"Goldilocks Effect" of Water in Lewis-Brønsted Acid and Base Catalysis

Different catalytic protocols were evaluated in the enantioselective Pd-catalyzed aza-Michael reaction involving monoprotected phenylenediamine (PDA) derivatives. The use of these nucleophilic amines leads to the poisoning of the (monomeric) Lewis acidic catalyst, and significant competitive formation of side products were observed. In contrast, good yields and enantioselectivities can be attained by employing the Brønsted basic-Lewis acidic dimeric Pd catalyst, in combination with PDA derivatives protonated by triflic acid. In this case, the presence of the right amount of water was found to be critical for success ("Goldilocks effect"). The results were rationalized on the basis of delicately balanced acid-base equilibria, dependent upon the nature of the catalyst and the amine.