Palladium-catalyzed Allylic Alkylation Research Articles

Relay PHOS: Ligands with Fluxional Chirality in Asymmetric Palladium-Catalyzed Allylic Alkylation.

A modular route toward the synthesis of P,N ligands containing a fluxional group along the pyrazoline ring core is described. The racemic ligands were accessed in three steps from commercially available fluoroacetophenone in overall yields ranging from 18 to 76%. The enantiopure ligands were obtained using semi-preparative chiral high-performance liquid chromatography and chiral enantioselective phase-transfer catalysis. The effectiveness of the new ligands was assessed in palladium-catalyzed allylic alkylation with diphenylpropenyl acetate and dimethylpropenyl acetate. Under optimized conditions, diphenylpropenyl acetate underwent alkylation with dimethyl malonate in 98% yield and 94% ee. In general, the enantioselectivity for the product correlates with the size of the ligand fluxional group; the larger the fluxional group, the higher the selectivity.

Direct synthesis of spirooxindoles enabled by palladium-catalyzed allylic alkylation and DBU-mediated cyclization: concept, scope and applications

A concise construction of spiro[indoline-3,2′-pyrrol]-2-one skeletons is reported. This reaction proceeded through a palladium-catalyzed decarboxylative allylic alkylation followed by a DBU-mediated intramolecular cyclization.

Palladium-catalyzed allylic alkylation enabled by ketone umpolung via Pudovik addition/[1,2]-phospha-Brook rearrangement

Palladium-catalyzed allylic alkylation enabled by ketone umpolung via Pudovik addition/[1,2]-phospha-Brook rearrangement

Non-nucleophilic Grignard Synthesis of Bridged Pyridine–Oxazoline Ligands and Evaluation in Palladium-Catalysed Allylic Alkylation

AbstractThe synthesis of eight pyridine-oxazoline ligands, five of which have never been reported previously, is described. The ligands were prepared in two efficient steps, initially preparing 2-pyridyl alkylnitriles, followed by their conversion into oxazolines ligands using chiral amino alcohols and zinc chloride. The 2-pyridyl nitriles are prepared via a novel SNAr alkylation reaction of 2-bromopyridine with alkyl nitriles using methylmagnesium chloride as a non-nucleophilic base in conjunction with an amine mediator. This methodology allows preparation of the existing gem-dimethyl motif and its elaboration beyond previously prepared ligands in fewer steps with simplicity and scalability. The toleration of variation in the nitrile, halopyridine, and amino alcohol starting materials allows for other novel bridging substitution which gives new ligands with enhanced reactivity. The library of bridging ligands was applied to the Pd-catalysed allylic alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate and afforded conversions of up to 100% and enantioselectivities of up to 68%.

Synthesis of Heterobenzyl Sulfoxides Enabled by Palladium-Catalyzed Allylic Alkylation of Sulfenate Anions with Allenamides.

A highly efficient palladium-catalyzed allenamide allylic alkylation of sulfenate anions under mild conditions is demonstrated. The present methodology provides a practical protocol to access various sulfoxide-containing heterocyclic compounds. The salient features of this transformation include a broad substrate scope, good functional group tolerance, easy to scale-up, and ready synthetic transformation.

Synergistic Organoboron/Palladium Catalysis for Regioselective N-Allylations of Azoles with Allylic Alcohols.

A method for regioselective palladium-catalyzed allylic alkylation of ambident nitrogen heterocycles, employing simple allylic alcohols as electrophile precursors, is described. An organoboron co-catalyst serves both to activate the azole-type nucleophile toward selective N-functionalization and to accelerate the formation of a π-allylpalladium complex from the allylic alcohol. The method can be applied to various heterocycle types, including 1,2,3- and 1,2,4-triazoles, tetrazoles, pyrazoles, and purines, and can be extended to substituted allylic alcohol partners.

