Easy Access to Esters with a Benzylic Quaternary Carbon Center from Diallyl Malonates by Palladium-Catalyzed Decarboxylative Allylation

Decarboxylative Allylic Alkylation of Phthalides: Stabilized Benzylic Nucleophiles for sp3-sp3 Coupling

A new family of stabilized benzylic nucleophiles for the palladium-catalyzed decarboxylative allylic alkylation reaction has been developed. Allyl esters derived from 3-carboxyphthalides were found to undergo palladium-catalyzed deallylation and decarboxylation under mild reaction conditions, a process facilitated by the formation of a stabilized aromatic anion. The regioselective allylic coupling of this intermediate afforded a variety of functionalized phthalides in 73-96% yields.

Palladium-Catalyzed Regiospecific Decarboxylative Allylation of (Cyclohexadienylidene)malononitriles: Access to α-Allyl-α-aryl Malononitriles

A palladium-catalyzed regiospecific decarboxylative ε-allylation of (cyclohexadienylidene)malononitriles is presented for the synthesis of functionalized α-allyl-α-aryl malononitriles. This reaction proceeds via a resonance-stabilized α-aryl malononitrile anion, resulting in a wide range of α-allyl-α-aryl malononitriles in high yields with excellent linear product selectivity. We have also shown that the resulting products can be transformed into valuable synthetic intermediates by decyanation and Mizoroki-Heck arylation. In addition, an enantioselective decarboxylative allylation reaction is also presented.

Recent Advances in Enantioselective Pd-Catalyzed Allylic Substitution: From Design to Applications

This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.

On the hydrolysis of diethyl 2-(perfluorophenyl)malonate

Diethyl 2-(perfluorophenyl)malonate was synthesized in 47% isolated yield by the reaction of sodium diethyl malonate and hexafluorobenzene. The resulting compound was considered as a starting material for synthesizing 2-(perfluorophenyl)malonic acid by hydrolysis. It was found that the desired 2-(perfluorophenyl)malonic acid could not be obtained from this ester by hydrolysis, neither under basic nor under acidic conditions. Nevertheless, hydrolysis of the ester with a mixture of HBr and AcOH gave 2-(perfluorophenyl)acetic acid in a good preparative yield of 63%. A significant advantage of this new approach to 2-(perfluorophenyl)acetic acid is that handling toxic substances such as cyanides and perfluorinated benzyl halides is avoided.

A borane-mediated palladium-catalyzed reductive allylic alkylation of α,β-unsaturated carbonyl compounds

The development of the palladium-catalyzed allylic alkylation of in situ generated boron enolates via tandem 1,4-hydroboration is reported. Investigation of the reaction revealed insights into specific catalyst electronic features as well as a profound leaving group effect that proved crucial for achieving efficient allylic alkylation of ester enolates at room temperature and ultimately a highly preparatively useful synthesis of notoriously challenging acyclic all-carbon quaternary stereocenters. The method demonstrates boron enolates as viable pro-nucleophiles in transition-metal catalyzed allylic alkylation, potentially opening up further transformations outside their traditional use.

Enantioselective Synthesis of Medium-Sized Lactams via Chiral α,β-Unsaturated Acylammonium Salts

Medium-sized lactams are important structural motifs found in a variety of bioactive compounds and natural products but are challenging to prepare, especially in optically active form. A Michael addition/proton transfer/lactamization organocascade process is described that delivers medium-sized lactams, including azepanones, benzazepinones, azocanones, and benzazocinones, in high enantiopurity through the intermediacy of chiral α,β-unsaturated acylammonium salts. An unexpected indoline synthesis was also uncovered, and the benzazocinone skeleton was transformed into other complex heterocyclic derivatives, including spiroglutarimides, isoquinolinones, and δ-lactones.

Non-stabilized enolates – versatile nucleophiles in transition metal-catalysed allylic alkylations

This review covers new developments in the transition metal-catalyzed allylic alkylations of non-stabilized enolates, preferentially generated from ketone, esters or amides.

One-pot synthesis of β-N-glycosyl imidazole analogues via a palladium-catalysed decarboxylative allylation

A concise and highly efficient strategy for the synthesis of N-glycosyl imidazole analogues is reported. This reaction is based on a palladium catalysed decarboxylative allylation and three steps, namely, carbamation, decarboxylation and allylation are involved. All the substrates can afford the desired products with excellent yields and selectivities.

Facile access to cis-2,6-disubstituted tetrahydropyrans by palladium-catalyzed decarboxylative allylation: total syntheses of (±)-centrolobine and (+)-decytospolides A and B

cis-2,6-Tetrahydropyran is an important structural skeleton of bioactive natural products. A facile synthesis of cis-2,6-disubstituted-3,6-dihydropyrans as cis-2,6-tetrahydropyran precursors has been achieved in high regio- and stereoselectivity with high yields. This reaction involves a palladium-catalyzed decarboxylative allylation of various 3,4-dihydro-2H-pyran substrates. Extending this reaction to 1,2-unsaturated carbohydrates allowed the achievement of challenging β-C-glycosylation. Based on this methodology, the total syntheses of (±)-centrolobine and (+)-decytospolides A and B were achieved in concise steps and overall high yields.

