Allyl enol carbonates

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. 

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]. 

Palladium-catalysed asymmetric a-allyl alkylation of acyclic ketones has been reported allyl enol carbonates of a wide range of ketones undergo allyl transfer in high yields and ees at room temperature.197... 

A regio- and enantio-selective palladium-catalysed allylic alkylation of ketones has been reported, using allyl enol carbonate chemistry in which a CO2 unit tethers the allylating agent to the nucleophile.198... 

Imidazole-containing compounds have been utilized as reagents for various synthetic transformations. A convenient access to substituted allyl enol carbonates was established through the reaction of ketone enolates with the complex of allyl l//-imidazole-l-carboxylates 74 and boron trifluoride etherate <07JOC9372>. Relatively mild and highly efficient Cul-catalyzed /V-arylation procedures for imidazoles with aryl and heteroaryl bromides or chlorides have been developed in the presence of ligands 75 and cesium carbonate <07JOC2737>. a, -Unsaturated 2-acyl imidazoles 76 are an alternative and practical class of dienophiles for the DNA-based catalytic asymmetric Diels-Alder reaction in... 

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. ... 

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. 

Formation of allylketones 567 from allyl -ksto esters 563 and allyl enol carbonates 566 is the Pd-catalyzed Carroll rearrangement. As a related reaction, Pd-catalyzed regioselective intramolecular allyladon of the allyl enol ether of 6-keto ester 574 occurred as shown by 575 in DMSO, and afforded a mixture of the endo- and eJco-bicyclo[3.2.1]octane frameworks 576 and 577 using DPPE as a ligand. PPh3 is not suitable [207]. 

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]. 

As a final set of examples, enantioselective allylic substitution of unstabilized eno-lates to form a new stereocenter at the enolate carbon have been developed through the decarboxylative reactions of allyl enol carbonates. - - These reactions are enantioselective versions of reactions closely related to those in Equation 20.18 and Scheme 20.4, and two examples are shown in Equations 20.60 and 20.61. In these cases, a new stereocenter is formed at the a-carbon of the enolate nucleophile. Most of these reactions have been conducted with allyl enol carbonates that generate cyclic ketone enolates, but enantioselective reactions of acyclic allyl enol carbonates have also been reported. Although allyl enol carbonates undergo decarboxylation faster than the 3-keto ester isomers, the 0-allyl p-keto esters are more difficult to prepare, and enantioselective allylations starting with p-ketoesters have been reported. - Decarboxylative reactions of amines and a-amino acids have been conducted to form allylic and homoallylic amines (Equation 20.62), respectively, and enantioselective decarboxylative allylations of amides have been reportedIridium-catalyzed enantioselective decarboxylative allylation of amides starting with 0-allyl imides has also been reported. ... 

Palladium-Catalyzed Asymmetric Tsuji Allylation of Allyl Enol Carbonates... 

Nucleophiles used in the seminal papers by Tsuji and co-workers were mostly stabilized carbon nucleophiles, and the method found an early synthetic application in a preparation of steroids." It soon became evident that many other types of nucleophiles could be used. In particular, hydride ion equivalents led to l-olefinsf ° " (see Sect. V.2.3.1), Silyl and stannyl enolates of simple ketones and aldehydes and esters can be aUylated, as well as allyl enol carbonates (see Sect. V.2.1.4), This is an indirect a-aUylation of ketones, aldehydes, and esters. Enol derivatives can take another reaction course under Pd(0) catalysis (Scheme 2). Thus, oxidation to a,/3-unsaturated carbonyl compounds ensues if reactions are performed in acetonitrile under precise sources of catalyst precursor. "" "" A full discussion on the dichotomy of allylation-oxidation has been published, as well as a comparison of the usefulness of several transition metals as catalysts in allylation of nucleophiles. ... 

A very diflferent application is the enantioselective protonation of the cyclic allyl enol carbonate of Scheme 61. Liberation of the enolate is followed by protonation in the presence of an enantiomerically pure base.t ° ... 

Yet another alternative that is complementary with that mentioned above involves the base-promoted reaction of ketones with allyl chloroformates to form the corresponding allyl enol carbonates followed by their Pd-catalyzed decomposition to give a-allylated ketones (Scheme 4). 

Not only silyl end ethers but also enol acetates prepared from saturated ketones give a,/3-unsaturated ketones by heating with allyl methyl carbonate in the presence of Pd(II)(OAc)2 and dppe with tributyltin methoxide as a bimetallic catalyst (Scheme 12). Regioselective generation of palladium(II) enolate intermediate is simply carried out by treatment of allyl enol carbonates, which are prepared by trapping of ketone enolates with chloroformate, with Pd(II)(OAc)2 in the presence of dppe (Scheme 13). 7r-Allylpal-ladium(II) enolates thus generated provide a,/3-nnsatnrated ketones. 

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,... 

We were able to envision a reasonable catalytic cycle for the use of silyl enol ethers in the asymmetric alkylation reaction, but there were several complications that could potentially lead to lower enantioinduction in the silyl enol ether reactions (Scheme 6). The generation of the enolate independent of the palladium(II) 7t-allyl complex and the presence of a tetrabutylammonium counter ion could shift the mechanism of the reaction. We had very httle proof at the time, but our working hypothesis was that the C-C bond-forming step occurred in the inner sphere of the palladium atom. We considered the possibihty that the conditions used in the silyl enol ether reactions might facilitate an outer sphere pathway resulting in lower ee products. In the event, we found that the ketone products generated from silyl enol ethers did not significantly differ in ee from the equivalent products of allyl enol carbonate reactions (e.g., Table 8 vs. Table 2). 

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). ... 

Allyl /3-keto carboxylates and allyl enol carbonates undergo a palladium-catalyzed decarboxylation-dehydrogenation to yield o, S-unsaturated ketones in usually high chemical yield and with good selectivity. Following this approach, it was possible to obtain 2-methyl-2-cyclopentenone in two steps from diallyl adipate in a procedure that could be convenient for large-scale preparations (eq 62). ... 

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

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

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