A Versatile Hexadiene Synthesis by Decarboxylative sp3–sp3 Coupling/Cope Rearrangement

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.

Promoting Thermodynamically Unfavorable [3,3] Rearrangements by Chemoselective Reduction

Herein described is a strategy for promoting thermodynamically unfavorable [3,3] Cope rearrangements. 3,3-Dicyano-1,5-dienes that are resistant to the thermal rearrangement can be promoted under reductive conditions. The reduced Cope products are versatile, bifunctional building blocks.

Transient [3,3] Cope rearrangement of 3,3-dicyano-1,5-dienes: computational analysis and 2-step synthesis of arylcycloheptanes

A simple and modular route to arylcycloheptene scaffolds is reported. The two-step route from Knoevenagel adducts and allylic electrophiles is made possible through the design of a Cope rearrangement that utilizes a "traceless" activating group to promote an otherwise thermodynamically unfavorable transformation. Experimentally, the [3,3] rearrangement occurrs transiently at room temperature with a computed barrier of 19.5 kcal mol-1, which ultimately allows for three-component bis-allylation. Ring-closing metathesis delivers the arylcycloheptane and removes the activating group. This report describes the design and optimization of the methodology, scope and mechanistic studies, and computational analysis.

Iridium-catalyzed direct asymmetric vinylogous allylic alkylation

A catalytic asymmetric vinylogous allylic alkylation of α,β-unsaturated lactones with an iridium catalyst was achieved with excellent regio- and enantioselectivity.

Stereoselective allylic reduction via one-pot palladium-catalyzed allylic sulfonation and sulfinyl retro-ene reactions

A tandem approach for the stereoselective allylic reduction has been developed based on a strategy combining the palladium-catalyzed S-allylation and the sulfinyl retro-ene reactions.

Decarboxylative and dehydrative coupling of dienoic acids and pentadienyl alcohols to form 1,3,6,8-tetraenes

Dienoic acids and pentadienyl alcohols are coupled in a decarboxylative and dehydrative manner at ambient temperature using Pd(0) catalysis to generate 1,3,6,8-tetraenes. Contrary to related decarboxylative coupling reactions, an anion-stabilizing group is not required adjacent to the carboxyl group. Of mechanistic importance, it appears that both the diene of the acid and the diene of the alcohol are required for this reaction. To further understand this reaction, substitutions at every unique position of both coupling partners was examined and two potential mechanisms are presented.

Deconjugative alkylation/Heck reaction as a simple platform for dihydronaphthalene synthesis

A simple platform for carbocycle synthesis by Knoevenagel adduct deconjugative alkylation/“pairing” reaction is described.

Iridium-catalysed asymmetric allylic alkylation of benzofuran γ-lactones followed by heteroaromatic Cope rearrangement: study of an unusual reaction sequence

The iridium-catalysed asymmetric allylic alkylation of benzofuran γ-lactones produces 1,5-hexadienes that were found to be excellent substrates for an unusual heteroaromatic Cope rearrangement.

Enantioselective γ-Alkylation of α,β-Unsaturated Malonates and Ketoesters by a Sequential Ir-Catalyzed Asymmetric Allylic Alkylation/Cope Rearrangement

A catalytic, enantioselective γ-alkylation of α,β-unsaturated malonates and ketoesters is reported. This strategy entails a highly regio- and enantioselective iridium-catalyzed α-alkylation of an extended enolate, and a subsequent translocation of chirality to the γ-position via a Cope rearrangement.

Palladium-Catalyzed Double-Decarboxylative Addition to Pyrones: Synthesis of Conjugated Dienoic Esters

An interceptive decarboxylative allylation protocol has been developed utilizing pyrone as a C4 synthon. This palladium-catalyzed transformation difunctionalizes the pyrone moiety by in situ generation and activation of both the electrophile and nucleophile via a double decarboxylation pathway. Ultimately, allyl carbonates react smoothly with 2-carboxypyrone under mild reaction conditions to generate synthetically useful acyclic dienoic esters, forming carbon dioxide as the sole byproduct.

