Synergic sedation of sensitive anions : alkali-mediated zincation of cyclic ethers and ethene

Deprotonation of alkyl and vinyl carbon-hydrogen bonds for synthetic purposes is often hindered not merely by the need for an exceptionally strong base, but by the inherent instability of the resultant anion. Metalation of cyclic ethers adjacent to oxygen, for example, has invariably initiated a ring-opening decomposition pathway. Here, we show that the use of a bimetallic base can overcome such instability through a cooperative combination of zinc-carbon and sodium-oxygen bonding. Both tetrahydrofuran and tetrahydropyran reacted cleanly over days at room temperature to yield alpha-zinc-substituted products that were sufficiently stable to be isolated and crystallographically characterized. A related zincation-anion trapping strategy, with sodium replaced by potassium, induced clean deprotonation of ethene to yield a stable product. Preliminary electrophilic quenching experiments with the alpha-zinc-substituted cyclic ethers and benzoyl chloride gave satisfactory yields of the tetrahydrofuran-derived ketone but only trace amounts of the tetrahydropyran-derived ketone