Fluorous-phase synthesis

Scheme 7.81 Fluorous phase synthesis of N,N -disubstituted hydantoins.

Scheme 7.85 Fluorous phase synthesis of imidazo[l,2-a]pyridine/pyrazine derivatives.

A fluorous phase Pummerer reaction was introduced by Procter. This report was based on the need to address problems associated with difficulties in optimization and monitoring of solid-phase processes. Good to excellent yields of heterocyclic scaffolds, mainly oxindoles, were reported after the development of a high-throughput fluorous phase synthesis strategy. 

A. Studer, S. Hadida, R. Ferritto, S.Y. Kim, P. Jeyer, P. Wipf, D.P. Curran, Fluorous synthesis — A fluorous-phase strategy for improving separation efficiency in organic synthesis. Science 275 823-826 1997. 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

Abstract Current microwave-assisted protocols for reaction on solid-phase and soluble supports are critically reviewed. The compatibility of commercially available polymer supports with the relatively harsh conditions of microwave heating and the possibilities for reaction monitoring are discussed. Instrmnentation available for microwave-assisted solid-phase chemistry is presented. This review also summarizes the recent applications of controlled microwave heating to sohd-phase and SPOT-chemistry, as well as to synthesis on soluble polymers, fluorous phases and functional ionic liquid supports. The presented examples indicate that the combination of microwave dielectric heating with solid- or soluble-polymer supported chemistry techniques provides significant enhancements both at the level of reaction rate and ease of purification compared to conventional procedures. 

Scheme 7.78 Fluorous-phase palladium-catalyzed synthesis of aryl sulfides.

Furthermore, multicomponent reactions can also be performed under fluorous-phase conditions, as shown for the Ugi four-component reaction [96], To improve the efficiency of a recently reported Ugi/de-Boc/cyclization strategy, Zhang and Tempest introduced a fluorous Boc group for amine protection and carried out the Ugi multicomponent condensation under microwave irradiation (Scheme 7.84). The desired fluorous condensation products were easily separated by fluorous solid-phase extraction (F-SPE) and deprotected by treatment with trifluoroacetic acid/tet-rahydrofuran under microwave irradiation. The resulting quinoxalinones were purified by a second F-SPE to furnish the products in excellent purity. This methodology was also applied in a benzimidazole synthesis, employing benzoic acid as a substrate. 

---