Silenes structure

Generalized silene structure Method of synthesis Common substituents Reference... 

Silenes with a wide range of structures have been synthesized over the years, from the simplest possible compound, H2Si=CH2, which has a transient existence, to species that contain four complex groups attached to the ends of the silicon-carbon double bond, e.g., (Me3Si)MesSi=C... 

Over the years a number of general synthetic pathways to silenes have been developed these have been employed by research groups to generate families of silenes closely related in structure. It seems useful to list the most common pathways, together with references, in Table I14-91 and then to comment on some of them in detail. 

A few other routes to new silenes involving photochemical processes have been reported, most of which have the structure RR Si=CH2110-112 and are listed in Table I. Photolysis of l,l-dimethyl-2-phenyl-l-silacyclo-but-2-ene 22 in a glass at 77 K led to the siladiene 23, which absorbed at 338 nm,113 as shown in Eq. (17) ... 

The stable silene Me2Si=C(SiMe3(SiMe(r-Bu)2) was first reported as a stable complex with THF,34 and its crystal structure showed the length of the silicon-carbon double bond as 1.747 A. Subsequently, it was possible to remove the THF and isolate the uncomplexed silene, which had a noticeably shorter Si=C bond length of 1.702 A.29 Further investigation showed that stable complexes of this or closely related silenes with trimethyl- or ethyldimethylamine, pyridine, and fluoride ion were also readily formed and moderately stable.31141... 

Jones179 pioneered in adding f-butyllithium to vinylhalosilanes as a route to several silenes of the general structure RR Si=CHCH2CMe3, as shown... 

Whether the [2+2] or [2+4] cycloaddition product is formed in the first reaction depends on the reaction conditions however, one product is convertible to the other via the silene intermediate. In the other reactions shown, the identity of the product formed, [2+2] or [2+4], seemed to be a function of the structures of both the silene and imine. In the last case, the [2+4] product appeared to be the kinetic isomer since conversion to the [2+2] isomer slowly occurred on standing in the dark, or faster if photolyzed, even at room temperature. 

Finally, it has recently been shown207 that some silenes can add sulfur or selenium to give silathiiranes 110, as confirmed by a crystal structure, or silaseleniranes 111, respectively (Eq. 37). Only sterically crowded silenes such as 112 gave stable solid adducts, and in the case of the selenium species the adducts visibly decomposed in the solid state over a few days. 

Sander26 generated the silene Me2Si=CHMe in an argon matrix doped with oxygen. Based on infrared data, the structures of two products were assigned, as shown in Eq. (45). 

Analogous behavior was followed by the phenyl-substituted silene 156. The initially formed silene 157 underwent 1,3-methyl migration to give the silene 158, which then dimerized in a head-to-tail manner to yield three different stereoisomeric dimers 159, two of which were characterized by crystal structures. Again, the exchange of trimethylsilyl and trimethylsi-loxy groups at the ends of the Si=C bond occurred, followed by 1,3-methyl silicon-to-silicon rearrangements. The steps are summarized in Eq. (54). 

As discussed in Section II. A, theoretical studies predicted that phospha-silenes with silyl substituents attached to phosphorus should have planar, trigonally coordinated silicon, with the Si—P w-bonds strengthened by the hyperconjugative influence of the silyl group.16 Recently, this was proved by a single-crystal X-ray structure determination of the derivative... 

Structures that come to mind for assignment to die observed spectra are those that would be expected from an initial disproportionation of two primary silyl radicals to a silene and a hydrosilane, followed by an addition of another silyl radical to the silene, producing a / silylated silyl radical. Repetition of the process would eventually lead to a highly sterically encumbered and undoubtedly persistent silicon-based radical carrying only silicons in its / positions ... 

Silenes and saturated silicon compounds that are coordinated by donors and that thus have higher coordination numbers at the silicon centres exhibit unusual reactivities [1]. An interpretation of these experimental findings is possible when the structural features of the stable model compounds [C6H4CH2N(CH3)2]2SiCl2 (1) [2], [C6H4CH2N(CH3)2]3SiH (4), and [C6H4CH2N(CH3)2]4Si (5) are considered [3]. 

In summary, it is clear that in the relatively few years since the synthesis of the first silene was reported, a wide variety of reliable routes has been developed which can lead cleanly to silenes that vary widely in structure and stability. 

Recently, Wiberg has reported the X-ray crystal structure of the tetrahydrofuran (THF) adduct of another stable silene, 2 (166). The... 

---