Synthesis of Fused Cyclopentenones through Palladium‐Catalyzed Cyclization of 2‐Iodoaryl Allenols

A versatile strategy for the synthesis of fused cyclopentenones, key structural motifs in biologically relevant compounds such as indenones and indole alkaloids, has been established successfully through regioselective palladium‐catalyzed cyclization of 2‐iodoaryl allenols.

     

Palladium on Carbon: An Efficient,Heterogeneous and Reusable Catalytic System for Carbonylative Synthesis of N‐Substituted Phthalimides

The application of palladium on carbon (Pd/C) as a heterogeneous recyclable catalyst was investigated for the double carbonylation of o‐dihaloarenes with amines providing excellent yield of N‐substituted phthalimides in shorter reaction time as compared to earlier reported homogeneous protocols. Furthermore, the scope of the developed protocol was applied for the synthesis N‐substituted phthalimides from o‐halobenzoates and o‐halobenzoic acid via a single step carbonylative cyclization reaction. The developed methodology describes an efficient one‐step approach for the synthesis of an important class of heterocycles and tolerates a wide variety of functional groups. It circumvents the use of phosphine ligands with an additional advantage of catalyst recyclability for up to eight consecutive cycles.     

Acid‐Catalyzed Synthesis of α,β‐Disubstituted Conjugated Enones by a Meyer–Schuster‐Type Rearrangement in Allenols

A novel, direct and simple methodology to gain access to α,β‐disubstituted conjugated enones from α‐allenols in a sustainable metal catalysis context, considering the inexpensiveness and environmentally friendliness of iron(III) species and protons, has been developed.

     

Platinum‐Catalyzed Divergent Reactivity of α‐Hydroxyallenes: Synthesis of Dihydrofurans and α,β‐Unsaturated Ketones

α‐Allenols were catalytically transformed into dihydrofurans in the presence of platinum dichloride. Notably, using platinum dichloride along with silver triflate as the catalytic system, α,β‐unsaturated ketones were obtained. Therefore, the role of the silver salt may not just consist in the activation of the platinum precatalyst.

     

Development of a New Spiro‐BOX Ligand and Its Application in Highly Enantioselective Palladium‐Catalyzed Cyclization of 2‐Iodoanilines with Allenes

In this communication, we report the synthesis of a new chiral spiro‐bisoxazoline ligand, i.e., β‐naphthylmethyl‐substituted spiro‐BOX [(R_a,S,S)‐ L7 ] and have successfully applied it to the palladium‐catalyzed enantioselective cyclization reaction of simple allenes with o‐aminoiodobenzenes, affording highly optically active 3‐alkylideneindolines in good yields with excellent enantiomeric excesses.     

Size‐Dependent Catalytic Activity of Supported Palladium Nanoparticles for Aerobic Oxidation of Alcohols

Silica‐alumina (SiO₂‐Al₂O₃)‐supported palladium catalysts prepared by adsorption of the tetrachloropalladate anion (PdCl₄²⁻) followed by calcination and reduction with either hexanol or hydrogen were studied for the aerobic oxidation of alcohols. The mean size of the Pd particles over the SiO₂‐Al₂O₃ support was found to depend on the Si/Al ratio, and a decrease in the Si/Al ratio resulted in a decrease in the mean size of the Pd nanoparticles. By changing the Si/Al ratio, we obtained supported Pd nanoparticles with mean sizes ranging from 2.2 to 10 nm. The interaction between the Pd precursor and the support was proposed to play a key role in tuning the mean size of the Pd nanoparticles. The Pd/SiO₂‐Al₂O₃ catalyst with an appropriate mean size of Pd particles could catalyze the aerobic oxidation of various alcohols to the corresponding carbonyl compounds, and this catalyst was particularly efficient for the solvent‐free conversion of benzyl alcohol. The intrinsic turnover frequency per surface Pd atom depended significantly on the mean size of Pd particles and showed a maximum at a medium mean size (3.6–4.3 nm), revealing that the aerobic oxidation of benzyl alcohol catalyzed by the supported Pd nanoparticles was structure‐sensitive.     

