Carbocation Catalyzed Bromination of Alkyl Arenes, a Chemoselective sp3 vs. sp2 C?H functionalization.

Article abstract of DOI:10.1002/adsc.201800788

The versatility of the trityl cation (TrBF₄) as a highly efficient Lewis acid organocatalyst is demonstrated in a light induced benzylic brominaion of alkyl-arenes under mild conditions. The reaction was conducted at ambient temperature under common hood light (55 W fluorescent light) with catalyst loadings down to 2.0 mol% using N-bromosuccinimide (NBS) as the brominating agent. The protocol is applicable to an extensive number of substrates to give benzyl bromides in good to excellent yields. In contrast to most previously reported strategies, this protocol does not require any radical initiator or extensive heating. For electron-rich alkyl-arenes, the trityl ion catalyzed bromination could be easily switched between benzylic sp³ C?H functionalization and arene sp² C?H functionalization by simply alternating the solvent. This chemoselective switch allows for high substrate control and easy preparation of benzyl bromides and bromoarenes, respectively. The chemoselective switch was also applied in a one-pot reaction of 1-methylnaphthalene for direct introduction of both sp³ C?Br and sp² C?Br functionality. (Figure presented.).