Thiosemicarbazone Salicylaldiminato‐Palladium(II)‐Catalyzed Mizoroki–Heck Reactions

Four tridentate thiosemicarbazone salicylaldiminato‐palladium(II) complexes of the general formula [Pd(saltsc‐R)PPh₃] [saltsc=salicylaldehyde thiosemicarbazone; R=H ( 1 ), 3‐tert‐butyl ( 2 ), 3‐methoxy ( 3 ), 5‐chloro ( 4 )], have been evaluated as catalyst precursors for the Mizoroki–Heck coupling reaction between a variety of electron‐rich and electron‐poor aryl halides and olefins. The palladium complexes (0.1–1 mol% loading) were found to effectively catalyze these reactions with high yields being obtained when aryl iodides and aryl bromides were utilized. The effects of base, catalyst loading, reaction temperature and reaction time on the catalytic activity of the most active complex were also investigated.     

Palladium(II) Complexes of C2‐Bridged Chiral Diphosphines: Application to Enantioselective Carbonyl‐Ene Reactions

(11bR,11′bR)‐4,4′‐(1,2‐Phenylene)bis[4,5‐dihydro‐3H‐dinaphtho[2,1‐c:1′,2′‐e]phosphepin] [abbreviated as (R)‐BINAPHANE], (3R,3′R,4S,4′S,11bS,11′bS)‐4,4′‐bis(1,1‐dimethylethyl)‐4,4′,5,5′‐tetrahydro‐3,3′‐bi‐3H‐dinaphtho[2,1‐c:1′,2′‐e]phosphepin [(S)‐BINAPINE], (1S,1′S,2R,2′R)‐1,1′‐bis(1,1‐dimethylethyl)‐2,2′‐biphospholane [(S,S,R,R)‐TANGPHOS] and (2R,2′R,5R,5′R)‐1,1′‐(1,2‐phenylene)bis[2,5‐bis(1‐methylethyl)phospholane] [(R,R)‐i‐Pr‐DUPHOS] are C₂‐bridged chiral diphosphines that form stable complexes with palladium(II) and platinum(II) containing a five‐membered chelate ring. The Pd(II)‐BINAPHANE catalyst displayed good to excellent enantioselectivities with ee values as high as 99.0% albeit in low yields for the carbonyl‐ene reaction between phenylglyoxal and alkenes. Its Pt(II) counterpart afforded improved yields while retaining satisfactory enantioselectivity. For the carbonyl‐ene reaction between ethyl trifluoropyruvate and alkenes, the Pd(II)‐BINAPHANE catalyst afforded both good yields and extremely high enantioselectivities with ees as high as 99.6%. A comparative study on the Pd(II) catalysts of the four C₂‐bridged chiral diphosphines revealed that Pd(II)‐BINAPHANE afforded the best enantioselectivity. The ee values derived from Pd(II)‐BINAPHANE are much higher than those derived from the other three Pd(II) catalysts. A comparison of the catalyst structures shows that the Pd(II)‐BINAPHANE catalyst is the only one that has two bulky (R)‐binaphthyl groups close to the reaction site. Hence it creates a deep chiral space that can efficiently control the reaction behavior in the carbonyl‐ene reactions resulting in excellent enantioselectivity.     

A Highly Efficient Tandem Reaction of 2‐(gem‐Dibromovinyl)phenols(thiophenols) with Organosilanes to 2‐Arylbenzofurans (thiophenes)

2‐Arylbenzofurans(thiophenes) were prepared through an efficient tandem elimination–intramolecular addition–Hiyama cross‐coupling reaction. In the presence of tetra‐(n‐butyl)ammonium fluoride (TBAF), palladium(II) acetate [Pd(OAc)₂] and triphenylphosphine (PPh₃), the reaction of 2‐(gem‐dibromovinyl)phenols(thiophenols) with phenyl(trialkoxy)silanes proceeded smoothly and generated the corresponding products with good yields in one‐pot. It should be noted that TBAF plays an important role in the tandem reaction.     

