Controlling divergent hydrogen atom transfer in vitamin B12-photocatalyzed oxetane ring opening

Vitamin B 12 (VB 12 ), one of nature's most versatile cofactors, performs a multitude of radical and polar steps carefully choreographed by its surrounding enzyme microenvironment. It is highly desirable to capture this variegated reactivity in synthetic applications as well; however, few design principles for controlling VB 12 reactivity have been enumerated. Here, we show proton availability to be a key factor in modulating VB 12 reactivity between hydrogen atom transfer (HAT) donation and hydrogen atom abstraction (HAA), with acidic, protic media favoring donation and aprotic media favoring abstraction. We demonstrate this principle in the modular transformation of oxetanes to saturated or allylic alcohols, depending on reaction media, via VB 12 photocatalysis. These reactions proceed with a wide scope and high yield, and mechanistic experiments are consistent with our postulated mechanisms. Together, these results demonstrate rational tuning of VB 12 reactivity by controlling the proticity of the reaction medium and highlight the synthetic opportunities offered by divergent reactivity.