Cage Alkyl Carbenes Provide Experimental Evidence for Isotope-Controlled Selectivity

We report the gas-phase synthesis and reactivity of adamantylidene and pentacyclo[5.4.0.02.6.03.10.05.9]undecanylidene. The latter, a previously unreported carbene, represents only the second alkyl carbene spectroscopically characterized to date. The singlet carbenes were generated through irradiation of their corresponding diazirine precursors followed by trapping the products in argon or nitrogen matrices at 3.5 K. Analyses using IR and UV/Vis spectroscopy, together with density functional theory computations provide strong evidence for the successful preparation of these reactive species. Adamantylidene (∆EST = –3.0 kcal mol–1) undergoes a slow hitherto unreported but theoretically predicted quantum mechanical tunneling (QMT) C–H-bond insertion and ring-closure to 2,4-dehydroadamantane. In contrast, pentacyclo[5.4.0.02.6.03.10.05.9]undecanylidene (∆EST = –5.2 kcal mol–1) remains unchanged under cryogenic conditions but rearranges to homohypostrophene upon = 627 nm irradiation. Attempts to prepare protoadamantylidene (∆EST = –5.1 kcal mol–1) in a similar fashion did not allow the direct observation of the free carbene, but enabled follow-up QMT reactions, whose selectivities are determined by the 1H and 2H isotopologs, thereby demonstrating isotope-controlled selectivity (ICS).