Achiral substrates with rhodium catalysts

Striking examples of this phenomenon are presented for allyl and homoallyl alcohols in Eqs. (5) to (7). The stereodirection in Eq. (5) is improved by a chiral (+)-binap catalyst and decreased by using the antipodal catalyst [60]. In contrast, in Eq. (6) both antipode catalysts induced almost the same stereodirection, indicating that the effect of catalyst-control is negligible when compared with the directivity exerted by the substrate [59]. In Eq. (7), the sense of asymmetric induction was in-versed by using the antipode catalysts, where the directivity by chiral catalyst overrides the directivity of substrate [52]. In the case of chiral dehydroamino acids, where both double bond and amide coordinate to the metal, the effect of the stereogenic center of the substrate is negligibly small and diastereoface discrimination is unsuccessful with an achiral rhodium catalyst (see Table 21.1, entries 9 and 10) [9]. 

More recently, Cramer and coworkers reported an asymmetric version of this chemistry to access functionalized chiral dihydrobenzofurans that possess a quaternary stereocenter using chiral rhodium catalyst (Scheme 6.13) [23]. They found that O-tethered substrate 54 was deuterated more quickly than meta-methyl-substituted derivative 55. Furthermore, the reaction will take more time for both substrates when the chiral complex with a 1,2-disubstituted cyclopen-tadienyl ligand was replaced with achiral complex with a more hindered Cp ligand. These differences between substrate 54 and 55 emphasize the significance of the alkoxy substituent as a secondary directing group for rhodium-catalyzed reactions. 

Diazoacetamides are also exceptional substrates for dirhodium carboxamidate-catalyzed reactions, although with these substrates a mixture of /3-lactam and y-lactam products are formed [8]. The rhodium carboxamidate catalyst can have a major effect on the ratio of products formed. A good synthetic example is the Rh2(4S-MPPIM)4)-catalyzed synthesis of (-)-hcliotridanc 11 (Scheme 5) [9]. The key C-H insertion step of 9 generated the indolizidine 10 in 86 % yield and 96 % de, whereas reaction of 9 with achiral catalysts tended to favor the opposite diaster-eomer. 

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