Ferrocenes undergo one-step carboxamidation by reaction with isocyanates in CF3SO3H solution. The chemistry is most efficient in excess superacid, and it has been accomplished with aryl and aliphatic isocyanates. In conversions with ferrocene carboxylic acids, isocyanates provide imides in good yields. A mechanism for this conversion is suggested involving carbamic acid anhydride formation and subsequent intramolecular reaction at the substituted cyclopentadienyl ring.

Michael addition has been achieved with a variety of amino acid esters and 2- or 4-vinylpyridine. Similar reactions were accomplished with an alkenyl-substituted pyrimidine, pyrazine, thiazole, quinoxaline, benzoxazole, and quinolone. In reactions at a prochiral center, modest diastereoselectivities were observed with the formation of the new stereogenic carbon. NMR experiments indicate that the addition reaction is reversible under acidic conditions.

A superacid-promoted method for the synthesis of 9,9-diarylfluorenes is described. The chemistry involves cyclizations and arylations with biphenyl-substituted heterocyclic ketones and a mechanism is proposed involving superelectrophilic intermediates. The key reactive intermediates–dicationic and trication fluorenyl cations have been observed by low-temperature NMR and the mechanism has been further studied using DFT calculations.

AbstractA series of 9-fluorenyl cations has been studied and it is shown that increasing charge on a heterocyclic substituent group enhances the anti-aromatic character of the carbocation system. Similarly, a series of dibenzosuberenyl cations has been studied and increasing charge on a substituent group is shown to enhance aromatic character in the carbocation system. These studies include the direct observations of dicationic and tricationic species using stable-ion conditions and low temperature NMR. The structures of these ions were further characterized using DFT calculations, confirming that highly charged organic ions may exhibit unusual distributions of π-electrons and delocalization of electrons in 4n or 4n+2 π-systems.

•A one step procedure has been developed for the synthesis of polychlorinated dibenzofurans.•The chemistry involves a superacid-promoted condensation reaction of o-chloranil and arenes.•An unusual carbocation cyclization is proposed for the conversion.Polychlorinated dibenzofurans have been prepared in one step by the condensations of o-chloranil with arenes in triflic acid. A mechanism is proposed involving formation of a monoprotonated quinone (carboxonium ion), electrophilic attack at the arene, and cyclization of a carbocation intermediate. The chemistry is further examined by spectroscopic and theoretical studies.

•Flash vacuum pyrolysis is used to generate two thiophene-based p-quinodimethanes linked by a tether.•A gas-phase cyclization occurs to give C15 macrocycles.•Product formation is thought to occur via a diradical mechanism.•The thiophene-containing macrocycles may be reduced to the saturated macrocycle by Raney nickel.Bis-2,5-dimethylene-2,5-dihydrothiophenes have been generated in the gas-phase by flash vacuum pyrolysis (FVP) of diester precursors. These thiophene-based bis-(p-quinodimethanes) are shown to undergo reactions leading to macrocycles. The conversions are consistent with a mechanism involving cyclic diradical intermediates followed by disproportionation of the radical centers.

A tetracationic electrophile has been generated in superacid and shown to undergo an arylation reaction with benzene. A cyclization product is also obtained in the absence of benzene, presumably from a tricationic intermediate. Using low-temperature NMR, the tetracationic species is directly observed from a FSO3H-SbF5-SO2ClF solution. Similar chemistry is described with a system involving penta- and tetracationic intermediates. These highly ionized structures and their chemistry were also examined by DFT calculation.

The intramolecular reactions of olefinic N-heterocycles have been studied. In triflic acid-promoted reactions, conjugate addition is observed with pyrazine-, 2-pyrimidine-, and 2-quinoxaline-based olefins and a phenyl group nucleophile. Markovnikov addition is observed with pyridine and 5-quinoxaline-based olefins. These results are in accordance with previous observations relating the type of addition—conjugate or Markovnikov—to the positions of olefinic substituents of the N-heterocycle.

In this Note, we describe superacid-promoted cyclodehydrations leading to functionalized indenes. The product indenes are synthesized having N-heterocyclic substituents, including pyridyl, imidazolyl, pyrimdyl, and other groups. A mechanism is proposed involving dicationic, superelectrophilic intermediates. The protonated N-heterocyclic rings are shown to dramatically lower the LUMO energy level of the carboxonium electrophile (compared to that of a similar uncharged system).

A series of pyridyl-substituted 3-hydroxy-2-oxyindoles have been prepared and reacted with arenes in superacid promoted Friedel–Crafts reactions. The product aryl-substituted 2-oxyindoles are formed in generally good yields. With substituted arenes such as toluene, bromobenzene, or ethyl salicylate, the Friedel–Crafts reactions occur with excellent regioselectivity. A mechanism is proposed involving dicationic, superelectrophilic species leading to the electrophilic aromatic substitution chemistry.

Electrophilic ring opening of trans-2-phenylcyclopropylamine·HCl occurs at the distal (C2–C3) bond. This is consistent with weakening of the distal bond by the σ-withdrawing ammonium group and charge–charge repulsive effects in the transition state.

Despite the relatively low reactivities of urea and thiourea functional groups towards nucleophilic attack, we have found conditions in which they are useful substrates in Friedel–Crafts reactions. The Brønsted superacid, triflic acid, promotes these reactions and a mechanism is proposed involving dicationic, superelectrophilic intermediates.

