Oligosilanes mechanism

We think two mechanisms take place at the same ime. Besides the P -elimination mechanism, which has been described above as a kind of disproportionation, an a-elimination like the Harrod mechanism seems also to take place. From the experimental results, the polymerization of trisilanes or tetrasilanes yields preferably the dimeric species, hexa- or octa-silanes respectively. In addition, oligosilanes with odd numbers of silicon atoms were formed. We do not yet understand why these hexa-and octasilanes were formed in the iso-forms. 

The photochemical addition of some cyclic oligosilanes to Ceo has also been found interesting. Scheme 8.8 shows some examples of such a transformation. Irradiation (X > 300 nm) of a toluene solution of disilirane 36 with Ceo afforded the fullerene derivative 37 in a 82% yield [37]. The reaction mechanism is still unknown. When toluene is replaced by benzonitrile the bis-silylated product of the solvent was obtained in good yields. In these experiments a photoinduced electron transfer between 36 and Ceo is demonstrated, indicating the role of Ceo as sensitizer [38]. The photoinduced reactions of disilirane 36 with higher fullerenes such as C70, Cv8(C2v)and CuiDi) have also been reported... 

Laser flash photolysis experiments showed that the (Reaction 8.17) reacts with cyclic oligosilanes 42, 43 and 44 in benzonitrile by an electron transfer mechanism [43]. The rate constant ( et) for the three-membered cyclic compound 42 is found to be 7.0 x 10 M s, whereas for the other two compounds it was more than two orders of magnitute lower, i.e., (1-2) X 10 M- s-. ... 

Much attention has been focused on the detailed mechanism of silylene extrusion from oligosilanes. The following three possible pathways (A-C, equations 4-7) should be first taken into account. 

A marked improvement of the yield of 141b (to 64%) was achieved by the dehalo-genation of an oligosilane with sodium in toluene at room temperature however, the mechanism of this reaction has not yet been elucidated (equation 34)137. 

The size of the ring formed in each case is strongly dependent on the reaction conditions. When KC8 is used instead of sodium for the dehalogenation of the oligosilane R3SiSiBr2Cl, the unsymmetrically substituted disilene 151 is obtained. 151 is the first synthesized cyclotrisilene (equation 37>137. The mechanism of formation of 151 is also still a matter of speculation. 

Acheson process, 566 Acid-initiated ring-opening polymerization comparison widi base-initiated process, 79 kinetics, 79-80 mechanism, 79-81 rate retardation by water, 81 relative reactivities of cyclosiloxanes, 79 step-growth process, 79 Acyclic oligosilanes, photochemical transformations, 432/-433/ Acylsilanes... 

Summary Reacticm of a-heteroatom-substituted oligosilanes with potassium alkoxides is of interest to obtain insight into the mechanism of the Wurtz type polymerization of halosilanes. It also provides access to building blocks with unique reactivity. Reactions with fluoro- and methoxy-substituted silanes exhibited initial formation of silylenoid species which can undergo self-condensation. This property is less pronounced with the alkoxysilanes, which allowed for the isolation and structural characterization of an a-methoxy-silyl potassium compound (3). 

Similar generation of palladium-silylene complexes may be involved in reaction of oligosilanes with aryl isonitriles giving four-membered rearranged products 95, although the mechanism has not yet been clarified (Eq.49) [81,86]. 

Several areas dealing with the photochemical processes in silicon containing compounds have also received renewed attention. Thus the photochemical reactivity of oligosilanes, polysilanes and silylenes has been the subject of a detailed review. Aspects of the mechanisms for the processes were highlighted. Other publications have been concerned with the photoreactions of silanes (cyclic and acyclic) disilanes, silenes, etc and the photoinduced electron-transfer reactions involving organosilicon compounds. ... 

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