New horizon of organosilicon chemistry

It is almost 150 years since the synthesis of tetraethylsilane as the first compound having a silicon–carbon bond; the birth of organosilicon chemistry. Although organosilicon chemistry was thought to be less hopeful in its infancy as remarked in the famous Kipping's Bakerian Lecture in 1936, it has come a long way to build up an important research field in both fundamental and applied aspects. Dramatic progress has been attained in the last three decades.

Comparison of the properties of silicon and carbon and their respective compounds has always been a main issue in organosilicon chemistry. Much attention has been focused on silicon congeners of organic key reactive intermediates like carbenes, carbocations, carbanions, and carbon radicals, that is, silylenes, silicocations, silyl anions, silyl radicals, respectively. Fifty years ago, multiply-bonded silicon compounds as well as those of germanium, tin, and lead were thought unrealistic; not long after their generation and detection as reactive intermediates was started. Today these low-coordinate silicon compounds are not reactive intermediates any more but are isolated as thermally stable compounds by introducing sterically bulky protecting substituents. Synthesis, spectroscopic properties, structure and reactivity of these compounds have shown remarkable distinction from those of their carbon analogues, attracting much attention from many experimental and theoretical chemists. Now this research area aims at developing in two directions in addition to the synthesis of new unsaturated silicon compounds; application of these unique electronic properties towards material science and reconstruction of a general theory of bonding and structure of heavy main group elements including silicon.

Silicon differs from carbon in its ability to form not only multiple bonds but hypercoordinate states. While stable hypercoordinate carbon compounds have been very rare until now, penta- and hexa-coordinate silicon compounds have been common for a long time. Design, synthesis, structural characterization and reactivity studies of hypercoordinate silicon compounds have been interesting and relevant to the aims of organosilicon chemistry.

Silicon forms relatively strong covalent bond to different elements. Among various such bonds, silicon–transition metal bonds have attracted much attention because of their potential role in catalytic reactions. The chemistry of silicon transition metal complexes having silylenes, silylynes disilenes and other unsaturated silicon ligands is a current topic in this area.

Many common organosilicon compounds are volatile, stable in air, highly soluble in organic solvents and easily handled, in contrast to inorganic silica and elemental silicon that are solid and chemically intractable. These facts would be among the driving forces behind the application of organosilicon compounds as reagents for organic synthesis and, pharmaceutical and biological purposes. A large industry producing silicone oil, rubber and resin has emerged through the invention of the direct synthesis of methylchlorosilanes by Rochow and Müller in 1940. Undoubtedly, their success has encouraged recent developments in silicone chemistry and related silicon-based materials science.

A compilation of six perspectives, 22 full articles and five communications in this themed issue features the diverse aspects of modern organosilicon chemistry. It is hoped that the readers will take an enjoyable journey towards its new horizon.

Mitsuo Kira

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