Cyanoguanidine is an inexpensive commodity chemical and it is found to be a useful reagent for the direct Friedel–Crafts carboxamidation of arenes. The reaction works best in an excess of Brønsted superacid, an observation suggesting the involvement of a superelectrophilic intermediate. Theoretical calculations indicate that the most stable diprotonated species involves protonation at the guanidine and cyano nitrogen atoms.a

The hydroxyalkylation reaction has been used to condense benzocrown ethers with various aldehydes and ketones. The condensation reactions are catalyzed by triflic or sulfuric acid. The products from the reactions are bis(benzocrown ethers) and they are formed in good yields (42–98%, 13 examples).

Friedel–Crafts acylation has been known since the 1870s, and it is an important organic synthetic reaction leading to aromatic ketone products. Friedel–Crafts acylation is usually performed with carboxylic acid chlorides or anhydrides, while amides are generally not useful substrates in these reactions. Despite being the least reactive carboxylic acid derivative, we have found a series of amides capable of providing aromatic ketones in good yields (55–96%, 17 examples). We propose a mechanism involving diminished C–N resonance through superelectrophilic activation and subsequent cleavage to acyl cations.

The superacid-promoted reactions of vinyl-substituted N-heterocycles have been studied. Diprotonated pyrimidines, quinoxalines, and quinazolines exhibit an unusual regioelectronic effect that controls the type of addition reaction observed. Depending on the ring position of the vinyl substituent, either conjugate addition or Markovnikov addition occurs. The mode of addition has been shown to correlate well to NBO calculated charges.

This Article presents the results from studies related to the chemistry of tricationic superelectrophiles. A series of triaryl methanols were ionized in Brønsted superacids, and the corresponding tricationic intermediates were formed. The trications are found to participate in two types of reactions; both are characteristic of highly charged organic cations. One set of reactions occurs through charge migration. A second set of reactions occurs through deprotonation of an unusually acidic site on the tricationic species. One of the tricationic intermediates has been directly observed by low temperature NMR spectroscopy. These highly charged ions and their reactions have also been studied using density functional theory calculations. As a result of charge migration, electron density at a carbocation site is found to increase with progression from monocationic to pentacationic structures.

A series of trifluoromethyl-substituted arenes were studied in their reactions with Brønsted superacids. The products from these reactions suggest the formation of reactive electrophiles, such as carbocations, acylium cations or equivalent electrophilic species. As such, Friedel–Crafts-type reactions occur between these species and arene nucleophiles. NMR studies were done, and the results suggest the formation of an acyl group from the trifluoromethyl groups in the superacid.

Benzamides with tethered acetal groups undergo reactions in CF3SO3H to give ring-fused isoindolinones by a cyclization cascade. The reaction initially forms an N-acyliminium ion which then gives the isoindolinone by the aza-Nazarov reaction. An unusual variant also cyclizes at the allylic position.

Fluoro-substituted aliphatic nitriles are shown to undergo the Houben–Hoesch reactions with arenes in CF3SO3H to give fluoro-substituted ketones in good yields. The fluorine substituents appear to enhance the reactivities of the nitriles (and the nitrilium ion intermediates) compared to similar aliphatic nitriles. Fluoro-substituted ketones are also shown to be accessible through the reactions of organometallic reagents and fluoro-substituted nitriles.

Trifluoromethyl-substituted superelectrophiles were generated in superacid (CF3SO3H), and their chemistry was examined. The strong electron-withdrawing properties of the trifluoromethyl group are found to enhance the electrophilic character at cationic sites in superelectrophiles. This leads to greater positive-charge delocalization in the superelectrophiles. These effects are manifested by the unusual chemo-, regio-, and stereoselectivities shown by the superelectrophiles in chemical reactions.

The chemistry of trifluoromethyl ketones has been studied. The work examines the condensation reactions of trifluoromethyl ketones with arenes in the superacid, including both synthetic and mechanistic aspects.Synthetic and mechanistic studies related to condensations at trifluoromethyl ketones.

Experimental and theoretical studies suggest the involvement of dicationic, superelectrophilic N-acyliminium ions in conversions catalyzed by superacids.

The hydroxyalkylation reactions of aceanthrenequinone (6) and acenapthenequinone (7) with a series of arenes have been studied. In reactions with the Brønsted superacid CF3SO3H (triflic acid), the condensation products are formed in good yields (58–99%, 10 examples) with high regioselectivity. Computational studies were also done to examine the structures and energies of mono- and diprotonated species from 6 and 7. The results from the condensation reactions are consistent with the formation of superelectrophilic species involving protosolvation of carboxonium ion intermediates.The hydroxyalkylation reactions of aceanthrenequinone (6) and acenapthenequinone (7) with a series of arenes have been studied. In reactions with the Brønsted superacid CF3SO3H, the condensation products are formed in good yields (58–99%, 10 examples) with high regioselectivity. The results from the condensation reactions and also theoretical calculations are consistent with the formation of superelectrophilic species.

Experimental and theoretical studies suggest the involvement of dicationic, superelectrophilic N-acyliminium ions in conversions catalyzed by superacids.

Despite the relatively low reactivities of urea and thiourea functional groups towards nucleophilic attack, we have found conditions in which they are useful substrates in Friedel–Crafts reactions. The Brønsted superacid, triflic acid, promotes these reactions and a mechanism is proposed involving dicationic, superelectrophilic intermediates.

A series of trifluoromethyl-substituted arenes were studied in their reactions with Brønsted superacids. The products from these reactions suggest the formation of reactive electrophiles, such as carbocations, acylium cations or equivalent electrophilic species. As such, Friedel–Crafts-type reactions occur between these species and arene nucleophiles. NMR studies were done, and the results suggest the formation of an acyl group from the trifluoromethyl groups in the superacid.