Copper-Free Oxime–Palladacycle-Catalyzed Sonogashira Alkynylation of Deactivated Aryl Bromides and Chlorides in Water under Microwave Irradiation

Palladium-catalyzed Heck alkynylation cross-coupling reactions between terminal alkynes and deactivated aryl chlorides and aryl bromides can be performed in the absence of copper cocatalyst with water as solvent at 130 °C under microwave irradiation. An oxime-derived chloro-bridged palladacycle is an efficient precatalyst for this transformation with 2-dicyclohexylphosphanyl-2′,4′,6′-triisopropylbiphenyl (XPhos) as ancillary ligand, pyrrolidine as base, and SBDS as surfactant. All of the reactions can be performed under air and with reagent-grade chemicals under low loading conditions (0.1–1 mol-% Pd).

Synthesis of Dihydroisobenzofurans via Palladium-Catalyzed Sequential Alkynylation/Annulation of 2-Bromobenzyl and 2-Chlorobenzyl Alcohols under Microwave Irradiation

The palladium-catalyzed synthesis of dihydroisobenzofurans has been performed by sequential Sonogashira cross-coupling/cyclization reactions between terminal alkynes and 2-(hydroxymethyl)bromo- and chlorobenzenes in methanol as solvent at 130 °C under microwave irradiation. A 4,4′-dichlorobenzophenone oxime-derived chloro-bridged palladacycle is an efficient pre-catalyst to perform this tandem process using 2-dicyclohexylphosphanyl-2′,4′,6′-triisopropylbiphenyl (Xphos) as ancillary ligand and potassium hydroxide as base in the absence of a copper cocatalyst. Under these conditions, functionalized 2-bromo- and 2-chlorobenzaldehydes are also suitable partners in the domino process affording phthalans in good yields. All the reactions can be performed under air and employing reagent-grade chemicals under low loading conditions (1 mol% Pd).

Microwave-Promoted Copper-Free Sonogashira–Hagihara Couplings of Aryl Imidazolylsulfonates in Water

Aryl imidazol-1-ylsulfonates have been efficiently cross-coupled with aryl-, alkyl-, and silylacetylenes in neat water under copper-free conditions at 110 °C assisted by microwave irradiation. Using 0.5 mol% of an oxime palladacycle as precatalyst, 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (SPhos, 2 mol%) as ligand, hexadecyltrimethylammonium bromide (CTAB) as additive, and triethylamine (TEA) as base, a wide array of disubstituted alkynes has been prepared in good to high yields in only 30 min.

Microwave-assisted palladium-catalyzed highly regio- and stereoselective head to head dimerization of terminal aryl alkynes in water

A highly regio- and stereoselective oxime palladacycle/imidazolinium-catalyzed head to head dimerization of terminal aryl alkynes in water is presented. The reaction, which is carried out at 130 °C under microwave irradiation in the presence of 1,3-bis-(2,6-diisopropylphenyl)imidazolinium chloride as ligand, triethylamine as base, and TBAB as surfactant, allows the synthesis of (E)-1,4-enynes as single stereoisomers in good isolated yields.

Microwave-Enhanced Asymmetric Transfer Hydrogenation of N-(tert-Butylsulfinyl)imines

Microwave irradiation has considerably enhanced the efficiency of the asymmetric transfer hydrogenation of N-(tert-butylsulfinyl)imines in isopropyl alcohol catalyzed by a ruthenium complex bearing the achiral ligand 2-amino-2-methylpropan-1-ol. In addition to shortening reaction times for the transfer hydrogenation processes to only 30 min, the amounts of ruthenium catalyst and isopropyl alcohol can be considerably reduced in comparison with our previous procedure assisted by conventional heating, which diminishes the environmental impact of this new protocol. This methodology can be applied to aromatic, heteroaromatic and aliphatic N-(tert-butylsulfinyl)ketimines, leading, after desulfinylation, to the expected primary amines in excellent yields and with enantiomeric excesses of up to 96 %.

Phosphane-free Suzuki-Miyaura coupling of aryl imidazolesulfonates with arylboronic acids and potassium aryltrifluoroborates under aqueous conditions

Aryl imidazole-1-sulfonates are efficiently cross-coupled with arylboronic acids and potassium aryltrifluoroborates using only 0.5 mol % of oxime palladacycles 1 under aqueous conditions at 110 °C. Under these simple phosphane-free reaction conditions a wide array of biaryl derivatives has been prepared in high yields. This methodology allows in situ phenol sulfonation and one-pot Suzuki arylation as well as the employment of microwave irradiation conditions.

Continuous flow nanocatalysis: reaction pathways in the conversion of levulinic acid to valuable chemicals

The selective production of 2-methyltetrahydrofuran from levulinic acid has been effectively conducted using designed Cu based catalysts and compared with a commercial Pd/C system under microwave irradiation. Optimised conditions for the most active catalysts Cu-MINT (>90% conversion, 75% selectivity to MTHF) and Pd/C (78% conversion, 92% selectivity to MTHF) were further translated into a continuous flow process using the proposed catalysts to find out the deactivation of Cu-MINT under flow conditions (79 vs. 13% conversion with a switch in selectivity to products after 30 min in flow), the high stability of Pd/C (73 vs. 70% conversion at stable selectivity under analogous conditions to those of Cu-MINT) but, most importantly, different relevant pathways to valuable products from levulinic acid depending on the type of catalyst employed.

