New Strategies in Radical-Based Aliphatic C-H Fluorination

In 2018, almost 50% of all small-molecule drugs approved by the FDA contained fluorine. The appeal of fluorine substitution in drug candidates can be attributed to the improved properties fluorine exerts on a molecule such as blocking undesired metabolism and increasing potency, absorption, or binding affinities. Incorporation of the radio isotope, [18F], to bioactive molecules also has applications in medical imaging as well as pharmacokinetic studies. Though countless methods exist for C-F bond formation, the realm of direct C-H fluorination is rather limited. Previous methods for direct fluorination employ fluorine gas, which is toxic and extremely reactive—requiring specialized handling. Rapid progress has been made in the last decade regarding arene fluorination through directed palladium catalyzed systems utilizing milder fluorinating reagents; however, the direct fluorination of unactivated C-H bonds remains largely underdeveloped. Carbon-centered radical generation has emerged as an effective strategy to fluorinate reactive aliphatic C-H bonds under mild conditions. Recent advances in this field attempt to combat the selectivity and reactivity issues related to aliphatic C-H fluorination under milder systems and expand these strategies to [18F] fluorination.