Progress on Visible-Light Promoted Addition Reactions of Inert C-H Bonds to Carbonyls

Secondary and tertiary alcohols are important structural motifs that often exist in a large number of bioactive natural products and pharmaceuticals. Moreover, they can also be used as upstream raw material for preparation of various highly valuable chemicals. Among the existing synthetic methodologies for preparation of these compounds, the nucleophilic addition of nucleophiles to aldehydes and ketones represents one of the most operative ways. However, pre-functionalized substrates are always needed for this protocol, resulting in low efficiency and poor atom economy. The growing area of photocatalysis has provided a mild and effective approach for inert C-H bond activation. As a result, the photocatalytic straight addition of inert C-H bond to carbonyls has been developed, which affords the synthesis of secondary and tertiary alcohols in a new manner. The visible-light induced addition reaction of inert C-H bond to carbonyls was classified by three different mechanisms: reductive radical-polar crossover (RRPCO), radical-radical cross coupling as well as radical addition, and reviewed, respectively. Finally, the limitations and future developments of this research field are discussed.