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In addition to CPA per se, metal phosphates function as achiral Lewis acid, and numerous kinds of enantioselective reactions were reported. Examples include Yb triphosphates by Inanaga et al. [28], lithium phosphates by Ishihara et al. [29], calcium phosphates by Masson and Zhu [30], and magnesium phosphates by Antilla [31].

Ishihara conducted a reinvestigation of the report of Terada [9] and pointed out the possible formation and contamination of BINOL‐derived phosphoric acid during purification on silica gel (Figure 2.5). They cautioned that such impurities may have a substantial influence on the catalytic activity [32]. Subsequently, Klussmann and List analyzed the impurities of CPA 6e bearing 2,4,6‐(i‐Pr)₃C₆H₂ groups at 3,3′‐positions of BINOL (TRIP) by inductively coupled plasma optical emission spectrometry (ICP‐OES) and found that TRIP contained various alkali and alkaline earth metals as major impurities. Because metal phosphates are generated during the preparation of CPA and/or the purification by silica gel chromatography, washing with hydrochloric acid is strongly recommended to obtain free phosphoric acid [33].

Figure 2.5. Metal salts of chiral phosphoric acid.