Intermolecular decarboxylative addition reactions

Polyfluoroalk-2-ynoic acids 7 readily undergo an intermolecular-intrarnolecular Michael addition reaction with a variety of bifunctional azanucleophiles and o-phenylenediamine. to give the corresponding carboxylated and/or decarboxylated 2-(polyfluoroalkyl)imidazolidine (e. g, 8 and 9. respectively), thiazolidine, and oxazolidine derivatives in moderate to good yields. The products formed depend strongly on the reaction conditions employed. For example, the decarboxylated derivative 9 is only formed under reflux conditions. ... 

Aliphatic aldehydes are also obtained with high selectivity. Selectivity is lower for primary aliphatic carboxylic acids because of a competing intermolecular decarboxylation reaction which results in ketone formation (Eq. 2) [16]. This ketonization activity of Zr02 was, substantially suppressed by addition of metal ions. 

A related strategy has been used for the conversion of 5,5-divinyl oxazoUdinones to highly substituted pyrrolidines [82]. For example, treatment of 107 with a Pd° catalyst in the presence of activated alkene 108 provides 109 in 95% yield (Eq. (1.45)). The reaction proceeds via oxidative addition followed by decarboxylation to afford the allylpalladium complex 110. Intermolecular conjugate addition to give 111, followed by intramolecular trapping of the allylpalladium moiety, affords the observed products. 

Considerable use has also been made of allyl carbonates as substrates for the allylation of Pd enolates.9 The reaction of Pd° complexes with allyl enol carbonates119,120 proceeds by initial oxidative addition into the allylic C—O bond of the carbonate followed by decarboxylation, yielding an allylpalladium enolate, which subsequently produces Pd° and the allylated ketone (equation 22). In like fashion, except now in an intermolecular sense, allyl carbonates have been found to allylate enol silyl ethers (equation 23),121 enol acetates (with MeOSnBu3 as cocatalyst) (equation 24),122 ketene silyl acetals (equation 25)123 and anions a to nitro, cyano, sulfonyl and keto groups.115,124 In these cases, the alkoxy moiety liberated from the carbonate on decarboxylation serves as the key reagent in generating the Pd enolate. 

The mechanisms proposed for these reactions are all quite analogous, and only the intramolecular cases will be considered in detail (Scheme 5). Oxidative addition by Pd° into the allylic C—O bond of the allyl 0-ketocaiboxylate produces an allylpalladium caxboxylate. This species then undergoes decarboxylation to yield an allylpalladium enolate (oxa-ir-allyl), which subsequently eliminates a 0-H to form the enone and provide an allyl-Pd-H. Reductive elimination from the allyl-Pd-H yields propene and returns Pd to its zero oxidation state. A similar mechanism can be imagined for the alkenyl allylcarbonate. Oxidative addition by the Pd° forms an allylpalladium carbonate, which decarboxylates again to give an allylpalladium enolate (oxa-ir-allyl). 0-Hydride elimination and reductive elimination complete the process. The intermolecular cases derive the same allylpalladium enolate intermediates, only now as the result of bimolecular processes. 

Although outside the scope of this section, the concurrent development of the Pd-catalyzed allylation of /3-ketocarboxylic acids via the formation and decomposition of allyl /3-ketocarboxylates is noteworthy (Sects. V.2.1.1 and V.2.1.2). The mechanism shown in Scheme 4, which involves (i) oxidative addition of allyl /3-ketocarboxylates, (ii) decarboxylation, and (iii) intermolecular enolate allylation was proposed and experimentally supportedUnfortunately, a-allylation with y-disubstituted allyl derivatives, such as geranyl carboxylates, proceeds in low yields, and there are some indications that the reaction may lack some specificity features, for example, stereospecificity of the allylic moiety. 

In addition to the 1,3-dienes, in situ formed allenes were applied as substrates and cyclized with 2-iodophenols as well. In 2007, Liang and co-workers reported a novel palladium-catalyzed intermolecular tandem reaction for the synthesis of tetracyclic compounds from propargylic compounds and 2-iodophenols. The corresponding tetracyclic compounds were isolated in moderate to good yields (Scheme 2.47). In respect of the reaction pathway, the possible reaction intermediate was isolated and applied. In detail, this transformation consists of two catalytic cycles. The steps are (a) initial decarboxylation of propargylic compound by palladium(O) to... 

In general, the PET decarboxylation of carboxylic acids exhibits low product selectivity, due to the subsequent radical coupling in solution however, the PET reaction between potassium propionate and the methyl ester of N-methyltrimellitic acid imide is highly regioselective to the para-addition product. This contrasts with other intramolecular and intermolecular PET reactions of quinolinic acid imides, which do not show a high degree of regioselectivity (Scheme S)." ... 

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