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 mechanisms for the reactions of isobutane and adamantane with polyhalogen electrophiles (HHal(2)(+), Hal(3)(+), Hal(5)(+), and Hal(7)(+), Hal = Cl, Br, or I) were studied computationally at the MP2 and B3LYP levels of theory with the 6-31G (C, H, Cl, Br) and 3-21G (I) basis sets, as well as experimentally for adamantane halogenations in Br(2), Br(2)/HBr, and I(+)Cl(-)/CCl(4). The transition structures for the activation step display almost linear C...H...Hal interactions and are characterized by significant charge transfer to the electrophile; the hydrocarbon moieties resemble the respective radical cation structures. The regiospecificities for polar halogenations of the 3-degree C-H bonds of adamantane, the high experimental kinetic isotope effects (k(H)/k(D) = 3-4), the rate accelerations in the presence of Lewis and proton (HBr) acids, and the high kinetic orders for halogen (7.5 for Br(2)) can only be understood in terms of an H-coupled electron-transfer mechanism. The three centered-two electron (3c-2e) electrophilic mechanistic concept based on the attack of the electrophile on a C-H bond does not apply; electrophilic 3c-2e interactions dominate the C-H activations only with nonoxidizing electrophiles such as carbocations. This was shown by a comparative computational analysis of the electrophilic and H-coupled electron-transfer activation mechanisms for the isobutane reaction with an ambident electrophile, the allyl cation, at the above levels of theory.
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