(Trifluoromethyl)sulfonyl group (Tf=CF₃SO₂) is one of the strongest electron-withdrawing substituents. A number of strongly acidic compounds bearing this substituent such as TfOH and Tf₂NH have been reported. During a past decade, structurally similar carbon(C-H)acids “Tf₂CHR” have attracted much attention as a new class of superacidic molecules, which show stronger acidity than sulfuric acid molecule, owning to excellent performance as acid catalysts in organic synthesis. Needless to say, development of such novel acids implies creation of highly stable anionic species as well. In recent years, the authors have been engaged in chemical research focusing on highly stabilised carbanions [Tf₂CR]^-. Here, the carbanion moiety is effectively stabilised by two Tf groups through electron-withdrawing inductive effect and negative hyperconjugation. Our effort revealed chemically inert, less coordinating, and lipophilic properties of this anion. In this account, we describe our achievements in the [Tf₂CR]^- chemistry including synthesis, structure, reactivity, and functions.

Immobilization of transition-metal catalysts on solid supports has practical benefits such as catalyst separation and reuse. However, in many cases, their catalytic activities were decreased compared to the homogeneous counterparts due to unfavorable interactions between the active site and the solid surface. To produce highly active heterogeneous transition-metal catalysts, we recently designed and synthesized multi-point solid-supported phosphines as supporting ligands. Owing to spatial isolation of the P centers on the solid supports, these immobilized phosphines allowed controlled metal coordination, which was effective to produce coordinatively unsaturated metal species. Thus, the multi-point solid-supported phosphines showed high ligand performance in various transition-metal catalysis, enabling challenging molecular transformations that were difficult with conventional homogeneous catalyst systems.

We herein describe a cobalt/Xantphos system enabling highly challenging catalytic nucleophilic functionalizations of low-reactive carbonyl compounds via C(sp³)-H bond cleavage. The combination of Co(acac)₂, Xantphos, and AlMe₃ was crucial to convert terminal alkenes into nucleophilic allylation reagents. With the aid of this system, CO₂ and various ketones were efficiently allylated using various allylarenes, 1,4-dienes, and simple α-olefins. The regioselectivity switched depending on the alkenes and ketones used. Furthermore, the intramolecular cyclization of ketoalkenes provided tertiary homoallylic cycloalkanols with good diastereoselectivity, leading to demonstration of a short-step synthesis of biologically active compound, tramadol.

Organosulfone is a chemically-stable and versatile reagent in organic synthesis. The sulfonyl group is a strong electron-withdrawing substituent that permits the rapid functionalization of adjacent positions via deprotonation/alkylation, arylation, or conjugation addition reactions. However, after these reactions, removal of the sulfonyl group using highly reducing reagents is virtually the main choice. Therefore, the development of new methods to introduce various functional groups in the place of sulfonyl group will provide tremendous opportunities for straightforward synthesis of complex molecules. Recently our group has developed several new transformations of sulfone derivatives as reacting templates through α-functionalization followed by carbon-sulfonyl (C-SO₂) bond activation. To activate C-SO₂ bonds, we have established a new strategy focusing on the development of new catalysis/reagents and the design of sulfonyl groups. These methods could provide facile access to a variety of multiply-arylated structures such as di-, tri-, and tetraarylmethanes from stable and readily available reagents. In addition, the rapid preparation of α-fluorinated arylmethane derivatives by combination with selective α-fluorination of benzyl triflones was achieved, which opens up avenues for the development of unexplored biomolecules. Importantly, this work highlights the unique property of organosulfone to functionalize sp³ carbon centers in a modular manner.

Various charged π-electronic systems have been synthesized for application in ion-pairing assemblies. Anion-responsive π-electronic systems, which form anion complexes as pseudo-π-electronic anions, have been functionalized by peripheral and core modifications. π-Electronic anions have also been constructed by the introduction of a negative charge to an aromatic system and the attachment of hydrogen-bonding-stabilized anionic units to the π-electronic systems via covalent linkages. Porphyrins have been found extended π-electronic systems that stabilize positively and negatively charged states. The combination of charged π-electronic systems enabled the formation of diverse ion pairs and their assemblies, exhibiting distinct assembling behaviors and properties depending on the constituent charged species.

As the mesityl group-substituted acridinium photocatalyst has an orthogonal structure, the π-systems are not conjugated, leading to the formation of a long-lived electron-transfer state depicted as Acr^•-Mes^•+ under photoirradiation conditions. Since this excited acridinium salt displayed a strong oxidizing ability (E^*_1/2=+2.06 V vs. SCE), single-electron oxidation of alkenes and arenes was accomplished. Applying this arene oxidation as key radical initiation protocol, direct C-H amination, cyanation, fluorination of electron-rich arenes and desulfurative [4+2] benzannulation were developed. Moreover, a single-electron reduced TICT state served as photoreductant in reductive dehalogenation of aryl halides and detosylation of N-tosylamines.