Cyclic guanidine organic catalysts: what is magic about triazabicyclodecene?

The bicyclic guanidine 1,5,7- triazabicyclo[4.4.0]dec-5-ene (TBD) is an effective organocatalyst for the formation of amides from esters and primary amines. Mechanistic and kinetic investigations support a nucleophilic mechanism where TBD reacts reversibly with esters to generate an acyl-TBD intermediate that acylates amines to generate the amides. Comparative investigations of the analogous bicyclic guanidine 1,4,6-triazabicyclo[3.3.0]oct-4-ene (TBO) reveal it to be a much less active acylation catalyst than TBD. Theoretical and mechanistic studies imply that the higher reactivity of TBD is a consequence of both its higher basicity and nucleophilicity than TBO as well as the high reactivity of the acyl-TBD intermediate, which is sterically prevented from adopting a planar amide structure.