Reactions via Transmetallation

Cross-coupling of allylic compounds occurs by transmetallation between 7i-allyl intermediates and organometallic compounds of Mg, Zn, B, Al, Si and Sn, and subsequent reductive elimination. Reaction of the allylic dithioacetal 180 with MeMgBr in the presence of an Ni catalyst affords alkenes 184 bearing a tert-butyl group [90]. In this reaction, generation of the 7i-allylnickel 181 by oxidative addition and subsequent transmetallation with MeMgBr afford 182. Then the methylated product 183 is formed by reductive elimination, and finally the dimethylated product 184 is formed by the sequence of similar reactions. 

Allylmetal compounds can be prepared by the Pd-catalysed allylation of dimetal compounds. The phenylallylboronate 195 is prepared by the reaction of 2-phenylallyl acetate with bis(pinacolate)diboron (194) catalysed by ligandless Pd in DMSO [95], A good synthetic route to the allylsilane 197 is the reaction of Me3SiSiMe3 with geranyl trifluoroacetate (196) catalysed by ligandless Pd via transmetallation of the n-allylpalladium intermediate at room temperature in DMF [96], 

Allylstannane 176 is formed by the reaction of allyl acetate with distannanes [97,98], In this reaction, umpolung of the electrophilic 7r-ally] palladium to the nucleophilic allylstannane occurs. Allylation of bromoindole 198 to give allylindole 199 involves the oxidative addition of 198 to Pd, transmetallation with the allylstannane 176, and final reductive elimination [99], 

Although simple ketones and esters can not be allylated by Pd catalysts, they are allylated with allyl carbonates via their enol ethers of Si and Sn. In the allylation of the silyl enol ether 202 with allyl carbonate 200, transmetallation of 202 with the n-allylpalladium methoxide 201, generated from allyl methyl carbonate (200), takes place to generate the Pd enolates 203 and 204. Depending on the reaction conditions, allyl ketone 205 is formed by the reductive elimination of 203 [100]. When the ratio of Pd Ph3P is small, the a,/i-unsaturated ketone 206 is obtained by -elimination [101]. For example, the silyl enol ether 208 of aldehyde 207 is allylated with allyl carbonate (200) to give a-allylaldehyde 210 via 209. The a-allyl carboxylate 213 is obtained by allylation of ester 211 with allyl carbonate (200), after conversion of ester 211 to the ketene silyl acetal 212 [102], As the silyl group is trapped in these 

In the reaction of allyl acetate 217 with ketene silyl acetal 218 of methyl acetate, using a Pd catalyst coordinated to DPPP, cyclopropane 220 is formed in addition to the expected allylacetate 219 [104], The cyclopropanation becomes main reaction when TMEDA, as a ligand, and thallium acetate are added [105]. The cyclopropanation can be understood by the attack of the enolate ion at the central carbon of 7r-allylpalladium to form the palladacyclobutane 221, followed by reductive elimination. 

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Another important reaction via transmetallation is carbon-metal bond formation by reaction with bimetallic reagents. This is a useful synthetic method for various main group organometallic reagents. 

Reactions via Transmetallation of Allenylpalladium Intermediates and Related Reactions (Type II)... 

Reactions with Orgunometullic Compounds of the Main Group Metals via Transmetallation... 

Tin enolates of ketones can be generated by the reaction of the enol acetate 733 with tributyltin methoxide[60i] and they react with alkenyl halides via transmetallation to give 734. This reaction offers a useful method for the introduction of an aryl or alkenyl group at the o-carbon of ketones[602]. Tin enolates are also generated by the reaction of siiyl enol ethers with tributyltin fluoride and used for coupling with halides[603]. 

