Formation of N‑Heterocyclic Carbene−Boryl Radicals through Electrochemical and Photochemical Cleavage of the B−S bond in N‑Heterocyclic Carbene−Boryl Sulfides Sofia Telitel, † Anne-Laure Vallet, ‡ Ste ́ phane Schweizer, § Bernard Delpech, ‡ Nicolas Blanchard, ⊥ Fabrice Morlet-Savary, † Bernadette Graff, † Dennis P. Curran, ∥ Marc Robert,* ,▽ Emmanuel Lacôte,* ,# and Jacques Laleve ́ e* ,† † Institut de Science des Mate ́ riaux de Mulhouse IS2M (UMR CNRS 7361), Universite ́ de Haute Alsace, 15 rue Jean Starcky, 68057 Mulhouse Cedex, France ‡ ICSN CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex France § Laboratoire de Chimie Organique et Bioorganique (EA 4566), Universite ́ de Haute-Alsace-ENSCMu, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France ⊥ Laboratoire de Chimie Molé culaire (UMR CNRS 7509), E ́ cole Europe ́ enne de Chimie, Polyme ̀ res et Mate ́ riaux, Universite ́ de Strasbourg, 25 rue Becquerel, 67087 Strasbourg, France ∥ Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 United States ▽ Laboratoire d’E ́ lectrochimie Molé culaire (UMR CNRS 7591), Universite ́ Paris Diderot, Sorbonne Paris Cite ́ , 15 rue Jean-Antoine de Baïf, 75013 Paris, France # Institut de chimie de Lyon, Universite ́ de Lyon, UMR 5265 CNRS-Universite ́ Lyon I-ESCPE Lyon, 43 Bd du 11 novembre 1918, 69616 Villeurbanne, France * S Supporting Information ABSTRACT: The B−S bond in N-heterocyclic carbene (NHC)−boryl sulfides can be cleaved homolytically to NHC−boryl or NHC−thioboryl and thiyl radicals using light, either directly around 300 nm or with a sensitizer at a longer wavelength (>340 nm). In contrast, the electrochemical reductive cleavage of the B−S bond is difficult. This easy photolytic cleavage makes the NHC−boryl sulfides good type I photopolymerization initiators for the polymerization of acrylates under air. ■ INTRODUCTION The complexation of boranes by N-heterocyclic carbenes (NHCs) generates stable adducts 1 that can be used as reagents for both organic 2−5 and polymer synthesis. 6−10 In radical-based transformations, the rich chemistry of NHC-complexed boryl radicals has found many uses. Such radicals can homolytically substitute mono- 3,7,11 and divalent 12 atoms, add to xanthates 2,13 and electron-poor olefins, 7,9 dimerize, 14−16 or enter redox processes. 17−19 In other words, NHC−boryl radicals undergo nearly all of the elementary steps open to carbon-centered radicals and thus have a high synthetic potential. Previous work on radical, ionic, and organometallic reactions of NHC−boranes has focused almost exclusively on Lewis pairs involving a variety of different NHCs and the parent borane (BH 3 ). Attention is now shifting to B-substituted NHC− boranes, and new reaction modes are being uncovered. For example, B-alkyl and B-aryl substituents can help stabilize boreniums, 19,20 generate frustrated Lewis pairs, 21 or lead to radical β-eliminations. 14 Also, B-substituents can be transferred to palladium complexes for Suzuki−Miyaura couplings. 22 NHC−boryl halides can be reduced by electron transfer to generate boryl anions, 17,18 borylenes, 23,24 and multiply bonded diboron compounds. 25 And NHC monoadducts of diboranes have a rich chemistry of their own. 14,26−28 We have shown that NHC−boranes react with diaryl disulfides to form NHC−boryl mono- and bis-sulfides. NHC−Boryl sulfides are a new family of B-substituted NHC−boranes with unexplored chemistry. 12 Here we describe the cleavage of the B−S bond in NHC− boryl mono- and bis-sulfides by irradiation with and without photosensitizers and by electrochemical stimulation. The Received: June 29, 2013 Published: October 10, 2013 Article pubs.acs.org/JACS © 2013 American Chemical Society 16938 dx.doi.org/10.1021/ja4066267 | J. Am. Chem. Soc. 2013, 135, 16938−16947