Highly stable Fe/γ‐Al2O3 catalyst for catalytic wet peroxide oxidation

BACKGROUND: A highly stable Fe/γ‐Al₂O₃ catalyst for catalytic wet peroxide oxidation has been studied using phenol as target pollutant. The catalyst was prepared by incipient wetness impregnation of γ‐Al₂O₃ with an aqueous solution of Fe(NO₃)₃· 9H₂O. The influence of pH, temperature, catalyst and H₂O₂ doses, as well as the initial phenol concentration has been analyzed. RESULTS: The reaction temperature and initial pH significantly affect both phenol conversion and total organic carbon removal. Working at 50 °C, an initial pH of 3, 100 mg L^?1 of phenol, a dose of H₂O₂ corresponding to the stoichiometric amount and 1250 mg L^?1 of catalyst, complete phenol conversion and a total organic carbon removal efficiency close to 80% were achieved. When the initial phenol concentration was increased to 1500 mg L^?1, a decreased efficiency in total organic carbon removal was observed with increased leaching of iron that can be related to a higher concentration of oxalic acid, as by‐product from catalytic wet peroxide oxidation of phenol. CONCLUSION: A laboratory synthesized γ‐Al₂O₃ supported Fe has shown potential application in catalytic wet peroxide oxidation of phenolic wastewaters. The catalyst showed remarkable stability in long‐term continuous experiments with limited Fe leaching, < 3% of the initial loading. Copyright © 2010 Society of Chemical Industry     

Catalytic Asymmetric Allylation of 3‐Aryloxindoles by Merging Palladium Catalysis and Asymmetric H‐Bonding Catalysis

A catalytic asymmetric allylic alkylation reaction of 3‐aryloxindoles was accomplished via a dual catalysis merging palladium catalysis and asymmetric H‐bonding catalysis for the first time. Using this approach, allylated oxindoles bearing chiral all‐carbon quaternary centers were produced in high yields with good enantioselectivities (up to 92 % yield and 96:4 er).

     

Palladium on Calcium Carbonate Combined to 2‐Hydroxypropyl‐α/β‐cyclodextrins: A Selective Catalytic System for Aqueous Heck Coupling and Hydroarylation

An efficient, selective and recoverable catalytic system for ligand‐free aqueous Heck reactions using hydroxypropylated cyclodextrins (HPCDs) and palladium on calcium carbonate (Pd/CaCO₃) is highlighted. Remarkably, stereo‐ and chemoselectivities could be tuned by the cavity size of cyclodextrins, exploiting the relevance of host‐guest interactions. UV‐Vis experiments have led to strong evidence concerning an interplay between Pd(II) and α‐HPCD, possibly ascribed to a reduction/stabilization effect of CDs. Unexpectedly, hydroarylation was the favored pathway with acrylonitrile which provided access to 3‐phenylpropionitrile derivatives without usual hydride donors. Finally, determination of soluble Pd(0/II) via AAS enabled the definition of a predominant homogeneous mechanism in which TONs over 5000 were observed.     

Chlorinative Cyclization of 1,6‐Enynes by Enantioselective Palladium(II)/Palladium(IV) Catalysis

A novel asymmetric catalysis via a palladium(II)/palladium(IV) cycle has been developed by utilizing a chiral spiro bis(isoxazoline) ligand (SPRIX). Intramolecular chlorinative cyclization of 1,6‐enynes catalyzed by a palladium‐SPRIX complex proceeded enantioselectively to afford α‐methylene‐γ‐lactone derivatives.     

Beyond the Limits: Palladium‐N‐Heterocyclic Carbene‐Based Catalytic System Enables Highly Efficient [4+2] Benzannulation Reactions

A highly efficient catalytic system for the palladium‐catalyzed [4+2] benzannulation reaction of enynes and enynophiles has been developed. The use of an N‐heterocyclic carbene‐based palladium precursor allowed us to achieve turnover numbers up to 1800. The new catalytic system has enabled an expansion of the scope of the [4+2] homo‐benzannulation reaction.     

Assisted Tandem Palladium(II)/Palladium(0)‐Catalyzed C‐ and N‐Arylations of Quinoxalin‐2(1 H)‐ones in Water

A straightforward assisted tandem palladium(II)‐ and palladium(0)‐catalyzed direct C‐3 and N‐4 arylation of quinoxalin‐2(1 H)‐ones with boronic acids and aryl halides in water as safe and cheap solvent is reported. This environmentally friendly catalytic protocol is compatible with a wide range of functional groups and allows construction of various biologically important quinoxalin‐2(1 H)‐one backbones.

     

Enantioselective Phase‐Transfer Catalytic α‐Benzylation and α‐Allylation of α‐tert‐Butoxycarbonyllactones

A new enantioselective α‐benzylation and α‐allylation of α‐tert‐butoxycarbonyllactones was devloped. α‐Benzylation and α‐allylation of α‐tert‐butoxycarbonylbutyrolactone and α‐tert‐butoxycarbonylvalerolactone under phase‐transfer catalytic conditions (50% cesium hydroxide, toluene, −60 °C) in the presence of (S,S)‐3,4,5‐trifluorophenyl‐NAS bromide (1 mol%) afforded the corresponding α‐substituted α‐tert‐butoxycarbonyllactones in very high chemical yields (up to 99%) and optical yields (up to 99% ee). The synthetic potential of this method has been successfully demonstrated by the asymmetric synthesis of unnatural α‐quaternary homoserines, 3‐alkyl‐3‐carboxypyrrolidine and 3‐alkyl‐3‐carboxypiperidine.     

