Electrochemical synthesis of cyclic biaryl λ3-bromanes from 2,2'-dibromobiphenyls

Abstract

The remarkable nucleofugality of bromoarenes in diarylbromonium species renders them particularly suitable for the generation of arynes for subsequent use in a wide range of synthetic applications. The common approach to generate cyclic biaryl λ³-bromanes is based on thermal decomposition of hazardous diazonium salts. Herein, we disclose a mild and straightforward approach to diarylbromonium species by direct anodic oxidation of 2,2'-dibromo-1,1'-biphenyl. The electrochemical method provides access to a range of symmetrically and non-symmetrically substituted cyclic biaryl λ³-bromanes in moderate yields.

Keywords: anodic oxidation; cyclic biaryl λ3-bromane; cyclic voltammetry; electrochemistry; hypervalent bromine.

Scheme 1

Synthesis of cyclic diarylbromonium compounds.

Scheme 2

Substrate scope. Reactions were performed on a 0.15 mmol scale. Yields were determined by ¹H NMR spectroscopy of the reaction mixture using 1,2,3,4-tetrafluorobenzene as an internal standard. Isolated yields are given in parentheses.

Scheme 3

A: Background and iR drop-corrected CVs of 5 mM 4a at different scan rates (solvent: HFIP, working electrode: glassy carbon, supporting electrolyte: 0.1 M TBA-BF₄). B: Plot of the peak current densities (j_p) vs v^0.5. C: Representative j_p vs v^0.5 slope values for oxidation of Martin’s bromane precursor 6 (ref. [17]). D: Plausible reaction mechanism.