Allylation with Lewis acids

Purification CHCI3 4) Allylation with Lewis acid Sc(CF3S03)3 and ... 

Scheme 25 Reaction sequence for the formation of a Nd allyl species by the reaction of Nd hydride with BD, subsequent chlorination of the reaction product, interaction of the chlorinated Nd allyl with Lewis acids and coordination of BD [178], reprinted with permission of Wiley-VCH Verlag GmbH Co. KGaA...

Therefore, in order to obtain a 1,4-addition of an allyl residue to an enone, two activation modes can be used reactions take place either under electrophilic conditions with Lewis acid promotion, or in the presence of fluoride ions. This is important as the stereochemical outcome often depends on the activation mode selected. 

Benzylic or allylic oxygen functions react with Lewis acids such as trifluoroacetic acid to generate allyl or benzylic cations which abstract a hydride from silanes such as triethylsilane 84 b to result in the removal of the oxygen function in a process which has been called ionic hydrogenation and which has been reviewed [34-38]. 

Reactions through chelated TS Reactions of a- or (3-oxy-substituted aldehydes often show chelation-controlled stereoselectivity with Lewis acids that can accommodate five or six ligands. Chelation with substituents in the allylic reactant can also occur. The overall stereoselectivity depends on steric and stereoelectronic effects in the chelated TS. 

Similar to the cycloaddition of allyl cations30, allenyl cations have been found to undergo cycloadditions with alkenes to afford bicyclic compounds31. The allenyl cations were generated from propargyl chlorides by treatment with Lewis acids. This reaction sequence proceeds via the cyclization 34 -------> 3532, in spite of the fact that... 

An alternative route to nonracemic a-alkoxy stannanes entails the reduction of acyl stannanes with chiral hydrides61 62. Accordingly, conjugated stannyl enones yield (S)-a-alkoxy allylic stannanes by reduction with (J )-(+)-BINAL-H. As expected, (S)-(—)-BINAL-H gives rise to the enantiomeric (7 )-a-alkoxy allylic stannanes (equation 29)61. Upon treatment with Lewis acids, these stannanes undergo a stereospecific anti 1,3-isomerization to the (Z)-y-alkoxy allylic stannanes61. 

The ene reaction is strongly catalyzed by Lewis acids such as aluminum chloride and diethylaluminum chloride204 Coordination by the aluminum at the carbonyl group increases the electrophihcity of the conjugated system and allows reaction to occur below room temperature, as illustrated in Entry 6. Intramolecular ene reactions can be carried out under either thermal (Entry 3) or catalyzed (Entry 7) conditions 205 Formaldehyde in acidic solution can form allylic alcohols, as in entry 1. Other carbonyl ene reactions are carried out with Lewis acid catalysts. Aromatic aldehydes and acrolein undergo the ene reaction with activated alkenes such as enol ethers in the presence of Yb(fod)3 206 Sc(03SCF3)3 has also been used to catalyze ene reactions.207... 

Allylic carboxylation. Diethyl oxomalonate (1) undergoes a thermal ene reaction with mono-, di-, and trisubstituted alkenes at 145 180°. The reaction is also subject to catalysis with Lewis acids, which can lead to a different ene product. The products are a-hydroxymalonic esters. The corresponding malonic acids are converted to carboxylic acids by bisdecarboxylation with NaI04 and a trace of pyridine- or with ceric ammonium nitrate (CAN). Diethyl oxomalonate then functions as an cnophilic equivalent of C02. 

The synthesis of the C20—C26 fragment started with a 4-alkylation of methyl aceto-acetate The first stereocentre was introduced by enantioselecuve catalytic hydrogenation with Noyort s (S)-binap rhodium complex (cf p 102f.) Stereoselective Frater-Seebach alkylation with allyl bromide introduced the second stereocentre in 90% yield (cf p 27) Stereospecifid introduction of the stereocentres C24 and C2 was achieved by a chelation controlled addition of an allylstannane to an aldehyde (see p 66f) After some experimentation with Lewis acid catalysts and reaction conditions a single diastereomer of the desired configuration was ob-... 

Simple alkylation can be accomplished by use of trialkylaluminums which, with Lewis acids, afford 1-C-alkyl products such as 90, available in 72% yield by reaction between di-O-acetyl-6-deoxy-D-galactal and trimethylaluminum in the presence of titanium tetrachloride in dichloro-methane at —78 °C.109 Importantly, the reaction can also be applied to 1-alkylglycals, also with high stereoselectivity, so that the doubly substituted compound 91 can, for example, be made by allylation of tri-O-acetyl-l-C-methyl-D-glucal. Its C-l epimer is available by C-methylation of the 1-C-allylglucal.110... 

In more detail these pathways and their coupling with Lewis-acid-catalyzed allylation using allyltrimethylsilane are shown in Scheme 14 <2000J(P1)3006>. 

On the other hand using LiLn(Allyl)4dioxane in combination with Lewis acids like AlBr3 or base like TMEDA or THF, where Ln = Ce, Nd, Sm, Gd gives yields in the range of 70-94 percent. When a Lewis base is involved, the vinyl content of the product is about 80 percent. 

The cycloadditions of (Ti -allyl)Fp complexes to alkenes proceed with high chemoselectivity. Only highly activated alkenes such as methylenemalonates, benzylidenemalononitrile, TCNE or I,2-di-cyano-4,5-dichloroquinone will participate in the reaction. With the less electrophilic a,p-unsaturated systems, cycloaddition can only be effected with Lewis acid activation. Thus the cycloadduct (6) is formed in reasonable yield (as a mixture of stereoisomers) from (3) and cyclohexenone using freshly sublimed AlBn (equation 4). The same reaction gives only a 8% yield with Aids. The exclusive formation of a ci.r-hydrindanone system can be explained by suprafacial attack of the enone on the allyl unit, which is then followed by another suprafacial ring closure of the dipolar intermediate, affording the thermodynamically preferi cyclic system (7a) and (7b). ... 

This article describes further progress in the chemistry of 7r-allylnickel compounds. First, preparative methods for 7r-allylnickel halides, alkoxides, amides, and alkyls are described. Next, some chemical properties of these compounds—e.g., a recently observed disproportionation reaction—are discussed. Then, the use of 7r-allylnickel halides as homogeneous catalysts is discussed. Whereas bis (7r-allyl) nickel is a catalyst for butadiene cyclotrimerization, 7r-allylnickel halides combined with Lewis acids, such... 

Allylstannanes work as allylating reagents under free-radical reaction conditions. Free-radical reactions have several advantageous features in organic synthesis -neutral reaction conditions, compatibility with Lewis acids, and no need to protect reactive functional groups such as hydroxy and amino groups. In these days, therefore, free radical allylation procedures have been widely used in organic synthesis and several reviews on free radical reactions are available [94]. 

The allylation of aldehydes with allylstannanes is effected with Lewis acids such as boron trifluoride etherate or titanium(IV) chloride (eq (104)) [99], in which diastereo-selectivity is dependent on the catalyst employed the most striking feature is that. n77-homoallyl alcohols are stereoselectively produced using boron trifluoride etherate. Although this allylation occurs without a catalyst, high reaction temperature must be used. 

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