NHC-Ligand Effectiveness in the Fluorine-Free Hiyama Reaction of Aryl Halides

1-Benzyl-3-(2-hydroxy-2-phenylethyl)imidazolium chloride (5), which is a precursor of an N-heterocyclic carbene ligand, in combination with palladium acetate, has been employed as an effective catalyst for the fluorine-free Hiyama reaction. A systematic study of the catalytic mixture, by a 32 factorial design, has revealed that both the amount of palladium and the Pd/NHC precursor ratio are important factors for obtaining good yields of the coupling products, indicating an interaction between them. The best catalytic system involves mixing 0.1 mol-% palladium acetate in a 1:5 ratio (Pd/salt 5), which allows the effective coupling of a range of aryl bromides and chlorides with trimethoxy(phenyl)silane. The Hiyama reactions are carried out in NaOH solution (50 % H2O w/w), at 120 °C under microwave irradiation during 60 min.

Palladium nanoparticles supported on graphene and reduced graphene oxide as efficient recyclable catalyst for the Suzuki–Miyaura reaction of potassium aryltrifluoroborates

Palladium nanoparticles supported on graphene platelets have been efficiently used as catalyst in the Suzuki–Miyaura coupling between aryl bromides and potassium aryltrifluoroborates using 0.1 mol% of Pd and potassium carbonate as base in MeOH/H2O as solvent at 80 °C. The reaction can be performed using conventional and microwave heating showing the catalyst high reusability, particularly with microwaves, where lower aggregation of Pd nanoparticles has been observed. A dissolution/re-deposition catalytic mechanism is proposed, based on the fact that palladium leaching to the solution is detected under microwave irradiation.

1,2-Functionalized Imidazoles as Palladium Ligands: An Efficient and Robust Catalytic System for the Fluorine-Free Hiyama Reaction

A variety of hydroxy- and amino-functionalized imidazoles were prepared from 1-methyl- and 1-(diethoxymethyl)imidazole by means of isoprene-mediated lithiation followed by reaction with an electrophile. These compounds in combination with palladium acetate were screened as catalyst systems for the Hiyama reaction under fluorine-free conditions using microwave irradiation. The systematic study of the catalytic system showed 1-methyl-2-aminoalkylimidazole derivative L1 to be the best ligand, which was employed under solvent-free conditions with a 1:2 Pd/ligand ratio and TBAB (20 mol-%) as additive. The study has revealed an interaction between the Pd/ligand ratio and the amount of TBAB. The established catalytic system presented a certain degree of robustness, and it has been successfully employed in the coupling of a range of aryl bromides and chlorides with different aryl siloxanes. Furthermore, both reagents were employed in an equimolecular amount, without an excess of organosilane.

Insights into the Active Species of Nanoparticle-Functionalized Hierarchical Zeolites in Alkylation Reactions

Supported iron oxide nanoparticles have been incorporated onto hierarchical zeolites by microwave-assisted impregnation and mechanochemical grinding. Nanoparticle-functionalised porous zeolites were characterised by a number of analytical techniques such as XRD, N2 physisorption, TEM, and surface acidity measurements. The catalytic activities of the synthesised nanomaterials were investigated in an alkylation reaction. The results pointed to different species with varying acidity and accessibility in the materials, which provided essentially different catalytic activities in the alkylation of toluene with benzyl chloride under microwave irradiation, selected as the test reaction.

Synthesis of 5-Carbapterocarpens by α-Arylation of Tetralones Followed by One-Pot Demethylation/Cyclization with BBr3

5-Carbapterocarpens, one of them displaying estrogenic activity, were prepared from α-aryltetralones in high yields through a one-pot, BBr3-promoted O-demethylation and cyclization sequence. The key α-aryltetralone intermediates were obtained by direct α-arylation of tetralones with o-alkoxybromoarenes in the presence of Pd2(dba)3 (2.5 mol-%) and tBu3PHBF4 (10 mol-%) as catalysts, together with 2.5 equiv. of KOH in dioxane/H2O (4:1), under microwave irradiation conditions (80 W, 100 °C, 40 min), leading to α-monoaryltetralones in good yields.

Microwave-Assisted Extraction of Phenolic Compounds from Almond Skin Byproducts (Prunus amygdalus): A Multivariate Analysis Approach

A microwave-assisted extraction (MAE) procedure to isolate phenolic compounds from almond skin byproducts was optimized. A three-level, three-factor Box–Behnken design was used to evaluate the effect of almond skin weight, microwave power, and irradiation time on total phenolic content (TPC) and antioxidant activity (DPPH). Almond skin weight was the most important parameter in the studied responses. The best extraction was achieved using 4 g, 60 s, 100 W, and 60 mL of 70% (v/v) ethanol. TPC, antioxidant activity (DPPH, FRAP), and chemical composition (HPLC-DAD-ESI-MS/MS) were determined by using the optimized method from seven different almond cultivars. Successful discrimination was obtained for all cultivars by using multivariate linear discriminant analysis (LDA), suggesting the influence of cultivar type on polyphenol content and antioxidant activity. The results show the potential of almond skin as a natural source of phenolics and the effectiveness of MAE for the reutilization of these byproducts.