Silanediyl

In the course of this development, knowledge about low valent (in the sense of formal low oxidation states) reactive intermediates has significantly increased [26-30]. On the basis of numerous direct observations of silylenes (silanediyles), e.g., by matrix isolation techniques, the physical data and reactivities of these intermediates are now precisely known [31], The number of kinetic studies and theoretical articles on reactive intermediates of silicon is still continuously growing... 

Fig. la. Structure of the complex bis( -butoxysilane-diyl)tetracarbonyliron(O) x HMPA 4. b. Molecular structure of the chromium complex bis( -butoxy-silanediyl)pentacarbonylchromium(O) x HMPA 9. c. Van der Waals surface of 9... 

As in the case of silanone 9, the reaction of the silylene bis[2-(dimethylamino-methyl) phenyl]silanediyl (8) with phenyl isothiocyanate was examined.29 In this reaction the expected silanethione 36 was obtained as a single product even in the presence of (Me2SiO)3, no insertion product of 36 into a Si-O bond of (Me2SiO)3 was observed (Scheme 10). 

The chemistry of the carbenes95 94 and their analogues, also referred to as silanediyls,95-97 germanediyls97,98 and stannanediyls97,99 (IUPAC nomenclature) have been reviewed extensively. 

Major advances in organometallic chemistry during the last years have been achieved in the area of silicon-metal multiple bonding and silicon with low coordination numbers. For late transition metals, new complexes have been synthesized such as silanediyl (A), silene (B), silaimine (C), disilene (D), silatrimethylenemethane (E), silacarbynes (F), cyclic silylenes (G), silacyclopentadiene (H) and metalla-sila-allenes (I) (Figure 3). 

The chromium complexes are proved to be silanediyl complexes, as shown by the silicon-transition metal bond lengths (Table 5) and by the extreme low field shift of the 29Si NMR signals (124.9 and 121.2) at 22 °C for R = H and CH2NMe2, respectively (Table 6). The 29Si NMR shifts of these complexes are temperature-dependent due to the hindered rotation of the phenyl ring and dynamic coordination of the nitrogens to the Si atom. 

J. O. Hoberg, Synthesis and chemistry of 4,6-0-di-(tert-butyl)silanediyl-D-glucal, Carbohydr. Res., 300 (1997) 365-367. 

Processes of this kind have some synthetic utility, since they permit exchange of silicon-containing groups from the readily prepared Si-Co compounds to other metal centres. The partial exchange of groups on silanediyl derivatives also provides a route to mixed-metal systems ... 

This process, shown as mode 2 in Fig. 2, involves the production and reaction of functionally substituted silyl and silanediyl species. Since this occurs while the silicon-metal bond remains intact, it is not surprising that most of the entries in Table XII relate to compounds with robust Si-Mn and Si-Fe bonds other examples involve bonds to Cr, Mo, W, Re, Pt, and even Co. The types of exchange reaction observed will be dealt with in turn. 

In the case of silanediyl derivatives, both metal fragments can be lost as anions, e.g. (entry 9),... 

There is no evidence that they will react with CO to form an open silanediyl derivative, although germanium analogs undergo the following reversible change (49, 263) ... 

Protection of the 2f-hydroxyl group of N acyl"3 5 -0-di(fm-butyl)silanediyl nucleoside derivatives was accomplished with SEMCl using rm-butylmagnesium chloride as the base [Scheme 4.306].556 Under the usual conditions (SEMCl, /-Pr2NEt), N-alkylation of the base was observed. 

Transition metal derivatives are known to stabilize unusual molecules in their coordination sphere. First studies on silanediyl- and silyl complexes of iron carbonyls have been carried out [7-9],... 

IR bands due to water have been crossed for silanediyl (4) a broad absorption band is recorded in the visible region of the spectrum ( ax= 500 nm)... 

Structural type D molecules can be synthesized in a two step reaction sequence K2[Cr(CO)5] (2) reacts with the h)q)ervalent trichlorosilane (2-Me2NCH2C5H4)SiCl3 (4) in THF at —25 °C to afford the chloro-functionalized silanediyl complex 5 in 55 % yield. 

A similarly distorted tetrahedral arrangement around the silicon atom is found in the intermolecular HMPA-stabilized acyclic heterobutadiene 3 (Fig. 4). Compound 3 crystallizes in the orthorhombic space group / i2. The Crl-Sil, Sil-Cll, and Sil-06 interatomic bond lengths of 3 are comparable with the distances found in other silanediyl compounds of type [R2Si=Cr(CO)5] HMPA [9]. 

SCHEME 18.2 Proposed mechanism for the formation of polysilanes by metallocene-catalyzed dehydrocoupling of alkylsilanes, RSiHj, via metallocene-silanediyl intermediates. ... 

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