Dirhodium tetraprolinates

The cyclopropanation of furan with methyl 4-bromophenyldizoacetate using the chiral dirhodium tetraprolinate was further studied, showing the initial bond formation occurred at the... 

Asymmetric activation of the C—H bonds in benzyl silyl ethers was achieved by using Hashimoto s A-phthaloyl-based Rh2((5)-PTTL)4 catalyst (Figure 5.6) in high diastereoselectivities and enantioselectivities (Scheme 5.15). The well-established dirhodium tetraprolinates such as Rh2((5)-DOSP)4 and Rh2((R)-DOSP)4 catalysts, which generally are excellent catalysts for asymmetric C—H bond activation, were not suitable catalysts in these reactions. 

In contrast to the asymmetric activation of C—H bonds in benzyl silyl ethers, the dirhodium tetraprolinate, Rh2(5-DOSP)2 (Figure 5.7), was found to be an efficient catalyst in an enantioselective C—H activation of acetals (Scheme 5.16). Interestingly, when the acetals had a methoxy substituent on the aromatic ring, the Stevens rearrangement was a main competing side reaction of the C—H activation of acetals. 

In contrast to the intermolecular cyclopropanation, the dirhodium tetraprolinates give modest enantioselectivities for the corresponding intramolecular reactions with the do-nor/acceptor carbenoids [68]. For example, the Rh2(S-DOSP)4-catalyzed reaction with al-lyl vinyldiazoacetate 32 gives the fused cyclopropane 33 in 72% yield with 72% enantiomeric excess (Eq. 4) [68]. The level of asymmetric induction is dependent upon the substitution pattern of the alkene cis-alkenes and internally substituted alkenes afford the highest asymmetric induction. Other rhodium and copper catalysts have been evaluated for reactions with vinyldiazoacetates, but very few have found broad utility [42]. 

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]. 

The use of chiral dirhodium tetraprolinate catalysts, such as 87 and 88, allows asymmetric induction resulting in formation of azabicyclo[3.2.2]nonane products with high enantiomeric excess <2002JOC5683>. 

Other reactions. iV-BOC-protected azocane 281 was reacted with aryl diazoacetate 282 in the presence of 1 mol% of dirhodium tetraprolinate catalyst to produce a very efficient C-H insertion with a high diastereoselectivity and enantioselectivity (Scheme 118 <2003JA6462>). The erythro C-H insertion product 283a was formed in 90% ee and 90% de, which infers that the flexibility of the eight-member ring sterically favors the accommodation of the transition state for the C-H activation. 

Davies HML, Walji AM, Nagashima T. Simple strategy for the immobilization of dirhodium tetraprolinate catalysts using a pyridine-linked solid support. J. Am. Chem. Soc. 2004 126 4271 280. 

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