Pyrolysis chemistry of cubane and methylcubane: The effect of methyl substitution on stability and product branching

Pyrolysis reactions of cubane and methylcubane were studied in a micro-flow tube reactor from room temperature to 1000 K. The composition of the flow tube eluent was probed by chemical ionization, followed by analysis in a tandem, guided-beam mass spectrometer. It was found that cubane and methylcubane have nearly identical breakdown temperature dependence, suggesting that methyl functionalization has minimal effect on the stability of cubane cage bonds. There is, however, a substantial effect of the methyl group on product branching, resulting in more stable isomers at intermediate temperatures, and a propensity to eliminate fragments containing the methyl group at high temperatures. Ab initio calculations performed to aid interpretation are discussed, along with possible mechanisms.