Cyclic silylenes complexes

Very recently, the coordination chemistry of low valent silicon ligands has been established as an independent, rapidly expanding research area. With the discovery of stable coordination compounds of silylenes [35-38], a major breakthrough was achieved. Within a short time a variety of stable complexes with a surprising diversity of structural elements was realized. Besides neutral coordination compounds (A, B) [35, 36, 38], and cationic compounds (C) [37], also cyclic bissilylene complexes (D) [39,40] exist. A common feature of the above-mentioned compounds is the coordination of an additional stabilizing base (solvent) to the silicon. However, base-free silylene complexes (A) are also accessible as reactive intermediates at low temperatures. 

By far more is known about the UV spectra of silylene-Lewis base complexes in matrices as well as in solution. To observe silylene-Lewis base complexes in matrices, an appropriate silylene precursor, in most cases a cyclic or acyclic oligosilane, is photolyzed at low temperature in an inert hydrocarbon or noble gas matrix, which is doped with small amounts (2-5%) of the respective Lewis base. In a few cases, 2-methyltetrahydrofuran (2-MeTHF), which serves as matrix material as well as a Lewis base, is used. When a rigid matrix such as a 3-methylpentane (3-MP) or 2-MeTHF matrix is used, the formation of the silylene complexes normally requires the matrix to be annealed in order to allow the molecules to diffuse through the softening matrix. In contrast, when the silylene is generated in an a priori soft matrix,... 

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). 

A 93% yield of a unique bis-silylene complex is obtained when a cyclic diaza derivative is combined with a metal carbonyl (b). 

In summary, the production of substituted silylenes and silyl radicals upon exhaustive irradiation at 254 nm of polysilane high polymers suggested that the polymer photochemistry resembled that previously reported for short chain acyclic and cyclic oligomers (39). More recent experiments, however, have suggested that the photochemical mechanism for the degradation of the high polymers is more complex than first envisioned (vide infra) (48). 

The silacyclopropanation of acyclic and cyclic alkenes with 169, catalyzed by AgOTf, occur at room temperature or even below to yield new cyclosilapropanes 173-177. In the case of chiral /3-pinene, the silacyclopropanation occurs enantioselectively (dr > 95 5) (Scheme 26).312 Mechanistic studies have been undertaken, which suggest that silyl silver complexes play an important role in the catalytic cycle of the silylene transfer.310... 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

Degradation of Larger Silicon Catenates. Although it seems clear that polymeric disilane derivatives photodecompose by bond homolysis to produce silyl radicals, model studies on larger silicon catenates indicate that their photochemistry may be more complex (Scheme IV). Cyclic silane derivatives seem to extrude monomeric silylenes upon irradiation to produce smaller cyclic silanes (52). The proposed silylene intermediates have been identified spectroscopically 49, 53), and trapping adducts have been isolated in solution. Exhaustive irradiation ultimately results in acyclic silanes, which... 

In reactions that are formally analogous to hydrosilylation, transition-metal complexes catalyze the insertion of unsaturated hydrocarbons into other Si-X (X = C, Si, Sn, etc.) bonds. Palladium catalysts seem to work best in many of these reactions. The work of Kumada and coworkers has already been referred to in connection with metal-catalyzed silylene transfer to alkynes (see equation 47)95"97. Sakurai s group has shown that the cyclic disilane 55 will add to alkynes in the presence of a palladium catalyst (equation 110, see also equation 80). The unstrained disilane Me3SiSiMe3 undergoes a similar reaction... 

Tsai et al. developed another reductive coupling method for the preparation of Zn-Zn-and Cd-Cd-bonded complexes (method xii) [29]. Cyclic dinudear M(II) compounds bearing two silylene-bridged diamide ligands were treated with KCg (for Zn) or KH (for Cd) to afford [LNNMMLNN] - (M = Zn, 14) or [L MLNNmMLNNmlnn]2- (M = Cd, 15), respectively (Scheme 12.8). Both products contain a direct M(I)-M(I) bond, and the latter contains two additional Cd(II) nudei. Calculations su ested that the formation of the M(I)-M(I) complexes is via mixed-valent M(II)—M(I) intermediates. 

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