Oxidative nucleophilic substitution of hydrogen in nitroarenes with an oxazoline-stabilized carbanion
Saverio Florio,*a Mieczyslaw Makosza,*b Patrizia Lorusso,c and Luigino Troisic
a Dipartimento Farmaco-Chimico, University of Bari, Via E. Orabona 4, 70126 Bari, Istituto di Chimica dei Composti Organometallici “ICCOM”, Sezione di Bari, Italy; b Institute of Organic Chemistry, Polish Academy of Sciences ul. Kasprzaka 44, 01-224 Warszawa 42, POB 58, Poland; c Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, University of Lecce,Via Prov. le Lecce-Monteroni, 73100 Lecce, Italy
E-mail: florio@farmchim.uniba.it; icho-s@icho.edu.pl
Dedicated to Professor Giuseppe Bartoli on the occasion of his 65th Anniversary
Abstract
The carbanion of 4,4-dimethyl-2-(1-phenylethyl)-4,5-dihydro-1,3-oxazole adds 1 to nitroarenes in the position para to the nitro group: oxidation of the produced sH adducts with dichlorodicyanobenzoquinone (DDQ) gives para-substituted nitroarenes whereas oxidation with dimethyldioxirane (DMD) leads to para- substituted phenols.
Keywords: Carbanions, nitroarenes, VNS reaction, ONSH reaction, oxazolines, phenols
Introduction
Nucleophilic substitution of hydrogen in nitroarenes and other electron-deficient arenes is a well recognized process of substantial value in organic synthesis.1,2 There are two main variants of this process: vicarious and oxidative nucleophilic substitution. The key step of both of these reactions is the formation of sH adducts via addition of nucleophiles to the electron-deficient aromatic rings in positions occupied with hydrogen. When the nucleophiles contain leaving groups L at the nucleophilic center, e.g., a-halocarbanions or alkyl hydroperoxide anions further transformation of the sH adducts proceeds with base-induced ß-elimination of HL giving products of vicarious nucleophilic substitution (VNS).3,4 On the other hand, oxidation of the sH adducts with external oxidants results in oxidative nucleophilic substitution of hydrogen (ONSH).5 Of particular value in organic synthesis is the nucleophilic oxidative alkylation of nitroarenes with Grignard reagents, pioneered by Bartoli.6,7 He has shown that alkyl Grignard reagents add to the electron-deficient rings of nitroarenes, while subsequent oxidation of the
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