Phenyl Bridging in Ring-Substituted Cumyloxyl Radicals. A Product and Time-Resolved Kinetic Study

Abstract

The reactivity of cumyloxyl radicals bearing cyclopropyl and 2,2-diphenylcyclopropyl groups in the para position has been investigated. Depending on radical structure, products deriving from C-C beta-scission and/or cyclopropyl ring-opening are observed, supporting the hypothesis that cumyloxyl (and, more generally, arylcarbinyloxyl) radicals exist in equilibrium with 1-oxaspiro[2,5]octadienyl radicals, in full agreement with the previously proposed mechanism for the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals.

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

“…This is in line with the estimated rate constants for the two processes, as it can be reasonably assumed that cyclopropyl ringopening occurs with the same rate in 4 Å and 2 Å (for which a value of k % 7:5 Â 10 8 s À1 has been estimated), 19 whereas that rate constants for O-neophyl shift in 1,1-diarylalkoxyl radicals bearing electron-releasing ring substituents have been shown to be 62.4 Â 10 6 s À1 , 14b and a similar (or lower) value can be reasonably predicted also for 3 Å . In conclusion, by means of a detailed product study, convincing experimental evidence in support of an equilibrium between 1,1-diarylalkoxyl radical 3…”

“…23 A rate constant k % 7:5 Â 10 8 s À1 has been instead estimated for the analogous rearrangement in 2 Å . 19 This value is about three orders of magnitude higher than the rate constants measured (in MeCN) for C-CH 3 b-scission of cumyloxyl radicals. 27.…”

“…In addition, the spectroscopic data are in excellent agreement with those obtained previously for the structurally related product 2,2-dimethyl-6-(3-hydroxy-3,3-diphenylpropyliden)-1-oxaspiro [2,5]octa-4,7-diene (A), obtained after visible light irradiation of 2-(4-(2,2-diphenylcyclopropyl)phenyl)-2-propanol in the presence of DIB and I 2 as described above (Scheme 3). 19 The formation of C can be rationalized in terms of cyclopropyl ring-opening in the intermediate- The failure to observe the product deriving from O-neophyl shift in 3 Å (cyclopropyl 4-(2,2-diphenylcyclopropyl)phenyl ketone (D)) indicates that in the presence of two phenyl substituents on the cyclopropyl group, 4…”

“…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.…”

“…19 A result that has been interpreted in terms of the existence of an equilibrium between 1 Å and 2 Å followed by a fast 2,2-diphenylcyclopropylcarbinyl?1,1-diphenyl-3-butenyl radical rearrangement in the latter radical. The failure to observe para-(2,2-diphenylcyclopropyl) acetophenone (B) deriving from C-CH 3 b-scission in 1 Å clearly indicates that in the presence of two phenyl substituents on the cyclopropyl group, this process does not compete with cyclopropyl ring-opening in 2 On the basis of these results, it seemed particularly interesting to extend the approach described above for the cumyloxyl radical also to 1,1-diarylalkoxyl radicals, in order to establish if a bridged 1-oxaspiro [2,5]octadienyl radical is actually an intermediate in the O-neophyl rearrangement, and moreover, if the existence of this equilibrium is a general feature of arylcarbinyloxyl radicals.…”