Development of Radical Reactions with Zirconocene Complexes as Electron Transfer Reagents

Bis(cyclopentadienyl)zirconium chloride hydride (Schwartz reagent) proved to be an efficient radical chain carrier for radical reduction of organic halides. Treatment of 1-bromoadamantane with Cp2Zr(H)Cl in THF at 25 °C in the presence of triethylborane furnished adamantane quantitatively. Radical cyclization of 2-haloalkyl allyl ethers afforded five-membered products under the same reaction conditions. Reduction with Cp2Zr(H)Cl generated in situ from Cp2ZrCl2 and sodium bis(2-methoxyethoxy)aluminium hydride (Red-Al) also proceeded smoothly. Moreover, the reduction could function by using a catalytic amount of Cp2ZrCl2. A zirconocene–olefin complex also induced reductive radical cyclization of 2-haloalkyl allyl ethers in THF. This complex served as a single electron transfer reagent to promote the radical cyclization. Furthermore, the cyclization reaction in DME afforded 3-tetrahydrofuranylmethylzirconium efficiently.