Nucleophilic displacement versus electron transfer in the reactions of alkyl chlorosilanes with electrogenerated aromatic anion radicals Saida Soualmi 1 , Mamadou Dieng 2 , Ali Ourari 3 , Diariatou Gningue-Sall 2 , Viatcheslav Jouikov * , 4 Molecular Chemistry and Photonics, UMR 6226, University of Rennes 1, 35042 Rennes, France ARTICLE INFO Article history: Received 12 November 2014 Received in revised form 25 January 2015 Accepted 28 January 2015 Available online 30 January 2015 Keywords: Chlorosilanes aromatic anion radicals silicophiles electrochemical reduction redox catalysis ABSTRACT Anion radicals of a series of aromatic compounds (C 6 H 5 CN, C 6 H 5 COOEt, anthracene, 9,10-dimethyl-, 9,10-diphenyl- and 9-phenylanthracene, pyrene and naphthalene) react with trialkyl chlorosilanes R 1 R 2 R 3 SiCl (R 1-3 = Me, Et; R 1,2 = Me, R 3 = t-Bu) in multiple ways, following classical bimolecular schemes. The ratio of one-electron transfer (ET) to a two-electron process (S N 2-like nucleophilic attack of the reduced form of mediator on the chlorosilane, with k 2 ffi 10 2 -10 8 M 1 s 1 ) is inversely related to the steric availability of Si for nucleophilic displacement reactions. The nucleophilic substitution pathway mainly results in mono- and disilylated aromatic products. Paralleling the electrochemical data with DFT calculations, the role of silicophilic solvent (DMF) in S N process was shown to be quite complex because of its involvement into coordination extension at silicon, dynamically modifying energetics of the process along the reaction coordinate. Although 2,2'-bipyridine also forms delocalized persistent anion radicals, they do not induce neither ET nor S N reactions in the same manner as aromatic mediators. Silicophilicity of 2,2'-bipyridine being superior to that of DMF, a R 3 SiClbipy complex of hypercoordinated silicon with electroactive ligand was formed instead, whose reduction requires about 1 V less negative potentials than bipyridine itself. ã 2015 Published by Elsevier Ltd. 1. Introductione Silyl protection-deprotection methodology as well as many synthetic procedures of organosilicon compounds are based on nucleophilic reactions exploiting large versatility of electrophilic properties and steric bulk of R 3 SiY (Y = Hal, AlkO, MeCOO, R 2 N, CN etc) precursors [1,2]. With all importance of the chemistry of introduction and removal of silyl groups, it is surprising that relatively few quantitative studies on the fundamentals of reactivity of chlorosilanes are available [2]. Corriu evoked the possibility of both single electron transfer (ET) and a pure nucleophilic pathway in nucleophilic reactions of chlorosilanes [3]. The first one involves one-electron transfer from an electron- rich nucleophile to an electrophilic chlorosilane, supposedly with the intermediacy of an anion radical of the chlorosilane [4]. Since the reduction of chlorosilanes requires very negative potentials [5] and—at least of those with aliphatic substituents—occurs via dissociative ET mechanism [6,7], the ET pathway seems less probable, though in the case of aryl chlorosilanes this possibility cannot be ruled out [7,8]. On the other hand, it is well documented that heterogeneous ET to alkyl chlorosilanes induces cleavage of the Si—Cl bonds leading to the reduction products with Si—Si or Si—H bonds [9]. Direct cathodic reduction of chlorosilanes, pioneered by Bobersky [10], Hengge [11] and Corriu [5,12], includes two- electron cleavage of the Si—Cl bond resulting in silyl anions. The nucleophilic attack of these latter on the starting molecule or on other chlorosilane, added to the solution [13–15], leads to stable final products via the formation of a Si—Si bond (Eq. (1)). The * Corresponding author. Tel.: +33 22 323 6293; fax: +33 22 323 6955. E-mail address: vjouikov@univ-rennes1.fr (V. Jouikov). 1 Permanent address: Faculté des Sciences et de la Technologie et Sciences de la Matière, Université de Tiaret, 14000 Tiaret, Algeria. 2 Permanent address: LCPOAI, Chemistry department, University Cheikh Anta DIOP of Dakar, BP5005 Dakar, Senegal. 3 Permanent address: Laboratory of Process Engineering, University of Setif, 19000 Setif, Algeria. 4 ISE member. http://dx.doi.org/10.1016/j.electacta.2015.01.182 0013-4686/ ã 2015 Published by Elsevier Ltd. Electrochimica Acta 158 (2015) 457–469 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta