| Budget Amount *help |
¥15,800,000 (Direct Cost: ¥15,800,000)
Fiscal Year 2005: ¥7,600,000 (Direct Cost: ¥7,600,000)
Fiscal Year 2004: ¥8,200,000 (Direct Cost: ¥8,200,000)
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| Research Abstract |
Palladium and nickel catalysts play a key role in modem organic synthesis. Cross-coupling reaction and Mizoroki-Heck reaction are among the most important carbon-carbon bond formation reactions. Normally aryl and vinyl halides are the choice of the substrates, since the use of alkyl halides having hydrogen at the β-position to the halide atom suffers from β-hydride elimination unless intensive screening of reaction conditions was performed. During the course of our study on transition-metal-catalysed reaction, readily available cobalt complexes were found to act as catalysts complementary to palladium and nickel in cross- coupling and Mizoroki-Heck reactions. The cobalt-catalysed reactions probably proceed via carbon- centred radicals as key intermediates that are generated by single electron transfer from electron-rich cobalt complexes to alkyl halides. The radicals enable fascinating transformations that conventional palladium and nickel can not catalyse.
Cross-coupling reactions of p
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henyl Grignard reagent with alkyl halides are rare. A cobalt complex was found to catalyse cross-coupling reaction of 6-halo-1-hexene derivatives with phenyl Grignard reagent, wherein radical cyclisation is involved prior to the cross-coupling. Treatment of bromo acetal with phenyl Grignard reagent in the presence of [CoCl_2(dppe)] yielded benzyl-substituted cyclic acetal in good yield. Cyclic acetals are useful building blocks of a variety of tetrahydrofuran derivatives. Not only oxacycle but also azacycle and carbocycle have become readily available. Other aromatic Grignard reagents such as 2-thienylmagnesium bromide could be employed. Intriguingly, DPPE is the choice of ligand, and other bidentate ligands such as DPPM, DPPP, DPPF, and triphenylphosphine considerably diminished the yield.
Trimethylsilylmethylmagnesium chloride was added to a mixture of styrene and bromocyclohexane in ether in the presence of [CoCl_2(dpph)] (DPPH = 1,6-bis(diphenylphosphino)hexane). The reaction mixture was heated at reflux to provide β-cyclohexylstyrene in 91% yield. Primary, secondary, and tertiary alkyl halides all participated in the reaction. It is worth noting that alkyl chlorides, which are usually less reactive in transition-metal-catalysed reactions, are good alkyl sources. Less
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