論文の公開元へStudies on Synthesis of Benzothiazoles via Redox Cyclization Using Elemental Sulfur
概要
Sulfur is one of the abundant elements on earth and is known to be the 15th Clarke number for sulfur.1 In nature, sulfur is found in sulfide minerals such as pyrite and gypsum, and in living organisms, sulfur-containing amino acids such as cysteine and methionine are important as one of the essential amino acids that make up proteins.
Elemental sulfur is in nature and is known for thousands of years. For example, sulfur has long been used to bleach cloth and as a raw material for pigments; around the 6th century, the Chinese developed gunpowder made by mixing combustible sulfur, with nitrous oxide and charcoal. More recently, it has been used as an important raw material in various fields, including as a vulcanizing agent for rubber, a basic material for agricultural chemicals, and an industrial source for sulfuric acid.2,3,4 Sulfur-containing compounds also play an important role as intermediates in pharmaceuticals.
Until the 1880s, sulfur was extracted from the volcanic soil in Sicily, but since the 1890s, the Frasch process has been used to extract sulfur from underground deposits; in the late 1950s, sulfur production was made possible by the Claus process, which desulfurizes the sulfur content in petroleum.5,6,7 In 2021, global production of elemental sulfur was approximately 80 million tons/year, of which 70% was produced from the refining of petroleum and only 1% by the old Frasch process.8 Sulfur recovered from oil refining is useful as a raw material for elemental sulfur, because its production costs are lower than those of other production methods.9
Although sulfur is located in the third row period of group 16 on the periodic table and is an element directly below oxygen, the chemical properties of sulfur differ from those of oxygen in many ways.10 The significant difference between oxygen and sulfur is the electronic configuration: sulfur has 3p electrons and empty 3d orbitals and its electron configuration is (1s22s22p63s23p4).11 The oxidation states of sulfur range from -2 to +6 (Figure 1-1). Thiol (R-SH), sulfenic acids (R-S- OH), thioethers (R-S-R’), and hydrogen sulfide (H2S) are examples of sulfur in the -2 oxidation state. Sulfur monoxide (SO) has sulfur in the +2 oxidation state while sulfur dioxide (SO2) formally has sulfur in the +4 oxidation state. Sulfuric acid, one of the most important industrial compounds, has an oxidation number of +6. Since sulfur has access to multiple oxidation states, it is likely to be an effective reagent in the redox reaction.
