Pergamon Tetrahedron Letters, Vol. 37, No. 41, pp. 7425-7428, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0040-4039/96 $15.00 + 0.00 PII: S0040-4039(96)01602-4 Alternating addition of carbon-centered radicals to unsaturated systems. Novel homolytic aromatic substitutions. Silvia Araneo, Riccardo Arrigoni, Hans-Ren~ Bjorsvik, Francesca Fontana, Francesco Minisci*, Francesco Recupero Dipartimentodi Chimicadel Politecnico; via Mancinelli, 7; 1-20131Milano, Italy Abstract. A novel type of homolytic aromatic substitutionis reported; it is based on the addition of alkyl radicals to maleic anhydride and on the subsequent attack of the electrophilicradical adduct to nucleophilic aromatics. Copyright © 1996 Elsevier Science Ltd The alternating effect in free-radical copolymerization has been one of the first significant • • 1 examples of polar effects in free-radical reactions. Several years ago we 2 have taken advantage of the same effect for synthetic purposes, by generating alkyl radicals in the presence of a pair of monomers (an electrophilic alkene and a nucleophilic conjugated diene or styrene derivative) and metal salt redox system (eqs. 1-3) R" + CH2=CHX --~ R-CH2-CHX (1) 1 3 R_CH2-CHX + CH2=CH-CH=CH2 D, -CH2-CHX-CH2-CH- CH :-: ~ C (2) 2 [~R-CH2-CHX-CH2-CH~-: CH-- CH21" -----~. Products (3) The synthetic success was related to polar and enthalpic effects so that the initial alkyl radical (R) adds with good selectivity to the electrophilic monomer (eq.1); the radical adduct 2, being electrophilic for the proximity of the polar substituent, selectively adds to the diene or styrene (eq.2). The allyl (2) or benzyl radicals are less reactive towards alkenes than are alkyl radicals, for enthalpic reasons 3 and they are selectively oxidized to the reaction products (eq.3) We report for the first time how this general concept can be applied to homolytic aromatic substitutions. Alkyl radicals, not bearing an electron-withdrawing group directly bonded to the radical center, can show nucleophilic character. Aryl radicals can also exhibit nucleophilic character, even if it is much less marked compared to alkyl radicals; one must always consider that the polar effect, being kinetic in nature, is not an intrinsic property of a free-radical, but it depends on the particular type of reaction involved 4. As the electrophilic alkene, we have chosen maleic anhydride because, as a cheap industrial intermediate, it can be suitable for industrial developments; besides, it has no tendency to homopolymerization. 7425