Cyclopropanation diazo compounds, dirhodium

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... 

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]. 

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 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... 

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. 

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... 

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]. 

Asymmetric cyclopropanation. The ability to effect ligand exchange between rhodium(II) acetate and various amides has lead to a search for novel, chiral rhodium(II) catalysts for enantioselective cyclopropanation with diazo carbonyl compounds. The most promising to date are prepared from methyl (S)- or (R)-pyroglutamate (1), [dirhodium(ll) tetrakis(methyl 2-pyrrolidone-5-carboxylate)]. Thus these complexes, Rh2[(S)- or (R)-l]4, effect intramolecular cyclopropanation of allylic diazoacetates (2) to give the cyclo-propanated y-lactones 3 in 65 S 94% ee (equation 1). In general, the enantioselectivity is higher in cyclopropanation of (Z)-alkenes. 

The incorporation of N-phthaloyl amino acids into the dirhodium(II) platform afforded excellent asymmetric cyclopropanation catalysts [81b, 97]. In contrast to other phthaloyl catalysts [97], the X-ray crystal structure of Rh2(S-PTTL)4 (2), reported by Fox et al. revealed that the four phthalimido groups are situated on one face of the catalyst in a chiral crown structure (Figure 9.10) [93]. The four Bu- groups are directed on the other face of the catalyst, and aU C- Bu bonds are parallel to the Rh-Rh bond. Compound 2 exhibits high diastereoselectivity and yields for cyclopropanation with a-alkyl-a-diazoesters (Table 9.2, entry 2). The enantiomeric excess (ee) increases with the a-alkyl diazoester substituent size, and the highest 99% ee and 95% yield were achieved in the reaction of styrene with ethyl-2-diazo-5-methylhexanoate [93]. 

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