Multiple bonding disilenes, stable

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

Through the use of bulky substituents a number of stable disilenes have been prepared in the recent past and X-ray diffraction studies are now available on three compounds containing Si=Si double bonds. Several recent review articles are relevant to this Section. The work of West and Michl in this field was briefly summarized in 198447,48 and 198549. Cowley has considered structural and theoretical aspects of disilenes, as part of two reviews of double bonds between heavier elements of groups 14 and 1550,51. Finally a very extensive review of all aspects of multiple bonding to silicon appeared in 198552. 

As silicon is the element most similar to carbon, compounds containing multiple bonds to Si have long been sought. Tetramesityldisilene, the first stable compound containing a silicon-silicon double bond, was initially prepared in 1981 by ultraviolet irradiation of 2,2-dimesityl-l,l,l,3,3,3-hexamethyltrisilane. Photolysis of the trisilane produces dimesitylsilylene, which dimerizes to form the disilene. 

Since several excellent reviews on the chemistry of multiple bonds to silicon [see the chapter written by G. Raabe and J. Michl in a previous volume of this series (1989) ] and also the reviews in Advances in Organometallic Chemistry, Vol. 39 (1995) (e.g. The Chemistry of Silenes by A. G. Brook and M. A. Brook , IminosUanes and Related Compounds—Synthesis and Reactions by I. Hemme and U. Klingebiel, Silicon-Phosphorus and Silicon-Arsenic Multiple Bonds by M. Driess and Chemistry of Stable Disilenes by R. Okazaki and R. West ) have appeared in recent years and as some parts of this chapter will be discussed in detail in related chapters of this book (e.g. Silenes and Iminosilanes by N. Auner and coworkers and Recent Advances in the Chemistry of Disilenes by H. Sakurai ), we will concentrate our attention in this chapter on the chemistry of silicon-chalcogen doubly-bonded compounds. 

---