Spiro-ammonium salts

In 1999, in consideration of the readily structural modifications and fine-tuning of catalysts to attain sufficient reactivity and selectivity, Maruoka and coworkers designed and prepared the structurally rigid, chiral spiro ammonium salts of type 1 derived from commercially available (S)- or (R)-1,1 -bi-2-naphthol as a new C2-symmetric chiral phase-transfer catalyst, and successfully applied this to the highly efficient, catalytic enantioselective alkylation of N-(diphenylmethylene)glycine tert-butyl ester under mild phase-transfer conditions (Scheme 5.1) [7]. 

Currently, the chiral phase-transfer catalyst category remains dominated by cinchona alkaloid-derived quaternary ammonium salts that provide impressive enantioselec-tivity for a range of asymmetric reactions (see Chapter 1 to 4). In addition, Maruoka s binaphthyl-derived spiro ammonium salt provides the best results for a variety of asymmetric reactions (see Chapters 5 and 6). Recently, some other quaternary ammonium salts, including Shibasaki s two-center catalyst, have demonstrated promising results in asymmetric syntheses (see Chapter 6), while chiral crown ethers and other organocatalysts, including TADDOL or NOBIN, have also found important places within the chiral phase-transfer catalyst list (see Chapter 8). 

Arai and Nishida described L-tartrate-derived single-center spiro ammonium salts 47 and 48 for the asymmetric Michael addition of glycine Schiff s base [42]. 

Currently, this area is not as well developed as the use of cinchona alkaloid derivatives or spiro-ammonium salts as asymmetric phase-transfer catalysts, and the key requirements for an effective catalyst are only just becoming apparent. As a result, the enantioselectivities observed using these catalysts rarely compete with those obtainable by ammonium ion-derived phase-transfer catalysts. Nevertheless, the ease with which large numbers of analogues - of Taddol, Nobin, and salen in particular- can be prepared, and the almost infinite variety for the preparation of new, chiral metal(ligand) complexes, bodes well for the future development of more enantioselective versions of these catalysts. 

The symmetrical quaternary ammonium salts 5-6 have also been shown to be effective reagents for the enantioselective alkylation of the Schiff base 3 under mild phase-transfer conditions [6]. Although results with the C2-symmetrical quaternary ammonium salt 5 were disappointing (entry 6, Table 1), the more rigid spiro ammonium salts 6 were much more effective catalysts. The rate and enantioselectivity of the benzylation of 3 was found to depend critically on the substituent R. With the ammonium salt 6c (R = / -Np), the reaction was complete within 30 min at 0 °C (entry 9, Table 1) and gave the amino acid derivative 4 with 95% ee, whereas the unsubstituted catalyst 6a (R = H) required a longer reaction time and gave 4 in only 79% ee (entry 7, Table 1). 

Fig. 4.1 The structurally rigid, chiral spiro ammonium salts.

When the optically active diazepinium s t (151) was treated with phenyllithium, only the (S)-amine (152) was obtained. However, the (S)-amine (152) consists of an equilibrating mixture of (+) and (-) dia-stereomeric amines which interconvert by mutarotation upon heating. Similarly, Brewster demonstrated that the pure (/ )-isomer of the spiro ammonium salt (153) was transformed into the alkaloid (155) having an (S) absolute configuration (Scheme 32). ... 

The same kind of associative event lies at the heart of the catalytic asymmetric epoxidation of enones using the interesting binaphthyl derived spiro ammonium salt 33, which serves as a phase transfer catalyst as well as chiral auxiliary. Using sodium hypochlorite in a biphasic system, this catalyst mediates the high-yielding epoxidation of a variety of electron-deficient trisubstituted and trara-disubstituted olefins with excellent enantioselectivity, as represented by the conversion of enone 34 to the corresponding epoxy ketone 35 <04JA6844>. 

Ooi and Maruoka developed an efficient phase transfer catalyst (46a-e), which consisted of chiral N-spiro ammonium salts with binaphthalene skeleton. 3,3 -(3,4,5-Trifluorophenyl)ammonium salt (46e) provided a perfect stereoselection in benzylation of benzophenone Schiffbase of glycine terf-butyl ester (47) (Scheme 5.13, Table 5.5) [19]. The perfect stereoselective alkylation is applicable for a variety of alkyl bromides in the presence of 1 mol% of the catalyst (46e). Not only monoalkylation but also the consecutive double alkylation of 49 was successful to give 50 in excellent enantioselectivities (Scheme 5.14) [20]. The protocol is useful for the enantioselective aldol reaction of 47 with aldehyde (51) [21] and a-imino ester [22], in which catalysts (46f) and (46g) were effective (Scheme 5.15) [23]. 

Scheme 5.16 Enantioselective Michael reactions catalyzed by A -spiro ammonium salts 106a and 106b.

On the other hand, axially chiral A-spiro ammonium salt 106d has been found to catalyze the Michael reaction of a serine nucleophile equivalent with ethyl... 

The introduction of a new catalyst system by Maruoka and coworkers using C2-symmetric binaphthyl-based chiral spiro ammonium salts 6 in 1999, paved the way for a new era in asymmetric phase-transfer catalysis. This PTC system was found to be highly effective for a variety of asymmetric transformations (e.g., Michael additions, a-amino acid syntheses, epoxidations. 

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