Synthesis of McGeachin bases and theoretical insights into reaction mechanisms

• A mild and metal-free method for preparing McGeachin bases from readily available starting materials was developed. • The method facilitates four intermolecular and intramolecular nucleophilic additions in a single operation. • The reaction mechanism of affording McGeachin bases was ascertained using DFT/M06-2X with def2-TZVPP basis set. • The reaction between 2-aminobenzaldehyde and aniline of generating ring-fused quinazoline derivative was also elucidated by DFT. McGeachin bases, the derivatives of 6,12-epiminodibenzo[ b,f ][1,5]diazocine, have potential applications in supramolecular chemistry and asymmetric catalysis due to its rigid Y-shape and chirality. The synthesis of McGeachin bases has attracted much attention, but they were rarely prepared by the reaction of 2-aminobenzaldehyde and arylamine. This paper set up an optimal protocol of affording McGeachin base by heating 2-aminobenzaldehyde and pyrimidin-2-amine in acetonitrile under the catalysis of p -toluenesulfonic acid. This method facilitates four intermolecular and intramolecular nucleophilic additions in a single operation under mild reaction condition, in addition to the advantages of readily available starting materials and metal-free catalyst. Aniline is also an arylamine like pyrimidin-2-amine, but the reaction between 2-aminobenzaldehyde and aniline affords an unexpected ring-fused quinazoline derivative instead of McGeachin base. The cascade cyclization mechanism of generating McGeachin base from 2-aminobenzaldehyde and pyrimidin-2-amine, and the reaction mechanism of affording ring-fused quinazoline derivative from 2-aminobenzaldehyde and aniline were both ascertained based on density functional theory (DFT). Substituent effects on the reactivity are distinct and can be reasonably explained according to the rate-determining step of the suggested reaction mechanism. The theoretical explorations are in good agreement with the experimental results.