Unconventional gas-phase synthesis of biphenyl and its atropisomeric methyl-substituted derivatives†

The biphenyl molecule (C₁₂H₁₀) acts as a fundamental molecular backbone in the stereoselective synthesis of organic materials due to its inherent twist angle causing atropisomerism in substituted derivatives and in molecular mass growth processes in circumstellar environments and combustion systems. Here, we reveal an unconventional low-temperature phenylethynyl addition–cyclization–aromatization mechanism for the gas-phase preparation of biphenyl (C₁₂H₁₀) along with ortho-, meta-, and para-substituted methylbiphenyl (C₁₃H₁₂) derivatives through crossed molecular beams and computational studies providing compelling evidence on their formation via bimolecular gas-phase reactions of phenylethynyl radicals (C₆H₅CC, X²A₁) with 1,3-butadiene-d₆ (C₄D₆), isoprene (CH₂C(CH₃)CHCH₂), and 1,3-pentadiene (CH₂CHCHCHCH₃). The dynamics involve de-facto barrierless phenylethynyl radical additions via submerged barriers followed by facile cyclization and hydrogen shift prior to hydrogen atom emission and aromatization to racemic mixtures (ortho, meta) of biphenyls in overall exoergic reactions. These findings not only challenge our current perception of biphenyls as high temperature markers in combustion systems and astrophysical environments, but also identify biphenyls as fundamental building blocks of complex polycyclic aromatic hydrocarbons (PAHs) such as coronene (C₂₄H₁₂) eventually leading to carbonaceous nanoparticles (soot, grains) in combustion systems and in deep space thus affording critical insight into the low-temperature hydrocarbon chemistry in our universe.