Trimethyl allylsilane

As shown in racemic series [220], the stereoselectivity of the allylation of chiral 1,3-dioxanes 6.20 depends upon the reaction conditions and upon the nature of the reagent (silane or stannane). The most selective reagent is CH2=CHCH2SnBu3. Most of the reactions have been carried out with trimethyl-allylsilane in the presence of TiC1.4/Ti(0/-Pr)4 under the conditions recommended by Johnson and coworkers [213], After oxidation of the products and treatment with a base, nonracemic homoallylic alcohols are obtained with excellent enantiomeric excesses (Figure 6.58). From CH2=C=CHCH2SiMe3, dienyl alcohols are similarly obtained [1228, 1229] (Figure 6.58). The TiC -catalyzed reactions of dioxanes 6.20 with alkynylsilanes also lead to nonracemic a-alkynyl alcohols [223],... 

The Julia alkene synthesis has been applied to allylsilane synthesis, including the synthesis of 2-cyclopentylidene- and 2-cyclobutylideneethyl(trimethyl)silanes30. 

Chelation control has been invoked to explain the stereoselectivity of the reaction between 2-methoxycyclohexanone and trimethyl(2-propenyl)silane promoted by titanium(IV) chloride59, and has been observed for addition of allylsilanes to a-amidoaldehydes60, although the stereoselectivity is very dependent upon the amount of Lewis acid used and the structure of the substrate, particularly when the Lewis acid can bind to more than one site61. 

To a mixture of vinyl bromide (40 mmol) and the catalyst dichloro-[(R)-Af,N-dimethyl-l-[(.S)-2-(diphenylphosphino)ferrocenyl]ethylamine]-palladium(n) (0.2 mmol) was added an ethereal solution of [a-(trimethyl-silyl)benzyl]magnesium bromide (0.6-1 m, 80 mmol) at —78 °C. The mixture was stirred at 30 °C for 4 days, and then cooled to 0 °C and hydrolysed with dilute aqueous HC1 (3 m). The organic layer was separated, and the aqueous layer was re-extracted with ether. The combined organic extracts were washed with saturated sodium hydrogen carbonate solution and water, and dried. Concentration and distillation gave the chiral allylsilane (79%, 66% ee), b.p. 55°C/0.4mmHg. 

THE CONVERSION OF ESTERS TO ALLYLSILANES TRIMETHYL(2-METHYLENE-4-PHENYL-3-BUTENYL)SILANE (Silane, trimethyi (2-methylene-4-phenyl-3-butenyl)-)... 

THE CONVERSION OF ESTERS TO ALLYLSILANES TRIMETHYL(2-METHYLENE-4-PHENYL-3-BUTENYL)SILANE... 

Photooxygenation of vinylsilane in the presence of acetic anhydride and pyridine furnishes a convenient procedure for the synthesis of a-trimethylsilylenones 185 in moderate to good yields (equation 156)278. Dye-sensitized photooxidations of allylsilane with singlet oxygen produces a mixture of (E)- and (Z)-3-trimethyl-silylallyl hydroperoxides 186 (equation 157)279. 

Stereochemically pure allylsilanes and stannanes can also be obtained by metallation of suitable olefins with the superbasic mixture of butyllithium and potassium /fV/-butoxide followed by reaction with trimethylchlorosilane and tributylchlorostannane, respectively. In a representative example, 6-(2-tetrahydropyra-nyloxy)hex-l-ene is transformed into Z-(6-hydroxyhex-2-enyl)trimethylsilane through metallation with trimethyl-... 

Epoxides can react smoothly with allylsilane reagents. For example, 1,3-bis-trimethylsilyl-Tpropene can react with one of a variety of epoxides under Lewis acid catalysis to generate a highly functionalized alkenol (Equation 35). Yields are modest, with best results being obtained using monosubstituted epoxides <1998TL529>. Trimethyl silyl... 

