Stereoelectronic effects in radical fragmentation: rearrangement of 3β,5-cyclocholestan-6-yl radical

Abstract

A stereoelectronic requirement in a radical fragmentation reaction is demonstrated by the specific fission of the 4,5-bond in the 3~,5-cyclocholestan-6-y1 radical.

Kinetics and Regioselectivity of Ring Opening of 1-Bicyclo[3.1.0]hexanylmethyl Radical

“…Fragmentation of the cyclopropylcarbinyl radical system is of particular interest and has been thoroughly studied by many groups. The prototype system holds a prominent place in radical kinetic investigations and has been accurately characterized across a temperature range spanning more than 250 °C with a fragmentation rate of 9.4 × 10 7 s -1 at 298 K. A wide breadth of substituted derivatives has also been studied and has provided much insight into the influence of substitution, resonance, and stereoelectronic effects . Unsymmetrically substituted derivatives have a bifurcated reaction pathway available and permit rate comparisons between the two modes of fragmentation.…”

“…Unsymmetrically substituted derivatives have a bifurcated reaction pathway available and permit rate comparisons between the two modes of fragmentation. Some aryl-substituted analogues have been shown 4d, to be exceptionally fast with unimolecular fragmentation rates exceeding 10 11 s -1 .…”

“…More recently, laser flash photolysis and the use of PhSeH have proven useful for the upper end of the time scale (>10 11 s -1 ) . Accurate rate expressions for the reaction of these reducing agents with carbon-centered radicals have been established and cross-calibrated with each other using several radical reactions. 3a,4g, …”

Reflex Carbocation Release from Covalent Molecules by Endogenous Free Radicals

“…It is worth mentioning the complementarity of these fragmentations to those of radical substrates that see anionic leaving groups, such as acetate anions or the loss of phosphate anions from deoxyribose radicals. 29 − 31 …”

“…Radicals generated six or seven ethyne units away from the cationic center were still effective in enhancing cation release. It is worth mentioning the complementarity of these fragmentations to those of radical substrates that see anionic leaving groups, such as acetate anions or the loss of phosphate anions from deoxyribose radicals. − …”