Bifunctional Metal Organic Framework Catalysts for Multistep Reactions: MOF‐Cu(BTC)‐[Pd] Catalyst for One‐Pot Heteroannulation of Acetylenic Compounds

A bifunctional metal organic framework catalyst containing palladium and copper(II) benzene‐1,3,5‐tricarboxylate – MOF‐Cu(BTC)‐[Pd] – has been prepared. This catalyst enables the performance of the tandem Sonogashira/click reaction starting from 2‐iodobenzylbromide, sodium azide and alkynes to produce 8H‐[1,2,3]triazolo[5,1‐a]isoindoles with good yields under mild reaction conditions.     

Studies on the Sorption of Palladium using Cross‐Linked Poly (4‐Vinylpyridine‐Divinylbenzene) Resins in Nitric Acid Medium

Various cross‐linked (4, 8, and 12%) gel‐type weak‐base poly(4‐vinylpyridine) (PVP) resins were studied for palladium recovery from nitric acid medium. The sorption of palladium was found to decrease with an increase in cross‐linkage of the resin. 8 and 12% PVP resins exhibited maximum D _Pd(II) values at 2–6 M HNO₃, whereas 4% PVP resin showed maximum D _Pd(II) values at lower acidities (0.1 M HNO₃). FT‐IR, SEM, and XPS techniques were used for the characterization of palladium‐loaded resins. Detailed studies were carried out with the resin of modest cross‐linkage i.e., 8% PVP resin. The sorption isotherm studies revealed that the maximum palladium loading approaches the theoretical capacity of the resin, presuming the sorption of palladium as divalent anion at 4 M HNO₃. The pseudo‐second order kinetics model yielded the best fit for the experimental data of sorption kinetics. An increase in temperature accelerates the rate of palladium extraction and also the addition of chloride ions increases the palladium uptake. Column studies were performed using 4 and 8% PVP resins in 2 and 4 M nitric acid concentrations. The loaded palladium could be eluted efficiently with acidic thiourea solution.     

Suzuki–Miyaura Cross‐Coupling under Solvent‐Free Conditions

A solvent‐free reaction protocol for Suzuki–Miyaura cross‐couplings was developed. (Hetero)aryl bromides and chlorides are coupled with pinacol arylboronates in high yields. The reaction is catalyzed by conventional bis(triphenylphosphine)palladium(II) chloride [(PPh₃)₂PdCl₂] and/or palladium(II) acetate/SPhos [Pd(OAc)₂/SPhos] under air.

     

“Click” Polymer‐Supported Palladium Nanoparticles as Highly Efficient Catalysts for Olefin Hydrogenation and Suzuki Coupling Reactions under Ambient Conditions

Complexation of palladium(II) acetate [Pd(OAc)₂] or dipotassium tetrachloropalladate [K₂PdCl₄] to “click” polymers functionalized with phenyl, ferrocenyl and sodium sulfonate groups gave polymeric palladium(II)‐triazolyl complexes that were reduced to “click” polymer‐stabilized palladium nanoparticles (PdNPs). Transmission electron microscopy (TEM) showed that reduction using sodium borohydride (NaBH₄) produced PdNPs in the 1–3 nm range of diameters depending on the nature of the functional group, whereas slow reduction using methanol yielded PdNPs in the 22–25 nm range. The most active of these PdNPs (0.01% mol Pd), stabilized by poly(ferrocenyltriazolylmethyl)styrene, catalyzed the hydrogenation of styrene at 25 °C and 1 atm hydrogen, with turnover numbers (TONs) of 200,000. When stabilized by the water‐soluble poly(sodium sulfonate‐triazolylmethyl)styrene, the PdNPs (0.01% mol Pd) catalyze the Suzuki–Miyaura coupling between iodobenzene (PhI) and phenylboronic acid [PhB(OH)₂] in water/ethanol (H₂O/EtOH) at 25 °C with TONs of 8,200. This high catalytic activity is comparable to that obtained with “click” dendrimer‐stabilized PdNPs under ambient conditions.     

