Cation radical-mediated semi-pinacol and n+2 ring expansions via organic photoredox catalysis

Abstract

In this report, we disclose a photoredox-catalyzed cation radical-mediated semi-pinacol (CRM-SP) rearrangement along with a similar, but mechanistically distinct, n+2 ring expansion with protocols for obtaining either dearomatized or rearomatized products. We present a substrate scope of >30 entries largely focused on the ring expansion of easily appended cyclobutanols and targeting medicinally relevant scaffolds, highlighting the utilization of the methodology for diversification. Gram scale reactions in batch and continuous flow were investigated. Additionally, the mechanism is probed both experimentally and computationally with results suggesting a substrate-dependent set of pathways featuring either (A) a concerted 1,2-shift or (B) a sequence of proton-coupled electron transfer (PCET), β-scission, Giese addition, and terminal hydrogen atom transfer (HAT).