The asymmetric cross-coupling was successfully applied to the synthesis of optically active allylsilanes [50,51] (Scheme 10). The reactions of a-(trimethyl-silyl)benzylmagnesium bromide (49) with vinyl bromide (4b), (E)-bromopro-pene (( )-50), and (R)-bromostyrene E)-8) in the presence of 0.5 mol % of a palladium complex coordinated with chiral ferrocenylphosphine, (R)-(S)-PPFA (10a), gave the corresponding (R)-allylsilanes (51) with 95%, 85%, and 95% ee, respectively, which were substituted with phenyl group at the chiral carbon center bonded to the siHcon atom. These allylsilanes were used for the S. ... 

The a-silyl ester (234) behaves as a typical allylsilane in TiCl -catalysed reactions with a variety of electrophiles (a-halo-ethers, aldehydes, Michael acceptors) to give generally good yields of the trimethyl esters (235). ... 

The Conversion of Esters to Allylsilanes Trimethyl(2-methylene-4-phenyl-3-butenyl)si lane... 

Asym. induction. 2 eqs. 5ec-butyllithium added to a soln. of startg. chiral allylsilane in ether at — 78° under argon, stirred for 15 min, 2 eqs. methyl iodide added, and worked up when reaction complete - l-[l-[2(S)-(methoxymethyl)pyrrolidinyl]]-2,2,3-trimethyl-2-sila-4-pentene. Y 94% (87% a-alkylation). a-Selectivity is effected by virtue of the metal-ion complexing facility of the substituent on silicon. F.e. and complexing functions s. R.F. Horvath, T.H. Chan, J. Org. Chem. 54, 317-27 (1989). 

Allyl acetates are readily converted to allylsilanes in good yields, when they are reacted with 0.5 equiv of tris(trimethylsilyl)aluminum etherate and 4-6 mol % of Pd(PPh3)4 in THF at 0-25 °C (Scheme 34).For the reaction with unsynunetrical allyl acetates, the trimethylsilyl group tends to attack the more substituted carbon of the allyUc system under Pd-catalyzed conditions. With 10 mol % of Mo(CO)e (toluene reflux for 2.5 h), on the other hand, the trimethylsilyl group selectively attacks the less substituted carbon as exemplified by the exclusive formation of geranyl(trimethyl)silane in 65% yield. Under the Pd-catalyzed conditions, many functional groups of synthetic importance (e.g., vinyl bromide, acetals, esters, and enones) remain intact. 

Radical Allylation Reactions. Trimethyl 2-[(tributylstannyl) methyl]-2-propenyl silane reacts with organic halides under photochemical conditions (irradiation with a medium pressure mercury lamp through a pyrex filter) at temperatures below 20 °C to give various allylsilanes (eq 7). The ease of the reaction was found to be halide dependent with those affording more electrophilic radicals generally behaving most effectively. 

Silicon.— Allylsilanes are outstandingly useful carbon nucleophiles as a result of their ability to react with electrophiles in a controlled and specific manner to form new carbon-carbon bonds. Furthermore, metallation to give a-(trimethyl-silyl)allylic carbanions introduces the possibility of an additional level of control over the mode of attack (either a or y) exhibited by the allylsilane. It is not surprising, therefore, to find the chemistry of these species occupying a prominent position in this year s organosilicon literature, and in addition to the many applications reported some new routes to these compounds have been described. One such example is shown in Scheme 13, in which the starting silyl sulphone is readily prepared by oxidation of the product obtained from reaction of thio-phenol with vinyltrimethylsilane." ... 

The structure of products obtained by the reaction of allylsilanes with a,P-unsaturated carbonyl compounds also heavily depends on the type of the carbonyl group and the size of silicon substituents of allylsilane reagents. Thus, the reaction of allyl(trimethyl)silane with 1-acetylcyclohexene gives mainly conjugate allylation products with a small amount of bicyclo[4.3.0]heptanes derived from [3 + 2]... 

---