Catalytic Asymmetric Synthesis. Группа авторов

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СКАЧАТЬ href="#uaf8360a1-c50c-51c4-92a0-23837f2da336">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].

      2.2.3. Effect of Metal Salts

      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.

2.3. NUCLEOPHILIC REACTIONS

      2.3.1. Reactions with Imines and Iminium Salts

BINOL‐derived CPA 6a is available in bulk amounts and is employed as a resolving reagent for amines [34]. In 2004, Akiyama reported a Mannich‐type reaction between aldimines bearing the 2‐hydroxyphenyl moiety on nitrogen and ketene silyl acetals catalyzed by CPA 6c, derived from (R)‐BINOL bearing 4‐nitrophenyl groups at 3,3′‐positions, which gave β‐amino‐α‐alkyl or α‐siloxy amino esters in preference of the syn isomer with 81–96% ee (Scheme 2.1a) [8]. The use of aldimines derived from 2‐hydroxyaniline is critical for the excellent enantioselectivity. Based on the theoretical study by Yamanaka, the Mannich‐type reaction is proposed to proceed through the protonation of imine with CPA followed by the nucleophilic attack via zwitterionic and nine‐membered cyclic transition state (TS‐1) [35]. Concurrently, Terada reported a direct Mannich reaction between N‐Boc aldimines and pentan‐2,4‐dione catalyzed by CPA 6j to give the adducts with 90–98% ee (Scheme 2.1b) [9].

Scheme 2.1. Mannich reaction by Akiyama (a)

      (Source: Based on [8])

      , and by Terada (b)

      (Source: Based on [9]).

Yamamoto reinvestigated the Mannich‐type reaction[8] and found that CPA 6k, bearing 2,4,6‐Me₃‐3,5‐(NO₂)₂C₆ moiety, was more effective for the Mannich‐type reaction, furnishing β‐amino esters with higher enantioselectivity in comparison with CPA 6b (Scheme 2.2a) [36]. The higher enantioselectivity (up to >99% ee) was ascribed to both steric and electronic effects from the three methyl groups. Furthermore, when phosphoramide 2a bearing 3,5‐(NO₂)‐4‐CH₃C₆H₂ groups was used, N‐phenyl aldimines were also found to be suitable substrates and use of N‐2‐hydroxyphenyl‐substituted aldimine was obviated without compromising the enantioselectivity (Scheme 2.2b).

Scheme 2.2. Mannich reaction with N‐(2‐hydroxyphenyl)‐imine (a) and with N‐phenyl imine (b).

      Lambert developed a novel chiral Brønsted acid 15, which is readily prepared from (–)‐menthol and 1,2,3,4,5‐pentacarbomethoxycyclopentadiene, and reported Mannich‐type reaction using as low as 0.01 mol% of 15 (Scheme 2.2a). A chiral anion pathway is proposed. It is noted that 15 can be synthesized inexpensively for about US$4/g because (–)‐menthol is a naturally occurring compound [37].

Ishihara developed the chiral bisammonium salt of (R)‐BINSA (1,1′‐binaphthyl‐2,2′‐disulfonic acid) 8b as a chiral Brønsted acid for the direct Mannich reaction [38]. Although CPA requires bulky 3,3′‐substituents for attaining high enantioselectivity, the BINSA ammonium salt achieved excellent enantioselectivity without 3,3′‐substituents using as low as 1 mol% of the catalyst loading (Scheme 2.3).

Scheme 2.3. Mannich reaction of 2,4‐pentandione and N‐Cbz‐imine.

List recently reported an enantioselective synthesis of unprotected β‐amino acids by the reaction between bis‐silyl ketene acetal and silylated aminomethyl ether, followed by hydrolytic workup using a confined IDPi 11a (Scheme 2.4) [39].

Scheme 2.4. Mannich reaction between bisi‐silyl ketene acetal and silylated aminomethyl ether.

      Source: Based on [39].

In 2006, List reported a Pictet‐Spengler reaction between substituted tryptamines and aldehyde catalyzed by CPA (S)‐6e, bearing 2,4,6‐(i‐Pr)₃C₆H₂ moieties at 3,3′‐positions, to furnish the corresponding tetrahydro‐β‐carbolines with 72–99% ee (Scheme 2.5a) [40]. The presence of bisethoxycarbonyl moieties is critical for the reaction. It is noted that CPA 6e is called TRIP, and is the most frequently used chiral phosphoric acid. You reported a Pictet‐Spengler reaction of indolyl СКАЧАТЬ