SolutionsAsymmetric autocatalysis: the Soai reaction
Exercise 1
A sample of the Soai alkanol has 0.00005% ee. What is its enantiomeric ratio? Why can a reaction catalysed by an ordinary chiral catalyst, with no nonlinear effect or autocatalysis, not give a product with a higher ee than that of the catalyst?
ee = 0.00005% means that the difference between the percentages of the two enantiomers is 0.00005: 50.000025% versus 49.999975%, about five molecules of difference in every ten million. If the two enantiomers of the catalyst act independently and with equal activity, each produces its enantiomer of the product in proportion to its amount: the ee of the product is at most the ee of the catalyst multiplied by its intrinsic selectivity (a linear relationship). Only if the catalyst enantiomers interact (poorly active heterochiral aggregates) or the product multiplies itself can the ee increase.
Exercise 2
In the addition of diethylzinc to benzaldehyde, (−)-DAIB of 15% ee gives (S)-1-phenylpropan-1-ol in 95% ee. Explain this positive nonlinear effect. What would be observed if the heterochiral dimer were less stable than the homochiral ones and also catalysed the reaction?
The zinc alkoxide of DAIB forms dimers. The heterochiral (+)/(−) dimer is very stable, does not dissociate and does not catalyse; it traps practically all of the minor enantiomer, (+)-DAIB, together with an equal amount of (−)-DAIB. The remaining 15% of (−)-DAIB is left as homochiral dimers that dissociate into the almost enantiopure active monomer, and the product is formed in 95% ee (although more slowly than with pure DAIB). If the heterochiral dimer were the most reactive and gave racemic product, the ee of the product would be lower than expected: a negative nonlinear effect.