Tsuji allylation silyl enol ethers

Ketone and ester enolates have historically proven problematic as nucleophiles for the transition metal-catalyzed allylic alkylation reaction, which can be attributed, at least in part, to their less stabilized and more basic nature. In Hght of these limitations, Tsuji demonstrated the first rhodium-catalyzed allylic alkylation reaction using the trimethly-silyl enol ether derived from cyclohexanone, albeit in modest yield (Eq. 4) [9]. Matsuda and co-workers also examined rhodium-catalyzed allylic alkylation, using trimethylsilyl enol ethers with a wide range of aUyhc carbonates [22]. However, this study was problematic as exemplified by the poor regio- and diastereocontrol, which clearly delineates the limitations in terms of the synthetic utihty of this particular reaction. 

The protocols for the utilization of ketone-derived silyl enol ethers in Tsuji-Trost reactions were preceded by a report of Morimoto and coworkers on the enantioselective allylation of sUyl ketene acetals 88. Without external activation, they reacted with the allylic substrate 19d in the presence of the palladium complex derived from the amidine ligand 89 to give y,5-unsaturated esters 90 in moderate chemical yield but high enantiomeric excess (Scheme 5.29) [46]. Presumably, the pivalate anion hberated during the oxidative addition functions as an activator of the silyl ketene acetal. The protocol is remarkable in view of the fact that asymmetric allylic alkylations of carboxylic esters are rare. Interestingly, the asymmetric induction originates from a ligand with an uncomplicated structure. The protocol seems however rather restricted with respect to the substitution pattern of allylic component and sUyl ketene acetal. 

To overcome the limitation of the high stability of the aluminum enolates, the oxygen atom has been transformed to silyl enol ethers, enol acetates, and allyl enol carbonates. Silyl enol ethers and enol acetates are precursors to lithium enolates. Enol acetates and allyl enol carbonates are precursors of cx-allylated adducts via the Tsuji-Trost rearrangement [75-77]. The silylation of aluminum enolates using TMSOTf is well established [78], although in some cases the isolation is difficult [33]. Silyl enol ethers allow further modification to be performed as they behave as lithium enolates (Scheme 15). A recent application can be found in the silylation of the conjugate addition adduct (/ )-((3-(but-3-en-l-yl)-3-methylcyclopent-l-en-l-yl)oxy)triethylsilane which allows aldol condensation to form an intermediate in the synthesis of Clavirolide C [79], a diterpene with a trans-bicyclo[9.3.0] tetradecane structure (Scheme 16) [80]. 

Silyl Enol Ethers A disadvantage of the aforementioned enantioselective Tsuji allylations was that they were performed in an intramolecular fashion, requiring the need to synthesize the allyl enol carbonate starting materials. Although their synthesis is not overly cumbersome, the... 

SCHEME 7.25. Intermolecular Tsuji allylation using silyl enol ethers. 

Tsuji J, Minami I, Shimizu I. Palladium-catalyzed aUyla-tion of ketones and aldehydes with allylic carbonates via silyl enol ethers under neutral conditions. Chem. Lett. 1983 1325-1326. 

Although it is mechanistically different from the Tsuji-Trost allylation, indirect allyla-tions of ketones, aldehydes, and esters via their enolates are briefly summarized here. Related reactions are treated in Sect V.2.1.4. Pd-catalyzed allylation of aldehydes, ketones, and esters with aUyhc carbonates is possible via the Tr-allylpaUadium enolates of these carbonyl compounds. Tr-AUylpalladium enolates can be generated by the treatment of silyl and stannyl enol ethers of carbonyl compounds with allyl carbonates, and the allylated products are obtained by the reductive elimination of the Tr-allylpalladium enolates. 

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