Alkenes vinyl silanes

Group 4 metallocene complexes can also be used as catalysts in the reduction of C=N bonds. Willoughby and Buchwald employed the titanium-based Brintzinger catalyst (3.54) for the asymmetric reduction of imines. The catalyst is activated by reduction to what is assumed to be the titanium(III) hydride species (3.55). The best substrates for this catalyst are cyclic imines, which afford products with 95-98% ee. Various functional groups including alkenes, vinyl silanes, acetals and alcohols were not affected under the reaction conditions. For example, the imine (3.56) was reduced with excellent enantioselectivity, without reduction of the alkene moiety. 

In an effort to identify a more stereoselective route to dihydroagarofuran (15), trimethylsilylated alkyne 17 was utilized as a substrate for radical cyclization (Scheme 2). Treatment of 17 with a catalytic amount of AIBN and tri-n-butyltin hydride (1.25 equiv) furnishes a mixture of stereoisomeric vinyl silanes 18 (72% combined yield) along with an uncyclized reduction product (13% yield). The production of stereoisomeric vinyl silanes in this cyclization is inconsequential because both are converted to the same alkene 19 upon protodesiiyiation. Finally, a diastereoselective di-imide reduction of the double bond in 19 furnishes dihydroagaro-... 

In addition to electron-deficient alkenes, under the catalysis of TiCLt, 1,2-allenylsi-lanes can react with aldehydes or N-acyliminium ion to afford five-membered vinylic silanes 71 and 72. Here the carbocations generated by a Lewis acid regiospecifically attack the C3 of the 1,2-allenylsilanes to produce a vinyl cation stabilized by hyper-... 

Cross-metathesis of enynes having various functional groups on the alkyne and an alkene gives dienes having useful functional groups such as vinyl silane or enol ether as the sole product ... 

Hydrosilylations of fluorine-containing alkenes are free radical reactions initiated by UV light or organic peroxides The direction of addition is the same as with fluonnated alkyl halides However, the reaction between hydrosilanes and fluorine-containing olefins catalyzed by platinum group metal complexes may result in bidirectional addition and/or formation of a vinylic silane, the latter by de hydrogenative silylation[/] The natures of both the silane and the catalyst affect the outcome of the reaction[7] A random selection of some typical new reactions of silanes are shown in Table 1 [1, 2, 3 4]... 

Acyl silanes can display disparate behaviour when treated with carbon nucleophiles, even of related types5,61149. For example, when aroyl silanes were treated with a Wittig reagent, none of the expected alkenes was obtained, and the only reaction products isolated were silyl enol ether and triphenylphosphine (Scheme 73)182,183. When alkanoyl silanes were treated with Wittig reagents, however, only the normal olefinated vinyl silane products were isolated (Scheme 74)182-184 Under soluble lithium salt conditions, Z-vinyl silanes were produced with very high selectivities the reaction was used to prepare a pheromone component (50) of the sweet potato leaf folder moth (Scheme 75)183. 

Alkenylsilanes, mainly vinyl silanes and allyl silanes or related compounds, being widely used intermediates for organic synthesis can be efficiently prepared by several reactions catalyzed by transition-metal complexes, such as dehy-drogenative silylation of alkenes, hydrosilylation of alkynes, alkene metathesis, silylative coupling of alkenes with vinylsilanes, and coupling of alkynes with vinylsilanes [1-7]. Ruthenium complexes have been used for chemoselective, regioselective and stereoselective syntheses of unsaturated products. 

Vinyl silanes offer a regio- and stereoselective route to alkenes... 

Vinyl silanes have C-Si bonds orthogonal to the p orbitals of the alkene—the C-Si bond is in the nodal plane of the Jt bond—so there can be no interaction between the C—Si bond and the Jt bond. Allyl silanes, by contrast, have C-Si bonds that can be, and normally are, parallel to the p orbitals of the n bond so that interaction is possible. 

Allyl silanes react with electrophiles with even greater regioselectivity than that of vinyl silanes. The cation 3 to the silyl group is again formed but there are two important differences. Most obviously, the electrophile attacks at the other end of the allylic system and there is no rotation necessary as the C-Si bond is already in a position to overlap efficiently with the intermediate cation. Electrophilic attack occurs on the face of the alkene anti to the silyl group. The process is terminated by loss of silicon in the usual way to regenerate an alkene. 

It is all true. Evidence that a silyl group stabilizes the S 2 transition state comes from the reactions of the epoxides of vinyl silanes. These compounds can be made stereospecifically with one equivalent of a buffered peroxy-acid such as m-CPBA. Epoxidation is as easy as the epoxidation of simple alkenes. You will see in a moment why acid must be avoided. 

The product is a vinyl silane as the M Si group is retained in this reaction of the anion. The reaction is stereoselective in favour of the E-alkene as might be expected. The alkene can be epoxidized... 

By itself, the Brook rearrangement is not very useful but, if the carbanion can do something else other than just get protonated, something useful may happen. We have seen what happens to the epoxides of vinyl silanes. Dihydroxylation of the same alkenes also gives interesting chemistry when the diols are treated with base. 

The low regioselectivity of the enc reaction of singlet oxygen with mono-alkenes can also be improved by incorporation of trialkylsilyl groups at a vinylic position. Thus, simple vinyl-silanes afforded, after photooxygenation and reduction, regioselectively, /1-silyl allylic alcohols35. The diastereoseleclivity of the titanium-catalyzed reaction of the /1-silyl allylic hydroperoxides is described in Section 4.9.4.2. 

Now the aldehyde is oxidized to an add and the acid chloride does an intramolecular, Lewis-ac . -catalysed, aliphatic Friedel-Crafts reaction on the vinyl silane. Notice attack at the ipso carbon give a stable P-sUyl cation intermediate and that this vinyl silane has to react with inversion as frans-alkene is impossible in the ring. 

Our preferred solution for the first compound is a Nazarov reaction as the starting material is symmetrical and it is not necessary to use a vinyl silane, though this would work well too. The starting alkene is easily made from ris-butenediol. The double bond in the product will much prefer to go inside the ring where it is more highly substituted. 

For sulfoxides with P-silyl substituents there is some preference for elimination to take place towards the silyl group. On protodesilylation both the allyl- and vinyl-silanes formed give the same alkene, so providing a regioselective and high yielding alkene synthesis (Scheme 5). If there is no P-hydrogen,... 

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