Chelation donor group variation

Fig. 10.20. Ensuring Felkin— Anh or Cram chelate selectivity in the addition of hydride donors to a-chiral carbonyl compounds by a combined variation of the hydride donor and the protecting group.

A 1H NMR study (6 ) found that the chemical shifts of the N-H protons were very sensitive to solvent with the shifts being related to the donor number (j) of the solvent, i.e. the ability of the solvent to donate electrons to form a hydrogen bond with the N-H protons. Further, the N-H protons that were essentially equatorial in the puckered five-membered chelate ring showed larger variations in their 6 values than the axial protons. The differences in the 6 values for the equatorial and axial protons for the NH2 groups were in the order ... 

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