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Mechanism of the Os-Catalyzed Oxidative Cyclization of 1,5-Dienes.

The oxidative cyclization of 1,5-dienes by metal oxo species is a powerful method for stereocon-trolled synthesis of tetrahydrofuran diols (THF-diols), structural motifs present in many bioactive natural products. Oxidative cyclization of (2E,6E)-octa-2,6-diene catalyzed by OsO4/NMO as has been studied using density functional theory (DFT) cal-culations (M06-2X/aug-cc-pVDZ/Hay-Wadt VDZ (n+1) ECP), highlighting the remarkable effect of acid on the fate of the first intermediate, an Os(VI) dioxoglycolate. Strong acid promotes cyclization of the Os(VI) dioxoglycolate, or its NMO complex, through protonation of an oxo ligand to give more electrophilic species. By contrast, in absence of acid reoxidation may occur to afford the Os(VIII) triox-oglycolate, which is shown to favor conventional "second cycle" dihydroxylation reactivity rather than cyclization. The results of the calculations are consistent with experimental results for reactions of OsO4/NMO with 1,5-dienes with acid (oxidative cyclization) and without acid (second cycle osmyla-tion / dihydroxylation). Detailed evaluation of po-tential catalytic cycles support oxidation of the cy-clized Os(IV) THF diolate intermediate to the corre-sponding Os(VI) species followed by slow hydroly-sis, and finally regeneration of OsO4.