The great utility of high-pressure technology to achieve the desired reactions which are not readily accessible by other means and its application in natural product or material synthesis have been reviewed. Newly developed methods especially atom-economic processes for functional group transformations, environment-friendly constructions of target molecules, and promotion of sterically-restricted organic reactions are described. These include (1) Diels-Alder reaction, (2) intramolecular Diels-Alder reaction, (3) hetero-Diels-Alder reaction, (4) 1, 3-dipolar [3+2] cycloaddition reaction, (5) aldol condensation, (6) Friedel-Crafts-type alkylation, (7) Michael addition reaction, (8) SN2-type substitution, (9) nucleophilic aromatic substitution, (10) epoxide ringopening reaction, (11) organometallic reaction, (12) Lewis acid-catalyzed reaction.
Vitamin B12-dependent enzymes, involving the cobalt species as a catalytic center, mediate various molecular transformations invivo. In order to use vitamin B12 derivatives (cobalamins) as green catalysts, various studies have been carried out such as a chemical modification of cobalamin or the fixation of B12 derivative on electrodes. The Co (I) species of vitamin B12 derivative is widely known as a supernucleophile that forms an alkylated complex by reaction with an alkyl halide. Such reaction is efficient for dehalogenation of various organic halides. Therefore, vitamin B12 derivatives can be used as catalysts for reductive degradation of organic pollutant such as DDT. In addition, the alkylated complex is a useful reagent for forming radical species as the cobalt-carbon bond is readily cleaved homolytically by photolysis or electrolysis. Therefore, the application of the alkylated complex to organic synthesis is quite interesting from the viewpoint of a radical-forming reagent. This review article summarizes recent progress in homogeneous and heterogeneous catalytic reactions mediated by vitamin B12 derivatives.
Benzothietes and benzoselenates were synthesized by the reaction of benzyne with thiones or selones. The reaction of di-tert-butyl selenoketone with propiolic acid gave 2H, 6H-1, 3-oxaselenin-6-one, whereas the reaction of di-tert-butyl thioketone with propiolic acid resulted in the recovery of the starting thioketone almost quantitatively. Other isolable thiones reacted with propiolic acid to give thiodioxenones in good yields. Thionation of thiones at room temperature afforded 1, 2, 4-trithiolanes via thiosulfine intermediates. Thionation of di-tert-butyl-thioketene S-oxide afforded 3, 3-di-tert-butyl-thiirane-2-thione (α-dithiolactone), which is the first example of an isolable α-dithiolactone. Sterically congested cis- and trans-1, 3, 4-selenadiazolines were isolated by one-pot reaction of ketone hydrazones with diselenium dibromide, which suggested the insituformation of selenoketone and diazoalkane intermediates. The reaction of acetophenone hydrazones with diselenium dibromide afforded 2, 5-diarylselenophenes in moderate yields.
Helical aromatics possessing a diazapentaphene skeleton were synthesized via one step in good quantity by solvent-free condensation reaction of N, N'-diphenyl-p-phenylenediamine and various carboxylic acids in the presence of a Lewis acid. Microwave irradiation greatly facilitated the condensation to furnish a helical aromatic with a 100% diastereo- and a 50% enantioselectivity, when a chiral carboxylic acid was utilized. The aromatic derived from 2-methylglutaric acid was quite stable, with no racemization taking place even at 200 °C. The assignment of the absolute configurations to the helical aromatics has been achieved by theoretical CD spectra calculated by time-dependent density functional theory.
Optically active diols are one of the useful groups of compounds for asymmetric reaction as a chiral auxiliary or ligand. Among them, cyclic 1, 2-diols are characteristic of the restricted conformation. In this review, we wish to report the use of these compounds for the development of asymmetric reactions such as ring transformations based on intramolecular and intermolecular aldol reactions, and carba-spirocyclization, in which cyclic 1, 2-diols are added as a chiral source in combination with a Lewis acid. In addition, the synthesis of optically active cyclic diene compounds starting from cyclic 1, 2-diols is also discussed.
A variety of seleno and telluroiminium salts were obtained by reacting the corresponding seleno and telluroamides with methyl triflate at room temperature for 30s. X-ray molecular structure analysis, 13C NMR spectra, and molecular orbital calculations of seleno- and telluroiminium salts suggested that the electrons on the chalcogen atom of the salts are somewhat delocalized with respect to the iminium group, and C-Se and C-Te bonds have a partial double-bond character. Aromatic thio- and selenoiminium salts reacted with a lithium acetylide at their iminium carbon atoms, whereas the aromatic telluroiminium salt reacted at the tellurium atom. This difference in reactivity was elucidated by molecular orbital calculations of the model compounds. Based on the reactivity test of a series of chalcogenoiminium salts, two types of new reactions were developed. Alkynylation of thio- and selenoiminium salts led to α, β-unsaturated ketones. The reaction involves the [1, 3] -shift of MeS- and MeSe-groups to give aminoallenes as intermediates. Alternatively, the sequential reactions with organolithium and -magnesium reagents selectively gave propargylamines. The wide applicability of these reactions is also shown.