In addition, a catalytic version of Tt-allylpalladium chemistry has been devel-oped[6,7]. Formation of the Tr-allylpalladium complexes by the oxidative addition of various allylic compounds to Pd(0) and subsequent reaction of the complex with soft carbon nucleophiles are the basis of catalytic allylation. After the reaction, Pd(0) is reformed, and undergoes oxidative addition to the allylic compounds again, making the reaction catalytic.-In addition to the soft carbon nucleophiles, hard carbon nucleophiles of organometallic compounds of main group metals are allylated with 7r-allylpalladium complexes. The reaction proceeds via transmetallation. These catalytic reactions are treated in this chapter. 

The stereochemistry of the Pd-catalyzed allylation of nucleophiles has been studied extensively[5,l8-20]. In the first step, 7r-allylpalladium complex formation by the attack of Pd(0) on an allylic part proceeds by inversion (anti attack). Then subsequent reaction of soft carbon nucleophiles, N- and 0-nucleophiles proceeds by inversion to give 1. Thus overall retention is observed. On the other hand, the reaction of hard carbon nucleophiles of organometallic compounds proceeds via transmetallation, which affords 2 by retention, and reductive elimination affords the final product 3. Thus the overall inversion is observed in this case[21,22]. 

The reaction of phenylzinc reagent proceeds with opposite stereochemistry, namely by retention of configuration at the final step via transmetallation. Both the (S)-( )- and (i )-(Z)-allylic acetates 4 and 9 afford the (/ )-( )-phe-nylated product II by overall inversion[23]. 

The allylstannane 474 is prepared by the reaction of allylic acetates or phosphates with tributyltin chloride and Sml2[286,308] or electroreduction[309]. Bu-iSnAlEt2 prepared in situ is used for the preparation of the allylstannane 475. These reactions correspond to inversion of an allyl cation to an allyl anion[3l0. 311], The reaction has been applied to the reductive cyclization of the alkenyl bromide in 476 with the allylic acetate to yield 477[312]. Intramolecular coupling of the allylic acetate in 478 with aryl bromide proceeds using BuiSnAlEti (479) by in situ formation of the allylstannane 480 and its reaction with the aryl bromide via transmetallation. (Another mechanistic possibility is the formation of an arylstannane and its coupling with allylic... 

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

Lee et al. [145] succeeded in preparation of Co-based bimetallic nanoparticles with core/shell structure via transmetalation reaction (Figure 3). The Co-core/Au-shell nanoparticles, e.g., were confirmed to be almost the same in particle size with the seeded Co nanoparticle, as shown in Figure 4. 

These reactions are believed to proceed via transmetallation. Configurational inversion occurs at both the transmetallation and addition steps, leading to overall retention of the allylic stereochemistry. 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

The basic mechanism of the Stille reaction involves transmetallation, either directly or via an organocopper intermediate, with a Pd(H) intermediate generated by oxidative addition from the aryl halide or triflate. 

To elucidate the reaction pathway, deuterium-labeled allenyl pinacol boronate 10 was prepared, and the addition reaction with hydrazonoester 6 was conducted in the presence of Bi(OH)3 and Cu(OH)2 (Scheme 4). In both Bi- and Cu-catalyzed cases, the reactions proceeded smoothly (in quantitative yields in both cases). In the Bi(OH)3-catalyzed reaction, a major product was allenyl compound 11, in which the internal position was deuterized. It was assumed that a propargyl bismuth was formed via transmetalation from boron to bismuth, followed by addition to hydrazonoester via y-addition to afford allenyl compound 11. Thus, two y-additions could selectively provide a-addition products [75, 76, 105, 106]. It was confirmed that isomerization of 10 did not occur. Recently, we reported Ag20-catalyzed anti-selective a-addition of a-substituted allyltributyltin with aldehydes in aqueous media [107], On the other hand, in the Cu(OH)2-catalyzed reaction, a major product was propargyl compound 12, in which the terminal position was deuterized. A possible mechanism is that Cu(OH)2 worked as a Lewis acid catalyst to activate hydrazonoester 6 and that allenyl boronate 10 [83-85] reacted with activated 6 via y-addition to afford 12. 

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