Synthesis of 3,4‐Disubstituted Quinolin‐2‐(1H)‐ones via Palladium‐Catalyzed Decarboxylative Arylation Reactions

The Pd‐catalyzed decarboxylative cross‐coupling reaction of 4‐substituted quinolin‐2(1H)‐one‐3‐carboxylic acids with (hetero)aryl halides is described. With palladium(II) bromide and triphenylarsine ligand as the catalyst system, a variety of 4‐substituted 3‐(hetero)aryl quinolin‐2(1 H)‐ones and related heterocycles, such as 4‐substituted 3‐arylcoumarins can be prepared in good to excellent yields.     

Palladium‐Catalyzed Hydroxymethylation of Aryl‐ and Heteroarylboronic Acids using Aqueous Formaldehyde

Cyclometallated NHC palladium complexes prepared from palladium(II) acetate [Pd(OAc)₂] and unsymmetrical 1,3‐diarylimidazolinium salts catalyzed the hydroxymethylation of (hetero)arylboronic acids using an excess amount of formalin to afford (hetero)arylmethanols in good to satisfactory yields.

     

Palladium‐Catalyzed Synthesis of (Z)‐3‐Arylthioacrylic Acids and Thiochromenones

The three‐component reaction of aryl halides, sodium sulfide pentahydrate (Na₂S⋅5 H₂O), and propiolic acid in the presence of 2.5% bis(triphenylphosphine)palladium chloride [Pd(PPh₃)₂Cl₂], 5% 1,4‐bis(diphenylphosphino)butane (dppb) and 2 equivalents of 1,8‐diazabicycloundec‐7‐ene (DBU) produces stereoselectively (Z)‐3‐arylthioacrylic acids in good yields. A study of the reaction pathway suggested that the C S bond formation between aryl halides and Na₂S⋅5 H₂O proceeded first, and the resulting intermediate reacted with propiolic acid to produce the desired product. In addition, when the resulting product was treated with acid, the respective thiochromenones were formed in good yields.     

Miyaura Borylation and One‐Pot Two‐Step Homocoupling of Aryl Chlorides and Bromides under Solvent‐Free Conditions

Solvent‐free protocols for Miyaura borylation and the one‐pot, two‐step homocoupling of aryl halides are reported for the first time. Bis(dibenzylideneacetone)palladium(0) [Pd(dba)₂] is an optimal source of palladium for Miyaura borylation, while for one‐pot two‐step homocoupling palladium(II) acetate [Pd(OAc)₂] gives highest yields. Aryl bromides are coupled most efficiently using the DPEphos ligand. Chlorides are coupled using XPhos. The developed protocols are robust, versatile and easily reproducible on a large scale.

     

Extraction of Palladium(II) from Nitric Acid Solutions with 1‐Benzoyl‐3‐[6‐(3‐benzoyl‐thioureido)‐hexyl]‐thiourea

The extraction of palladium (II) from HNO₃ solutions with 1‐Benzoyl‐3‐[6‐(3‐benzoyl‐thioureido)‐hexyl]‐thiourea (Ia) and several monodentate thiourea derivatives in 1,2‐dichloroethane has been studied. The effect of HNO₃ concentration in the aqueous phase and that of the extractant in the organic phase on the Pd(II) extraction is considered. The stoichiometry of the extracted complexes has been determined. The increasing number of thioamide groups in the molecule of Ia increases its extraction efficiency towards Pd(II). The potentialities of a polymeric resin impregnated with compound Ia for selective extraction of Pd(II) from nitric acid solutions are demonstrated.     

