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The metalation of fluorene by means of the Et2Mg–HMPA system was investigated. The reaction scarcely occurred at all when the mole ratio of HMPA: Et2Mg fell below 1:1, but it proceeded easily when the ratio was over 1:1, and a solvent-separated ion pair of magnesium fluorenide was formed. When the ratio was 2:1, the order of reaction was unity in both fluorene and Et2Mg. It was demonstrated by the kinetic and NMR studies that the active species of metalation in such a case was not Et2Mg but the Et2Mg–HMPA complex, in which two molecules of HMPA are coordinated to Et2Mg.
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Tomoi et al. (1973) studied this question.
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