Stannanes allyl halide reagents

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

Scheme 57 illustrates an example of this process coupling of the lithio reagent derived from vinyl stannane 237 with epoxide 242 leads to the C-disaccharide derivative 243 in good yield. A double transmetallation to give an organocopper reagent 244 can facilitate the C-glycosylation reaction with allyl halides [80] (Scheme 58). 

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. 

Readily available functionalized aryl siloxanes are also viable cross-coupling partners for Pd(dba)2-catalyzed allylic arylations. A mixture of 5% Pd(dba)2, allylic halide, and in situ-generated aryl zinc reagent produces allylated arenes in high yield. Aryl boronic acids have been converted to allylated arenes as well. Diastereoselective intramolecular Stille-type coupling of two allylic moieties (aUylic acetate and allylic stannane) has been performed in high yield to produce the key intermediate in the synthesis of racemic 10-ep/-elemol (eq 19). ... 

All types of electrophiles have been used with 2-lithio-l,3-dithiane derivatives, including alkyl halides, sulfonates, sulfates, allylic alcohols, arene-metal complexes, epoxides, aziridines, carbonyl compounds, imines, Michael-acceptors, carbon dioxide, acyl chlorides, esters and lactones, amides, nitriles, isocyanates, disulfides and chlorotrialkylsilanes or stannanes. The final deprotection of the dithioacetal moiety can be carried out by means of different types of reagents in order to regenerate the carbonyl group by heavy metal coordination, alkylation and oxidation184 or it can be reduced to a methylene group with Raney-nickel, sodium or LiAIII4. 

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