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CPA is, in principle, a Brønsted acid, but the phosphoryl oxygen plays a crucial role as a basic site (Figure 2.4). The bifunctional nature of CPA and other chiral Brønsted acids is, in many cases, critical for the high enantioselectivity and reactivity. CPA is considered to be “proton‐bearing chiral counteranion.” List expanded the concept of chiral Brønsted acid to “asymmetric counteranion‐directed catalysis” (ACDC) [23]. Toste also proposed “chiral anion catalysis” [24]. The concept will be discussed in detail in Chapter 4 [25]. Metal phosphates are also employed as Lewis acid catalysts [26]. Cooperative and/or relay catalyst system of combining chiral Brønsted acids with metal‐based catalysts or photoredox catalysts have also attracted much attention lately [27].