1.4- Disilyl-2-alkenes

Exocyclic bis-silylated olefins have been constructed through the Pd(OAc)2-catalyzed reaction of alkynes with a tethered disi-lanyl group. The reactions are carried out in the presence of a tert-alkyl isocyanide, although the precise role of this ligand is unclear. Diimide reduction of the disilylated alkene so-formed followed by Fleming-Tamao-type oxidation of the two C-Si bonds in the saturated product then affords 1,2,4-triols in a stereoselective manner (eq 83).l ... 

This method can be used for the silylation of a variety of alkynes. It provides a practical route for the preparation of m-disilylated alkenes with two different silyl groups, which could be used for further transformation such as cross-coupling reactions. 

Platinum(O) phosphine complexes undergo a variety of oxidative addition reactions with compounds containing Group 14 elements. These reactions are of widespread interest because similar processes are probably involved in the catalysis by platinum complexes of reactions such as the hydrosilation of alkenes and the disilylation of dienes and alkenes. 

The reactions of styrene, 4-chlorostyrene, 4-methoxystyrene, and methyl acrylate with l,2-bis(dimethylsilyl)benzene (14) catalyzed by Pt(CH2=CH2)(PPh3)2 give the corresponding dehydrogenative double silylation products 15 in good to high yields (equation 7)14. When dimethyl maleate is employed, benzo-l,4-disilacyclohexene 16 (R1 = R2 = C02Me) is obtained as the major product (15/16 = 22/78) (equation 7). In the reactions of 1-alkenes, i.e., ethylene and 1-octene, the formation of monosilylated products is also observed (13-57% yield). On the basis of the fact that no deuterium is incorporated into the products when l,2-dideuterio-l,2-bis(dimethylsilyl)benzene is used, disilyl-Pt metallacycle 17 is proposed to be the key intermediate of this process (equation 7). 

In a series of papers West and coworkers described the reactions of disilene 9 with several epoxides to give the disilyl enol ethers 125, the five-membered ring compounds 126 — the formal insertion products, as well as the products of epoxide deoxygenation, namely alkenes and the disilaoxirane 62 (equation 29)80,83,124 125. 

As pointed out in the introduction, a particular feature of hydrosilylation reactions is that they require catalysis. Arguably the most valuable of enantioselective synthetic methods are those in which asymmetric induction occurs from small quantities of enantiomerically pure catalysts. It is natural, therefore, that considerable effort has been directed towards the catalytic enantioselective hydrosilylation-oxidation of C —C double bonds. Some degree of success has been met in the hydrosilylation of simple alkenes and 1,3-dienes, and in intramolecular hydrosilyla-tions. Also, as discussed at end of this section, a catalytic enantioselective disilylation (effectively the same as a hydrosilylation) has been developed for a,)3-unsaturated ketones. 

Titanocene dichloride catalyzes the reduction of alkyl, aryl, and vinyl bromides, aryl chlorides, alkoxy- and halosilanes ketones, esters, and carboxylic acids with alkyl Grignard reagents. This Cp2TiCl2/RMgX system can also be used for the hydromagnesation of alkynes, dienes, and alkenes (Section 3.2.5). Kambe et al. have reported a new type of titanocene-catalyzed transformation with vinyl Grignard reagents and chlorosilanes to furnish l,4-disilyl-2-butenes, as shown in Scheme 3.43 [31]. 

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