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Size Matters: Room Temperature P−C Bond Formation Through C−H Activation in m-Terphenyldiiodophosphines

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posted on 07.04.2008, 00:00 by Armando A. Diaz, Bryan Buster, Daniel Schomisch, Masood A. Khan, J. Clayton Baum, Rudolf J. Wehmschulte
m-Terphenyl- and biphenyl-2-diiodophosphines, TerphPI2 and BiphPI2, have been obtained by halide exchange from the chloro derivatives TerphPCl2 and BiphPCl2 and excess LiI in a benzene solution at room temperature. Whereas BiphPI2 compounds are stable, the TerphPI2 species undergo intramolecular C−H activation at room temperature and cyclize to form unsymmetrical 9-iodo-9-phosphafluorenes 1-(3,5-dimethylphenyl)-6,8-dimethyl-9-iodo-9-phosphafluorene, 4; 1-(4-t-butylphenyl)-7-t-butyl-9-iodo-9-phosphafluorene, 5; and 1-(2-methylphenyl)-5-methyl-9-iodo-9-phosphafluorene, 6, albeit the latter reaction is rather slow due to unfavorable steric interactions. Cyclization of the alkyl-substituted 4,4′-di-tert-butyl-biphenyl-2-diiodophosphine, 11, is slow in refluxing benzene solution, but faster than that for the parent biphenyl-2-diiodophosphine. Ab initio and density functional theory calculations are in agreement with an electrophilic aromatic substitution mechanism that is facilitated by steric strains in the terphenyl compounds 2,6-(3,5-Me2C6H3)2C6H3PI2, 1; 2,6-(4-t-BuC6H4)2C6H3PI2, 2; and 2,6-(2-MeC6H4)2C6H3PI2, 3. All new compounds have been characterized by multinuclear NMR spectroscopy and direct analysis in real time mass spectrometry. 9-Iodo-9-phosphafluorene, 12, was also analyzed by X-ray diffraction.