Chiral catalytic phase transfer

Lemaire, C., Guillouet, S., Plenevaux, A., et al. (1999) The synthesis of 6-[18F]fluoro-L-dopa by chiral catalytic phase-transfer alkylation. J Labeled Comp. Radiopharm., 42, S113-S115. 

Very recently, the Siva group successfully reported a chiral multisite phase-transfer catalytic Michael addition by the use of 2,4,6-(triscincho-niummethyl)phenyl-l,3,5-triazines as new polymeric chiral quaternary ammonium PTC catalysts. Catalyst 12c showed a higher catalytic efficiency than the corresponding monomeric catalyst 8q in terms of chemical yield and enantioselectivity due to the trimeric reaction sites of 12c, which can promote efficient ion-pair interactions between the a-carbon of the diethyl-malonate and trimeric catalysts due to steric factors (Scheme 16.22). ... 

Catalytic asymmetric synthesis with participation and formation of heterocycles (including asymmetric phase transfer reactions and asymmetric reactions with chiral Lewis catalysts) 93MI1. 

Early work on the use of chiral phase-transfer catalysis in asymmetric Darzens reactions was conducted independently by the groups of Wynberg [38] and Co-lonna [39], but the observed asymmetric induction was low. More recently Toke s group has used catalytic chiral aza crown ethers in Darzens reactions [40-42], but again only low to moderate enantioselectivities resulted. 

Chiral thioureas have been synthesized and used as ligands for the asymmetric hydroformylation of styrene catalyzed by rhodium(I) complexes. The best results were obtained with /V-phenyl-TV -OS )-(l-phenylethyl)thiourea associated with a cationic rhodium(I) precursor, and asymmetric induction of 40% was then achieved.387,388 Chiral polyether-phosphite ligands derived from (5)-binaphthol were prepared and combined with [Rh(cod)2]BF4. These systems showed high activity, chemo- and regio-selectivity for the catalytic enantioselective hydroformylation of styrene in thermoregulated phase-transfer conditions. Ee values of up to 25% were obtained and recycling was possible without loss of enantioselectivity.389... 

An enantioselective synthesis of both (R)- and (5)-a-alkylcysteines 144 and 147 is based on the phase-transfer catalytic alkylation of fert-butyl esters of 2-phenyl-2-thiazoline-4-carboxylic acid and 2-ort/ro-biphenyl-2-thiazoline-4-carboxylic acid, 142 and 145 <06JOC8276>. Treatment of 142 and 145 with alkyl halides and potassium hydroxide in the presence of chiral catalysts 140 and 141 gives the alkylated products, which are hydrolyzed to (R)- and (S)-a-alkylcysteines 144 and 147, respectively, in high enantioselectivity. This method may have potential for the practical synthesis of chiral a-alkylcysteines. 

Arai et al.51 reported that by using a catalytic amount of chiral quaternary ammonium salt as a phase transfer catalyst, a catalytic cycle was established in asymmetric HWE reactions in the presence of an inorganic base. Although catalytic turnover and enantiomeric excess for this reaction are not high, this is one of the first cases of an asymmetric HWE reaction proceeding in a catalytic manner (Scheme 8-20). 

Abstract Phase transfer catalysts including onium salts or crown ethers transfer between heterogeneous different phases and catalytically mediate desired reactions. Chiral non-racemic phase transfer catalysts are useful for reactions producing new stereogenic centers, giving chiral non-racemic products. Recent developments in this rapid expanding area will be presented. 

E. J. Corey, F. Xu, M. C. Noe, A Rational Approach to Catalytic Enantioselective Enolate Alkylation Using a Structurally Rigidified and Defined Chiral Quaternary Ammonium Salt under Phase Transfer Conditions , J. Am. Chem. Soc, 1997,119,12414-12415. 

Shioiri and co-workers developed a catalytic asymmetric synthesis of allenes by isomerization of the alkyne 240 to allene 242 under the control of a chiral phase-transfer catalyst 241 (Scheme 4.62) [98], Although the enantiomeric excess is not high (35% ee), this is the first example of the asymmetric isomerization of alkynes under phase-transfer catalyzed conditions. 

Acyloxy-l-cyanoalkanes [45, 46], which can be used as precursors for ketones [47], a-hydroxy ketones [48] and 1,4-dicarbonyl compounds [47], are prepared in one pot from the appropriate aldehyde, sodium or potassium cyanide, and the acylating agent under phase-transfer catalytic conditions [47-49]. Attempts to synthesize chiral cyanhydrins using chiral phase-transfer catalysts have been unsuccessful (see Section 12.3). 

Attempts to produce chiral cyanhydrins under phase-transfer catalytic conditions (3.3.9) using ephedrinium or cinchoninium catalysts has been singularly unsuccessful [21,22]. Optical purities varying from 0 to 60% have been recorded [22], but verification of the reproducibility of the higher values is needed. Similarly, nucleophilic attack on a carbonyl group by the trichloromethyl anion under phase-transfer catalytic conditions (see Section 7.4) in the presence of benzylquininium chloride produces a chiral product, but only with an enantiomeric excess of 5.7% [23]. The veracity of this observation has also been questioned [24],... 

