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

The biphenyl molecule (CH) 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 (CH) along with -, -, and -substituted methylbiphenyl (CH) derivatives through crossed molecular beams and computational studies providing compelling evidence on their formation bimolecular gas-phase reactions of phenylethynyl radicals (CHCC, XA) with 1,3-butadiene- (CD), isoprene (CHC(CH)CHCH), and 1,3-pentadiene (CHCHCHCHCH). The dynamics involve de-facto barrierless phenylethynyl radical additions submerged barriers followed by facile cyclization and hydrogen shift prior to hydrogen atom emission and aromatization to racemic mixtures (, ) of biphenyls in overall exoergic reactions.