Asymmetric Activation of Chirally Flexible Tropos Catalysts

Davies, Renaud, and Sibi independently reported the chiral relay approach to control the enhanced steric extension inside a substrate to achieve increased asymmetric induction. However, as our study proves, the asymmetric activation of a tropos catalyst clearly differs from the chiral relay approach, in which substrate conformational control is utilized, since asymmetric activation controls the chiral environment of a tropos catalyst by the addition of a chiral external source (a chiral activator). 

The effect of the tropos complex 31b with (S,.S)-DPEN can be observed in the hydrogenation of I -acetonaphthone 16 as compared with the results using ( )-10b/(S,S)-DPEN bearing racemic and atropos XylBlNAP (Table 8.10).  

A higher enantioselectivity is obtained at a lower temperature. The enantioselectivity by 31b/(5,5)-DPEN is higher than that by ( )-10b/(5,5)-DPEN at the same temperature and pressure. The tropos 31b/(5,5)-DPEN catalyst affords (f )-l-(l-naphthyl) ethanol with 92% ee in quantitative yield (entries 3 and 4). 31b/(5,5)-DPEN is also effective for o-methylacetophenone 22a to afford (7 )-l-(l-o-methylphenyl)ethanol quantitatively with 88% ee, higher than 86% ee obtained by ( )-10b/(5,5)-DPEN (entries 5 and 6). 

The metal complexes even with tropos ligands can thus be used as asymmetric catalysts for carbon-carbon bond-forming reactions in the same manner as atropos catalysts. The single diastereomer (7 )-32/(/ )-DABN can be employed as an activated asymmetric catalyst for the Diels-Alder reaction at room temperature (Table 8.11). The high chemical yield and enantioselectivity (62%, 94% ee) in the Diels-Alder reaction of ethyl glyoxylate with 1,3-cyclohexadiene are obtained 

The treatment of (5)-34/(5)-BINOL complex (95 5) with concentrated HCl followed by recrystallization leads to the enantiopure (5)-35 complex with retention of 

---