Asymmetric induction has been noted [64] when ethyl glycine, protected as its imine by (S)-menthone, is allowed to react with ethyl acrylate under phase-transfer catalytic conditions using tetra-n-butylammonium bromide. An overall yield of 43% was achieved with 46% ee. The stereoselectivity of the reaction was not enhanced when A-benzylquininium or cinchoninium chloride were used and, unlike reactions catalysed by chiral catalysts, the enantiomeric excess increased, when a more polar solvent was used. 

Stereoselective ring cleavage and monoesterification of chiral Meldrum s acid derivatives has been achieved in high yield with a 34% enantiomeric excess under phase-transfer catalytic conditions in the presence of A-benzylquininium chloride [29]. A similar asymmetric ring-opening of prochiral (meso) acid anhydrides with... 

Catalytic asymmetric methylation of 6,7-dichloro-5-methoxy-2-phenyl-l-indanone with methyl chloride in 50% sodium hydroxide/toluene using M-(p-trifluoro-methylbenzyDcinchoninium bromide as chiral phase transfer catalyst produces (S)-(+)-6,7-dichloro-5-methoxy-2-methyl-2--phenyl-l-indanone in 94% ee and 95% yield. Under similar conditions, via an asymmetric modification of the Robinson annulation enqploying 1,3-dichloro-2-butene (Wichterle reagent) as a methyl vinyl ketone surrogate, 6,7 dichloro-5-methoxy 2-propyl-l-indanone is alkylated to (S)-(+)-6,7-dichloro-2-(3-chloro-2-butenyl)-2,3 dihydroxy-5-methoxy-2-propyl-l-inden-l-one in 92% ee and 99% yield. Kinetic and mechanistic studies provide evidence for an intermediate dimeric catalyst species and subsequent formation of a tight ion pair between catalyst and substrate. 

There are only a few reports on chiral phase transfer mediated alkylations". This approach, which seems to offer excellent opportunities for simple asymmetric procedures, has been demonstrated in the catalytic, enantioselective alkylation of racemic 6,7-dichloro-5-methoxy-2-phenyl-l-indanone (1) to form ( + )-indacrinone (4)100. /V-[4-(tnfluoromethyl)phenylmethyl]cinchoninium bromide (2) is one of the most effective catalysts for this reaction. The choice of reaction variables is very important and reaction conditions have been selected which afford very high asymmetric induction (92% cc). A transition state model 3 based on ion pairing between the indanone anion and the benzylcinchoninium cation has been proposed 10°. 

Hori, K., Tamura, M., Tani, K., Nishiwaki, N., Ariga, M. and Tohda, Y. Asymmetric Epoxidation Catalyzed by Novel Azacrown Ether-type Chiral Quaternary Ammonium Salts under Phase-transfer Catalytic Conditions. Tetrahedron Lett. 2006, 47, 3115-3118. 

ASYMMETRIC EPOXIDATION CATALYZED BY NOVEL AZACROWN ETHER-TYPE CHIRAL QUATERNARY AMMONIUM SALTS UNDER PHASE-TRANSFER CATALYTIC CONDITIONS... 

Quaternary ammonium salts of heterocyclic compounds have been used in liquid-liquid phase-transfer syntheses. When these compounds are achiral, they show a behavior very similar to that of other quaternary ammonium salts. For example, 2-dialkylamino-l-alkylpyridinium tetrafluoroborates have been used by Tanaka and Mukayama282 in the alkylation of active methylene compounds PhCH2CN, PhCH(Et)CN, and PhCH(Me)COPh. However, comparative studies of the efficiency of the catalysts show that alkylpyridinium bromides283 or N-alkyl-Af-benzyl-piperidinium chloride284 have a smaller catalytic activity compared to tetraalkylammonium halides. McIntosh285 has described the preparation of azapropellane salts 186 as potential chiral phase transfer catalysts. 

The group of Arai and Nishida investigated the catalytic asymmetric aldol reaction between tert-butyl diazoacetate and various aldehydes under phase-transfer conditions with chiral quaternary ammonium chloride 4c as a catalyst. The reactions were found to proceed smoothly in toluene, even at —40°C, when using 50% RbOH aqueous solution as a base, giving rise to the desired aldol adducts 23 with good enantioselectivities. The resulting 23 can be stereoselectively transformed into the corresponding syn- or anti-P-hydroxy-a-amino acid derivatives (Scheme 2.20) [42],... 

In the Park-Jew group s systematic investigation, two types of catalyst - the 1,3-phenyl- and 2,7-naphthyl-based dimeric ammonium salts - were selected as an efficient skeleton of chiral PTCs for the catalytic asymmetric phase-transfer alkylation... 

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