Organoboron compounds are useful reagents in organic synthesis. Recently, the transition-metal catalyzed addition reactions of boron-heteroatom bonds to unsaturated organic compounds (diboration, borylstannation, borylsilylation, and thioboration) have been reported. These are useful in introducing simultaneously two functional groups into organic compounds. In this review we will describe these reactions by classifying into following features : (i) addition reactions to organic compounds containing C-C triple bond (alkynes, diynes, and enynes), (ii) addition reactions to organic compounds containing C-C double bond (alkenes, 1, 3-dienes, allenes, unsaturated carbonyl compounds, methylenecyclopropanes, and vinylcyclopropanes), (iii) addition reactions to other organic compounds.
Photochemistry of aromatic compounds has received much attention from both the mechanistic and synthetic perspectives, and the intra- and intermolecular photochemical cycloaddition of carbon aromatics is well documented. Furthermore, the photocycloaddition of alkenes to benzene derivatives is also a useful method for synthesizing natural products. However, in contrast, the photocycloaddition reaction of aza-aromatics, such as pyridines, pyridazines, pyrazines, or pyrimidines, is an underdeveloped area of research. We found that the induction of suitable substituents to appropriate position of aza-aromatic ring made the photoreactivity considerably high. Many new photochemical reactions for pyridine derivatives have been found in photodimerization, photocycloaddition with alkenes, furan, benzofuran or cyanonaphthalene. These reactions provide not only new photochemical aspect of aza-aromatics, but also useful synthetic method of polycyclic heterocycles.
β-Alkoxyalkenyllithiums show potential versatility as precursors of various novel compounds in synthetic organic chemistry. We have described several applications using β-alkoxyalkenyllithiums. One is an α-sulfenyl or α-selenenyl formylation of aldehydes and ketones, which was based on the Wittig-type olefination using β-alkoxyalkenyllithiums. The process is simple and comprises two steps : electrophilic addition of the β-alkoxyalkenyllithium with aldehydes and ketones and hydrolysis of the allylic alcohols. The tandem formylation gave several kinds of photo-sensitive hepta-2, 4, 6-trienals, photo-reactions of which underwent the new tandem cyclization to afford 2-oxa- or 2-aza-bicyclo [3.3.0] octa-3, 7-dienes. On the other hand, β-perfluoroalkyl-β-alkoxy-alkenyllithiums afforded α, β-unsaturated perfluoroalkyl ketones, which are novel precursors in the preparation of fluorine-containing bioactive compounds. Another application is a Lewis acid-promoted C-C bond formation reaction of 2-sulfenyl- or 2-selenenyl-2-allylic alcohols or their acetals with soft nucleophiles, which provided many kinds of products in good to high yields. Furthermore, we have found new allylic cations generated from 2, 3-bis (chalcogene) -substituted allylic alcohols.
New reactions and molecular self-assemblies based on intra- and intermolecular interactions, especially σ*-n orbital interactions, between chalcogen atoms are described. The article includes the following topics. (1) Intramolecular interaction between sulfonium and sulfide sulfur atoms in the reaction of 1, 2-bis (benzylthio) benzene monooxide with triflic anhydride to generate benzodithiete via a dithia dication. (2) Intermolecular interaction between sulfonium and sulfide sulfur atoms in the reaction of p-bis (benzylthio) -aromatic monooxide with trifluoroacetic anhydride to form cyclic bis (disulfide) dimer or cyclic tetrakis (disulfide) tetramer via a cyclic bis (dithia dication) dimer. (3) Interaction of tellurium atom of cationic tetraaryl ditelluroxane with oxygen atom of diaryl telluroxide to produce oligotelluroxane with hypervalent Te-O apical linkages in the main chain. (4) Interaction of tellurium atom of cationic tetraaryl ditelluroxane with oxygen atom of phthalate to form macrocyclic multi-tellurane with hypervalent Te-O apical linkages.
Novel dendritic polyphenylazomethines (DPAs) were synthesized up to the fourth generation by the convergent method via dehydration of aromatic ketones with aromatic amines in the presence of titanium (IV) tetrachloride. DPAs show high thermal stability and high solubility unlike the con-ventional linear polyphenylazomethines. Novel cyclic phenylazomethine trimers (CPAs) were syn-thesized in a one-step dehydration of the 4-aminobenzophenone derivatives in the presence of TiCl4 or p-toluenesulfonic acid (PTS). When using TiCl4 as the dehydration agent, the introduction of bulky substituents as the α-position of the substrate enhanced the yields of the CPAs. On the other hand, PTS served as an effective catalyst for the synthesis of the phenyl-substituted CPA. Cyclization in hyperbranched polymer synthesis was first controlled on the basis of a steric effect using a Lewis acid with bulky ligands. A novel dendrimer with a cyclic structure was quantitatively obtained via controlled cyclization using a monomer with a bulky dendron.
Helicity induction on optically inactive polymers bearing functional groups via non-covalent bonding interactions with chiral compounds is described, especially focusing on poly (phenylacetylene) and poly (organophosphazene) derivatives. Stereoregular, optically inactive poly (phenylacetylene) s bearing a carboxy, an amino, a phosphono, or a boronic acid group respond to the chirality of various optically active compounds capable of interacting with the functional groups and the complexes exhibit a characteristic induced circular dichroism in the UV-visible region in organic solvents as well as in water. These polymers form a dynamic, one-handed helical conformation upon complexation with chiral compounds and can be considered as a promising candidate for a novel probe to determine the absolute configuration of various chiral compounds. Moreover, the induced macromolecular helicity can be memorized by replacing the chiral compounds with achiral ones. Poly (organophosphazene) bearing carboxy groups also forms a dynamic, one-handed helical conformation upon complexation with optically active amines. Other examples of helicity induction on optically inactive polymers and oligomers are also briefly described.
Development of synthetic methodologies using titanium reagents in either a catalytic or stoichiometric amount has been the research subject of the author over a number of years. The synthetic reactions developed by the author involve 1) hydroalumination reaction of olefins with LiAlH4 in the presence of a catalytic amount of TiCl4, 2) Cp2TiCl2-catalyzed hydromagnesiation reaction of conjugated dienes and alkynes with an alkyl Grignard reagent, 3) synthetic reactions using a η3-allyl titanium complex and 4) synthetic reactions mediated by a Ti (O-i-Pr) 4 / 2i-PrMgX reagent.