Cyclization triggered by deprotonation: the gas-phase acidity of 1,8-chalcogen-bridged naphthalenes.

High-level density functional theory computations have been used to estimate the gas-phase (intrinsic) acidities of the complete series of 1,8-chalcogen-bridged naphthalene derivatives. The existence of a chalcogen-chalcogen bond in chalcogen-bridged naphthalene derivatives plays a crucial role in the intrinsic acidity of the system. For 1,8-naphthalenedilylbis(oxy), where this bond does not exist, the para C-H group is the most acidic site, whereas for the remaining compounds, deprotonation of the ortho CH groups is the most favorable process. Deprotonation of the aromatic rings has a large effect on the strength of the bonds of the five-membered ring. These effects depend on the nature of the heteroatoms forming the X-Y bridge, and modulate the acidity of the molecule. Also importantly, when one of the heteroatoms is oxygen, ortho and para deprotonation lead to cleavage of the X-Y bridge. This bond fission favors the formation of a CYC (Y = S, Se, Te) three-membered ring that enhances the stability of the anion and, therefore, increases the acidity of these compounds. We have shown that, whereas this cyclization process is energetically favorable for oxygen-containing compounds, it is not favorable for the remaining derivatives.