Si-phenyl bonds

The Isometric ion plots of Figures A and 5 indicate that evolution of benzene from the silicone-epoxy samples occurs in two distinct stages, with the low temperature peak attributable to residual solvent species. Above 200°C, thermal degradation processes involving scission of the Si-phenyl bond occur and account for the increased formation rate of benzene. The other high temperature volatile products are similar to those observed for the novolac epoxy samples, and are attributed to decomposition of the epoxy fraction of samples D and E. 

The linker was used in combination with alkylation, acylation, and Mitsunobu reactions according to Scheme 37, and the Si-phenyl bond was cleaved with either TFA vapor, neat TFA [93], or TFA/CH2CI2 (1 1) [94],... 

The replacement of some of the methyl groups in polydimethylsiloxane by phenyl groups improves the thermal stability but reverses the dependence of stability on MW noted above in addition, benzene is produced in small amounts and insolubility develops, even at low temperatures. A process of OH-assisted cleavage of Si— phenyl bonds, leading to chain branching, has been proposed, " which accompanies a similar process of cyclic oligomer formation to that given in Scheme 40 for polydimethylsiloxane. 

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 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). 

Functionalization of polysilanes by chemical modification (post-polymerization) was covered in COMC II (1995) (chapter Organopolysilanes, p 101), where the formation of precursor polysilanes with potentially functionalizable side groups such as chloride, type 34 (via HCI/AICI3 chlorodephenylation of PMPS), 6 triflate, type 35 (via triflate replacement of phenyl groups)135,137 or alkyl halide (via chloromethylation of phenyl groups,138,139 type 36, or addition of HC1 or HBr to double bonds140) was discussed. Four other precursor polysilanes, which utilize the reactivity of the Si-Cl or Si-H bond, have been successfully applied in functionalization since COMC (1995) perchloropolysilane, 17 (see Section 3.11.4.2.2.(i) for synthesis),103 poly[methyl(H)silylene-f >-methylphenylsilylene],... 

A different methodology using the reactivity of the Si-H bond in 38 was reported in two communications, as COMC (1995) was being completed. Precursor poly[phenyl(H)silylene] 38 is conveniently prepared by the... 

Silanes can react with acceptor-substituted carbene complexes to yield products resulting from Si-H bond insertion [695,1168-1171]. This reaction has not, however, been extensively used in organic synthesis. Transition metal-catalyzed decomposition of the 2-diazo-2-phenylacetic ester of pantolactone (3-hydroxy-4,4-dimethyltetrahydro-2-furanone) in the presence of dimethyl(phenyl)silane leads to the a-silylester with 80% de (67% yield [991]). Similarly, vinyldiazoacetic esters of pantolactone react with silanes in the presence of rhodium(II) acetate to yield a-silylesters with up to 70% de [956]. 

It should be pointed out that the Si—C bond length is influenced by the hybridization state of the carbon atom. The Si—C distance in vinyl-, ethinyl- and phenyl-silanes is shortened as compared with that of corresponding alkylsilanes [examples13 H3SiCH3 (186.7 pm), H3SiCH=CH2(185.3 pm), H3SiC=CH(182.6 pm)]. 

Additional to Si—N, Si—0 and Si-halogen bonds, silicon can also form partial double bonds [a + (p - d)ff] to carbon. However, such 7r-interactions are only possible, if the carbon atom is a part of an unsaturated group (sp2- or sp-hybridization), e.g. in phenyl, vinyl and keto groups, etc. This is shown by the following resonance structures of vinylsilanes [Eq. (2)] and a-silylketones [Eq. (3)] ... 

The data also suggest that phenyl substitution weakens Si—H bonds, but only by a small amount (ca 4-8 kJ mol-1) per phenyl group. The difficulty here is that since methyl... 

Ab initio computations at the Hartree-Fock level were carried out on two conformations of silylbenzene17 and the Si—C(phenyl) bond length was calculated to be 1.8729 A. Longer Si—C(aryl) bonds have been found in only a few compounds, such as 3357 (1.941, 1.954 and 1.972 A) and 3458 (1.910 and 1.993 A). In both cases the lengthening may be attributed to steric congestion. 

---