Kinetic and product studies on the side chain fragmentation of 1-arylalkanol radical cations in aqueous solution: Oxygen versus carbon acidity

A kinetic and product study of the side-chain fragmentation reactions of a series of 1-arylalkanol radical cations (4-MeOC6H4CH(OH)R.+) and some of th eir methyl ethers was carried out; the radical cations were generated by pu lse radiolysis and gamma radiolysis in aqueous solution. The radical cation s undergo side-chain fragmentation involving the C-alpha-H andior C-alpha-C -beta bonds, and their reactivity was studied both in acidic (pH14) and bas ic (pH 10-11) solution. At pH 4, the radical cations decay with first-order kinetics, and the exclusive reaction is C-alpha-H deprotonation for 1(.+) 2(.+), and 3(.+) (R = H, Me, and Et, respectively) but C-alpha-C-beta bond cleavage for 5(.+)-, 6(.+), and 7(.+) (R = tBu, CH(OH)Me, and CH(OMe)Me, re spectively). Both types of cleavage are observed for 4(.+) (R = iPr). The r adical cations of the methyl ethers 8(.+), 9(.+), and 10(.+) (R = H, Et, an d iPr, respectively) undergo exclusive deprotonation, whereas C-C fragmenta tion predominates for 11(.+) (R = tBu). Large C-alpha deuterium kinetic iso tope effects (4.5 and 5.0, respectively) were found for 1(.+) and its methy l ether 8(.+). Replacement of an alpha-OH group by OMe has a very small eff ect on the decay rate when the radical cation undergoes deprotonation, but a very large, negative effect in the case of C-C bond cleavage. It is sugge sted that hydrogen bonding of the alpha-OH group with the solvent stabilize s the transition state of the C-C bond fragmentation reaction but not that of the deprotonation process; however, other factors could also contribute to this phenomenon. The decay of the radical cations is strongly accelerate d by HO-, and all the alpha-OH substituted radical cations react with HO- a t a rate (approximate to 10(10) M-1 s(-1)) very close to the limit of diffu sion control and independent of the nature of the bond that is finally brok en in the process (C-H or C-C). The methyl ether 8(.+), which exclusively u ndergoes C-H bond cleavage, reacts significantly slower (by a factor of ca. 50) than the corresponding alcohol 1(.+). These data indicate that 1-aryla lkanol radical cations, which display the expected carbon acidity in water, become oxygen acids in the presence of a strong base such as HO- and under go deprotonation of the O-H group; diffusion-controlled formation of the en counter complex between HO- 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 radic al zwitterion (which undergoes intramolecular electron transfer) or directl y (electron transfer coupled with deprotonation). The latter possibility se ems more in line with the general base catalysis (beta approximate to 0.4) observed in the reaction of 5(.+), which certainly involves O-H deprotonati on. The benzyloxyl radical can then undergo a beta C-C bond cleavage to for m 4-methoxybenzaldehyde and R-. or a formal 1,2-H shift to form an alpha-hy droxybenzyl-type radical. The factors of importance in this carbon/oxygen a cidity dichotomy are discussed.