Aldehydes reactions with allyl metal compounds

Finally, Thomas has reported that allylstannanes (247) and (248) possessing stereocenters at C-4 undergo moderately diastereoselective reactions with p-nitrobenzaldehyde (Scheme 46). The origin of asymmetric induction in these cases is probably related to that presented in Section 1.1.3.1 for the reactions of achiral allyl metal compounds and chiral aldehydes (e.g. Scheme 27). 

There are two main synthetic applications where the reaction of an allyl system with electrophiles is accompanied by an allylic rearrangement. One consists of the use of heteroatom-substituted allylic anions as homoenolate anion equivalents and the other represents a synthetically valuable alternative to the aldol reaction by addition of allyl metal compounds to aldehydes. 

For earlier reviews of the reactions of allyl metal compounds with aldehydes and imines, see (a) Y. 

Both stoichiometric and catalytic reactions of allylic compounds via 7r-allyl complexes are known. Reactions of nucleophilic 71-allyl complexes with electrophiles involve oxidation of metals and hence constitutes stoichiometric reactions. 7i-Allyl complexes of Ni, Fe, Mo, Co and others are nucleophilic and undergo the stoichiometric reaction with electrophiles. However, electrophilic 71-allyl complexes react with nucleophiles, accompanying reduction of metals. For example, 71-allylnickel chloride (2) reacts with electrophiles such as aldehydes, generating Ni(II), and hence the reaction is stoichiometric. In contrast, electrophilic 7i-allylpalladium chloride (3) reacts with nucleophiles such as malonate and Pd(0) is generated. Thus repeated oxidative addition of allylic compounds to Pd(0) constitutes a catalytic reaction. 

Allyl ketones can be prepared by trapping acylpalladium intermediates with main group metal compounds. The allyl ketone 251 can be prepared from the allyl ester 248 by trapping acylpalladium 249 with the alkylzinc compound 250 at room temperature and 1 atm [115]. Reaction of geranyl chloride (252) with the furylstannane 253 under CO pressure afforded the ketone 254, which was converted to dendrolasin (255) [93]. Aldehydes are prepared by trapping with a metal hydride. The /l,y-unsaturated aldehyde 257 is prepared by the carbonylation of the allyl chloride 256 in the presence of Bu3 SnH [116,117],... 

Benzyl methyl ether or allyl methyl ethers can be selectively metalated at the benzylic/allylic position by treatment with BuLi or sBuLi in THF at -40 °C to -80 C, and the resulting organolithium compounds react with primary and secondary alkyl halides, epoxides, aldehydes, or other electrophiles to yield the expected products [187, 252, 253]. With allyl ethers mixtures of a- and y-alkylated products can result [254], but transmetalation of the lithiated allyl ethers with indium yields y-metalated enol ethers, which are attacked by electrophiles at the a position (Scheme 5.29). Ethers with ft hydrogen usually undergo rapid elimination when treated with strong bases, and cannot be readily C-alkylated (last reaction, Scheme 5.29). Metalation of benzyl ethers at room temperature can also lead to metalation of the arene [255] (Section 5.3.11) or to Wittig rearrangement [256]. Epoxides have been lithiated and silylated by treatment with sBuLi at -90 °C in the presence of a diamine and a silyl chloride [257]. 

The direct allylation of aromatic compounds with allylic chlorides is achieved in the presence of a catalytic amount of indium metal. According to the authors, indium is considered to act as a Lewis acid (Equation (101)).399 Trialkylindiums react with acid chlorides to give ketones. When trialkylindiums are treated with air prior to the reaction with acid chlorides, esters are formed (Scheme 121).400 The reaction of cyclopentadienylindium(i) with aldehydes in aqueous media gives mixtures of the isomeric coupling products (Equation (102)).401... 

When Lewis acids such as SnCU and TiCU are used to promote additions of allylic trialkyltin reagents to aldehydes several reaction outcomes are possible, depending on stoichiometry and the mode of addition. If the Lewis acid is added to the aldehyde followed by the allylic stannane, the typical product (syn for crotylstannanes) derived from an acyclic transition state is formed. If, however, the stannane and Lewis acid are premixed and left to equilibrate, metathesis can occur forming the allylic halome-tal compound which reacts with the subsequently added aldehyde to give products (anti for crotyl) consistent with a cyclic transition state (Eq. 22). The initially formed allylic halostannane gives rise to the linear adduct, but if aldehyde addition is delayed, this initial secondary allylic metal halide can equilibrate to the primary isomer which then reacts with the aldehyde to afford the branched product. 

Silver(I) compounds are often used to generate cationic metal complexes from the corresponding metal halides. Suzuki and coworkers found that -hexylzirconocene chloride (61), derived from 1-hexene and Schwartz reagent 60, can react with aldehydes in the presence of a catalytic amount of AgAsFs to give secondary alcohols [27]. The reaction with hydrocinnamaldehyde, for example, provides the alcohol 62 in 95 % yield (Sch. 14). Allylic alcohols are also obtainable by a similar procedure using 1-hexyne as a starting material. 

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