Donor‐Strength‐Guided Formal [2 + 2] Cycloaddition–Retroelectrocyclization: A Click‐Type Route to Push–Pull Chromophores and Lewis Acid Promoted Post‐Transformations

In this work, we explore the formal [2 + 2] cycloaddition–retroelectrocyclization (CA‐RE) reactivity of substrates bearing a 1,4‐diethynyl‐2,5‐dimethoxybenzene core. Methoxy substituents mesomerically increase electron density, facilitating [2 + 2] CA‐RE. This effect is particularly important in determining whether the CA‐RE occurs at one or both alkyne units, yielding mono‐ or bis‐[2 + 2] CA‐RE products, which can be tuned through strategic modifications of the donors. Beyond a detailed study of the optical properties of the synthesized chromophores, we demonstrated that certain substrates can be converted into benzofurans in the presence of SiO 2 or LiClO 4 . Although both conditions afforded the desired transformations, LiClO 4 generally provided higher yields and was therefore selected for the final optimization of the reactions. These post‐transformations of [2 + 2] CA‐RE products, which are typically rare, are anticipated to provide a significant contribution to the field. The proposed mechanism for the formation of benzofurans was investigated using density functional theory (DFT) calculations. The findings indicate that the donor strength of the substituents on the framework exerts a pronounced effect on the reaction pathway. Benzofuran formation is proposed to proceed predominantly via the mono‐[2 + 2] CA‐RE product. Subsequent protonation of this compound, cyclization through the resulting allene intermediate, and a final demethylation yield the corresponding benzofurans.