Si-C cleavage

The photochemistry of benzyl-substituted digermanes is thus basically the same as that observed for vinyl- and styryl-substituted digermanes reported in Section in. In comparison, the equivalent disilane compounds exhibit only Si—Si bond homolysis, with no Si—C cleavage or silylene formation39. 

As shown for 47 in Scheme 9, different methods were applied for the syntheses of these compounds (48 synthesized according to methods a-c). All reactions were carried out in acetonitrile at room temperature and compounds 47-jCH3CN and 48 were isolated as crystalline solids. The benzoin-mediated Si-C cleavage reactions (methods b-d in Scheme 9) are especially remarkable. 

As illustrated for the synthesis of 81 in Scheme 19, compounds 81-84 were prepared by different methods (82 was obtained according to methods a and b 83 and 84 were synthesized by method a). All syntheses were carried out in acetonitrile at room temperature and the products isolated in good yields as crystalline solids. Method b involves a remarkable Si-C cleavage reaction. 

While method A furnished only dimethyl(vinyl)silane (see Section IILC.l.a), the Wurtz reaction (method B) in an inert solvent gave a mixture of products which stem either from the a-elimination generating 3a or from the C,C coupling process leading to 158 (equation 38)55. It is assumed that 3a forms 1,1-dimethylsilirane by 1,3-C,H insertion (cf equation 23) and that C,C and Si,C cleavage of this sensitive ring system by (trimethylsi-lyl)methyl sodium is responsible for the formation of 159-161 (combined yield 32%). It is obvious that, due to the complexity of this reaction, method B cannot serve as a versatile entry into the chemistry of 3a. 

These reactions, shown in Table VII, are chiefly a special type of oxidative addition process. Most important are those involving insertion of "Fe(CO)4 this is thought to occur via electrophilic Si-C cleavage in an essentially concerted process. Experiments with substituted silacyclobutanes have shown that the process is both regio- and stereospecific (135). Other metals seem reluctant to undergo this reaction the manganese analog in entry 23 is thermally very unstable. 

The unusual products are only formed with alkyl- or predominantly alkyl-substituted phosphines [Pt(C2H4)2(PR3)](R3 = Ph3, Ph2Me) are not capable of inducing Si—C cleavage. 

Organosilane radical cations undergo alcohol-assisted nucleophilic Si-C cleavage that is rapid enough to compete with return ET to the sensitizer in polar solvents, and that raises quantum yield in nonpolar solvents when a nucleophile is tethered in proper position for the nucleophilic cleavage [91], Photo-ET allows cleavage of a trimethylsilylated enol ether in the presence of a trimethylsilylated saturated alcohol, although other products are also formed, so the deprotection is not as clean as desired [92],... 

No adsorption effects or Si-C cleavage are observed even in the trace range. The idea that an unacceptable blank is produced by even a small amount of cleavage of dimethyldifluorosilane to methyltrifluorosilane during the depolymerization can thus be ruled out. 

Keywords lithium, Si-C cleavage, silyl anions, silyllithium compounds, tetraorganosilanes... 

Stereoselective intramolecular alkene hydrosilylation followed by Si-C cleavage is a valuable route to diols both relative - and absolute stereochemistries may be controlled. The rates of the fundamental steps in the [Rh(diphosphine)] catalysed reactions are controlled to some extent by the nature of the diphosphine. From deuterium-labelling studies a silyl insertion mechanism becomes apparent. Whether such mechanisms are applicable to C=0 hydrosilylation versions of this reaction is not yet known. Other highly enantioselective (83-96% ee) C=0 silylations use 13,162 but attempts to use readily available (-)-sparteine as a rhodium ligand are much less successful (5-8% ee).i Further details of the spectacularly effective MOP ligand 14 have appeared.i Optical purities of 93-96% ee are realised using 0.01 mol% Pd2( x-Cl)2( n-C3H5)2 and 0.02 mol% 14 for terminal alkenes and norbomene. 

The first reaction carried out in this way was with (Cl3Si)2CH, and no Si—C cleavage was observed [99] ... 

The enhancement of Si—H substitution and Si—C cleavage due to the use of more polar solvents confirms that changes of polarization in the molecule determine the course of reaction. In carbosilanes such as (H3Si—CH2)2SiH2, partial charge... 

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