Allyl enol carbonates, decarboxylation

Decarboxylation of Carboxylic Acids and Derivatives.—A palladium-catalysed decarboxylation-dehydrogenation of allyl P-keto-carboxylates and allyl enol carbonates occurs in good yield for ten examples however, mixtures of isomers are obtained in some cases (Scheme 32).  

Enol carbonates react with alkylating agents in the presence of a palladium catalyst. The decarboxylative alkylation of allyl enol carbonates to the corresponding aUylcyclohexanone derivatives is known as the Tsuji alkylation. An asymmetric version of this reaction has been reported. The same reaction can be done using enolate anion and aUylic acetates with a palladium catalyst.  

Whereas allyl enol carbonates are prochiral starting materials, the fi-keto esters are frequently chiral but can be used as racemic compounds because the stereogenic center vanishes in the course of the decarboxylative allylic alkylation, precisely during the formation of the enolate. In general, both enantiomers of chiral fi-keto esters 91 react without enantiodifferentiation with the palladium(O) catalyst to give the enolate, a course of the reaction that was termed stereoablative 

Allyl enol carbonates derived from ketones and aldehydes undergo Pd-cat-alyzed decarboxylation-elimination, and are used for the preparation of o, /3-unsaturated ketones and aldehydes. The reaction is regiospecific. The regio-isomeric enol carbonates 724 and 726, prepared from 723, are converted into two isomeric enones, 725 and 727. selectively. The saturated aldehyde 728 can be converted into the a,/3-unsaturated aldehyde 730 via the enol carbonate 729[459]. 

The decarboxylation-allylation of allyl enol carbonates proceeds smoothly[450]. The isomeric enol carbonates 699 and 701 of the enone 698 undergo regiospecific allylation, giving the regioisomers 700 and 702 selectively. 

Scheme 5.44 Enantioselective formation of butenolides 133 by palladium-catalyzed decarboxylative allylic alkylation of furan-derived enol carbonate 132.

Scheme 5.32 Enantioselective decarboxylative allylic alkylations, starting from allyl enol carbonates 101 (reported by Stoltz) and allyl enol carbonates 103 (reported by Trost).

As for the original allylation reaction, decarboxylative allylation may be made asymmetric by the inclusion of chiral ligands (Scheme 9.64). Similar results are obtained using either the 3-ketoester 9.238 or the enol carbonate 9.237 (Scheme 9.65).  

Trost BM, Bream RN, Xu J. Asymmetric allylic alkylation of cyclic vinylogous esters and thioesters by Pd-catalyzed decarboxylation of enol carbonate and beta-ketoester substrates. Angew. Chem. Int. Ed. 2006 45 3109-3112. 

Scheme 5.30 Mechanistic pathways in the palladium-catalyzed decarboxylative allylic alkylation, starting from allyl p-keto esters 91 or allyl enol carbonates 92.

In order to achieve regio- and stereoselective a-allylation of a cyclohexanone derivative, Paquette and Nicolaou carried out the Pd-catalyzed decarboxylation-allyladon of the allyl enol carbonate 578 at room temperature to give the allyl ketone 579 in 58% yield and a regioisomer (24%) using PPh3 [208]. 

According to a tentative mechanism as represented by the above (simplified) catalytic cy-de, the propargyl carbonate is transformed to a Pd-intermediate 90, which decarboxylates to the allenic moiety 93 after addition of the enolate of acetoacetate ( 92) and isomerization (92 91), the iT-allyl-Pd complex 91 cyclizes to the product 89. Highly functionalized tetrasubstituted furans can be prepared by a novel Pd-catalyzed three-component cycliza-tion of 2-alkinyl-2-alken-l-ones with an allyl chloride and an alcohol [30]. 

As an alternative to prior deprotonation of the parent carbonyl compounds, enolates can also be generated in situ from allyl /(-koto carboxylates or allyl enol carbonates by decarboxylation with simultaneous production of a 7i-allylpalladium complex 1-12. A similar utilization of / -keto acids has also been described13. The following diagram illustrates the reaction course for an allyl jS-keto carboxylate. 

In addition, unstabilized enolate nucleophiles have been generated by decarboxylation of (3-ketocarboxylates. In this case, no additives are required to activate the nucleophile, but the highest yields and selectivities were obtained in the presence of two equivalents of DBU [82]. Although reactions of allylic carbonates containing aromatic, heteroaromatic, and aliphatic substituents occurred, only reactions to form aryl ketone products were published. 

The most common nucleophiles employed in the allylic alkylation are soft species such as malonate esters. However, Trost and Schroeder have discovered that high ees can be achieved in the addition of cyclic lithium enolates using ligand (10.54) with one equivalent of trimethyltin chloride, which may act to soften the nucleophilic species by transmetaUation to the tin enolate. Enantioselective additions of nonstabilised ketone enolates can also be achieved using an alternate palladium-catalysed decarboxylation protocol. In this approach an allyl 3-ketoester (10.72) or allyl vinyl carbonate (10.73) undergoes decarboxylation in the presence of Pd(0) to 

The use of palladium(II) 7i-allyl complexes in organic chemistry has a rich history. These complexes were the first examples of a C-M bond to be used as an electrophile [1-3]. At the dawn of the era of asymmetric catalysis, the use of chiral phosphines in palladium-catalyzed allylic alkylation reactions provided key early successes in asymmetric C-C bond formation that were an important validation of the usefulness of the field [4]. No researchers were more important to these innovations than Prof. B.M. Trost and Prof. J. Tsuji [5-10]. While most of the early discoveries in this field provided access to tertiary (3°) stereocenters formed on a prochiral electrophile [Eq. (1)] (Scheme 1), our interest focused on making quaternary (4°) stereocenters on prochiral enolates [Eq. (2)]. Recently, we have described decarboxylative asymmetric allylic alkylation reactions involving prochiral enolates that provide access to enantioenriched ot-quatemary carbonyl compounds [11-13]. We found that a range of substrates (e.g., allyl enol carbonates, 

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