Regulation of orthogonal functions in a dual catalyst system. Subservient role of a nonchiral lewis acid in an asymmetric catalytic heteroatom Diels-Alder reaction

  • Newman C
  • Antilla J
  • Chen P
 et al. 
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A catalytic asym. heteroatom Diels-Alder reaction of unactivated imines with Danishefsky's diene is described which gives high asym. induction for N-benzhydryl imines derived from a variety of aldehydes. The catalyst is derived from B(OPh)3 and the VAPOL ligand and gives good induction, but the reaction stalls and does not give high conversion (∼50%). In the presence of both the chiral catalyst and excess amts. of B(OPh)3 the reaction proceeds to completion and gives high yields of the dihydropiperidinone product. Despite the presence of large quantities of the nonchiral Lewis acid B(OPh)3, the asym. induction of the product remains const. (90% ee) as the amt. of B(OPh)3 is steadily increased and does not drop off until the ratio of B(OPh)3 to VAPOL is 100:1 (82% ee). These observations are interpreted as involving highly sepd. and different activities for the chiral and nonchiral Lewis acids present in the reaction. Specifically, the excess B(OPh)3 serves to bind to the product and release the chiral catalyst to turnover more starting material. The B(OPh)3 does not compete in turning over the starting material, and binding studies reveal that this is likely due to a combination of two factors. The binding studies reveal that the chiral catalyst binds to the starting imine 7 times more strongly than does B(OPh)3. However, to explain the const. asym. induction obsd. despite the addn. of increasing amts. of B(OPh)3, the rate of the reaction of the imine complexed with the chiral catalyst must be at least 10 times faster than the reaction of the imine complexed with B(OPh)3. Finally, a catalyst generated from BINOL and B(OPh)3 does not show this phenomenon. [on SciFinder(R)]

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