Gas-phase synthesis of naphthalene through an unconventional thermal alkyne–alkene [2 + 2] cycloaddition mechanism

Exotic cycloaddition entrance channels were discovered for the bimolecular gas-phase reactions of the phenylethynyl radical (C₆H₅CC, X²A₁) with ethylene-d₄ (C₂D₄) and propylene (C₃H₆) as explored under single-collision conditions utilizing the crossed molecular beams technique combined with electronic structure and statistical calculations. Connecting the concepts of barrierless entrance channels, excited states, and facile non-photochemically activated cycloadditions, the reaction pathway features an unconventional thermal [2 + 2] cycloaddition forming a four-membered ring collision complex followed by multiple isomerizations prior to unimolecular decomposition <i>via</i> atomic hydrogen loss to (un)substituted naphthalenes-naphthalene-d₄ (C₁₀H₄D₄) and 1-/2-methylnaphthalene (C₁₁H₁₀). The small energy gap between the singly-occupied a₁ highest occupied molecular orbital (HOMO) with a σ-character and the underlying doubly-occupied b₁ molecular orbital with a π-character allows a facile promotion of an electron. This in turn enables a versatile low-temperature reactivity of phenylethynyl, where the end-on and side-on barrierless approaches of ethylene are due to its interaction with the σ and π orbitals, respectively, thus suggesting this mechanism as a possible method for tuning substituents in polycyclic aromatic hydrocarbon (PAH) formation and highlighting its versatility as a probe of fundamental carbon chemistry <i>via</i> counterintuitive cycloaddition reactions under single-collision conditions.