Pyrroles fluoroalkylation

The synthetic paths of fluorinated porphyrins (fluorine atoms or fluoroalkyl chains) are numerous and depend on the fluorination site (periphery or meso position). These syntheses are performed through the use of fluoroalkylated pyrroles, through DAST fluorination, or through direct perfluoroalkylation. Some examples are given next. 

The oldest methods for synthesis involve the condensation-cyclization of trifluor-omethyl or fluoroalkyl pyrroles in the presence of a metal salt. These reactions afford tetrakis(fluoroalkyl) porphyrins. The electrophilic trifluoromethylation of porphyrins is selective and leads to /I-CF3 and meso-CVT, porphyrins. While condensation of meio-trifluoromethyl-dipyrromethane with an aldehyde in acidic medium is rather difficult, it proceeds with better yields and permits a selective introduction of trifluoromethyl groups in meso The Ruppert reagent (CF3TMS) has been used to... 

Pyrroles, furans, and thiophenes react preferentially with radicals at the 2-position. Thus, reaction of pyrrole with benzyl radicals gives 2-benzylpyrrole. In rather better yields, pyrrole and indole treated with per(poly)fluoroalkyl chlorides (RfCI) in the presence of sodium dithionite in DMSO produce 2-perfluoroalkyl products <2001JFC(111) 107>. Furans trap aryl radicals, generated from the Mn(OAc)3 oxidation of arylboronic acids <2003JOC578> or... 

Only 2-fluoroalkyl derivatives 573 were obtained when pyrrole and indole were treated with per(poly)fluoroalkyl chlorides (RpCl) in the presence of sodium dithionite in DMSO (Equation 137) <2001JEC(111)107>. 

Since a review on Dewar heterocyclic compounds has been published recently102), in this chapter only some typical reactions of valence-bond isomers of heteroaromatic compounds will be discussed. The emphasis will be placed on the chemistry of trifluoromethylated Dewar thiophene and pyrroles, both of which are rare examples of stable valence-bond isomers of heterocyclic compounds. Since the isolation of valence-bond isomers of heteroaromatic compounds is mostly limited to the per-fluoroalkylated isomers, those postulated as intermediates are also included. 

Akiyama et al. [33] were the first to introduce chiral phosphoric acids to the FCA reaction of nitroalkenes, and they found that the addition of 3-A molecular sieves dramatically improved both the reactivity and selectivity of the reaction between (3-nitrostryrene and indole (Scheme 9.13). A catalytic working model was proposed that phosphoric acid performed as a bifunctional catalyst to activate both indole and nitroalkene. This methodology was further extended to the reactions with pyrroles as nucleophiles [34] and fluoroalkylated nitroalkenes as electrophiles [35]. 

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