Silane, vinyl synthesis

We have seen that vinyl silanes can be prepared by hydrosilylation of alkynes by three different mechanisms giving good control over geometry of these inevitably terminal vinyl silanes. Vinyl silanes are stable compounds and can be isolated, unlike most of the vinyl metals we have seen so far, and other ways of making vinyl silanes allow the more-or-less controlled synthesis of mono-or trisubstituted compounds with reasonable control over selectivity. These include the Peterson reaction with two SiMe3 groups on the same carbon atom 183 and, more relevant to this chapter, reactions of vinyl lithiums with silyl chlorides.44... 

In addition to the numerous examples of the McMurry couplings described above, the following examples illustrate the power of the transformation in the synthesis of a broad range of natural products and non-natural molecules. Of particular note in the following examples is the impressive functional group tolerance of the McMurry conditions. In general, alcohols, tosyl alcohols, alkyl ethers, silyl ethers, alkyl silanes, vinyl silanes, amines, sulfides, and alkenes are inert to McMurry conditions. Acteals, halides, alkynes, nitriles, and carboxylic acids are semi-compatible. 

Gross-Linking. A variety of PE resins, after their synthesis, can be modified by cross-linking with peroxides, hydrolysis of silane-grafted polymers, ionic bonding of chain carboxyl groups (ionomers), chlorination, graft copolymerization, hydrolysis of vinyl acetate copolymers, and other reactions. 

The synthesis of PDMS macromonomers with vinyl silane end-groups and their free-radical copolymerization with vinyl acetate, leading to poly(vinyl acetate)-PDMS graft copolymers, was described 346). The copolymers produced were later hydrolyzed to obtain poly(vinyl alcohol)-PDMS graft copolymers. 

Entries 11 to 13 are examples of iminium ion and acyliminium ion reactions. Note that in Entries 11 and 12, vinyl, rather than allylic, silane moieties are involved. Entries 14 and 15 illustrate the synthesis of (3,-y-unsaturated ketones by acylation of allylic silanes. 

Allylsilanes have been found to be more reactive than vinyl silanes toward electrophiles and they are being studied more intensively in recent years because they hold considerable promise in organic synthesis. In allylsilanes, the geometry of carbon-silicon bond can be more favourably be oriented for efficient stabilization of a developing positive change P to silicon. 

A more recent synthesis of 197 [365] is shown in Fig. 9. Enders introduced the stereogenic centre of (S)-lactic acid into the crucial position 10 in 197. The vinylsulfone B, readily available from lactic acid, was transformed into the planar chiral phenylsulfonyl-substituted (q3-allyl)tetracarbonyliron(+l) tetra-fluoroborate C showing (IR,2S,3 )-configuration. Addition of allyltrimethyl silane yielded the vinyl sulfone D which was hydrogenated to E. Alkylation with the dioxolane-derivative of l-bromoheptan-6-one (readily available from 6-bro-mohexanoic acid) afforded F. Finally, reductive removal of the sulfonyl group and deprotection of the carbonyl group furnished 197. A similar approach was used for the synthesis of 198 [366]. 

Analogously to the carbocycle and oxycycle synthesis, cyclic amines can be obtained by the hydrosilylation of a suitable enyne, such as 46 (Reaction 7.54), which gave the six-membered ring via a 6-endo cyclization of the vinyl radical onto the C=N bond [63]. In another example, the isothiocyanide functionality of compounds 47 or 48 reacts with silane under radical conditions... 

The ability to provide highly functionalized reagents, such as unsaturated silanes and vinyl boronates, starting from terminal olefins is one of the most attractive attributes of CM, particularly when traditional methods for the preparation of such compounds are not synthetically straightforward. Therefore, significant research has been undertaken to determine the broad-spectrum chemoselectivity of olefin metathesis catalysts. In many cases, the use of a CM protocol in reagent synthesis is completely orthogonal to alternative methods of preparation. 

We have previously reported on the synthesis of a series of mono- and bifunctional poly(DMS) having a variety of reactive end groups, such as silan (Si-H), vinyl, hydroxyalkyl, dimethylamino and carboxyllic acid groups.7 We have also described already on telechelic poly(DMS) having tosylate end group, lb and l b, where the hydrosilation reaction of poly(DMS) having silan end group was performed with allyl alcohol in the presence of Pt/C catalyst, followed by the tosylation reaction with tosyl chloride in the presence of dimethylaminopyridine.9... 

Apart from the Takai method and titanium reagents such as 15, silyl reagents 16 and 17 frequently find application in the synthesis of vinylic silanes from carbonyl compounds. Reagent 16 can be utilized with aldehydes and non-enolizable ketones in a reaction analogous to the Peterson olefination Reagent 17 also reacts successfully with enolizable ketones.6... 

Palladium-mediated addition of silyl stannane reagents to alkynyl ethers has been employed for the synthesis of aliphatic acyl silanes in very good yields via the intermediate a-alkoxy-/J-stannyl vinyl silanes (enol ethers of acyl silanes)82. In a second palladium-catalysed step, the vinyl stannane moiety could be coupled to suitable halides before hydrolysis to the acyl silanes with trifluoroacetic acid (Scheme 11). 

Hydrosilylation of divinyl ether has been applied for the synthesis of silacyclopentane 12 using Speier s catalyst (Scheme l)13. One of the two carbon-carbon double bonds was hydrosilylated first with a dialkyl(ethoxy)silane, giving silylethyl vinyl ether 10 in 53-59% yield, which was reduced with LiAlELr to hydrosilane 11. The intramolecular hydrosilylation of 11 affords silacyclopentane 12 in moderate yields (Scheme 1). The reaction with HSiEt2(OEt) gives 12a exclusively in 45% yield, while silacyclohexane 13b is formed as the minor product when HSiMe2(OEt) is used as the hydrosilane (12b/13b = 2.3/1 50% total yield)13. Other intramolecular hydrosilylation reactions useful in organic syntheses will be discussed in the section n.C. (vide infra). 

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