Construction of Quaternary Allylic Amino Acid Derivatives through Palladium-Catalyzed Allylic Alkylation Reaction of Azlactones with Vinyl Aziridine

AbstractA highly efficient Pd-catalyzed allylic alkylation reaction of azlactones with vinyl aziridine has been achieved for the first time to access­ functionalized quaternary allylic amino acid derivatives (17 examples, up to 89% yield and 80% ee). Moreover, the broad scope and easy transformations of the products reinforce the value of this approach, which can enrich the chemistry of quaternary allylic amino acid derivatives.

Cooperativity between the Substrate and Ligand in Palladium-Catalyzed Allylic Alkylation Using 1-Aryl-1-propynes.

A monoprotected amino acid Bz-Gly-OH assists in the allylic alkylation of a variety of ketones, β-keto esters, aldehydes, etc., during enamine-palladium catalysis. Density functional theory calculations reveal that Bz-Gly-OH assists in the formation of an enamine that attacks the π-allylpalladium complex via an outer sphere mechanism. The preliminary result points to an asymmetric allylic alkylation under a new mode of bifunctional catalysis.

Palladium-catalyzed allylic alkylation of hydrazones with hydroxy-tethered allyl carbonates: synthesis of functionalized hydrazones

Pd-catalyzed allylic alkylation of hydroxy-tethered allyl carbonates and hydrazones worked well without an external base to afford various E configurations of functionalized hydrazones, which were successfully transformed into pyridazines.

Pd-Catalyzed Formal [3 + 3] Heteroannulation of Allylic gem-Diacetates: Synthesis of Chromene-Based Natural Products and Exploration of Photochromic Properties

Palladium-catalyzed allylic alkylation reactions of allylic gem-diacetates are rarely explored. This work unveils an unusual chemical reactivity pattern of allylic gem-diacetates and establishes them as new prototypes to synthesize complex benzo[f]chromene systems. Under the reaction conditions, the diacetates behave as 1,3-dicationic equivalents and undergo a [3 + 3] heteroannulation with 2-naphthols (and meta-substituted phenols) to produce novel polycyclic chromenes possessing spiro-, tri-, and tetrasubstituted carbon centers. The versatility of the method is demonstrated in the synthesis of several chromene-based bioactive natural products. Further, interesting photochromic properties of the new classes of benzo[f]chromenes are also discovered.

Self-Adaptable Tropos Catalysts.

ConspectusBiological systems have often served as inspiration for the design of synthetic catalysts. The lock and key analogy put forward by Emil Fischer in 1894 to explain the high substrate specificity of enzymes has been used as a general guiding principle aimed at enhancing the selectivity of chemical processes by optimizing attractive and repulsive interactions in molecular recognition events. However, although a perfect fit of a substrate to a catalytic site may enhance the selectivity of a specific catalytic reaction, it inevitably leads to a narrow substrate scope, excluding substrates with different sizes and shapes from efficient binding. An ideal catalyst should instead be able to accommodate a wide range of substrates—it has indeed been recognized that enzymes also are often highly promiscuous as a result of their ability to change their conformation and shape in response to a substrate—and preferentially be useful in various types of processes. In biological adaptation, the process by which species become fitted to new environments is crucial for their ability to cope with changing environmental conditions. With this in mind, we have been exploring catalytic systems that can adapt their size and shape to the environment with the goal of developing synthetic catalysts with wide scope.In this Account, we describe our studies aimed at elucidating how metal catalysts with flexible structural units adapt their binding pockets to the reacting substrate. Throughout our studies, ligands equipped with tropos biaryl units have been explored, and the palladium-catalyzed allylic alkylation reaction has been used as a suitable probe to study the adaptability of the catalytic systems. The conformations of catalytically active metal complexes under different conditions have been studied by both experimental and theoretical methods. By the design of ligands incorporating two flexible units, the symmetry properties of metal complexes could be used to facilitate conformational analysis and thereby provide valuable insight into the structures of complexes involved in the catalytic cycle. The importance of flexibility was convincingly demonstrated when a phosphine group in a privileged ligand that is well-known for its versatility in a number of processes was exchanged for a tropos biaryl phosphite unit: the result was a truly self-adaptive ligand with dramatically increased scope.

Palladium-catalyzed allylic alkylation dearomatization of β-naphthols and indoles with gem-difluorinated cyclopropanes.

