The reactions of a series of structurally related large-ring propellanes with iodine monochloride were studied experimentally and computationally. In the case of 1,3-dehydroadamantane (1) and [3.3.1]propellane (2) free-radical addition was observed. [3.3.2]Propellane (3) and 3,6-dehydrohomoadamantane (4), which are less prone to radical attack, selectively form products of formal double nucleophilic (oxidative) addition, e.g., dichloro (in ICl/CH2Cl2), dimethoxy (in ICl/CH3OH), and diacetamino (in ICl/CH3CN) derivatives under otherwise identical conditions. Single-electron transfer pathways involving the alkane radical cations are proposed for the activation step for aliphatic hydrocarbons with relatively low oxidation potentials such as cage alkanes. Similar mechanisms are postulated for the activation of the tertiary C−H bonds of adamantane based on H/D-kinetic isotope effect data. The latter compare well to the k H/k D value for hydrogen atom loss from the adamantane radical cation (measured 2.78 ± 0.21 and computed 2.0) and differ considerably from the kinetic isotope effects for electrophilic C−H bond activations (i.e., hydride abstraction) or for loss of a proton from a hydrocarbon radical cation (k H/k D = 1.0−1.4; computed 1.4). Hence, the reactions of alkanes with elementary halogens and other weak electrophiles (but strong oxidizers) do not necessarily involve three-center two-electron species but rather occur via successive single-electron oxidation steps. Upon C−C or C−H fragmentation, the incipient alkane radical cations are trapped by nucleophiles.

The oxidation of 3,6‐dehydrohomoadamantane (1) was achieved under chemical (NO+BF4–/EtOAc, NO+OAc–/Ac2O, and NO+BF4–/CH3CN), photochemical (photoexcited 1,2,4,5‐tetracyanobenzene), and electrochemical (Pt anode, CH3CN, NH4BF4) conditions. Supporting ab initio [density functional theory (BLYP) and Møller–Plesset perturbation theory (MP2)] computations utilizing standard basis sets, 6–31G* (optimizations) and 6–311+G* (single‐point energy evaluations), agree with the experimental results implicating the involvement of the same radical cation intermediates in the activation processes. Isomeric radical cations formed from different precursors can equilibrate with low barriers (2.0–11.7 kcal mol–1) and lead to common products. The computed and experimental adiabatic ionization potential of adamantane shows that activation with NO+BF4– is also likely to occur through the adamantyl radical cation. Hence, the bonds need not be attacked directly by the electrophile in the C–H or C–C activation of alkanes with relatively low ionization potentials.

The rearrangement of the cubane radical cation (1*+) was examined both experimentally (anodic as well as (photo)chemical oxidation of cubane 1 in acetonitrile) and computationally at coupled cluster, DFT, and MP2 [BCCD(T)/cc-pVDZ//B3LYP/6-31G* ZPVE as well as BCCD(T)/cc-pVDZ//MP2/6-31G* + ZPVE] levels of theory. The interconversion of the twelve C2v degenerate structures of 1*+ is associated with a sizable activation energy of 1.6 kcalmol(-1). The barriers for the isomerization of 1*- to the cuneane radical cation (2*+) and for the C-C bond fragmentation to the secocubane-4,7-diyl radical cation (10*+) are virtually identical (deltaH0++ = 7.8 and 7.9 kcalmol(-1), respectively). The low-barrier rearrangement of 10*+ to the more stable syn-tricyclooctadiene radical cation 3*+ favors the fragmentation pathway that terminates with the cyclooctatetraene radical cation 6*+. Experimental single-electron transfer (SET) oxidation of cubane in acetonitrile with photoexcited 1,2,4,5-tetracyanobenzene, in combination with back electron transfer to the transient radical cation, also shows that 1*+ preferentially follows a multistep rearrangement to 6*+ through 10*+ and 3*+ rather than through 2*+. This was confirmed by the oxidation of syn-tricyclooctadiene (3), which, like 1, also forms 6 in the SET oxidation/rearrangement/electron-recapture process. In contrast, cuneane (2) is oxidized exclusively to semibullvalene (9) under analogous conditions. The rearrangement of 1*+ to 6*+ via 3*+, which was recently observed spectroscopically upon ionization in a hydrocarbon glass matrix, is also favored in solution.

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