Catalytic Asymmetric Synthesis. Группа авторов
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Название: Catalytic Asymmetric Synthesis
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119736417
isbn:
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2 ASYMMETRIC ACID ORGANOCATALYSIS
Takahiko Akiyama
Department of Chemistry, Gakushuin University, Tokyo, Japan
2.1. INTRODUCTION
The advent of chiral Brønsted acids in the early 2000s followed extensive studies on chiral Lewis acid catalysts [1]. While a range of compounds can be considered chiral Brønsted acids (Figure 2.1) [2], including thiourea 1 [3], 2 [4], TADDOL (α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol) 3 [5], 1,1′‐bi‐2‐naphthol (BINOL) derivatives 4 [6], and bis‐quinoline acid salt (HQuin‐BAM) 5 [7], we will primarily focus on more acidic BINOL‐derived phosphorus‐ and sulfur‐containing organic acids, such as phosphoric acid (CPA [chiral phosphoric acid]) 6 [8, 9], dicarboxylic acid 7 [10], disulfonic acid (BINSA) 8 [11], disulfonimide (DSI) 9 [12], imidodiphosphate (IDP) 10 [13], and imidodiphosphorimidate (IDPi) 11 [13]. This chapter will provide a compilation of transformations that have been mediated by chiral Brønsted acids [15, 16], and is organized by the type of reactions starting from nucleophilic reaction, cycloaddition reaction, Michael reaction, reduction, addition to alkenes, substitution, rearrangements, and others.
2.2. FEATURES OF CHIRAL BRØNSTED ACIDS
2.2.1. Acidity of Chiral Brønsted Acids
The pK as of common chiral Brønsted acids in dimethyl sulfoxide (DMSO) have primarily been interrogated computationally. Cheng found using SMD/M06‐2x/6‐311++G(2df,2p)//B3LYP/6‐31+G(d) method that chiral phosphoric acids and carboxylic acids, derived from BINOL, were the least acidic among those studied, with pK as of 3.37 and 3.97, respectively, and that bis(sulfonic acid) 8a was the most acidic (Figure 2.2) [17]. In comparison to CPA 6, derived from BINOL, those derived from H8‐BINOL 12 [18] and SPINOL 13 [19, 20] are less acidic with pK as of 4.61 and 4.20, respectively. N‐Triflyl chiral phosphoramide 14 [21] is much more acidic with pK a –3.40. Dicarboxylic acid 7 is slightly less acidic than phosphoric acid, whereas sulfonimide 9 is more acidic. These values broadly agree with experimental values of related compounds [22]. A follow‐up study revealed that 3,3′‐substitution in chiral phosphoric acids enables modulation of acidity by nearly two orders of magnitude: from 2.63 for CPA 6b with 3,5‐(CF3)2C6H3 groups to 4.20 for CPA 6i with Ph3Si groups (Figure 2.3).
Figure 2.1. Examples of chiral Brønsted acids.
Figure 2.2. pKa values of chiral Brønsted acids in DMSO by calculation.
Source: [17].
Figure 2.3. Effect of 3,3′‐substituents of BINOL‐derived phosphoric acid on the pK a values in DMSO by calculation.
Figure 2.4. Function of chiral phosphoric acid.
2.2.2. Mode of Activation of Chiral Phosphoric Acids and Related Compounds
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 СКАЧАТЬ