A palladium-catalyzed allylic alkylation dearomatization of β-naphthols and indoles with gem-difluorinated cyclopropanes has been developed. This reaction provided an efficient route to access 2-fluoroallylic β-naphthalenones and indolenines bearing quaternary carbon centers in good yields with high Z-selectivity via C-C bond activation, C-F bond cleavage and the dearomatization process, benefiting from the wide substrate scope and good functional group tolerance. Moreover, 2-fluoroallylic furanoindoline and pyrroloindolines were achieved in good efficiency via cascade allylic alkylation, dearomatization and cyclization processes in the presence of Et3B.

Effective synthesis of bicyclodienes via palladium-catalyzed asymmetric allylic alkylation and ruthenium-catalyzed cycloisomerization.

[n.3.0]Bicycles (n = 3–6) can be synthesized using palladium-catalyzed asymmetric allylic alkylation followed by ruthenium-catalyzed cycloisomerization. New types of triarylphosphino-1,2-diaminooxazoline ligands show the same high levels of enantioselectivity observed with Trost ligand when employed in Pd-catalyzed allylic alkylation reactions. The enyne products of these allylic alkylation reactions were further elaborated using a Ru-catalyzed redox isomerization process, for which a mechanism is proposed.

Ion-paired Ligands for Palladium-catalyzed Allylic Alkylation under Base-free Conditions

The effectiveness of a catalytic amount of bromide ions for significantly accelerating the palladium-catalyzed allylation of carbonyl compounds with allylic carbonates is reported. In particular, t...

Palladium-Catalyzed Enantioselective Decarboxylative Allylic Alkylation of Acyclic α-N-Pyrrolyl/Indolyl Ketones.

The synthesis of fully substituted α-N-pyrrolyl and indolyl ketones via enantioselective palladium-catalyzed allylic alkylation is described. The acyclic ketones are alkylated in high yields with high enantioselectivities through the use of an electron-deficient phosphinooxazoline ligand, furnishing a highly congested and synthetically challenging stereocenter. The obtained alkylation products contain multiple reactive sites poised for additional functionalizations and diversification.

Palladium-Catalyzed Allylic Alkylation Reaction of α-Substituted Benzyl Nitriles with Branched Allyl Carbonates

Palladium-Catalyzed Allylic Alkylation Reaction of <i>α</i>-Substituted Benzyl Nitriles with Branched Allyl Carbonates

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.

Sequential Palladium-Catalyzed Allylic Alkylation/retro-Dieckmann Fragmentation Strategy for the Synthesis of α-Substituted Acrylonitriles.

A straightforward synthesis of α-substituted acrylonitriles is described using 4-cyano-3-oxotetrahydro-thiophene (c-THT) as an acrylonitrile surrogate. This unprecedented two-step sequence featuring a palladium-catalyzed allylic alkylation (Pd-AA) and a retro-Dieckmann fragmentation provides a general entry into diversely substituted 1,4-dienes.

Formal Synthesis of Indolizidine and Quinolizidine Alkaloids from Vinyl Cyclic Carbonates.

Cyclic carbonates have long been considered relatively inert molecules acting as protecting groups in complex multistep synthetic routes. This study shows that a concise, yet modular synthesis of indolizidine and quinolizidine alkaloids can be developed from vinyl-substituted cyclic carbonate (VCC) intermediates. Through a highly stereoselective palladium-catalyzed allylic alkylation reaction, these alkaloid motifs can be assembled in four synthetic and only two column purification steps. The combined results help to further advance functionalized cyclic carbonates as useful and reactive intermediates in natural product synthesis.

Ligand's electronegativity controls the sense of enantioselectivity in BIFOP-X palladium-catalyzed allylic alkylations

Palladium-catalyzed allylic alkylations of Na(CH(CO2Me)2 with 1,3-diphenylallyl acetate, employing BIFOP-X (X = H, D, Cl, CN, N3) ligands, yield the C–C coupling product (up to 91% yield, 70% ee). A NBO effect reveals a change of stereochemistry.