Silanes, homoallylic

Reaction of allylic silanes with enantiomerically pure 1,3-dioxanes has been found to proceed with moderate enantioselectivity.104 The homoallylic alcohol can be liberated by oxidation followed by base-catalyzed (3-elimination. The alcohols obtained in this way are formed in 70 5% e.e. 

The 121/Cl3SiH combination selectively cross-couples alkenes with alkynes intermolecularly to give acyclic homoallylic silanes 127 and 128 (Eq. 22) [73]. 

Homoallylic alcohols.1 In the presence of fluoride ion these silanes react with aldehydes to form homoallylic alcohols in high yield (equation I). Crotyltrifluo-rosilanes undergo a similar reaction with aldehydes, but in this case the stereose-... 

The development of new methods for the synthesis of homoallylic amine derivatives is an important area of synthetic efforts. Homoallylic amines are extremely important compounds as biologically active molecules [1], The Lewis acid-mediated reactions of imines with allyl silanes are among the most efficient for... 

A combination of C0CI2 and l,6-bis(diphenylphosphino)hexane catalyzes a similar reaction of an alkyl bromide with a 1,3-diene and trimethylsilylmethyhnagnesium chloride, giving homoallylic silanes (equation 108) . 

As one might expect, the diastereoselectivities with homoallylic alcohols are quite substrate-dependent. One successful example involves the cyclopropanation of allyl silanes which could be cyclopropanated with a high level of diastereocontrol (equations 65 and 66). 

Rh-diphosphine complex, [Rh(Chiraphos, 106)](C104)2, was used as a catalyst for the intramolecular hydrosilylations of homoallylic silane 107 and silyl allyl ether 108 in acetone at 25°C to give the corresponding 1,4-diol 109 (60% ee, R) in 84% yield and 1,3-diol 110 (with 56% ee, R) in 96% yield, respectively [64] (Scheme 2.7). The Rh-Chiraphos catalyzed reaction of l-(3-phenylpropen-2-yloxy)silacyclohexane (111a) gave diol 112 with 74% ee (R) in 61%... 

The reaction of allyl silanes with aldehydes and ketones activated as electrophiles by Lewis acids is a very useful method for preparing homoallylic alcohols. Since allyl silanes are only modestly nucleophilic, strong electrophiles are needed to ensure a good reactivity match. 

The addition of trimethyl (2-methylallyl)silane 28 to acetal 27 was chosen as the key step. The reaction proceeded smoothly and generated homoallylic ether 29 with high diastereoselectivity. The desired homoallylic alcohol 30 could subsequently be obtained, in high enantiomeric purity, by oxidative deprotection of the chiral template (Scheme 13.12). 

Upon treatment with TiCl3(OCHMe2), compound 32 reacts with allyltrimethyl-silane to form ether 33 in good yield and selectivity. The chiral template is then removed by treatment with an excess of LDA affording the desired homoallylic alcohol 34 in 80-94% ee. 

Ketones were also reacted under these conditions, leading to tertiary ethers. Thus, by mixing equimolar quantities of a carbonyl (aldehyde or ketone), allyl-silane and a silylated alcohol, followed by the addition of a catalytic amount of TMSOTf, homoallylic ethers can be obtain in good yields via a three-component coupling reaction (Scheme 13.22). 

Scheme 7.21. Formation of silylmethyl allylic silanes from gem-disubstituted homoallylic ethers.

The accelerated rate for alcoholysis with le, which was observed for the 10 % Pd/C catalytic system, was also seen with the Mn(CO)sBr catalyst. Reactions of le with primary, secondary or tertiary alcohols resulted in moderate yields of the corresponding silyl ketals after 2 h (Table 8 and 9). When mono-alkoxy silane from 3-hydroxy butyrate (lg) was treated with homoallyl alcohol in the presence of Mn(CO)sBr as the catalyst under the standard conditions, 76 % of the silyl ketal was obtained. These silyl ethers possess neighboring carbonyl groups that can participate in the reaction by forming a more reactive pentacoordinated silicon center upon addition of the silane to the metal center.. 

The reaction presumably goes through the addition of a radical (formed from RX) to the diene, followed by the reaction of the formed allyl radical with [NiU2Ar] complex. A combination of CoCl2 and l,6-bis(diphenylphos-phino)hexane catalyzes a similar reaction of an alkyl bromide with a 1,3-diene and trimethylsilylmethylmagnesium chloride, yielding homoallylic silanes in good to excellent yields.212 For both cases, the same catalytic cycle was proposed (Scheme 67). 

Indium-mediated allylation of trialkyl(difluoroacetyl)silane 70 in aqueous media gives homoallylic alcohol 71 exclusively (Scheme 60). Both water and THF are essential for the allylation reaction. It is worth noting that homoallylic alcohol 71 is formed exclusively under these reaction conditions. On the contrary, enol silyl ether 72 is a major product of the fluorinated acylsilanes reaction with other organometallic compounds than indium via a Brook rearrangement and defluorination. Indium-mediated allylsilylation of carbonyl compounds provides a facile route to 2-(hydroxyethyl)allylsilanes. The allene homologs are similarly prepared (Scheme 61).244,244a... 

Allyl silanes will also attack carbonyl compounds when they are activated by coordination of the carbonyl oxygen atom to a Lewis acid. The Lewis acid, usually a metal halide such as TiCLj or ZnCl2, activates the carbonyl compound by forming an oxonium ion with a metal-oxygen bond. The allyl silane attacks in the usual way and the (3-silyl cation is desilylated with the halide ion. Hydrolysis of the metal alkoxide gives a homoallylic alcohol. 

A closely related reactive oxonium ion can be prepared by Lewis-acid-catalysed breakdown of the corresponding acetal. Alternatively, especially if the acetal is at least partly a silyl acetal, the same oxonium ion can be produced in situ using yet more silicon in the form of TMSOTf as the Lewis acid catalyst. All these intermediate oxonium ions act as powerful electrophiles towards allyl silanes producing homoallylic alcohols or ethers. 

Addition of oiganditfaiuin and organomagnesium reagents to such vinyl(alkoxy)silanes, followed by catalyzed coupling of the new organometallic with either vinyl or allyl bromide, leads,after oxidative cleavage, to allylic or homoallylic alcohols, respectively (Scheme 9). 

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