Intramolecular configurational match/mismatch

When the allene moiety of 2,3-allenylamines was substituted with Br, an intramolecular nucleophilic substitution reaction led to a chiral 2,3-ds-ethynylaziridine 323. The diastereoselectivity depends on the absolute configuration of the allene moiety, i.e. typically for a matched-mismatched pair the S,aR-isomer afforded the product with much higher stereoselectivity [155, 156],... 

As an efficient bifunctional catalyst, proline has been used as a Br0nsted acid in combination with a nucleophilic Lewis base catalyst in the asymmetric BH reaction. Miller and co-workers [119] disclosed that in the L-proline-catalyzed BH reaction of MVK and electron-deficient aldehydes the imidazole-tailed peptide 67 was an efficient co-catalyst. A matched/mismatched phenomenon of two chiral catalysts was observed in this reaction. Furthermore, Zhou and co-workers [120] synthesized various chiral amines and screened them as co-catalysts of L-proline in the BH reaction of MVK and aldehydes, revealing that chiral benzodiazepine 68 and aminoalcohol 69 were efficient catalysts. Interestingly, the intramolecular reaction shown in Scheme 9.34 could be directly catalyzed by L-proline in DMF solvent, while the addition of imidazole resulted in enhancement of the enantioselectivity with opposite configurational product [121]. A similar process was realized in the intramolecular reaction shown in Scheme 9.35 with 70 as co-catalyst of iV-methylimidazole. Moreover, Cordova and co-workers [122] reported in 2007 the first example of asymmetric aza-BH reaction between (3-mono- or disubstituted acroleins and aldimines. By utilizing L-proline as catalyst in combination with... 

Kinetic resolution (enantiomer differentiation) of cycloalkenyl diazoacetates has been achieved (for example, according to Eq. 3) [34]. In these cases one enantiomer of the racemic reactant matches with the catalyst configuration to produce the intramolecular cyclopropanation product in high enantiomeric excess, whereas the mismatched enantiomer preferentially undergoes hydride abstraction from the allylic position [35] to yield the corresponding cycloalkenone. With acyclic secondary allylic diazoacetates the hydride abstraction pathway is relatively unimportant, and diastereoselection becomes the means for enantiomer differentiation [31]. 

The combination of the geometrical preference of the tether and the stereochemical preference of the dipolarophile substituent can be seen in the intramolecular cycloadditions of alkyl nitronates, (Scheme 2.6) (99). When the tether is restricted to two atoms, only the endo approach of the tether is observed in up to a 100 1 ratio, independent of the configuration of the disubstituted dipolarophile. However, in the case of a three-atom linker, there exists a matched and mismatched case with respect to the observed stereoselectivities. With a (Z)-configured dipolarophile, only the exo isomer was observed since the ester moiety also approaches on the exo to the nitronate. However, with an ( )-configured dipolarophile, the ester group is forced to approach in an endo manner to accommodate an exo approach of the tether, thus leading to lower selectivity. 

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