Boron trifluoride allylsilane reactions

Lewis acids used for acylation of allylsilanes with acid chlorides include titanium and tin tetrachlorides, aluminum, gallium and indium trichlorides and zinc dichloride, and boron trifluoride for reactions with acid anhydrides. Although gallium and indium chlorides have been used successfully in catalytic (ca. 2 mol %) amounts, most frequently aluminum chloride has been used as catalyst, with titanium tetrachloride also commonly employed, both in molar quantities. With these catalysts, acylations can often be carried out at low temperatures, and with short reaction times. 

The reaction between a-alkoxyaldehydes and allylsilanes is highly stereoselective in favor of chelation-controlled products if tin(IV) chloride is used as the Lewis acid, whereas boron trifluoride gives modest stereoselectivity in favor of the nonchelation-controlled product58. 

A one-pot reaction between a tryptophan ester, benzotriazole, and 2,5-dimethoxytetrahydrofuran in acetic acid gives the diastereomeric benzotriazolyl tetracycles, 349, in good yield. Substitution of the benzotriazole by reaction with silyl enol ethers and boron trifluoride etherate gives the corresponding ketones 350 and 351, and reaction with allylsilanes gives the corresponding alkenes 352 and 353. If the boron trifluoride etherate is added to the mixture before the silane, elimination of benzotriazole from 349 is also observed (Scheme 83) <1999T3489>. 

The Lewis acid-catalyzed reaction of 4-acetoxy-l,3-dioxanes with nucleophiles has been reviewed <2001TCC(216)51>. The principles of the reaction are displayed in (Equation 44). The combination of boron trifluoride and organozinc compounds was found to be very efficient <1997JOC6460, 1999JOC2026> but allylsilanes react as well <19%SL536, 1998TL6811>. 

In the absence of Lewis acids, allylsilanes react only with very electrophilic ketones like hexafluo-roacetone, when they undergo ene reactions rather than electrophilic replacement of the silyl group. In the presence of Lewis acids, allylsilanes react with aldehydes and ketones with clean allylic transposition (Scheme 1) and the formation of homoallylic alcohols. The range of Lewis acids used is wide, but titanium tetrachloride and boron trifluoride etherate are the most common. Typically, reaction takes place somewhere between -78 °C and 0 C in dichloromethane solution, and a molar proportion of Lewis acid is used. 

Acetals and ketals are exceptionally good electrophiles for allylsilanes, often giving better yields than the corresponding aldehyde or ketone, because the products are less prone to further reaction. Typically the reactions are catalyzed by stoichiometric amounts of titanium tetrachloride in dichloromethane at -78 °C, and the regiospecific reaction is complete in a few minutes (Scheme 37). Other Lewis acids used are boron trifluoride etherate, tin(IV) chloride and, in catalytic amounts, trimethylsilyl tri-flate, - trityl perchlorate and montmorillonite clays. With the unsymmetrical acetal (53), the meth-oxyethoxy group departs selectively (Scheme 38), presumably because it chelates the Lewis acid. ... 

The reaction of acetals and ketals with enol silyl ethers, usually catalyzed with the more traditional Lewis acids such as boron trifluoride etherate or titanium tetrachloride, can be carried out with trimethylsilyl triflate catalysis50. In the example shown in equation 42 the resulting allylsilane was annulated with titanium tetrachloride. 

The reaction of propargyl alcohols with dicobalt octacarbonyl to give the complex salts 148 (X = BF4 or PF6) and synthetic uses of the latter have been reviewed. The salts react with electron-rich aromatic compounds ArH, such as anisole, phenol or N,N-dimethylaniline, to yield substitution products 149 after oxidative demetallation with an iron(III) or cerium(I V) salt with j5-diketones or j -keto esters the corresponding propargyl-substituted compounds 150 are obtained k Acetone reacts in an analogous fashion to give 151. The action of the cobalt complexes 148 on allylsilanes 152 leads to enynes 153. Indole reacts with the complex 148 (R = H R = R = Me) in the presence of boron trifluoride etherate to give 154, which was converted into 155 by the action of iron(III) nitrate " ... 

Note that the aldehyde approaches the alkene from the direction anti to the silicon atom. Therefore, when a chiral allylsilane or allylstannane with a substituent in the a-position is used, chirality transfer takes place, to generate the homoallylic alcohol with essentially no loss in enantiomeric purity. For example, reaction of the aldehyde 157 with the chiral allylsilane 158, using boron trifluoride etherate as the catalyst, gave predominantly the syn product 159 (1.151). The absolute stereochemistry can be determined by using a model in which the hydrogen atom on the a-carbon of the allylsilane eclipses the alkene (the so-called inside hydrogen effect ) in order to minimize steric interactions (1.152). 

Nncleophilic Addition and Substitution Reactions. Allylsilane (3) has been shown to undergo conjugate addition to enones in the presence of tetra- -butylammonium fluoride (eq 6). The reaction demonstrates high regioselectivity, as no products arising from 1,2-addition to the enone or attack at the /-position of the allylsilane were isolated. Stereoselective C-glycosidation can be effected by reaction of allylsilane (3) with D-mannopyranoside derivatives in the presence of boron trifluoride etherate (eq 7). The a-C-glycoside arises from axial addition to the pyranoside oxonium ion. 

Kopecky and Rychnovsky also targeted leucascandrolide A to demonstrate their Mukaiyama Aldol-Prins (MAP) method (Scheme 38) [78]. The MAP reaction generates an oxocarbenium intermediate from an enol ether and an aldehyde, which is capable of imdergoing a Prins cyclization with the pendant allylsilane. Aldehyde 278 (from Scheme 73, Eq. 1) in the presence of enol ether 142 and boron trifluoride etherate underwent the MAP cascade without incident. The reaction mixture was treated with sodium borohydride to reduce any of the unreacted aldehyde and simplify purification. Desired alcohol 143 was then isolated in 78 % yield as a 5.5 1 mixture of diastereomers. 

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