Mechanism of the Pd-catalyzed decarboxylative allylation of α-imino esters: decarboxylation via free carboxylate ion

The Pd-catalyzed decarboxylative allylation of α-(diphenylmethylene)imino esters (1) or allyl diphenylglycinate imines (2) is an efficient method to construct new C(sp(3))-C(sp(3)) bonds. The detailed mechanism of this reaction was studied by theoretical calculations [ONIOM(B3LYP/LANL2DZ+p:PM6)] combined with experimental observations. The overall catalytic cycle was found to consist of three steps: oxidative addition, decarboxylation, and reductive allylation. The oxidative addition of 1 to [(dba)Pd(PPh(3))(2)] (dba = dibenzylideneacetone) produces an allylpalladium cation and a carboxylate anion with a low activation barrier of +9.1 kcal mol(-1). The following rate-determining decarboxylation proceeds via a solvent-exposed α-imino carboxylate anion rather than an O-ligated allylpalladium carboxylate with an activation barrier of +22.7 kcal mol(-1). The 2-azaallyl anion generated by this decarboxylation attacks the face of the allyl ligand opposite to the Pd center in an outer-sphere process to produce major product 3, with a lower activation barrier than that of the minor product 4. A positive linear Hammett correlation [ρ = 1.10 for the PPh(3) ligand] with the observed regioselectivity (3 versus 4) supports an outer-sphere pathway for the allylation step. When Pd combined with the bis(diphenylphosphino)butane (dppb) ligand is employed as a catalyst, the decarboxylation still proceeds via the free carboxylate anion without direct assistance of the cationic Pd center. Consistent with experimental observations, electron-withdrawing substituents on 2 were calculated to have lower activation barriers for decarboxylation and, thus, accelerate the overall reaction rates.

Transition metal-catalyzed decarboxylative allylation and benzylation reactions

A review. Transition metal catalyzed decarboxylative allylations, benzylations, and interceptive allylations are reviewed.

Mechanistic origin of the stereodivergence in decarboxylative allylation

A stereochemical test has been used to probe the mechanism of decarboxylative allylation. This probe suggests that the mechanism of DcA reactions can change based on the substitution pattern at the alpha-carbon of the nucleophile; however, reaction via stabilized malonate nucleophiles is the lower energy pathway. Lastly, this mechanistic proposal has predictive power and can be used to explain chemoselectivities in decarboxylative reactions that were previously confounding.

Palladium-catalyzed decarboxylative benzylation of diphenylglycinate imines

General reaction conditions for the Pd-catalyzed decarboxylative benzylation of benzyl diphenylglycinate imines are described. The overall procedure requires a simple catalyst/ligand combination to form a new Csp(3)-Csp(3) bond. Microwave irradiation greatly accelerated the transformation. Moreover, various heteroaromatic moieties are tolerated in both the imine and ester components.

Palladium-catalyzed decarboxylative rearrangements of allyl 2,2,2-trifluoroethyl malonates: direct access to homoallylic esters

Homoallylic esters are obtained in a single transformation from allyl 2,2,2-trifluoroethyl malonates by using a Pd(0) catalyst. Facile decarboxylation of allyl 2,2,2-trifluoroethyl malonates is attributed to a decrease in pK(a) compared to allyl methyl malonates. Subsequent reduction of the homoallylic 2,2,2-trifluoroethyl ester provides a (hydroxyethyl)cyclopentenyl derivative that represents a key intermediate in the synthesis of carbocyclic nucleosides. A select allyl 2,2,2-trifluoroethyl malonate undergoes a decarboxylative Claisen rearrangement to provide a regioisomeric homoallylic ester.

Tandem C-C bond-forming processes: interception of the pd-catalyzed decarboxylative allylation of allyl diphenylglycinate imines with activated olefins

Interception of the Pd-catalyzed decarboxylative allylation of allyl diphenylglycinate imines with appropriately functionalized Michael acceptors, followed by Heck cyclization, allows for the efficient construction of relatively complex organoamine frameworks in one reaction vessel. The initial intercepted decarboxylative allylation is remarkably insensitive toward solvent and catalyst, typically proceeding under ambient conditions.

Palladium-catalyzed decarboxylative sp3-sp3 coupling of nitrobenzene acetic esters

Allylic esters of nitrobenzene acetic acids undergo facile palladium-catalyzed decarboxylative coupling. Both mono- and dinitroarene substrates give high yields of the coupled products. Moreover, the rates of the reactions suggest that decarboxylation is rate-limiting and substrates that sterically disfavor attainment of the reactive conformation for decarboxylation are not viable. Finally, reduction of the product nitroarenes to the corresponding anilines provides access to a variety of heterocycles including quinolines and dihydroquinolones.

Homoallylic ketones and pyrroles by way of copper-catalyzed cascade additions of alkyl-substituted vinyl Grignard reagents to esters

Exploring the scope of the copper-catalyzed cascade addition of vinyl Grignard reagents to carboxylic esters, a set of substituted homoallylic ketones (γ,δ-unsaturated ketones) have been synthesized in 11%–94% yields from treatment of methyl 4-methoxybenzoate and methyl N-Boc-β-alaninate with different methyl-, dimethyl-, and phenyl-substituted vinyl Grignard reagents in the presence of catalytic amounts of CuCN in THF. The respective 2,4-di-, 2,3,5-tri-, and 2,3,4,5-tetrasubstituted pyrroles were obtained in 47%–93% yields from the homoallylic ketones by a sequence featuring ozonolysis followed by Paal–Knorr condensation with ammonium formate.Key words: copper-catalyzed cascade addition, homoallylic ketone, alkyl-substituted vinyl Grignard reagents, ozonolysis, pyrrole, γ,δ-unsaturated ketone.