Lewis Acids promoted reactions

The diastereofacial selectivity of Lewis acid promoted reactions of allylsilancs with chiral aldehydes has been thoroughly investigated58. Aldehydes with alkyl substituted a-stereogenic centers react with a mild preference for the formation of Cram products, this preference being enhanced by the use of boron trifluoride-diethyl ether complex as catalyst58. 

An interesting and stereoselective synthesis of 1,3-diols has been developed which is based on Lewis acid promoted reactions of /f-(2-propenylsilyloxy (aldehydes. Using titanium(IV) chloride intramolecular allyl transfer takes place to give predominantly Ag/r-l,3-diols, whereas anti-1,3-diols, formed via an / / /-molecular process, are obtained using tin(IV) chloride or boron trifluoride diethyl ether complex71. 

Lewis acids, particularly the boron trifluroride diethyl ether complex, are used to promote the reaction between allyl(trialkyl)- and allyl(triaryl)stannanes and aldehydes and ketones52-54. The mechanism of these Lewis acid promoted reactions may involve coordination of the Lewis acid to the carbonyl compound so increasing its reactivity towards nucleophilic attack, or in situ transmetalation of the allyl(trialkyl)stannane by the Lewis acid to generate a more reactive allylmetal reagent. Which pathway operates in any particular case depends on the order of mixing of the reagents, the Lewis acid, temperature, solvent etc.55- 58. 

The stereoselectivity of Lewis acid promoted reactions between 2-butenylstannanes and aldehydes has been widely studied, and several very useful procedures for stereoselective synthesis have been developed. In particular syn-products are formed stereoselectively in reactions between trialkyl- and triaryl(2-butenyl)stannanes, and aldehydes induced by boron trifluoride-diethyl ether complex, irrespective of the stannane geometry66. 

G. Majetich, K. Hull, D. Lowery, C. Ringold,. 1. Defauw in Selectivities in Lewis Acid-Promoted Reactions, D. Schinzer, Ed., Kluwer, Dordrecht 1989. 

The Lewis acid promoted reactions of styrene with 7V-phenylsulfonyl-l,4-benzoquinone monoimine to the 2-aryIbenzofuranoid ring system have been reported previously. This offers a new route to henzofinanoid neoUgnans (( )-hcarin B, eupomatenoid-1, eupomatenoid-12) (Scheme 25, <96TL6969>). 

Schinzer, D., Ed. Selectivities in Lewis Acid Promoted Reactions 1989, Kluwer Academic Dordrecht, Netherlands. 

A novel method is reported to convert 1,3,5-oxathiazine-A-oxides 116 into 1,2,4-oxathiazoles 117 under thermal conditions. Lewis acid-promoted reaction of compounds 117 furnishes 1,2,4-thiadiazoles 118 and the final step is a type A synthesis (see Section 5.08.9.2) (Scheme 15) <2004HAC175>. 

Reaction with 2,3-epoxy alcohols. Both (CH3)2CuLi and (CH3)2CuCNLi2 react with trans-2,3-epoxy alcohols (with a simple alkyl group at C4) in ether to give about 1 1 mixtures of 1,2- and 1,3-diols. Addition of TMEDA or an imida-zolidinone (DMI, 11, 202) promotes reaction at C2 to give 1,3-diols, whereas addition of a Lewis acid promotes reaction at C3 to give 1,2-diols. 

The mechanism of the Lewis acid promoted reaction of acetals has been investigated extensively. For example, Denmark demonstrated the presence of... 

The alkoxycarbenium ions generated by the cation pool method react with various carbon nucleophiles such as substituted allylsilanes and enol silyl ethers to give the corresponding coupling products in good yields. It should be noted that the reactions of alkoxycarbenium ion pools with such nucleophiles are much faster than the Lewis acid promoted reactions of acetals with similar nucleophiles. A higher concentration of the cationic species in the cation pool method seems to be responsible. 

Schinzer D (1989) Selectivities in Lewis acid promoted reactions. Kluwer Academic, Dordrecht... 

The first two large terms in braces are for an unpromoted reaction, the second two for Lewis acid promoted reaction. The fact that the rate dependence (Section V,B,4) is linear with [A] is consistent with a very large Kk (as indicated in Section II,E) and means that the first two terms make a negligible contribution in this case. We also find that the last term is very... 

The reactivity of the five-membered heterocycles pyrrole, furan, thiophen and imidazole (Fig. 8-10) is characterised by interactions with electrophilic reagents. The precise nature of these reactions depends upon the particular ring system. Thiophens undergo facile electrophilic substitution, whereas the other compounds exhibit a range of polymerisation and other Lewis acid-initiated reactions upon treatment with electrophiles. We saw a number of examples of Lewis acid-promoted reactions of furans and pyrroles in Chapter 6. Although reactions of complexes of five-membered heterocyclic ligands have not been widely investigated, a few examples will illustrate the synthetic potential. 

Barbier-type reactions, 9, 358 catalystic asymmetric reactions, 9, 359 Lewis acid-promoted reactions, 9, 358 Allenyltrichlorosilanes from 1,3-enynes, 9, 309 preparation, 9, 309... 

Intramolecular Lewis acid-promoted reaction of coordinated propargylic ether with the silyl enol ether in 158 has been applied successfully to the construction of the highly strained 10-membered cyclic enediyne system 159, present in esperamycin and calicheamycin [39,40]. The enediyne system 157 was prepared by the Pd-catalysed Sonogashira coupling of (Z)-l,2-dichloroethylene (154) with two different terminal alkynes 155 and 156. 

The Lewis acid-promoted reactions of acrylates and propiolates with allylsilanes usually afford [2 + 2]-cycloadducts rather than [3 + 2]-cycloadducts (vide infra). The ratio of the two kinds of cycloadducts depends on the reaction temperature.166 Particularly, the product ratio in the reaction of alkylidene malonates and their derivatives is markedly temperature dependent.167,168 Cyclobutanes are major products at low temperature, and [3 + 2]-cycloaddi-tion proceeds predominantly at higher temperature (Equation (42)). 

The Lewis acid-promoted reaction of aldehydes with a-substituted allylsilanes forms 3-silyltetrahydrofurans in good to high yields.169-172 The use of homochiral allylsilanes is very valuable for highly diastereo- and enantio-selective syntheses of tri- and tetrasubstituted tetrahydrofurans (Equation (43)). Catalytic asymmetric [3-1-21-cycloaddition of a-substituted allenylsilanes to aldehydes can be achieved by a chiral scandium complex.173... 

The formation of dialkyl benzo[f]furan-l-yl phosphonates by Lewis acid-promoted reaction of o-phthalaldehyde with trialkyl phosphites has been described (Equation 127) <2006S4124>. 

A general approach to 2-aryl-7-alkoxy-benzofuranoid neolignans is based on a Lewis acid-promoted reaction of styrenes with A -phenylsulfonyl-l,4-benzoquinone monoimines. Regioselective formation of the 2-arylbenzofuranoid ring system is followed by conversion of the aromatic A -phenylsulfonyl moiety into a propenyl substituent (Equation 147) <1996TL6969>. 

Scheme 10 Chem selectivity control in Lewis acid promoted reactions of alkyl halides with alkenes.

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