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Adding Aliphatic C–H Bond Oxidations to Synthesis

Oxidations of aliphatic C–H bonds, known since the 1800s, have only recently been considered for use in organic synthesis. Aliphatic C–H bonds are among the least reactive in organic chemistry, yet enzymes have evolved that not only oxidize them, but can discriminate between individual tertiary (3°) and secondary (2°) C–H bonds in complex molecules. Chemists considered reactivity differences between these inert bonds too minor for a small-molecule catalyst to discriminate (1); bio-inspired catalysts with intricate binding pockets were thought to be the only viable solution (2). This view pervaded because the reported examples of selective aliphatic C–H oxidations [halogenations (3), alkylations (4), aminations (5), hydroxylations (6), and dehydrogenations (7)] were not sufficiently high in yield or predictable in site selectivity to be useful in synthesis, except in some special cases such as steroids (see below). In the past several years, however, aliphatic C–H oxidations and simple rules for predicting their selectivities have been emerging prominently in synthetic planning. Powerful small-molecule catalysts have been invented that furnish high enough yields (>50%) to be preparatively useful and can predictably discriminate between aliphatic C–H bonds even within complex molecules that have many possible sites of oxidation.