Though the synthetic reactions of C-C multiple bonds directly having functional groups are more effective to give functionalized compounds which are easily converted to useful compounds like natural products and drugs, the applicability of this process to functionalized C-C multiple bonds is limited, probably because of the poor compatibility of the reactive metallic portion and the functional group. We focused on the interaction between transition metals and functionalized C-C multiple bonds, and have developed new synthetic methods using the above interaction and its synthetic applications. We discussed the new synthetic methods disclosed by as will be described using iron reagents in detail in this paper.
Formyl compounds represent one of synthetic integrants essential for the construction of complex organic molecules. In general, their activation at the carbonyl carbon by a Brønsted or Lewis acid has been used for the new bond formation at the carbon with a wide variety of nucleophiles. On the other hand, transition metal, especially late transition metal, complexes often activate a formyl C-H bond to generate an acylmetal hydride species (RC(=O)-M-H), which can react with a C-C unsaturated bond, leading to the hydroacylation. Furthermore, the C-H bond cleavage, followed by the migratory extrusion of the carbonyl moiety, generates a metal carbonyl species (M-CO) along with the release of R-H. This is recognized as the decarbonylation. The account describes a novel method for synthesis of carbonyl compounds, in which the decarbonylation process of formyl compounds is used as a carbonyl-donating process (CO gas-free carbonylation). It provides more reliable and accessible carbonylation reactions for synthetic organic chemists.
This article describes the most recent developments in the synthesis of three dimensional (3D) π-conjugated molecules and the elucidation of their properties accomplished by the authors’ group. Cycloparaphenylenes (CPPs) of different sizes and a cage-like 3D molecule were synthesized based on the platinum-mediated assembly of π-units and subsequent reductive elimination of platinum. The assembly of π-units mimics the self-assembly process for the formation of supramolecular transition metal-ligand complexes with 3D cages and polyhedral structures. Furthermore, reductive elimination of platinum successfully took place with high efficiency, despite the high strain energy of the target molecule. Several size-dependent properties of CPPs, namely the photophysical, redox, and host-guest chemistries, were also clarified. These results are of use for a molecular-level understanding of CNT physical properties as well as fullerene peapods. Theoretical and electrochemical studies suggest that small CPPs and their derivatives should be excellent lead compounds for molecular electronics.
Liquid-crystalline semiconductors exhibit good carrier transport properties as well as solution-processability. The applications to field-effect transistors, light-emitting diodes, and solar cells have been studied. Liquid-crystalline perylene tetracarboxylic bisimide derivatives bearing oligosiloxane moieties were designed. They exhibit columnar phases at room temperature and do not crystallize even when they are cooled to −100°C. The mesomorphic columnar structures are stabilized by nanosegregation promoted by the interaction between the oligosiloxane moieties. The electron mobility exceeds 0.1 cm2V−1s−1 at room temperature in the ordered columnar phase. The analysis of the electron transport characteristics in the columnar phases indicates closed aggregation of the π-conjugated cores, resulting in high electron mobilities.
Acyl radicals prefer to attack at the nitrogen end of an imine (C=N) group although they are commonly thought to be nucleophilic. We have been interested in this intriguing selectivity of acyl radicals and sought recourse in ab-initio and DFT calculations to shed light on the chemistry of these radicals. In this paper, the origin of the N-philicity of acyl radicals is discussed using (1) geometry of the transition states (TS) involved in the addition reaction, (2) energy barriers for the reaction, (3) the multi-orbital interactions and (4) the NBO analysis of the TS. Calculations revealed that α,β-unsaturated acyl radicals are even more N-philic, supported by experimental results in which α,β-unsaturated acyl radicals react with imines to give 4-8 membered rings exclusively. Our computational investigations successfully established a reasonable mechanistic pathway for the reaction of α,β-unsaturated acyl radicals with amines involving 1,4-hydrogen shift and homolytic substitution reaction at nitrogen. Calculations of other types of reactions involving acyl radicals, such as homolytic substitution at group XIV atoms, halogen atom transfer, and reduction with borohydride, are also described.
Organic synthesis using organobismuth compounds has considerably advanced during the last 15 years. One of the bottlenecks using organobismuth compounds in organic synthesis was their limited accessibility because of lack of variety of synthetic methods. Several new synthetic methods for organobismuth compounds have recently been developed, which allowed the access to wider variety of organobismuth compounds useful in organic synthesis. Considerable advancement has been achieved in cross-coupling reactions forming C-C, C-O, C-N, C-S and C-Se bonds using organobismuth reagents. Not only the arylation reaction, but also alkenylation and alkylation reactions became possible. Stereoselective allylation reaction of carbonyl compounds as well as 1,4-addition reaction of α,β-unsaturated carbonyl compounds have also been developed. There was also noticeable advancement in oxidation reaction, in which highly reactive and selective oxidation reagents have been developed. Catalysis by organobismuth compounds including CO2 fixation reaction, cross-condensation reaction, radical and cationic polymerization reaction has also been achieved.
An electrophilic halogen is known to behave a halogen bonding donor for Lewis base or anion to interact through a non-covalent bonding both in solid state and in solution. Halogen bonding shows strong interaction with high directionality as well as hydrogen bonding. Lergons reported DABCO·(C8F17I)2 is a recyclable catalyst for Morita-Baylis-Hillman reaction. Huber developed a neutral multidentate halogen bond donor in analogy to anion-binding thiourea organocatalyst. Although there are a few reports employing halogen bonding for organic reactions, the halogen bonding will be a key tool for the design and development of novel catalysts and reactions.
Asymmetric C-H functionalization is an efficient synthetic method for highly enantioenriched molecules. Recently, highly enantioselective arylations of unactivated C(sp3)-H bonds have been established using chiral palladium catalyst systems. In these reactions, chiral monodentate ligand and carboxylic acid act as key elements to achieve high catalyst activity and enantioselectivity.