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Abstract

We report the first X-ray structure of a spiroaminal hydrochloride. The chiral spiroaminal crystallizes as a racemic hydrochloride in the monoclinic space group P2₁/n and adopts the thermodynamically most stable conformation. Density functional calculations on several spiroaminals were used to establish correlations between trends in conformational energies, steric repulsions, and anomeric effects and to reveal the mechanism of the ring-opening tautomerization reaction. In the unsubstituted and backbone-substituted spiroaminals, the aminal tautomer is thermodynamically preferred. N-Substituted spiroaminals favor the amine/imine form for steric reasons, except for those with bridging N,N′ groups. The tautomerization from the aminal to the amine/imine is endergonic and kinetically hindered in the neutral species but quite facile after protonation. Anomeric effects lower the barriers but are less important than steric factors for relative energies.