Fluorides chiral quaternary ammonium

The enantioselective addition of a nucleophile to a carbonyl group is one of the most versatile methods for C C bond formation, and this reaction is discussed in Chapter 2. Trifluoromethylation of aldehyde or achiral ketone via addition of fluorinated reagents is another means of access to fluorinated compounds. Trifluoromethyl trimethylsilane [(CF SiCFs] has been used by Pra-kash et al.87 as an efficient reagent for the trifluoromethylation of carbonyl compounds. Reaction of aldehydes or ketones with trifluoromethyltrime-thylsilane can be facilitated by tetrabutyl ammonium fluoride (TBAF). In 1994, Iseki et al.88 found that chiral quaternary ammonium fluoride 117a or 117b facilitated the above reaction in an asymmetric manner (Scheme 8-42). 

Scheme 8-42. Asymmetric trifluoromethylation of carbonyl compounds by chiral quaternary ammonium fluorides.

A. Ando, T. Miura, T. Tatematsu, T. Shioiri, Chiral Quaternary Ammonium Fluoride. A New Reagent for Catalytic Asymmetric Adol Reactions , Tetrahedron Lett. 1993, 34,1507-1510. 

K. Iseki, T. Nagai, Y. Kobayashi, Asymmetric Trifluoro-methylation of Aldehydes and Ketones with Trifluoro-methyltrimethylsilane Catalyzed by Chiral Quaternary Ammonium Fluorides , Tetrahedron Lett. 1994, 35, 3137-3138. 

Chiral Quaternary Ammonium Fluorides for Asymmetric Synthesis... 

In-Situ Generation of Chiral Quaternary Ammonium Fluorides... 

In 1978, Wynberg and coworkers reported the first example of a chiral quaternary ammonium fluoride-catalyzed Michael addition of nitromethane to chalcone (Scheme 9.1) [3]. The reaction was performed in toluene at 20 °C with 10mol% of chiral ammonium salt 1 or 3a and excess potassium fluoride (KF, 15 equiv.), yielding the y-nitro ketone with 10-23% enantiomeric excess (ee). The requisite chiral... 

The usefulness of the present system was then demonstrated by its application to the in-situ generation of structurally rigid, C2-symmetric chiral quaternary ammonium fluorides of type 6 (X = F) from the corresponding hydrogen sulfate 6 (X = H S04), and their direct use for the asymmetric aldol reactions (Scheme 9.3). For instance,... 

Further application of the in-situ generation of chiral quaternary ammonium fluorides from the corresponding hydrogen sulfates has also been shown in the facile preparation of optically active esters via the alkylative kinetic resolution of secondary alkyl halides. For example, simple stirring of the mixture of 3-phenylpropionic acid, l-(l-bromoethyl)naphthalene, (S,S)-6b (X = HS04 2 mol%) and KF-2H20 (5 equiv.)... 

Table 9.1 Asymmetric Mukaiyama-type aldol reactions of a glycine derivative catalyzed by in situ-generated chiral quaternary ammonium fluoride.

Chiral Quaternary Ammonium Fluorides Preparation and Application to Organocatalytic Asymmetric Reactions... 

Corey and Zhang utilized chiral quaternary ammonium fluoride 4d possessing a 9-anthracenylmethyl group on nitrogen for the face-selective nitroaldol reaction of nitromethane with protected (S)-phenylalaninal. This was directed toward the... 

Currently, it is fair to say that asymmetric synthesis using chiral quaternary ammonium fluorides remains an underdeveloped field, and the various useful stereoselective carbon-carbon bond-forming processes described in this chapter are only the start of an exploration of the vast synthetic potential of this process, particularly when combined with current knowledge of organosilicon chemistry. It seems that the key issue to be addressed is the rational molecular design of chiral quaternary ammonium cations with appropriate steric and electronic properties. These are expected to be readily tunable to impart not only a sufficient reactivity but also an ideal chiral environment to the requisite nucleophile involved in a desired chemical transformation. Clearly, the continuous accumulation of information related to the structure of fluoride salts and their reactivity and selectivity should create a solid basis for this field, offering - in time - a unique yet reliable tool for sophisticated bond construction events with rigorous stereocontrol, under mild conditions. 

Homogeneous Catalysis with Chiral Quaternary Ammonium Fluorides 123... 

Worth mentioning are chiral gold complexes [20d, e] as well as chiral quaternary ammonium fluorides [21], which are used successfully as catalysts in the asymmetric aldol reaction. 

Palomo C, Oiarbide M, Garcia JM. Current progress in the asymmetric aldol addition reaction. Chem. Soc. Rev. 2004 34 3-15. Ooi T, Maruoka K. Asymmetrix organocatalysis of structurally well-defined chiral quaternary ammonium fluorides. Acc. Chem. Res. 2004 37 526-533. 

Reviews on phase-transfer catalysis for the syntheses of a-amino acids, (a) Ooi, T. and Maruoka, K. (2004) Asymmetric organocatalysis of structurally well-defined chiral quaternary ammonium fluorides. Acc. Chem. Res., 37, 526-533 (b) Maruoka, K. and Ooi, T. (2003) Enantioselective amino acid synthesis by chiral phase-transfer catalysis. Chem. Rev., 103, 3013-3028 (c) Ooi, T. and Maruoka, K. (2003) Enantioselective synthesis of a-amino acids by chiral phase-transfer catalysis. Yuki Gosei Kagaku Kyokaishi (J. Synth. Org. Chem.) 61, 1195-1206. 

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