Conversion of Alkenes Into Simple or Functionalized Aldehydes by Reaction With an Ambiphilic Masked Formyl Radical.

The synthetic application of formyl radicals and other masked formyl radicals is constrained by their weak electrophilicity (ω = 1.17 eV for the formyl radical, ω = 0.58 eV for the •CH(OEt)2 radical), which often results in a mismatched reactivity with unactivated alkenes. Herein, we develop two novel formylating reagents that overcome this limitation by generating an ambiphilic masked formyl radical, enabling the masked formylation of both activated and unactivated alkenes. The reaction initiates from the N─O bond homolysis of (benzoyloxy)-(methylsulfonyl)methyl oxime ester reagents under the photocatalytic energy-transfer strategy, releasing CO2 to form a persistent iminyl radical and a transient ambiphilic (benzoyloxy)-(methylsulfonyl)methyl radical as the masked formyl radical. Consequently, the aminoformylation, heteroarylformylation, and hydroformylation of alkenes can be achieved by adding the masked formyl radicals to olefins, followed by quenching of the formed carbon-radicals by iminyl radicals, heteroarenes, and solvents, respectively. The resulting masked aldehydes are readily deprotected under mild basic or acidic conditions, affording simple or functionalized free aldehydes. Density functional theory (DFT) calculations confirm that the (benzoyloxy)-(methylsulfonyl)methyl radical has superior electrophilicity (ω = 1.82 eV) compared to conventional formyl radical equivalents.