This article highlights our recent studies on chiral dirhodium(II) complex-catalyzed asymmetric hetero-Diels-Alder (HDA) reactions. Rh2(S-BPTPI)4, a dirhodium(II) carboxamidate complex which incorporates (S)-3-(benzene-fused-phthalimido)-2-piperidinonate as bridging ligands, has been identified as a highly efficient Lewis acid catalyst for endo-selective and enantioselective HDA reactions of aldehydes with electron-rich 1,3-butadienes such as Danishefsky-type dienes, monooxygenated dienes, Rawal’s dienes and 2-aza-3-silyloxy-1,3-butadienes.
In recent years, dehydrogenative aromatization has emerged as an efficient strategy for the synthesis of important aromatic compounds, such as phenols and anilines, from ubiquitous saturated six-membered carbocyclic compounds. Starting with the report of homogeneously Pd-catalyzed dehydrogenative aromatization of cyclohexanones to phenols using molecular oxygen as the terminal oxidant by Stahl and co-workers, to date numerous homogeneously Pd, Cu, or Ir-catalyzed systems have been developed for the synthesis of phenols, anilines, and aryl ethers. On the other hand, heterogeneous supported metal catalysts have traditionally been utilized to dehydrogenate cyclohexane to benzene under harsh (gas-phase) conditions, but it is rarely known that they can be applied to liquid-phase fine chemicals synthesis under relatively mild conditions. Quite recently, we have successfully developed efficient supported Pd or Au-Pd alloy nanoparticles-catalyzed dehydrogenative aromatization reactions; for example, conversion of cyclohexanols/cyclohexanones to phenols, selective conversion of cyclohexylamines to either primary anilines or diarylamines, and one-pot synthesis of unsymmetrically substituted diarylamines. The metal nanoparticle-support cooperation is the key point for efficiently promoting dehydrogenation and realizing high selectivity to the desired products. The proposed reactions are expected to be novel green synthesis methods for important chemicals such as phenols and anilines because they can use molecular oxygen as the terminal oxidant or even proceed under oxidant-free conditions. The catalyses for these reactions are truly heterogeneous, and the catalysts can be easily retrieved from the reaction mixture and reused several times without significant loss of their high catalytic performance.
Surface-assisted bottom-up synthesis of graphene nanoribbons, which consists of the radical polymerization of precursors followed by dehydrogenation, attracted much attention due to capable of controlling their edges and widths. Although these reactions on metal surface have been believed to be catalytic, the mechanism has remained unknown. We demonstrate a new concept of ‘conformation-controlled surface catalysis’; the two-zone chemical vapor deposition of the ‘Z-bar-linkage’ precursor, which represents two terphenyl units are linked like a ‘Z’, exhibiting flexible geometry that allows it to adopt chiral conformations with height-asymmetry on a Au(111) surface, results in the efficient formation of acene-type graphene nanoribbons (GNRs) with a width of 1.45 nm through optimized cascade reactions. These cascade reactions on surface include the production of self-assembled homochiral polymers in a chain with a planar conformation, followed by efficient stepwise dehydrogenation via a conformation-controlled mechanism. Our proposed concept of bio-inspired surface catalytic reactions analogous to the biological catalyst, enzyme, is useful for the fabrication of new nanocarbon materials. Additionally, we report ‘Z-bar-linkage’ precursor, which was extended to quarter phenyl branch from terphenyl branch formed the unique nanostructures on surface.
The Mitsunobu reaction is widely used for transformation of hydroxy groups into various functional groups and inversion of the stereochemistry of secondary alcohols in organic synthesis. The Mitsunobu reaction, however, has some serious problems that inhibit its application of the reaction to practical synthesis on large scales. Especially, the reaction requires hazardous azo reagents such as diethyl azodicarboxylate (DEAD) and produces large amounts of undesired waste that sometimes complicate purification of the desired products. We have recently developed catalytic Mitsunobu reactions using new azo reagents recyclable by iron-catalyzed aerobic oxidation. Ethyl 2-(3,4-dichlorophenyl)hydrazinecarboxylate and ethyl 2-(4-cyanophenyl)hydrazinecarboxylate (or their azo forms) were identified as good catalysts by our systematic investigation. The modified catalytic reaction using these azo reagents was applicable to broad scope of substrates. High thermal stability of the reagents was shown by SC-DSC analysis, suggested low explosive nature of these reagents.
Highly fluorinated solvents (fluorous solvents) have unique characteristics: immiscibility with water or organic solvents, and higher density than conventional solvents. These characteristics are cleverly applied in the phase-vanishing (PV) method. Fluorous solvents are used as liquid membranes to regulate reactions by passive diffusion of reagents through the media. Herein, we summarize second generation PV systems, where active species are generated from reagents diffused into the fluorous phase, which subsequently react with substrates to afford the desired products. For example, in photo-irradiative PV reactions, diffused Br2 was reacted with organic solvents in the fluorous phase to afford HBr by photoirradiation, and subsequently the generated HBr added to alkenes. In Grignard-type PV reactions, diffused alkyl iodide was reacted with magnesium at the border between the fluorous and organic phases to afford alkylmagnesium iodide, which added to carbonyl compounds. In a PV reaction with acetylene evolution, diffused water was reacted with calcium carbide to generate acetylene gas, which underwent several reactions such as Sonogashira coupling. These PV reactions can be called spontaneous “molecular level” two-step flow-reaction systems. The influence of fluorous phase characteristics such as surface area and viscosity on the bromination of alcohol with Br2/PPh3, and PV reactions using improved reaction vessels are also described.
A number of organocatalytic methods for the synthesis of axially chiral biaryl compounds have been reported to date, and most of them are predicated on desymmetrization of achiral biaryl precursors or dynamic kinetic resolution of stereo-labile biaryl precursors. This mini-review highlights the alternative approaches utilizing the aromatization of alicyclic intermediates derived through the stereoselective organocatalytic reactions.
Catellani reaction enables multi-functionalization of aromatic rings. Originally, Catellani reaction starts from oxidative addition of aryl halide to zerovalent palladium species. This reaction pattern restricts a variation of reactant. In this review, recently developed palladium(II)-initiated borono-Catellani reactions are discussed.
A phototriggered functionalization reaction of a carboxy group using a tetrazole was recently reinvestigated as a useful method for functionalization of proteins and polymer materials in aqueous media. Irradiation of a tetrazole generates a highly reactive nitrile imine in high efficiency. The generated nitrile imine reacts with a carboxylic acid to produce an acyloxy hydrazone which undergoes an acyl-rearrangement to afford a stable diacyl hydrazide. This review summarizes basic information about the reaction and application of the reaction to photoaffinity labeling of proteins and preparation of functionalized materials.
Peptides and proteins have received a great deal of attention in the chemical biology field due to their unique biological and physical properties as well as their enormous potential applications such as catalysts and drugs. However, their chemical and structural complexity hinders the progress of researches. This mini-review focuses on the selective synthetic modification methods of peptides and proteins targeting specific amino acid residues for future applications.
Primary amines are naturally abundant and readily available feedstock chemicals, and a number of methods have been developed to exploit them as synthetic intermediates. However, there have been no effective methods for C(sp3)-N bond transformation of primary amines with unactivated alkyl groups. This mini-review focused on development of C-C and C-B bond forming reactions from primary alkyl amines via bench-stable Katritzky pyridinium salts available in a single step from corresponding amines.