UNIT 7:CATION RADICALS
Cation radicals are species which are formed from neutral molecules by the removal (ionization) of a single electron. The resulting species therefore has a unit of positive charge and a unit of spin density (Scheme 1). The HOMO of the neutral species becomes the SOMO of the cation radical. Remember that the SOMO completely controls the spin density distribution. Also, in alternant hydrocarbon (nonpolar) systems, it also controls the charge density, i.e., spin and charge travel together. This means that wherever the spin is large the charge is also large. Consequently, radical reactivity and ionic reactivity tend to occur at the same atoms.
Scheme 1. Illustrating the formation of a cation radical by the removal of an electron from the HOMO of a neutral molecule.
Recall, also, that the HOMO and the LUMO of 1,3-butadiene are paired, since it is an alternant system (has no odd-membered rings). Consequently, the spin and charge distribution in the cation radical are the same as in the anion radical. From the coefficients of the HOMO and LUMO of 1,3-butadiene (given earlier), you should be able to calculate the spin and charge distribution of both the anion and cation radical, and to verify that they are the heaviest on the terminal carbons (C1 and C4).
There is one other thing I’d like for you to notice about both the anion radical and the cation radical, viz., that the bond order between C2-C3 is increased in both relative to the neutral molecule, making it much more resistant to rotation, and therefore making it more difficult to interconvert the s-cis and s-trans conformations (Scheme 2). This is especially easy to see In the anion radical, because the extra electron is going into a LUMO which is bonding (has coefficients of the same sign) between C2-C3. But in a similar way, in the cation radical an electron is being removed from the HOMO of butadiene, and in this MO the interaction between C2-C3 is antibonding. So an antibonding interaction is being removed, resulting in a larger net bonding between C2-C3.
Scheme 2. Upon either cation radical formatioin or anion radical formation, the C2-C3 bond order is sharply increased, resulting in s-cis and s-trans conformations of the ion radicals which are not readily interconverted, in contrast to the easy conversion in the neutral diene.
Like neutral radicals and anion radicals, cation radicals can be isolably stable if there is sufficient provision for extensive conjugative stabilization and also steric effects which hinder dimerization. An excellent, and useful, example of a stable cation radical is the tris(4-bromophenyl)aminium hexachloroantimonate salt, a shelf-stable, commercially available, deep blue salt (Scheme 2). This salt is prepared by the oxidation (using antimony pentachloride) of tris(4-bromophenyl)amine. The removal of an unshared electron pair from nitrogen generates a cation radical moiety, which is delocalized over all three phenyl rings, primarily at the ortho and para positions of the ring. The bromo substituent in the para position is necessary in order to prevent the cation radical from dimerization at the para position.
Scheme 3. A Stable Cation Radical Salt: Tris(4-bromophenyl)aminium hexachloroantimonate
Since a bromine substituent is an electron withdrawing group (an EWG), it destabilizes positive charge (which is located in part at the para position), This cation radical is therefore less thermodynamically stable than the corresponding unsubstituted one would be, even though it is more highly stabilized (sterically) against dimerization. It is especially useful for acting as a rather reactive one electron oxidant, i.e., for removing an electron from certain substrates in order to transfer the cation radical moiety to the substrate. This is illustrated in Scheme 4, where the cation radical of 1,3-cyclohexadiene is generated using this aminium salt, and it undergoes an extremely fast Diels-Alder reaction with a molecule of the neutral diene. This represents a powerful way for catalyzing the Diels-Alder reaction, and it works best with relatively easily ionizable substrates which would not undergo the normal, thermal Diels-Alder reaction very efficiently.
Many relatively easily ionizable alkenes can undergo efficient cyclodimerization to yield cyclobutanes (cyclobutanation) via a similar cation radical mechanism. See if you can write out the cation radical chain mechanism for the cyclobutadimerization of trans-anethole (Scheme 5). Recall that thermal combination of two alkene moieties to give a cyclobutane is extremely difficult.
Scheme 4. The Cation Radical Diels-Alder Reaction
Scheme 5. Cation Radical Chain Cyclobutadimerization
Interestingly, anion radical chain reactions can accomplish analogous cyclobutanations. In this case, the substrate must be readily able to accept an electron, i.e., have not too negative a reduction potential. This is provided by conjugation in combination with strongly electron withdrawing groups (Scheme 6). See if you can draw out an an anion radical chain mechanism for cyclobutanation which is analogous to that for cation radical chain cyclobutanation.
Scheme 7. Anion Radical Chain Cyclobutanation