Rapid synthesis of polyprenylated acylphloroglucinol analogs via dearomative conjunctive allylic annulation

Polyprenylated acylphloroglucinols (PPAPs) are structurally complex natural products with promising biological activities. Herein, we present a biosynthesis-inspired, diversity-oriented synthesis approach for rapid construction of PPAP analogs via double decarboxylative allylation (DcA) of acylphloroglucinol scaffolds to access allyl-desoxyhumulones followed by dearomative conjunctive allylic alkylation (DCAA).

Toward a symphony of reactivity: cascades involving catalysis and sigmatropic rearrangements

Catalysis and synthesis are intimately linked in modern organic chemistry. The synthesis of complex molecules is an ever evolving area of science. In many regards, the inherent beauty associated with a synthetic sequence can be linked to a certain combination of the creativity with which a sequence is designed and the overall efficiency with which the ultimate process is performed. In synthesis, as in other endeavors, beauty is very much in the eyes of the beholder. It is with this in mind that we will attempt to review an area of synthesis that has fascinated us and that we find extraordinarily beautiful, namely the combination of catalysis and sigmatropic rearrangements in consecutive and cascade sequences.

Asymmetric conjugate addition to α-substituted enones/enolate trapping

An NHC-Cu complex catalyzed the asymmetric conjugate addition (ACA) of various Grignard reagents to nonactivated α-substituted cyclic enones to give 2,3-dialkylated cyclopentanones and cyclohexanones. The Michael addition features the formation of a magnesium enolate intermediate. One-pot diastereoselective trapping of this enolate by alkyl, propargyl, allyl, and benzyl halides led to ketones with contiguous α-quaternary and β-tertiary centers.

The reaction mechanism of the enantioselective Tsuji allylation: inner-sphere and outer-sphere pathways, internal rearrangements, and asymmetric C-C bond formation

We use first principles quantum mechanics (density functional theory) to report a detailed reaction mechanism of the asymmetric Tsuji allylation involving prochiral nucleophiles and nonprochiral allyl fragments, which is consistent with experimental findings. The observed enantioselectivity is best explained with an inner-sphere mechanism involving the formation of a 5-coordinate Pd species that undergoes a ligand rearrangement, which is selective with regard to the prochiral faces of the intermediate enolate. Subsequent reductive elimination generates the product and a Pd(0) complex. The reductive elimination occurs via an unconventional seven-centered transition state that contrasts dramatically with the standard three-centered C-C reductive elimination mechanism. Although limitations in the present theory prevent the conclusive identification of the enantioselective step, we note that three different computational schemes using different levels of theory all find that inner-sphere pathways are lower in energy than outer-sphere pathways. This result qualitatively contrasts with established allylation reaction mechanisms involving prochiral nucleophiles and prochiral allyl fragments. Energetic profiles of all reaction pathways are presented in detail.

Mechanism of silver- and copper-catalyzed decarboxylation reactions of aryl carboxylic acids

Silver- and copper-catalyzed decarboxylation reactions of aryl carboxylic acids were investigated with the aid of density functional theory calculations. The reaction mechanism starts with a carboxylate complex of silver or copper. Decarboxylation occurs via ejecting CO(2) from the carboxylate complex followed by protodemetallation with an aryl carboxylic acid molecule to regenerate the starting complex. Our results indicated that the primary factor to affect the overall reaction barriers is the ortho steric destabilization effect on the starting carboxylate complexes for most cases. Certain ortho substituents that are capable of coordinating with the catalyst metal center without causing significant ring strain stabilize the decarboxylation transition states and reduce the overall reaction barriers. However, the coordination effect is found to be the secondary factor when compared with the ortho effect.

Transition metal-catalyzed decarboxylative allylation and benzylation reactions

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