CF3-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

"The extraordinary instability of such an "ion" accounts for many of the peculiarities of organic reactions" - Franck C. Whitmore (1932). This statement from Whitmore came in a period where carbocations began to be considered as intermediates in reactions. Ninety years later, pointing at the strong knowledge acquired from the contributions of famous organic chemists, carbocations are very well known reaction intermediates. Among them, destabilized carbocations - carbocations substituted with electron-withdrawing groups - are, however, still predestined to be transient species and sometimes considered as exotic ones. Among them, the CF3-substituted carbocations, frequently suggested to be involved in synthetic transformations but rarely considered as affordable intermediates for synthetic purposes, have long been investigated. This review highlights recent and past reports focusing on their study and potential in modern synthetic transformations.

TfOH-Promoted Reactions of TMS-Ethers of CF3-Pentenynoles with Arenes. Synthesis of CF3-Substituted Pentenynes, Indenes, and Other Carbocyclic Structures

Trimethylsilyl ethers of 1,5-diaryl-3-(trifluoromethyl)-pent-1-en-4-yn-3-oles [Ar-C≡C-C(CF₃)(OSiMe₃)-CH═CH-Ar'] in the superacid TfOH give rise to reactive conjugated CF₃-allylic-propargylic cations [Ar-C≡C-C⁺(CF₃)-CH═CH-Ar']. These species react with arenes in the presence of 1.5 equiv of TfOH forming regio- and stereoselectively E-1,1,5-triaryl-3-(trifluoromethyl)-pent-2-en-4-ynes [Ar-C≡C-C(CF₃)═CH-CHAr'(Ar″)] in good yields. In the excess of TfOH, these CF₃-pentenynes are further intramolecularly cyclized into CF₃-bicyclic dihydroanthracene derivatives ("helicopter"-like molecules). The CF₃-pentenynes may also react with arenes, as external nucleophiles, leading to CF₃-indenes. These two main reaction pathways depend on internal nucleophilicity of aryl substituents in CF₃-pentenynes and external nucleophilicity of aromatic molecules. Plausible cationic reaction mechanisms have been discussed. CF₃-bicyclic dihydroanthracene derivatives have been studied regarding their cytotoxicity and virus-inhibiting activity against influenza virus A/Puerto Rico/8/34 (H1N1) in MDCK cell line.

Generation and NMR Study of Short-Lived and Reactive Trifluoroalkyl Carbocations of the α-Halogenothiophene Series in Brønsted Superacids: Reactions of the Cations with Arenes

Protonation of oxygen in the side chain of the Me₃SiO group (followed by the elimination of Me₃SiOH) and protonation of the thiophene ring in 2-chloro(or bromo)-5-(1'-Me₃SiO-1'-trifluoromethyl-alkyl)thiophenes in Brønsted superacids (CF₃SO₃H, FSO₃H) gave rise to short-lived and reactive trifluoroalkyl carbocations of the thiophene series. These cations were studied by low-temperature NMR spectroscopy in the superacids, which shed light on their reactivity and reaction mechanisms. The cations may react with (hetero)aromatic π-nucleophiles in various directions, depending on their structures as well as the reaction temperature and time. These transformations resulted in the formation of novel fluoro-organics of the thiophene family, namely, products of arylation of both the thiophene system and its side chain, hydrodehalogenation of halothiophenes, or electrophilic "dimerization".

TfOH-promoted transformation of TMS-ethers of diarylsubstituted CF3-allyl alcohols with arenes into CF3-indanes

Novel synthesis of 1,3-diaryl-1-trifluoromethyl indanes, having predominantly a trans-configuration of aryl groups, was developed on the basis of the reaction of TMS-ethers of 2,4-diaryl-1,1,1-trifluorobut-3-en-2-ols with arenes in superacid TfOH.

HUSY zeolite-promoted reactions of trifluoromethylated propargyl alcohols with arenes: synthesis of CF3-indenes and DFT study of intermediate carbocations

Reaction of CF₃-propargyl alcohols with arenes under the action of HUSY zeolite CBV-720 at 100 °C for 1 h affords CF₃-indenes.

TfOH-Promoted Reaction of 2,4-Diaryl-1,1,1-Trifluorobut-3-yn-2-oles with Arenes: Synthesis of 1,3-Diaryl-1-CF₃-Indenes and Versatility of the Reaction Mechanisms

The TfOH-mediated reactions of 2,4-diaryl-1,1,1-trifluorobut-3-yn-2-oles (CF₃-substituted diaryl propargyl alcohols) with arenes in CH₂Cl₂ afford 1,3-diaryl-1-CF₃-indenes in yields up to 84%. This new process for synthesis of such CF₃-indenes is complete at room temperature within one hour. The synthetic potential, scope, and limitations of this reaction were illustrated by more than 70 examples. The proposed reaction mechanism invokes the formation of highly reactive CF₃-propargyl cation intermediates that can be trapped at the two mesomeric positions by the intermolecular nucleophilic attack of an arene partner with a subsequent intramolecular ring closure.

Reactions of 1,5-Diaryl-3-(trifluoromethyl)pent-1-en-4-yn-3-yl Cations with Benzene in TfOH. Synthesis of CF3-"Helicopter"-Like Molecules

Trimethylsilyl ethers of 1,5-diaryl-3-(trifluoromethyl)pent-1-en-4-yn-3-oles in superacid CF₃SO₃H (TfOH) give rise to the corresponding intermediate CF₃-pentenynyl cations. These species react with benzene to afford conjugated CF₃-pentenynes, which undergo subsequent cyclization, first, into CF₃-cycloheptadienes and, finally, into unusual CF₃-"helicopter"-like bicyclic structures.