Cage Alkyl Carbenes Provide Experimental Evidence for Isotope-Controlled Selectivity in Competing Tunneling Reactions

We report the synthesis and reactivity of adamantylidene (1) and pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecanylidene (2) under matrix isolation conditions. The latter previously unreported carbene is persistent under cryogenic conditions and has been characterized spectroscopically. 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. Carbene 1 (ΔE_ST = -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 (4). In contrast, 2 (ΔE_ST = -5.2 kcal mol^-1) remains unchanged under cryogenic conditions but rearranges to homohypostrophene (9) upon λ = 627 nm irradiation. Attempts to prepare protoadamantylidene (3) (ΔE_ST = -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).