Electron-transfer Reactions of Cycloalkanes

Among the simple cycloalkanes, we first discuss electron transfer from the three-to eight-membered cycloalkane prototypes to electron holes generated by radiolysis in different matrices, giving rise to the simple cycloalkane radical cations. Because of the significant interest they have attracted, the electron-transfer reactions of cyclopropane and, to a lesser extent, cyclobutane derivatives will be treated separately. Finally, electron transfer from some bicyclic hydrocarbons and the resulting radical cations will be discussed in a separate section (Section 2.4). 

Electron-transfer reactions of higher cycloalkanes were also studied. Electron transfer from C-C7H14 to unstable holes generated by radiolysis in Freon-113 gave rise to a stable radical cation, c-Ci A f its spectrum was interpreted in terms of a twisted chair form with C2 symmetry [37]. Finally, radiolysis of c-CgHie in a Freon-113 matrix generated a Jahn-Teller-active radical cation, c-CgHie, with three sets of non-equivalent protons [37]. A detailed discussion of these species exceeds the scope of this review. 

It has long been speculated that the high-mobility solvent holes exist in hydrocarbons other than the four cycloalkanes. Recently, high-mobility solvent holes were observed in 2,6,10,15,19,23-hexamethyltetracosane (squalane) [24] and in cyclooctane [27]. In the squalane, rapid electron-transfer reactions of solvent holes with low-IP solutes were observed using transient absorbance spectroscopy and magnetic resonance [24]. Fast diffusion and high-rate... 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

The photoaddition of alkanes onto electron-poor alkynes (e.g., propiolate or acetilendicarboxylate esters) can be accomplished by a radical conjugate addition reaction [7]. Radicals have been generated either via hydrogen abstraction from cycloalkanes or via electron transfer from 2-alkyl-2-phenyl-l,3-dioxolanes. In the first case, the irradiation was pursued on an alkane solution of an aromatic ketone (used as the photomediator) and the alkyne. Under these conditions, methyl propiolate was alkylated upon irradiation in the presence of 4-trifluoromethylacetophenone to form acrylate 48 in 97% yield (E/Z= 1.3 1 Scheme 3.31) [78]. 

SET photochemistry is involved in the reaction between the enones (371) and the a-stannyl ethers (372) in methanol. The products are the 3-sub-stituted cycloalkanes (373) which arise from addition of aryloxymethyl radicals to the enones. Irradiation (X > 400 nm) of the stannanes (374) in the presence of the ketones and aldehydes (375) affords two products identified as (376) and (377). The former of these is dominant and the reaction arises by an electron transfer from the stannane to the ketone. The resultant stannane radical-cation undergoes fission to yield an alkoxy allyl radical and the tin cation. The alkoxyalkyl radical adds to the carbonyl radical-anion with a preference for... 

From conductivity studies, it is known that the cycloalkane holes rapidly react with various solutes, typically by electron or proton transfer [7-19]. These scavenging reactions establish the identity of the high-mobility cations as the solvent holes Rapid generation of aromatic radical cations (A +) in reactions of the holes with aromatic solutes (A) was observed using pulse radiolysis - transient absorption spectroscopy [4,5,6,20,23-25] and, more recently, using pulse-probe laser-induced dc conductivity [26]. Rapid decay of the conductivity and transient absorbance signals from the cycloalkane holes was also observed [4-25]. 

Photoexcited aromatic hydrocarbons undergo hydrogen abstraction and 1,4-addition reactions with cycloalkanes and hydroxylic compounds. These reactions are believed to take place through an excited singlet state of arene to form an exciplex or ion pair as an intermediate, which on back electron transfer dissociates into triplet diradical and undergoes proton abstraction from solvent or amines, followed by addition of an alkyl or aryl unit to give the products [37]. The following examples of this reaction are illustrative ... 

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