Palladium/Brønsted Acid‐Catalyzed α‐Allylation of Aldehydes with Allylic Alcohols

A simple, highly efficient, and readily scalable direct α‐allylation of aldehydes with allylic alcohols that is co‐catalyzed by palladium and a Brønsted acid has been developed.     

Palladium(II)‐Catalyzed Aerobic Intermolecular Cyclization of Acrylic Acid with Alkenes to α‐Methylene‐γ‐butyrolactones

The methodology in this article is a palladium(II)/copper(II)‐ or palladium(II)‐catalyzed intermolecular cyclization of acrylic acid with alkenes to produce α‐methylene‐γ‐butyrolactone derivatives using molecular oxygen as an environmentally benign oxidant. In this system, the carboxylato, especially trifluoroacetato, or trimethylacetato ligand, plays a quite important role to afford a high catalytic activity by suppressing the deposition of palladium(0) black.     

Palladium‐Catalyzed Intramolecular Coupling of Aryl Halides and Ketone Enolates: Synthesis of Hexahydro‐2,6‐methano‐1‐2

The palladium‐catalyzed intramolecular coupling of 2‐haloanilines and ketone enolates is a useful methodology for the synthesis of hexahydro‐2,6‐methano‐1‐2 derivatives. A study about the reaction conditions of the process is reported.     

Progress in the Palladium‐Catalyzed α‐Arylation of Ketones with Chloroarenes

Non‐ and deactivated chloroarenes can be coupled with a wide range of ketones to yield the corresponding arylmethyl ketones in good to excellent yields using a palladium(II) acetate/n‐BuPAd₂ catalyst system. Depending on the ketone, the chloroarene/ketone ratio and the base, mono or diarylation can be effected selectively.     

Palladium(II)‐Catalyzed Hydration of Alkynylphosphonates to β‐Ketophosphonates

A highly efficient palladium(II)‐catalyzed hydration of a wide range of alkynylphosphonates to the corresponding β‐ketophosphonates has been developed to give high yields at 80 °C in 1, 4‐dioxane, with no acidic or alkaline cocatalysts required. The described catalytic system should provide an efficient alternative to highly toxic mercury‐catalyzed methodologies and be useful in synthetic programs.     

Preparation and Use of Polystyryl‐DABCOF2: An Efficient Recoverable and Reusable Catalyst for β‐Azidation of α,β‐Unsaturated Ketones in Water

Novel solid fluorides were prepared to optimize the β‐azidation of α,β‐unsaturated ketones. The higher loading of these catalysts compared to that of commercially available fluorides has allowed the use of a smaller mass of catalyst helping the mixing of the reaction mixture. Porous polymeric supports have proved to be more efficient in the presence of water as reaction medium. Water has played a crucial role showing a beneficial effect on the reactivity by improving dispersion of the reaction mixture and also by avoiding organic fouling caused by the retention of the reaction mixture within the polymeric matrix. This has facilitated the recovery of the products from the catalyst. The protocol reported has allowed a significant reduction in the organic solvent required for the complete recovery of the pure product whilst leaving the catalyst clean and reusable. E‐factors are in the range of 5.9–10.5 and therefore ca. 3 times smaller than previous procedures operating under solvent‐free conditions. To further improve the efficiency of our approach we have developed a protocol operating in a continuous‐flow manner that has allowed us to achieve an E‐factor of 1.7–1.9, with a reduction of ca. 80% of the corresponding batch conditions. The continuous‐flow protocol has allowed us to minimize the use of trimethylsilyl azide making the recovery and reuse of water and catalyst 5f very efficient and simple. Finally, a novel reduction system using palladium on alumina (5 mol%) and equimolar amount of formic acid has been used in the presence of 1 equivalent of di‐tert‐butyl pyrocarbonate to set a multistep protocol operating in continuous‐flow conditions for the preparation of two representative N‐Boc‐β‐amino ketones starting from the corresponding enones with E‐factors of 3.2 and 2.7, respectively.     

Gold(I)‐Catalyzed Carboaminations of Allenes by N‐Allyltetrahydro‐β‐carbolines: An Experimental and Theoretical Study

N‐Allyltetrahydro‐β‐carbolines bearing a pendant allene undergo unprecedented gold(I)‐catalyzed cyclizations proceeding with an allyl migration from the nitrogen to the allene moiety. The initial product of the gold‐catalyzed cyclization evolves either via isomerization or Cope rearrangement, leading to different scaffolds. Density function theory (DFT) calculations established that the allyl migration occurs in an asynchronous suprafacial manner.