High selectivities and exceptional substrate scope prompted us to undertake mechanistic investigations summarized in Figure . In the proposed mechanism, the spectator pyridine ligand in 41 opens a site for binding of amide 42 to molybdenum complex 41 (Figure ). The catalyst likely "samples" various amide positions and establishes bidentate configuration 43 with N-H bonding that can assist in rotation of the amide group into the required conformation for 5-exo-trig cyclization resulting in oxazolidine/thiazolidine 45. The pyridine ring in this step plays an important role of a proton carrier from the N-H to the carbonyl's oxygen atom and reorganization into another H-bonded intermediate 46, which then releases oxazoline 48 and a molecule of water. This proposal is consistent with reactions of O=C-labelled amides (95% O incorporation) catalyzed by 2h, which furnished the expected azolines together with recovered molybdenum catalyst 2h with <5% incorporation of O as confirmed by high-resolution mass-spectrometry analysis (for details, see SI). This data shows that the oxo ligands may play a role maintaining high Lewis-acidic character of Mo but remain intact throughout the entire catalytic cycle. Along similar lines, Hammett analysis of cyclodehydration of substituted benzamides 49 with first-order kinetics revealed accumulation of negative charge at the carbonyl group (Figure ). This result is supportive of with the hypothesis that breakage of the carbonyl group through addition of a nucleophile is the rate-determining step (conversion of 43 into 45).