidine carboxylate dirhodium(II) catalyst, Rh2[(45, 5)-BSPIM]4. An excellent result, depicted in Scheme 6.16, was obtained resulting from a double diastereoselection. 

Chiral dirhodium(II) catalysts with carboxylate or carboxamidate ligands have recently been developed to take advantage of their versatility in metal carbene transformation, and these have now become the catalysts of choice for cyclopropanation. Chiral carboxylate ligands 195,103 196,104 and 197105 have been used for tetrasubstitution around a dirhodium(II) core. However, the enantioselectivity in intermolecular reactions with simple ketenes is marginal. 

Dirhodium(II) carboxylate catalysts have been used extensively for the catalysis of carbene insertions. In many cases, impressive selectivities have been achieved (19-21). In an effort to find selective catalysts for carbenoid insertions, Moody screened a series of dirhodium(II) carboxylate catalysts for their ability to catalyze carbenoid Si-H insertion (22). The authors surveyed the commercially available carboxylic acids, -10,000 of which are chiral. The members of this group that contained functionality that is incompatible to the reaction were culled out. The remaining chiral carboxylic acids (-2000 compounds) were then grouped into 80 different clusters. There is no discussion presented for the criteria used in the grouping of the acids. A representative acid from each cluster was then chosen for... 

Intermolecular amination experiments described by Muller using 02NC,5H4S02N=IPh (NsN=IPh) as the nitrene source underscore the value of certain rhodium(II) catalysts for C-H insertion (Scheme 17.5) [12, 34—36]. In accord with Breslow s finding, dirhodium carboxylates were demonstrated to catalyze the amination of allylic, benzylic, and adamantyl substrates. Notably, structurally related tetracarboxamide dimers fail to give... 

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. 

The high degree of enantiocontrol in C-H insertion reactions with such a diversity of diazoacetates suggests unique advantages for chiral dirhodium(II) catalysts derived from pyrrolidone-5-carboxylates. Both lactone enantiomers are accessible from a single diazo ester, and the absence of by-products of similar composition allows convenient product isolation. 

Dirhodium(II) catalysts that possess chiral 2-pyrrolidone-5-carboxylate ester ligands (mepy) are the most effective among those of dirhodium or copper for highly diastereoselective and enantioselective intermolecular cyclopropenation reactions between l-alkynes and diazoesters (eq. (9)). Product yields are moderate, and enantiomeric excesses range from 40 to 98 %. Interestingly, the (R) or (5) catalyst produces the cyclopropene-l-carboxylate respectively with the (/ ) or (5) configuration [26]. 

Doyle et al. reported that the intramolecular cy-clopropanation of the phenyldiazoacetate 690 in the presence of the chiral 2-oxaazetidine-4-carboxylate-ligated dirhodium(II) catalyst Rh2(4S-MEAZ)4 provided the bicyclic lactones 691 in high yields with high enantioselectivities (Scheme 2 1 4).290d... 

The use of soluble rhodium(II) carboxylate catalysts in the cyclopropanation of olefins is the origin of a very detailed crystallographic and spectrophotometric SF study of dirhodium(II) tris- and tetrakis(tritolylbenzoate), and tetrakis(pivalate)/ The Rh(II)-Rh(II) moiety at the Rh2(OsC)4 core resides inside a parallelepipedic box formed by the carboxylate ligands such that smaller ligands, such as pyridine, can coordinate at either end of the dirhodium axis. The formation... 

The same research group has also demonstrated a catalytic enantioselective tandem carbonyl yhde formation-cycloaddition of the a-diazo-j8-ketoester 91 using 0.5 mol% of Rh2(R-DDBNP)4 95, as catalyst to afford the cycloadduct 93 in good yields (Scheme 28) with up to 90% ee [ 109]. A detailed study on enantioselective reaction using a series of dirhodium tetrakiscar-boxylate and tetrakisbinaphtholphosphate catalysts under different solvent conditions has been described [56]. These studies indicate that dirhodium tetrakisbinaphtholphosphate catalysts are superior to the more commonly used carboxylates and carboxamidates in asymmetric transformations. Typically, the reaction [58] of the nitrophenyl-substituted diazodione 96 and phenyl acetylene in the presence of the binaphthyl catalyst 95 at 0 °C afforded the cycloadduct 97 with 76% ee (Scheme 29). 

In 2012, Du Bois and Zare published details of a novel method to observe intermediates in the CH amination reactions catalyzed by dirhodium carboxylate complexes [88]. In this study, desorption electrospray ionization (DESl) was coupled to mass spectroscopy to capture transient intermediates from solution having very short lifetimes (ca. nanoseconds to microseconds). Rh2(esp)2 is a catalyst for the amination of alkanes, such as adamantane, as shown in Scheme 24. 

The functionalization of C—H bonds through a transition metal carbenoid insertion has been known for over a century and has become a powerful method to achieve new C—C bonds. In most cases, these transformations have been completed with dirhodium (II) carboxylate catalysts. The development of chiral dirhodium (II) complexes has allowed the enantioselective version of these reactions and has led to a straightforward method for the preparation of chiral natural products and dmgs. 

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... 

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

Desymmetrization strategy in enantioselective oxonium ylide formation/[l,2]-shift reaction has been reported by Doyle and co-workers.With dirhodium(ii) tetrakis[methyl l-(3-phenylpropanoyl)-2-oxoimidazolidine-4(3 )-carboxylate] [Rh2(43 -MPPIM)4] as the catalyst, up to 88% ee is obtained (Equation (7)). 

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. 

Enantiocontrol with 21-23 is lower than that achieved with chiral copper catalysts for reactions of diazoacetates with styrene and a few other alkenes examined thus far [68], but the carboxamidates display far greater stereocontrol than do the dirhodium(II) carboxylates for the same reactions [69]. However, Hashimoto has reported the use of chiral piperidinonate 24 and found exceptional enantiocontrol in the cyclopropanation of styrene and both mono- and... 

Dirhodium(II) compounds are reported to be the most suitable catalysts for insertion. Selectivity is higher and yields are greater with dirhodium(II) carboxylates or carboxamidates than with copper catalysts, whereas Ru catalysts are not known to facilitate C-H insertion. As expected by a process that is basically electrophilic, electron-donating substituents that are adjacent to the site of insertion activate that center for C-H insertion ril4]. In addition to electronic influences, however, conformational effects that are basically steric in origin can also control reaction selectivity [115]. 

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