Alkenes allylic silanes

Metallation of alkenes with rc-BuLi-KOC(CH3)3 provides a route that is stereoselective for Z-allylic silanes.68 (See p. 632 for discussion of this metallation method.)... 

Most reactions of alkenyl and allylic silanes require strong carbon electrophiles and Lewis acid catalysts are often involved. The most useful electrophiles from a synthetic standpoint are carbonyl compounds, iminium ions, and electrophilic alkenes. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

As the intermediate formed in a polyene cyclization is a carbocation, the isolated product is often found to be a mixture of closely related compounds resulting from competing modes of reaction. The products result from capture of the carbocation by solvent or other nucleophile or by deprotonation to form an alkene. Polyene cyclizations can be carried out on reactants that have structural features that facilitate transformation of the carbocation to a stable product. Allylic silanes, for example, are stabilized by desilylation.12... 

The intramolecular addition of carbon nucleophiles to alkenes has received comparatively little attention relative to heterocyclization reactions. The first examples of Pd-catalyzed oxidative carbocyclization reactions were described by Backvall and coworkers [164-166]. Conjugaled dienes with appended al-lyl silane and stabilized carbanion nucleophiles undergo 1,4-carbochlorination (Eq. 36) and carboacetoxylation (Eq. 37), respectively. The former reaction employs BQ as the stoichiometric oxidant, whereas the latter uses O2. The authors do not describe efforts to use molecular oxygen in the reaction with allyl silanes however, BQ was cited as being imsuccessful in the reaction with stabihzed car-banions. Benzoquinone is known to activate Ti-allyl-Pd intermediates toward nucleophilic attack (see below. Sect. 4.4). In the absence of BQ, -hydride eUm-ination occurs to form diene 43 in competition with attack of acetate on the intermediate jr-allyl-Pd" species to form the 1,4-addition product 44. 

A dinuclear Rh(II) complex, Rh2(pfb)4 (pfb = perfluorobutanoate), effectively catalyzes the hydrosilylation of 1-alkynes to give (Z)-l-silyl-l-alkenes as the major product when a trialkylsilane is added to the mixture of a 1-alkyne and the catalyst in CH2CI2111. However, the reverse addition, i.e. addition of the 1-alkyne to the mixture of the hydrosilane and the catalyst, affords an allylic silane in place of the silylalkenes in high yield (vide infra)111. 

As described above, Rh2(pfb)4 and ILPlCL effectively catalyze the hydrosilylation of 1-alkynes to give (Z)-l-silyl-l-alkenes and (E)-l-silyl-l-alkenes, respectively, as the major products. However, the exclusive formation of allylic silane 150 (68-73% yield as a mixture of E and Z isomers) is observed when the order of the addition of reactants is reversed, i.e. slowly adding 1-alkyne to the solution of excess hydrosilane (2 equiv.) in CH2CI2 at 25 °C over a period of 1 h (equation 59)111. The results imply that two different catalyst species, one for hydrosilylation and the other for the formation of allylic silane, are generated just by changing the order of the addition111. 

Table 9 Regioselectivities in the Photo-Oxygenation of Allylic Silanes, Alkenes, and Germanes...

In the presence of Lewis acids allyl silanes and stannanes react with epoxides generally at the sterically less demanding carbon atom. Other electron-rich alkenes, such as ketene acetals, can also be used as nucleophiles. The strong Lewis acids required might, however, also lead to rearrangement of the epoxide before addition of the nucleophile can occur (last reaction, Scheme 4.72). 

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. 

The carbon-silicon bond has two important effects on the adjacent alkenc. The presence of a high-energy filled CT orbital of the correct symmetry to interact with the n system produces an alkene that is more reactive with electrophiles, due to the higher-energy HOMO, and the same ff orbital stabilizes the carbocation if attack occurs at the remote end of the alkene. This lowers the transition state for electrophilic addition and makes allyl silanes much more reactive than isolated 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. 

Vinyl and aryl silanes react with electrophiles at the same (ipso or a) atom occupied by silicon. Allyl silanes react at the end of the alkene furthest from silicon (y). In both case a (J-silyl cation is an intermediate. 

Dipolar cycloaddition of alkenes with carbonyl ylides generated in situ is a versatile method for tetrahydrofuran synthesis. The synthetic potential of such transformations has been reviewed <2005JOM(690)5533, 2003BMI6-253>. In addition, the stereoselective [3 + 2] annulation of allyl silanes has become a reliable protocol for the synthesis of tetrahydrofurans as demonstrated in several total syntheses . Such a [3 + 2] annulation, for example, affords the tetrahydrofuran product 11 as a single stereoisomer (Scheme 15) <2002OL2945>. Lanthanide salts serve as efficient Lewis acid catalysts in similar [3 + 2] cycloaddition reactions . 

Hi. Carbon-silicon bonds. Following the earlier reports mentioning the palladium-catalysed addition of organosilylstannanes to alkynes or isonitriles , Mori and coworkers realized tandem transmetallation-cyclization reactions with bifunctional halogeno triflates and Bu3SnSiMe3 18. The reactivity of 18 under palladium catalysis was used for the silylstannylation of alkenes or the synthesis of allylic silanes via a three-component (aryl iodide - - diene - - 18) coupling reaction. Recently, a similar... 

The radical anions of carbonyl groups can also be generated via PET from activated alkenes, e.g. allylic silanes or stannanes. Triplet excited aromatic ketones, a-dicarbonyls and Michael systems are suitable substrates for oxidizing allylic Group 14 organometallic compounds with subsequent formation of homoallylic alcohols or S-allylated ketones (Scheme 32) [120-122]. 

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