Abstract
ABSTRACT
Photo‐induced hydrogen atom transfer (HAT) catalysis has become a powerful platform for the direct and selective functionalization of ubiquitous C─H bonds. Traditionally, the outcome of these transformations is dictated by the interplay between C─H bond dissociation energies and polar effects, enabling predictable yet inherently substrate‐controlled selectivity. Recently, a fundamentally new strategy has emerged that relies on the catalytic and reversible modulation of substrate electronics to transiently enhance the reactivity of targeted C─H bonds. Notably, catalytic full or partial deprotonation of oxygen‐ and nitrogen‐containing functional groups has proven remarkably effective in selectively activating adjacent C─H bonds toward HAT, a phenomenon known as the anionic substituent effect. Rather than relying solely on the intrinsic properties of a substrate, this concept allows chemists to reprogram C─H bond reactivity in situ, unlocking previously inaccessible levels of selectivity in the direct functionalization of abundant functional groups under mild photocatalytic conditions. In this minireview, we discuss the mechanistic principles underpinning catalytic anionic substituent effects (CASE), the diverse strategies developed to generate them, and their impact on photocatalytic HAT‐mediated C–H functionalization. By highlighting recent advances and emerging opportunities, we illustrate how transient electronic activation is reshaping the design of selective C–H functionalization reactions.