Alkenes vinylsilanes

Carbonylation of alkenes bearing suitable functional groups proceeds regio-selectively. Carbonylation of vinylsilane and / -substituted vinylsilanes pro-... 

Computational investigations of vinylsilanes indicate that there is a groimd-state interaction between the alkene n oibital and the carbon-silicon bond which raises the energy of the n HOMO and enhances reactivity. Furthermore, this stereoelectronic interaction favors attack of the electrophile anti to the silyl substituent. 

Different substrate geometries can even result in alternate reaction pathways operating. The reactions between trans-a, (3-epoxytrimethylsilane 115 and organo-metals (metal = Li, Ce, or La) give predominantly trans-alkene 116 in high yields (Scheme 5.25) [38]. In contrast, treatment of cis-115 with some of the same organo-metals produces (Z)-vinylsilanes. The use of a bulkier substituent on silicon (e. g.,... 

Finally, in a study of Lewis-acid-catalysed intramolecular attack of acetals on vinylsilanes, to produce allylically unsaturated oxacyclics, it has been found (75) that the alkene stereochemistry can control the mode of cyclization in an exo- or endocyclic sense, as shown here ... 

These alkene isomers are separately available (4) by treatment of threo-S-trimethylsilyloctan-4-ol, prepared by reduction of the corresponding ketone with DIBAL in pentane at —120°C, with base or acid. The preparation of 5-trimethylsilyloctan-4-one itself illustrates three general procedures the addition of alkyl lithium reagents to vinylsilanes to generate a-lithiosilanes, the preparation of complex /5-hydroxysilanes, as diastereoisomeric mixtures, and the oxidation of such compounds to /8-ketosilanes... 

High levels of diastereocontrol in an ISOC reaction were induced by a stereogenic carbon center that bears a Si substituent (Scheme 23) [55]. For instance, conversion of nitro alkenes (e.g., 199) to j3-siloxyketones (e.g., 203) has been accomplished via a key ISOC reaction-reduction sequence with complete control of 1,5-relative stereochemistry. The generality of the ISOC reaction of a silyl nitronate with a vinylsilane was demonstrated with seven other examples. Corresponding INOC reaction proceeded with lower stereoselectivity. 

Hydroalumination of terminal alkenes using EtjAl as the hydride source must be carried out with titanium catalysts [24], since zirconium compounds lead to the formation of alumacyclopentanes [60, 61] (Scheme 2-11) and carbometallated products [62]. Suitable substrates for hydroalumination include styrene, allylnaphthalene and vinylsilanes. Only one of the ethyl groups in EtjAl takes part in these reactions, allowing the synthesis of diethylalkylalanes, which are difficult to obtain by other methods. 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

In contrast, ruthenium catalysts gave the best results for the cross-metathesis reactions of vinylsilanes with a range of unfunctionalised alkenes [8] (a typical example is shown in Eq. 4). 

Among the latter group, iridium complexes (though less common than rhodium) and perhaps also ruthenium play crucial roles in many of the above-mentioned transformations of silicon compounds, leading to the creahon of sihcon-carbon bonds. Examples include the hydrosilylation or dehydrogenahve silylation of alkenes and alkynes, the hydroformylahon of vinylsilanes, and the silyhbrmylation of alkynes as well as activation of the sp C—H of arenes (by disilanes) and alkenes (by vinylsilanes). 

An interesting variation of the dehydrogenative silylation system involves the platinum complex-catalyzed reaction of 1-alkenes with disilanes to produce vinylsilanes.40 In this system, one H atom and a silyl group are released by the reactants to yield the alkenylsilane product, rather than the two hydrogens released in reactions of hydrosilanes [Eq. (6)]. 

However, the directing influence of silicon can be overcome if the vinylsilane contains another substituent that can stabilize a carbocation more strongly than silicon. For example, when the silyl group is attached to C-2 of a terminal alkene, reaction occurs to give the more substituted carbocation 82 (equation 44)107. Similarly, if the silicon is bound to the same carbon atom as a phenyl group, reaction occurs via the benzyl cation to give the product shown in equation 45108. 

For alkyl(silyl)carbenes where the alkyl contains an a-C—H bond, 1,2(C C) hydride shift leading to a vinylsilane is the common reaction pathway. Vinylsilane formation has been observed for free (photochemically or thermally generated) carbenes (equations 4384, 4448.49.50 and 45 85,86) but also in carbenoid reactions. In the latter case, the configuration of the alkene could be controlled to a large extent by the choice of the catalyst The -alkene was formed nearly exclusively with copper(I) chloride as catalyst87, whereas rhodium(II) pivalate88 gave mainly the Z-alkene (equation 46). 

Vinylsilanes react with boron trichloride to give the corresponding borodesilylation products in good yield which, in turn, can be transformed into boronic esters 124 by alcoholysis (equation 102). The initial dichloroorganoborane products can be used directly in the Suzuki-Miyaura cross-coupling reaction192. Replacement of a carbon-silicon bond by a carbon-tin bond in fluorinated alkenes (e.g. 125) can be achieved by the reaction of silanes with Bu3SnCl and KF in DMF under mild conditions (equation 103)193. It is... 

Disproportionation between vinylsilanes and monosubstituted alkenes catalyzed by ruthenium complex (equation 104) has been suggested to occur via a /1-silyl group elimination. The intermediate silylruthenium complex, RuCl(CO)(PPh3)2(SiMe2R1), has been characterized by spectroscopic means194. 

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