Brønsted acid catalysis has received considerable attention in modern organic synthesis. However, the utility of these Brønsted acid catalysts are somewhat limited toward applicable substrates due to its relatively lower reactivities of these Brønsted acids. With these perspectives, it is highly desirable to develop Brønsted acids demonstrating both high reactivities and selectivities. In this feature article, we will describe our achievement in the design and development of Lewis acid assisted Brønsted acid catalysts (LBA), Brønsted acid assisted Brønsted acid catalysts (BBA), strong Brønsted acid catalysts, and their applications.
We found that (NO)- and (aqua)ruthenium-salen complexes are efficient catalysts for the asymmetric aerobic oxygen atom transfer reactions such as epoxidation and sulfoxidation at ambient temperature. (NO)ruthenium-salen complexes 2 and 3 could catalyze oxidation of sulfides and the epoxidation of conjugated olefins with good to high enantioselectivity using dioxygen as oxidant, albeit under visible light-irradiation, respectively. On the other hand, (aqua)ruthenium-salen complex 5 was found to catalyze highly enantioselective epoxidation in air even without irradiation. Although the mechanism of this ruthenium-catalyzed aerobic oxidation has not been completely elucidated, water that is bound to the ruthenium ion has been considered to play a critical role in proton coupled electron transfer, a key step for oxygen activation, and to be regenerated via oxo-hydroxo tautomerization. We also found that (di-µ-hydroxo)iron-salan complexes catalyzes asymmetric dehydrogenative oxidation reactions such as 2-naphthol coupling, alcohol oxidation, and dearomatization, using air as oxidant.
The “cation pool” method involves the generation and accumulation of highly reactive organic cations in solution. These can serve as powerful carbon and heteroatom electrophiles in organic synthesis. Recent developments in the cation pool method including the indirect cation pool method, cation chain reactions, and integrated electrochemical-chemical reactions are described.
The proton is the smallest group in synthetic organic chemistry. Moving a proton within a catalytic cycle in an enantioselective manner is a formidable task that has attracted the attention of many chemists. Furthermore, catalytic enantioselective protonation is an atom economical method of generating chiral carbon centers. Variations of this kind of reaction catalyzed by transition metals catalysts have been well investigated. On the other hand, a complementary approach to this reaction, the use of chiral Brønsted base catalysts is relatively less explored. We have developed several protonation reactions using chiral bicyclic guanidine as the Brønsted base catalyst. In this review, we will report these reactions and other related examples recently described by other groups.
Xanthanolides were synthesized with intramolecular acylation of organolithium forming a seven-membered carbocycle and one-pot acylation-Wittig lactonization as key steps. The Cu(II) complex efficiently catalyzed the acylation of thioester in Wittig lactonization under neutral conditions. Using the Cu(II) catalyst, symmetric dithiomalonates were converted into dissymmetric S,O-malonates via selective monoacylation. The key step in this reaction was the thermal formation of an acylketene, the stability of which would contribute to selectivity.
Carbon dioxide (CO2) is an abundant, inexpensive, relatively nontoxic, and renewable C1 source for organic synthesis, fixations of which have been intensively studied over the past decade. For the purpose of preparing valuable commodity chemicals from easily accessible starting materials through CO2 incorporation, we have made many efforts toward the development of α-amino acid synthesis from CO2 and imines. By changing the latent polarity of imino carbon by treatment of a stannyl or a silyl anion derived from CsF and bismetal reagents (Me3Si-SnBu3 or PhMe2Si-Bpin) with imine, we successfully developed a one-pot synthesis of N-Boc-α-amino acids from gaseous CO2 and N-Boc-α-amido sulfones (stable N-Boc-imine precursors). In addition, we have developed a one-pot α-amino acid synthesis from three basic components, CO2, an aldehyde, and a sulfonamide, promoted by Bu3Sn-SnBu3 and CsF. Intermediates of these α-amino acid syntheses are imines and α-amino stannanes or α-amino silanes.
Combining the concepts of supramolecular chemistry with material science has led to the development of supramolecular polymer chemistry. Although a large number of host-guest motifs have been produced, only a limited number of recognition motifs have been utilized as supramolecular connections within polymeric assemblies. In this account, we describe the molecular recognition of host molecules based on a calixarene and a bisporphyrin, demonstrating unique guest encapsulations; subsequently, these host-guest motifs were applied to the synthesis of supramolecular polymers that display polymer-like properties in both solution and solid states. In addition, we disclose that bisresorcinarenes form supramolecular polymers that are connected via a hydrogen-bonded rim-to-rim dimeric structure, which is composed of two resorcinarene moieties.
The direct functionalization of inactive bonds is an ideal transformation in organic synthesis because the introduction of activating groups, such as halogens and metals, to substrates is unnecessary and by-products derived from them are not formed. This report describes several types of cationic iridium-catalyzed reactions that occur through C-H, N-H, or C-O bond cleavage. The carbonyl-directed sp2 C-H bond activation of aryl ketones realized new approaches to benzofulvene, benzofuran and indole synthesis, as well as the C2-position-selective alkylation of N-acyl-protected indoles. The enantioselective sp3 C-H alkylation of 2-(alkylamino)pyridines and the intermolecular enantioselective hydroamination of heteroaromatic amines gave various chiral amines. O-to-N-alkyl migration in 2-alkoxypyridines gave N-alkylpyridones via C-O bond cleavage.
Heteroaryl triflones are potential building blocks for the synthesis of novel biologically active compounds as well as functional materials. However, their synthesis has received relatively little attention. In this account, we describe our recent development of three novel synthetic strategies for the regioselective synthesis of heteroaryl triflones: direct trifluoromethanesulfonylation of heteroaromatic compounds, anionic Fries rearrangement of heteroaryl triflates, and transformation from triflyl-containing building blocks.