Quaternary ammonium salts, derived from Cinchona Alkaloids, have become the most privileged class of chiral PTCs.
Asymmetric phase-transfer catalysis has been recognized as a “green” alternative to many homogeneous synthetic organic transformations and is nowadays one of the major catalytic principles in the synthesis of chiral molecules.
Its operational simplicity, mild reaction conditions, environmentally friendly reaction procedure, ready availability, easy recovery / reuse and the low toxicity of these catalysts makes it applicable to large-scale synthesis of complex and multi-functionalized organic molecules in industrial processes. In particular, the use of water as a solvent, heavy transition metal-free conditions, and low energy consumption due to employing room temperature or 0 °C as the reaction temperature make phase-transfer catalysis suitable as a sustainable green chemistry.
Catalytic asymmetric epoxidation is for instance one of the most important chemical conversions used to synthesize numerous valuable chiral intermediates in the pharmaceutical, agrochemical and fine chemical industry.
The same catalyst can be used for at least 10 reaction cycles without any loss of enantioselectivity or loss of significant yield. The reaction is applicable to a wide range of substrates.
Key Features of Chiral Phase Transfer Catalysts derived from Cinchona Alkaloids
- enable reactions between reagents in two immiscible phases such as e.g. water/toluene
- achieve highly enantioselective transformations under mild phase-transfer conditions
- can be used for variety of C─C, C─O and C─N bond forming reactions
- are available as pseudoenantiomers providing the required (R) or (S) stereoisomer
- simple experimental procedure, applicable to large scale industrial processes
- enhanced reactivity of the anion in organic phase and increased reaction rates
- green chemistry: non-hazardous solvents, simple reagents, mild temperature conditions, Cinchona PTC recyclable
- use of simple and inexpensive reactants such as NaOH instead of NaH, t-BuOK, etc.
- Cinchona PTCs are relatively inexpensive and environmentally friendly reagents
Mono- and Biscation Phase Transfer Catalysts (PTCs)
Excellent enantioselectivities can be obtained for a broad variety of reactions using the chiral Cinchona alkaloid backbone.
Stereoselective C–C bond-forming reactions and asymmetric α-heterofunctionalization of prochiral nucleophiles (i.e. enolates) became one of the privileged application fields for this catalysis concept. Multiple examples can be found in our Chiral Catalyst Search Engine:
Apart from these applications quaternary N-Benzyl mono-cation catalysts are also excellent resolving agents for the separation of BINOLS and their derivatives.
Novel bis-quaternary Cinchona Alkaloid PTCs have been synthesized by Buchler providing a more active and efficient process. Compared to mono-quaternary catalysts, often less catalyst loading, higher efficiency rates and asymmetric products in higher enantiomeric excess are obtained.
Please do not hesitate to contact us if you will require a special modified chiral PTC for your asymmetric application. Sixteen slightly different substituted Cinchona Alkaloid starting materials are available.