Allyl carbonates transmetallation

Some organosilicon compounds undergo transmetallation. The allylic cyanide 461 was prepared by the reaction of an allylic carbonate with trimethylsi-lyl cyanide[298]. The oriho esters and acetals of the o. d-unsaturated carbonyl compounds 462 undergo cyanation with trimefhylsilyl cyanide[95]. 

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... 

Pd(O) species to an allyl chloride, transmetalation of TMSCN with the resulting tz-allylpalladium chloride, conjugate addition of the cyanide ligand of the rr-alIylpalladium cyanide to an activated alkene, and reductive elimination of the carbon ligands. 

Allylation of ketone enolates is treated in Chapter 4.2.3. The chemistry of tt-allylpalladium enolates, which is related, but more versatile than that of simple ketone enolates, is treated in this section. 7r-Allylpalladium enolates are generated in two ways. One is the transmetallation of silyl and tin enolates with TT-allylpalladium alkoxide formed from allyl carbonates. The other is Pd-catalyzed reaction of allyl j6-keto esters [199]. 

Formation of 7r-allylpalladium enolates by the reaction of the silyl enolate 539 with allyl carbonate is summarized as follows. The enolate of cyclohexanone is treated as a model compound here. Transmetallation of 7r-allylpalladium methoxide 538 with the silyl enolate 539 generates either Pd enolates 540 or a-palladake-tone 541. Reductive elimination affords allylcyclohexanone 542 in THF [200]. At higher temperature, cyclohexanone 543 is obtained by elimination using ligandless Pd catalyst in MeCN [201]. 

In the allylation of the silyl enol ethers with allyl carbonates, the transmetallation of the silyl enol ethers with the 7r-allylpalladium methoxides, generated from allyl methyl carbonates, gives the 7r-allylpalladium enolates. Depending on the reaction conditions, allyl ketones are formed by reductive elimination (Scheme When a ratio... 

Transition Metal-Catalyzed Allylie Alkylation. Chelated amino acid ester enolates were found to be suitable nucleophiles for palladium-catalyzed allylie alkylations (eq 25). They were conveniently prepared by deprotonation of a glycine derivative with LHMDS followed by transmetallation with zinc chloride. The palladium-catalyzed allylie alkylation then takes place in the presence of allyl carbonates to produce the desired anti amino acid derivative. ... 

Various allylic compounds react with hard carbon nucleophiles > 25) including organometallic compounds of Zn, B, Al, Sn and Si via a transmetallation pathway (see 1.1.8.3). For example, the reaction of allylic lactone 46 with phenylzinc chloride gave the product 48 in excellent yield. Note the inversion of the stereochemistry at the allylic carbon (see 1.1.8.3). 

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]. 

Despite the great success of the transmetalation process in the enantiose-lective arylation of ketones, its extension to allylation or alkynylation reactions failed, providing the corresponding tertiary alcohols with enantiomeric excesses never higher than 50% ee. On the other hand, more success has been found in the alkenylation of ketones. The process started with the hydrozirconation of terminal alkynes to give the corresponding alkenylzirconium intermediates, which were transmetalated by reaction, in this case, with various ketones in the presence of the HOCSAC ligand. This protocol tolerated the presence of other carbon-carbon multiple bonds on the alkyne, as well as different functionalities and achieved excellent results for alkyl ketones, a,(3-unsaturated ketones and even dialkylketones, as shown in Scheme 4.22. 

The first carbon—carbon bond-forming reaction of zirconacyclopentadienes involving a copper transmetalation was the double allylation reaction shown in Eq. 2.33 [6]. 

Assuming that the transmetallation and subsequent reaction with allyl bromide do not alter the configuration of the carbon-zinc bond47, the stereochemistry of the intermediate is that shown for 85. Hence it follows that one of the two carbon-metal bonds in 84 has... 

---