Electron-spin distribution in radical anions of aryl methyl ethers vs reactivity and regioselectivity of their unimolecular fragmentation

Abstract

Seven different aryl methyl ethers (1-7) were reduced by Li, Na, K, and Cs in THF, DME, or mixtures of THF and DME. The resulting solutions were studied by ESR and, in some cases (5-7), by ENDOR and TRIPLE. With the exception of 3 and 4, the resulting ROAr*~M+ radicals could be observed. The experimental electron-spin distributions (including those published for other ethers) were compared with the results of the Hückel-McLachlan and INDO calculations. These combined results and the temperature variation for 6"M+ allowed us to establish the conformations of ROAr*" and the localization of M+. The persistence of ROAr" toward OAr bond scission increases with the corresponding ir-bond order and the oxygen metal distance. The orthogonal conformation favors RO-bond scission, the planar one OAr-bond scission. A new way to understand the influence of structure, conformation, counterion, and solvent on the reactivity and regioselectivity in unimolecular ROAr"M+ fragmentation is presented in terms of a single-step mechanism and by considering the lowering effect of the coulombic field of M+ on the energies of the various species and states involved.Recent publications on alkyl aryl ether cleavage by alkali metals1,2 and by electrolytical reduction,3•4 as well as unpublished material and earlier literature, were reviewed by Maercker,5 who also presents some new mechanistic views on this reaction. The first step leads to radical anions ROAr", known since 1968 from ESR studies.6 Dianions were also been discussed in the past as intermediates,7 *9"12 although the more recent literat 11ure1•5•13"19 shows that in most cases this is an unnecessary hypothesis. Intermediates

Reactivity in Cleavage of Dimethoxybenzenes by Sodium in Liquid Ammonia

“…Somewhat similar behavior is shown by 3,4-dimethoxytoluene on exposure to Na/K alloy in THF or dimethoxyethane at very low temperature (ref ). Cleavage to p -methoxytoluene (aryl−oxygen scission!)…”

“…The species that rupture during cleavage of alkyl aryl ethers by alkali metals are radical anions of the ethers. − Two steps are involved: transfer of an electron to the ether from an electron donor, generally an equilibrium process, and scission of the radical anion. Although solvated electrons in ammonia have high negative redox potential, studies of Birch reduction kinetics indicate that neither benzene nor anisole is fully electronated in liquid ammonia solutions of alkali metals.…”

“…There are suggestions also in studies of ESR spectra of radical anions that a methyl group is almost as strong a π-donor as is a methoxy group; see refs and .…”

Effect of Topologically Controlled Coulombic Interactions on the Dynamic Behavior of Photoexcited Nitrophenyl Alkyl Ethers in the Presence of Tertiary Amines with Limited Motion Freedom

“…has been traditionally attributed to Lewis acid complexation, ignoring the important associated electrostatic effect. Only recently has this electrostatic effect been recognized as responsible for the lowest energy conformation of the radical anion/cation pair in alkyl aryl ethers and the acceleration of the electrocyclic reactions by metal cation complexation …”

Metal-Induced Reductive Cleavage Reactions: An Experimental and Theoretical (MNDO) Study on the Stereochemical Puzzle of Birch and Vinylogous Birch Processes

“…Remarkably, no attention has been paid in these studies to the possible role played by the metal center in the cleavage of radical anions, in spite of the well-known effect of the metal on the hyperfine coupling observed in the ESR spectra of radical anion/cation species . Only a recently published investigation claimed, on the basis of experimental (ESR, ENDOR, and triple studies) and low-level theoretical (Hückel and INDO) calculations, that the counterion actually determines the lowest energy conformation of the radical anion/cation pair . Furthermore, even though solvation, concentration, and temperature are known to affect the structure of the radical anion salts (ketyls) resulting from metal reduction of aromatic ketones, counterion and solvation effects in redox reactions remain “sunk at unknown depths”, especially for those processes which involve two electron transfer steps with concomitant cleavage of a C−X bond in a position α or β to an existing π system …”