Silylative dimerization

Another method for reductive dimerization has been developed in hy-drosilylation. NiCl2-SEt2 is an effective catalyst in silylative dimerization of aromatic aldehydes with a hydrosilane (Scheme 12) [40]. A catalytic thiolate-bridged diruthenium complex [Cp RuCl(/ 2-SPrI)2RuCp ][OTf] also induces the conversion to 1,2-diaryl-1,2-disiloxyethane [41]. A dinuclear (siloxyben-zyl)ruthenium complex is considered to be formed, and the homolytic Ru - C bond fission leads to the siloxybenzyl radicals, which couple to the coupling product 14. 

A pinacol-type silylative dimerization of various aromatic aldehydes promoted by a cationic thiolate-bridged diruthenium complex has been reported by Flidai and co-workers (Equation (28)).344 l,2-Diaryl-l,2-disiloxyethanes 153 were isolated as the major products along with the corresponding arylmethyl silyl ethers as minor products. 

Bis-silylation of 1,3-dienes has been studied extensively, showing that 1,4-addition occurs selectively in the presence of palladium catalysts (Equation (40)). Not only the simple 1 1 addition, but also bis-silylative dimerization of... 

The first attempted silylation of nitromethane resulted in the production of a silylated dimeric nitronate due to self-condensation (17). Subsequent investigations, however, have shown that sUyl nitronates are stable compounds that can be isolated and distilled at reduced pressures, Table 2.1 (18-21). Bulkier silyl groups help increase the stability of the resulting nitronate (21). 

Ruthenium. In 1995, Hidai reported that a diruthenium complex can catalyze the silylative dimerization of a wide variety of aromatic aldehydes (Eq. 3.29) [49, 50]. The analogous reaction of acetophenone proceeds more slowly and furnishes only 33% of the desired coupling product. It is postulated that this ruthenium-catalyzed... 

In sharp contrast to the predominant formation of 1 1 adducts, reactions of 1,3-dienes with various disilanes in the presence of phosphine-free palladium catalyst provide bis-silylative dimerization products 63 in high yields (Eq. 30) [58,59]. In comparison with the Pd(OAc)2 and PdCl2(ArCN)2 catalysts originally found [58], the use of Pd(dba)2 (dba dibenzalacetone) in DMF was found to be more effective in enabling the reaction to proceed at room temperature [59]. Worthy of note is that head-to-head coupling products 63 having carbon-carbon double bonds with E-geometry are exclusively formed in these reactions. 

Allylsilanes, which are accessible by bis-silylation of 1,3-dienes and silylation of allylic substrates, are useful allylation reagents in organic synthesis [99, 100] ( , )-l,8-bis(Trimethylsilyl)octa-2,6-diene 63a, prepared by palladium-catalyzed bis-silylative dimerization of 1,3-butadiene, was successfully applied to the synthesis of dZ-muscone (Scheme 8) [58]. A key feature of the synthesis is regi-oselective reaction with acetaldehyde at the positions y to the silyl groups. 

Successive insertions into a Li—Si bond are believed to form the basis of a novel dimerization of isocyanides Reaction of a range of isocyanides with phenyldimethylsilyllithium, followed by treatment with trialkylchlorosilane, offered silylated dimers of the isocyanide. 

Trimethylsilane in the presence of palladium gives 1,4-dihydro-1-trimethylsilylpyr-idine, together with silylated dimer titanium-catalysed hydrosilylation produces a tetrahydro-derivative cleanly. ... 

In sharp contrast to the predominant formation of l l-adducts, reactions of 1,3-dienes with various disilanes in the presence of phosphine-free palladium catalyst provide bis-silylative dimerization product in high yield (Scheme... 

Silylsubstituted oligothiophenes have also been prepared to investigate the effect of dn Pn conjugation with the terminal silicon atom. The a-a and a,co silylated dimer, trimer and tetramer (Scheme 14.8) were directly prepared via lithiation of the parent bi, ter and quaterthiophene [20]. 

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