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Radical Substitution at Boron in Small Cobaltacarboranes. Conversion of η5-C5Me5 to η4-C5Me5R Complexes1

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journal contribution
posted on 2002-11-05, 00:00 authored by J. Monte Russell, Michal Sabat, Russell N. Grimes
As an alternative approach to metal-promoted cross-coupling reactions for the preparation of B-substituted organo derivatives of small cobaltacarboranes, synthesis via radical substitution reactions has been explored. Reduction of neutral Cp*CoIII(2,3-Et2C2B4H3-5-I) (1) and Cp*CoIII(2,3-Et2C2B4H4) (4) to generate the respective 19-electron anionic Co(II) complexes 1- and 4-, followed by radical reactions with nucleophiles and electrophiles, led to substitution at boron and/or the Cp* (η5-C5Me5) ligand. Reaction of 1 with Rieke Mg* in THF formed Cp*Co(Et2C2B4H3)-5-O(CH2)4-(η4-C5Me5)CoH(Et2C2B4H3-5-I) (2), whose two cobaltacarborane units are linked through a tetramethyleneoxy linking group. In contrast, 1- and 4- reacted with MeOSO2CF3 to afford (η4-C5Me6)CoH(Et2C2B4H3-5-I) (3) or (η4-C5Me6)CoH(Et2C2B4H3-5-Me) (5) and (η4-C5Me6)CoH(Et2C2B4H4) (6). The somewhat reactive 1- was stabilized with the introduction of 18-crown-6 to form the paramagnetic salt K(18-crown-6)+Cp*CoII(Et2C2B4H3-5-I)- (10). Treatment of 1 and 10 with the nucleophile LiNMe2 in the presence of K(Hg) gave respectively Cp*Co(Et2C2B4H3-5-NMe2) (8) and Cp*Co(Et2C2B4H3-7-NMe2) (11). Compound 8 was decapped on exposure to air to give Cp*Co(Et2C2B3H4-5-NMe2) (9); its apically substituted isomer 11 cannot undergo deborylation. A tert-butoxide group was introduced at B(5) in moderate yield by reaction of K O-t-Bu with 1-, affording 12. Treatment of 12 with acidified methanol produced the B(5)−OH derivative 14. X-ray diffraction analyses confirmed the structures of 2, 10, and 11.

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