ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Reactivity of substrates with multiple competitive reactive sites toward NBS under neat reaction conditions promoted by visible light

Blaž Grjol and Marjan Jereb

Kilobaser GmbH, Graz, Austria

E-mail: marjan.jereb@fkkt.uni-lj.si

Received: 23 December 2020  Accepted: 21 May 2021

Abstract:

Regioselectivity of visible-light-induced transformations of a range of (hetero)aryl alkyl-substituted ketones bearing several competitive reactive sites (α-carbonyl, benzyl and aromatic ring) with N-bromosuccinimide (NBS) was studied under solvent-free reaction conditions (SFRC) and in the absence of inert-gas atmosphere, radical initiators and catalysts. An 8-W energy-saving household lamp was used for irradiation. Heterogeneous reaction conditions were dealt with throughout the study. All substrates were mono- or dibrominated at the α-carbonyl position, and additionally, some benzylic or aromatic bromination was observed in substrates with benzylic carbon atoms or electron-donating methoxy groups, respectively. Surprisingly, ipso-substitution of the acyl group with a bromine atom took place with (4-methoxynaphthyl) alkyl ketones. While the addition of the radical scavenger TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxy) decreased the extent of α- and ring bromination, it completely suppressed the benzylic bromination and α,α-dibromination with NBS under SFRC.

Keywords: Halogenation; Photochemistry; Solvent-free reaction conditions; NBS; Regioselectivity

Full paper is available at www.springerlink.com.

DOI: 10.1007/s11696-021-01711-x

Chemical Papers 75 (10) 5235–5248 (2021)