DABCO and DMAP - A Comparison

Significance

The benzhydrylium ion method (see comment) in combination with laser flash techniques allowed the determination of the nucleophilicities of tertiary amines DABCO (1) and quinuclidine 2. The nucleophilicities and the corresponding carbon basicities (derived from equilibrium constants) of 1 and 2 were compared with those values obtained for DMAP (3) (Chem. Eur. J. 2007, 13, 336). The authors found that both 1 and 2 are significantly better nucleophiles (by a factor of 10³) as well as much better nucleofuges (by a factor of 10⁶ and 10⁵, respectively) than 3. Since the relative reactivities of nucleophiles towards benzhydrylium ions resemble those towards typical Michael acceptors, they related these findings to organocatalysis and reasoned that 1 and 2 will be superior catalysts to 3 in MBH reactions, if the rates of step A or B control the reactivity. However, depending on the substrates, a high equilibrium concentration of C can be crucial for the reactivity. In this case, due to its higher carbon basicity, 3 will be the better catalyst as for example in MBH reactions with cycloalkenones and acrylates.