Catalytic C−C Coupling via Transfer Hydrogenation: Reverse Prenylation, Crotylation, and Allylation from the Alcohol or Aldehyde Oxidation Level

We report byproduct-free carbonyl reverse prenylation, crotylation, and allylation from the alcohol oxidation state via alcohol−allene hydrogen autotransfer. Specifically, exposure of alcohols 1a−6a to 1,1-dimethylallene, methylallene, and allene in the presence of [Ir(cod)(BIPHEP)]BARF (5−7.5 mol %) delivers reverse prenylation products 1c−6c, crotylation products 1d−3d, and allylation product 1e. Similarly, under the conditions of transfer hydrogenation employing isopropanol as terminal reductant, aldehydes 1b−6b are converted to the very same adducts 1c−6c, 1d−3d, and 1e. Isotopic labeling studies corroborate a mechanism involving hydrogen donation from the reactant alcohol or sacrificial alcohol (i-PrOH). The ability to achieve carbonyl addition directly from the alcohol oxidation level circumvents the redox manipulations so often required to convert alcohols to aldehydes. Further, through hydrogen autotransfer, there resides the potential to develop myriad byproduct-free carbonyl additions wherein alcohols and π-unsaturated compounds are exploited as coupling partners.