Dirhodium catalysts, chiral

Fig. 14.3 Predictive models for asymmetric induction by (a) (R)-panto-lactone as a chiral auxiliary (b) (S)-prolinate dirhodium catalysts...

The C-H activation of allylic and benzylic C-H bonds has considerable application in organic synthesis. Studies by Muller [131] and Davies [130] on reactions with cyclohexene revealed that Rh2(S-DOSP)4 in a hydrocarbon solvent is the optimum system for high asymmetric induction (Tab. 14.13). Although this particular example gives a mixture of the C-H activation product 179 and cyclopropane 180, similar reactions with ethyl diazoacetate gave virtually no C-H activation product. Some of the other classic chiral dirhodium catalysts 181 and 182 were also effective in this chemistry, but the en-antioselectivity with these catalysts (45% ee and 55% ee) [131] was considerably lower than with Rh2(S-DOSP)4 (93% ee) [130]. 

By far the greatest advances in enantiocontrolled C-H insertion reactions have been provided by Doyle and co-workers with chiral dirhodium(II) carboxamidate catalysts [7,10]. The key development here is the creation of chiral imidazolidinone-ligated dirhodium catalysts 22 to control diastereoselectivity and enhance enantiocontrol [122]. A significant example of the power of this methodology is the insertion reactions of cycloalkyl diazoacetates. With cyclohexyl diazoacetate, for example, four products are possible via C-H insertion constituted in two pairs of diastereoisomers (Eq. 5.28). 

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... 

Chiral dirhodium catalysts confined in porous hosts... 

In this study chiral dirhodium catalysts (Rh2(MEPY)4) and Rh2(BNOX)4), developed by Doyle [10] (see Scheme 1) were immobilised. It can be anticipated that the spatial constraints induced by the carrier (MCM-41 or silica), and especially by the pores of MCM-41, are able to increase the influence of the chiral ligands. Earlier research [11,12] showed that enantioselective reduction catalysed by a palladium complex immobilised inside the pores of MCM-41 resulted in a threefold increase in enantioselectivity compared to the homogeneous palladium complex. In order to immobilise the homogeneous catalysts on the surface, an organic linker group was... 

Siegel and Schmalz [79] reported a new approach to optically active ferrocenes with planar chiraHty, based on a Cu-catalyzed C-H insertion (Scheme 17). Both a five- and a six-membered ring could be formed in this way. Using a copper-bisoxazoline catalyst, good yields and ees in the range of 60-80% could be obtained, whereas chiral dirhodium catalysts proved to be ineffective in this case. 

Fig. 3 Chiral dirhodium catalysts discussed in this chapter...

In an extension of this work, Hashimoto and co-workers tested several chiral dirhodium catalysts for the intramolecular amidation reactions in 2006. Under the catalysis of Rh2(S-TFPTTL)4 as the optimal catalyst,... 

The rhodium carbenoid (55) generated from diazocompound (50) has been shown to react efficiently with 1,3-dipolar nitrones in a formal [3 + 3] manner to produce heterocyclic cycloadducts (56) in high yields. Using chiral dirhodium catalysts such 0 as (51) or equivalent (R = methyl instead of adamantyl), the process exhibits a very high level of enantioinduction. 

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. 

Figure 9.9 Common chiral dirhodium catalysts for cyclopropanation.

The chirtil dirhodium catalysts 14-18 (Figure 9.12), with phthaloyl-terf-leucinate based ligands on the dinuclear scaffold, were tested for asymmetric cyciopropenation reactions [105]. It was found that 14 exhibits the best catalytic activity for enantioselective cyciopropenation of 1-alkynes with 2,4-dimethyl-3-pentyl a-alkyl-a-diazoacetates, with high enantioselectivity (up to 99% ee). The high selectivity of 14 is attributed to the C4-symmetry-like chiral crown conformation [105]. 

As in the case of C-H insertion, Si-H bond functionalization in aryl- and vinyl-diazoacetates takes place in the presence of dirhodium catalysts, and chiral ones result in asymmetric induction [60a]. In pioneering studies by Doyle et al, enantioselective Si-H bond insertion with chiral dirhodium catalysts was achieved [90]. Recently, Ball etal reported innovative studies with bis-acetate dirhodium complexes, bearing chelating nona-peptides (see Section 9.2.3.2), which catalyzed the enantioselective carbenoid insertion into Si-H bonds (Scheme 9.18) [46]. The optimization of the peptide bound to the dirhodium unit or the presence of a phosphite additive significandy improved the enantiose-lectivity of the silane products [46]. 

Davies HML, Walji AM. Universal strategy for the immobilization of chiral dirhodium catalysts. Org. Lett. 2005 7 2941-2944. 

Dirhodium tetra(A-arylsulfonylprolinates) as chiral catalysts for asymmetric transformations of vinyl and aryldiazoacetates 99EJ02459. 

Abstract The dirhodium(II) core is a template onto which both achiral and chiral ligands are placed so that four exist in a paddle wheel fashion around the core. The resulting structures are effective electrophilic catalysts for diazo decomposition in reactions that involve metal carbene intermediates. High selectivities are achieved in transformations ranging from addition to insertion and association. The syntheses of natural products and compounds of biological interest have employed these catalysts and methods with increasing frequency. 

Chiral dirhodium(II) carboxamidate catalysts are, by far, the most effective for reactions of allylic diazoacetates [44, 45] and allylic diazoacetamides [46]. Product yields are high, catalyst loading is low (less than 1 mol%), and enan-tioselectivities are exceptional (Scheme 6). The catalysts of choice are the two... 

Addition to a carbon-carbon triple bond is even more facile than addition to a carbon-carbon double bond, and there are now several reports of intermolec-ular [71] and intramolecular reactions [72-74] that produce stable cyclopropene products with moderate to high enantioselectivities. One of the most revealing examples is that shown in Scheme 9 [72] where the allylic cyclopropanation product (30) is formed by the less reactive Rh2(MEPY)4 catalyst, but macrocy-clization is favored by the more reactive Rh2(TBSP)4 and Rh2(IBAZ)4 catalysts and, as expected, the highest enantioselectivities are derived from the use of chiral dirhodium(II) carboxamidate catalysts. 

The use of dirhodium(II) catalysts to generate ylides that, in turn, undergo a vast array of chemical transformations is one of the major achievements in metal carbene chemistry [1,103]. Several recent reviews have presented a wealth of information on these transformations [1, 103-106], and recent efforts have been primarily directed to establishing asymmetric induction, which arises when the chiral catalyst remains bound to the intermediate ylide during bond formation (Scheme 11). 

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... 

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... 

McKervey and Ye have developed chiral sulfur-containing dirhodium car-boxylates that have been subsequently employed as catalysts for asymmetric intramolecular C-H insertion reactions of y-alkoxy-ot-diazo-p-keto esters. These reactions produced the corresponding ci -2,5-disubstituted-3(2H)-furanones with diastereoselectivities of up to 47% de. Moreover, when a chiral y-alkoxy-a-diazo-p-keto ester containing the menthyl group as a chiral auxiliary was combined with rhodium(II) benzenesulfoneprolinate catalyst, a considerable diastereoselectivity enhancement was achieved with the de value being more than 60% (Scheme 10.74). 

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. 

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