Direct site-selective C(sp3)–H functionalization of unprotected non-activated alcohols

Free alcohols are ubiquitous in nature, drugs, bulk chemicals, and various complex molecules. The abundantly available free aliphatic alcohols, being inexpensive, are sustainable starting materials in organic synthesis to construct value-added scaffolds. Direct C(sp³)–H functionalization of inert bonds in aliphatic alcoholic chain is challenging due to their inactive nature towards chemical reactions and also multiple similar C-H bonds are available for functionalization reaction at a time. Barton process, which is a classical method for generating alkoxyl radicals, has been widely used for the remote functionalization of alcoholic chain using the strategy of 1,5-hydrogen atom transfer (1,5-HAT). However, the requirement for the pre-activation of alcohols not only adds extra and tedious steps but also, these precursors are challenging to prepare and handle. Generating desired alkoxyl radicals from these precursors is a difficult task and also affects the atom economy of reactions. Numerous methods covering the C(sp³)–H functionalization of unprotected non-activated alcohols at diverse (α-, β-, γ-, δ-) positions to access functionalized alcohols are elaborated here. A precise and focused compilation will help to understand what has been established and how new methodologies can be developed for the future needs.
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