Foreword

Group XVI elements (oxygen, sulfur, selenium, tellurium, and polonium—the chalcogens) possess fascinating properties. Of these elements, oxygen usually takes center-stage as it is the most widely-studied and its importance in biological processes is common knowledge. In contrast, the chemistries of sulfur, selenium, and tellurium are lesser known. However, new discoveries have meant that oxygen is increasingly sharing the spotlight with other Group XVI members. Inorganic chemists are continuously discovering interesting new chalcogen complexes, organic chemists are expanding their synthetic toolboxes with chalcogen-containing reagents, and biochemists are finding new roles for the chalcogens in biological processes. This book focuses on the chemistries of three important chalcogen elements—sulfur, selenium, and tellurium—areas which are developing rapidly.

Sulfur occurs in its elemental form in many parts of the world, and has been known since prehistoric times. Under the ancient name of brimstone, it is frequently mentioned in the Bible. In fact, many religions and traditions use this element to depict their version of hell—imagery inspired by the vile odors and toxicity of the readily formed hydrogen sulfide, sulfur dioxide, and other foul-smelling sulfur compounds. Organic compounds containing sulfur are also famously malodorous. It was therefore perhaps somewhat surprising that sulfur was detected in plants in 1781, and then later in the bile and blood of animals in 1813. Of course, many of us will have discovered firsthand the presence of hydrogen sulfide in rotten eggs! Nowadays, the importance of sulfur in metabolic processes is firmly established. Since the beginning of the nineteenth century sulfur compounds have been widely used in the chemical industry. In addition, many pharmaceutical compounds contain sulfur atoms within their structures and several reagents used in organic synthesis are sulfur-containing compounds. As a result, the field of sulfur chemistry is well established, being thoroughly described in undergraduate inorganic and organic chemistry textbooks. Overall, the importance of this element is clearly apparent.

In contrast to sulfur, selenium was only identified in 1817, when Berzelius discovered selenium deposits formed in the lead chambers of his sulfuric acid plant in Gripsholm, Sweden. Berzelius named the element selenium to honor Selene, the goddess of the moon in Greek mythology. Berzelius realized the similarity between sulfur and selenium—organosulfur compounds have a bad smell, but selenium analogs smell even worse! Then, for more than a century, selenium chemistry was a neglected field. It was only in 1955 that the first extensive review work on selenium chemistry was published, as a chapter of Houben-Weyl, by Professor H. Rheinboldt, a German chemist who came to Brazil in 1934 to escape Nazi persecution. He was contracted by the recently founded São Paulo University. During his lifetime in Brazil, Rheinboldt trained a number of PhD students in the fields of selenium and tellurium chemistry. In 1973, I started my graduate studies in the laboratory of Petragnani in São Paulo, and witnessed firsthand the difficulty in manipulating organic selenium compounds. This was due to not only their bad smell, but also the poor reproducibility of reported results, owing to the limited knowledge on the chemistry of this element at that time.

Another problem associated with selenium compounds is its reputation as being highly detrimental to health. Around 1930, it was observed that when selenium is present in the soil, it is taken up by the vegetation, causing damage to animals grazing on the toxic plants. Around the same time, it was shown that feeding selenium-containing cereals to hens caused malformation or death in their chicks. These findings deterred, and continues to deter, chemists from studying selenium chemistry.

Finally, in 1957, something positive about selenium was discovered — selenium was identified to be an essential trace element in the animal diet, and the absence of selenium causes disease in both animals and humans. In 1973, it was discovered that selenium is present in the reactive site of mammalian glutathione peroxidase (GPX-1) enzyme, as a component of the 21st amino acid, selenocysteine. GPX-1 is involved in removing peroxides and serves as part of the antioxidant defense system of organisms. In this book, Professor Flohé describes this important discovery in more detail.

The above-mentioned discoveries garnered the attention of the scientific community to the chemistry of selenium. There is currently intense activity in the area of selenium compounds and how they relate to biological systems.

In 1970, the serendipitous discovery of the selenoxide syn elimination drew the attention of the organic chemistry community toward the synthetic potential of organoselenium compounds, and this branch of chemistry enjoyed an explosive growth in popularity during the years that followed. Various synthetic transformations using selenium were found to be superior to the corresponding sulfur versions, clarifying the chemistry of selenium as a field in its own right, and not a mere extension of the chemistry of sulfur. As a direct result, selenium-containing intermediates have been intensely used in the synthesis of complex molecules. Nowadays, chemical transformations using selenium compounds can be found as standard in undergraduate organic chemistry textbooks.

The history of the element tellurium is older than that of selenium. Tellurium was discovered by von Reichenstein in 1782, in mineral ores in the gold district of Transylvania. Its name is derived from the Latin “Tellus”, meaning earth. The preparation of the first organotellurium compound, the volatile and malodorous diethyl telluride, was reported by Whöler in 1840. Whöler described the smell of this compound as “obnoxious”. Indeed, the low molecular weight tellurium compounds possess a terrible and nauseating smell, which I experienced firsthand when dimethyl telluride was prepared in my laboratory. Whöler's comments contributed to a long hibernation period in the study of the organic chemistry of tellurium. For example, only about 50 papers were published on tellurium chemistry between 1910 and 1950. Around a century after Whöler's original paper, the first comprehensive account of the inorganic and organic chemistry of this element appeared in the literature as part of the aforementioned chapter in Houben-Weyl by H. Rheinboldt. Rheinboldt's group showed that several classes of organic tellurium are high melting point solids. I can myself testify, from as early as the beginning of my graduate studies, that several organic derivatives of tellurium, especially those containing Te(iv), are almost odorless.

After the growth of selenium chemistry during the 1970s and 1980s, the attention of the chemistry community turned to the chemistry of tellurium. It was soon realized that the reactivities of the organic compounds of selenium and tellurium are quite different. For example, the telluroxide syn elimination is much more difficult than the corresponding transformation of selenoxides. In this sense, it is more interesting to explore the differences in reactivity between the compounds of these elements, than to explore their similarities. A more synthetically useful transformation of tellurides proved to be tellurium/metal exchange, which occurs more readily than selenium/metal exchange. Many organometallic intermediates used in organic synthesis can be prepared using this reaction. The first total synthesis of a complex natural product using a telluride as an intermediate was reported in 2002. Enantiomerically-enriched hydroxyl tellurides were obtained by enzymatic kinetic resolution, and were subsequently transformed into the corresponding dianions and then used in the synthesis of enantiomerically-enriched natural products. Contrary to early statements, organotellurium compounds can be handled in the presence of the air and under ceiling light, although the exposure of organotellurium compounds to air for prolonged periods should be avoided, especially when in solution.

There are few reports in recent literature on the interaction of tellurium compounds with biological systems, although it has been described that organic tellurides possess antioxidant properties, and that tellurium(iv) derivatives are potent inhibitors of some important enzymes, such as cathepsins. Recent studies on the toxicity of tellurium compounds are scarce.

A more detailed account on the history of the development of selenium and tellurium chemistry has been published (J. V. Comasseto, J. Braz. Chem Soc., 21 (2010) 2027–2031).

It is with great pleasure that I introduce this book—whose chapters are written by distinguished scientists—describing the recent advances in the area of sulfur, selenium, and tellurium chemistry. I hope you enjoy reading about these latest discoveries.

João V. Comasseto

São Carlos, Brazil