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Unprecedented pentacoordinate chelate-type boron and aluminum intermediates (see equation) have been observed for the first time, and successfully utilized in several chelation-controlled nucleophilic and electrophilic reactions of various substrates. Hence, contrary to previous belief, boron and aluminum reagents can be now classified as chelating rather than non-chelating Lewis acids.
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Maruoka et al. (1999) studied this question.
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