Spectral Properties and Absolute Rate Constants for β-Scission of Ring-Substituted Cumyloxyl Radicals. A Laser Flash Photolysis Study

Abstract

A laser flash photolysis study of the spectral properties and beta-scission reactions of a series of ring-substituted cumyloxyl radicals has been carried out. All cumyloxyl radicals display a broad absorption band in the visible region of the spectrum, which decays on the microsecond time scale, leading to a strong increase in absorption in the UV region of the spectrum, which is attributed to the corresponding acetophenone formed after beta-scission of the cumyloxyl radicals. The position of the visible absorption band is red-shifted by the presence of electron-donating ring substituents, while a blue-shift is observed in the presence of electron-withdrawing ring substituents, suggesting that + R ring substituents promote charge separation in the excited cumyloxyl radical through stabilization of the partial positive charge on the aromatic ring of an incipient radical zwitterion. Along this line, an excellent Hammett-type correlation between the experimentally measured energies at the visible absorption maxima of the cumyloxyl radicals and sigma(+) substituent constants is obtained. A red-shift is also observed on going from MeCN to MeCN/H(2)O for all cumyloxyl radicals, pointing toward a specific effect of water. The ring substitution does not influence to a significant extent the rate constants for beta-scission of the cumyloxyl radicals, which varies between 7.1 x 10(5) and 1.1 x 10(6) s(-1), a result that suggests that cumyloxyl radical beta-scission is not governed by the stability of the resulting acetophenone. Finally, k(beta) increases on going from MeCN to the more polar MeCN/H(2)O 1:1 for all cumyloxyl radicals, an observation that reflects the increased stabilization of the transition state for beta-scission through increased solvation of the incipient acetophenone product.

The O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. Experimental evidence for the formation of an intermediate 1-oxaspiro[2,5]octadienyl radical

“…In this context, it is, however, important to point out that a recent product and time-resolved kinetic study carried out by some of us has clearly shown that the para-(2,2-diphenylcyclopropyl)cumyloxyl radical (1 Å ) exists in equilibrium with a 2,2-dimethyl-1-oxaspiro [2,5]octadienyl radical (2 Å ). 19 Cumyloxyl radicals are known to undergo C-CH 3 b-scission as the exclusive unimolecular reaction (Scheme 2, path a), 6f,6g, 20,21 and the failure to observe the O-neophyl rearrangement (Scheme 2, path b) reasonably reflects the lower stability of the 2-phenoxy-2-propyl radical as compared to the 1-phenoxy-1-phenylalkyl one displayed in Scheme 1.…”

Kinetic Solvent Effects in Organic Reactions

“…This result was explained by a transition state effect, where the transition state is more polar than the radical and is thus more stabilized by interactions with polar and polarizable solvents. Furthermore, the β-scission of cumyloxyl radicals measured using laser flash photolysis also showed a rate increase with increasing solvent polarity 63 Figure 7. The E T parameter represents the charge-transfer absorption of the solvent in pyridinium N -phenolbetaine and serves as a different measure of solvent polarity than the dielectric constant 66 .…”

Reaction Pathways of Alkoxyl Radicals. The Role of Solvent Effects on C–C Bond Fragmentation and Hydrogen Atom Transfer Reactions

“…16 In acetonitrile solution, small differences in the rate constant were measured after ring substitution, with the k β values for the different radicals ranging between 7.1 × 10 5 and 1.1 × 10 6 s -1 (Table 1). This behavior suggests that substituent effects operate in the same direction on both the cumyloxyl radical and the transition state for β-scission that is characterized by an incipient acetophenone-like character.…”

“…12, 16 Very interestingly, a red-shift in λ max (vis) was also observed for cumyloxyl and ring-substituted cumyloxyl radicals on going from acetonitrile to protic solvents such as methanol, 5e 2,2,2-trifluoroethanol (TFE) 15 and water, 16,18 with the shift increasing in the order: MeOH < TFE < H 2 O. Compared to acetonitrile, no variation in the position of λ max (vis) was observed in other protic solvents such as acetic acid and tertiary alkanols, and in a variety of aprotic solvents.…”

“…The spectrum recorded 160 nanoseconds after the laser pulse (black circles) shows a broad absorption band in the visible region centered at 485 nm and a weaker band in the UV region around 310 nm, which are assigned to the cumyloxyl radical. 12, 16 The decay of these bands [inset (a), showing the decay of absorption at 485 nm] is accompanied by a corresponding buildup of absorption at 255 nm [inset (b)] assigned to acetophenone (an isosbestic point is visible at 295 nm), formed after C-methyl β-scission in the cumyloxyl radical (Scheme 6). …”