Structure–reactivity studies are performed to explore the reaction mechanism of the guanidine‐catalyzed intramolecular aldol reaction of ketoaldehydes. A large number of guanidine and guanidine‐like catalysts are synthesized and their properties studied. Kinetic profiles and pKa values of the catalysts are measured and correlated to reaction barriers calculated using density functional theory (DFT). The DFT calculations show that structural rigidity influences the pKa of the guanidines. Although the basicity is a very important factor in the catalysis, it is not sufficient to fully account for its catalytic efficiency. The availability of two aligned nitrogen reaction sites for proton shuttling in the transition state is an essential feature that helps to rationalize the reactivity pattern and the activation mode for this family of organocatalysts.
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