Kinetic and Product Studies on the Side-Chain Fragmentation of 1-Arylalkanol Radical Cations in Aqueous Solution: Oxygen versus Carbon Acidity

Abstract

A kinetic and product study of the side-chain fragmentation reactions of a series of 1-arylalkanol radical cations (4-MeOC 6 H 4 CH(OH)R . ) and some of their methyl ethers was carried out; the radical cations were generated by pulse radiolysis and g radiolysis in aqueous solution. The radical cations undergo side-chain fragmentation involving the C a À H and/or C a À C b bonds, and their reactivity was studied both in acidic (pH 4) and basic (pH 10 ± 11) solution. At pH 4, the radical cations decay with first-order kinetics, and the exclusive reaction is C a ÀH deprotonation for ) very close to the limit of diffusion control and independent of the nature of the bond that is finally broken in the process (CÀH or CÀC). The methyl ether 8. , which exclusively undergoes CÀH bond cleavage, reacts significantly slower (by a factor of ca. 50) than the corresponding alcohol 1 . . These data indicate that 1-arylalkanol radical cations, which display the expected carbon acidity in water, become oxygen acids in the presence of a strong base such as HO À and undergo deprotonation of the OÀH group; diffusion-controlled formation of the encounter complex between HO À and the radical cation is the rate-determining step of the reaction. It is suggested that, within the complex, the proton is transferred to the base to give a benzyloxyl radical, either via a radical zwitterion (which undergoes intramolecular electron transfer) or directly (electron transfer coupled with deprotonation). The latter possibility seems more in line with the general base catalysis (b % 0.4) observed in the reaction of 5 . , which certainly involves OÀH deprotonation. The benzyloxyl radical can then undergo a b CÀC bond cleavage to form 4-methoxybenzaldehyde and R . or a formal 1,2-H shift to form an a-hydroxybenzyl-type radical. The factors of importance in this carbon/ oxygen acidity dichotomy are discussed.

Unraveling the flavin-catalyzed photooxidation of benzylic alcohol with transient absorption spectroscopy from sub-pico- to microseconds

“…46,61 According to our model, flavin and MBA are in contact-or at least close proximity-after the initial ET. Therefore, the decay of the ion pair should not depend on the substrate concentration.…”

The Deprotonation of Benzyl Alcohol Radical Cations: A Mechanistic Dichotomy in the Gas Phase as in Solution

“…[15] c GB(A . ) 1/b (2) The relationship between the kinetics (the reaction efficiency) and the thermodynamics (the free energy change: GB(A . ) À GB(B)) of the proton-transfer reaction was derived for the simple kinetic scheme of Equation (3) and provides a quantitative analysis of the so-called bracketing method, also named the thermokinetic method.…”

“…Only 4-MeO-C 6 H 4 CD 2 OH was prepared by a previously described procedure. [2] Ar and the compounds used as reference bases were also obtained from commercial sources and used as received.…”

“…Whereas, in aqueous acid solutions, these species behaved, as expected, as carbon acids, in the presence of OH À they exhibited kinetic oxygen acidity, that is, the key step in their decay involved deprotonation at the OH group. [2] This result is of considerable interest since simple calculations based on a thermochemical cycle show that from a thermodynamic point of view, a-carbon acidity of these radical cations is much stronger than oxygen acidity.…”

Fragmentation reactions of radical cations

“…Thus, 4 .þ undergoes C-H deprotonation as the exclusive reaction with k ¼ 4.6 Â 10 4 s À1 , a value that is significantly higher than those measured for the deprotonation of both 1-(4methoxyphenyl)ethanol (5 .þ ) and 1-(4-methoxyphenyl)propan-1-ol (6 .þ ) radical cations under analogous experimental conditions (k ¼ 7.0 Â 10 3 and 5.4 Â 10 3 s À1 , respectively). 27 With both 5 .þ and 6 .þ , the most stable conformation is not the one most suitable for C-H bond cleavage and energy has to be spent to reach the alignment between the C-H bond and the p-system.…”