Diastereoselective Synthesis of Six‐Membered Carbocyclic Spirooxindoles via 6π‐Electrocyclization of 3‐Dienylidene‐2‐ oxindoles

The Wittig reaction of isatin derivatives with Morita–Baylis–Hillman bromides of cinnamaldehydes afforded 3‐dienylidene‐2‐oxindoles. These trienes were converted into the corresponding spirooxindoles in a stereoselective manner in refluxing toluene in good yields. The diastereomeric spirooxindoles could be obtained stereoselectively by adding a catalytic amount of palladium(II) acetate via the palladium‐catalyzed isomerization of EEE‐trienes to ZEE‐trienes followed by a more facile 6π‐electrocyclization process. The obtained spirooxindoles could be further functionalized by palladium‐catalyzed oxidative arylation, thionation with Lawesson’s reagent, catalytic hydrogenation and Friedel–Crafts‐type reaction.

     

Selective Aryl α‐Diimine/Palladium‐Catalyzed Bis‐Alkoxy‐ carbonylation of Olefins for the Synthesis of Substituted Succinic Diesters

Aryl α‐diimine derivatives have been used, for the first time, as efficient new ligands for the palladium‐catalyzed oxidative bis‐alkoxycarbonylation reaction of olefins. The most active catalyst was formed in situ from bis(9‐anthryl)‐2,3‐dimethyl‐1,4‐diazabutadiene and palladium(II) trifluoroacetate [Pd(TFA)₂]. This catalytic system was able to selectively convert olefins into succinic diesters in good yields (up to 97%) and low catalyst loading (up to 0.5 mol%) under mild reaction conditions [4 bar of carbon monoxide (CO) at 20 °C in the presence of p‐toluenesulphonic acid as additive and p‐benzoquinone as oxidant]. The optimized conditions could be successfully applied to both aromatic and aliphatic olefins, by using methanol, benzyl alcohol or isopropyl alcohol as nucleophiles.

     

A Density Functional Theory Study of the Stille Cross‐Coupling via Associative Transmetalation. The Role of Ligands and Coordinating Solvents

An associative mechanism has been computationally characterized for the Stille cross‐coupling of vinyl bromide and trimethylvinylstannane catalyzed by PdL₂ (L=PMe₃, AsMe₃) with or without dimethylformamide as coordinating ligand. All the species along the catalytic cycles that start from both the cis‐ and the trans‐PdL(Y)(vinyl)Br complexes (Y=L or S; L=PMe₃, AsMe₃ or PH₃; S=DMF) have been located in the gas phase and in the presence of polar solvents. Computations support the central role of species trans‐PdL(DMF)(vinyl)Br which react by ligand dissociation and stannane coordination in the rate‐limiting transmetalation step via a puckered four‐coordinate (at palladium) transition state comprised of Pd, Br, Sn and sp² C atoms. A donating solvent may enter the catalytic cycle assisting isomerization of cis‐PdL₂(vinyl)Br to trans‐PdL(DMF)(vinyl)Br complexes via a pentacoordinate square pyramidal Pd intermediate. In keeping with experimental observations, the activation energies of the catalytic cycles with arsines as Pd ligands are lower than those with phosphines. Polytopal rearrangements from the three‐coordinate T‐shaped Pd complexes resulting from transmetalation account for the isomerization and the C C bond formation on the reductive elimination step.     

Microwave‐Assisted Synthesis of N‐Heterocyclic Carbene‐ Palladium(II) Complexes

The microwave‐assisted synthesis of two different types of N‐heterocyclic carbene‐palladium(II) complexes, (NHC)Pd(acac)Cl (NHC=N‐heterocyclic carbene; acac=acetylacetonate) and (NHC)PdCl₂(3‐chloropyridine), has been carried out. A drastic reduction in reaction times (20 to 88 times faster, depending on the complex) was observed when compared to the previously described, conventionally‐heated synthesis of these complexes. The protocol also allowed for the synthesis of (IPr)Pd(acac)Cl [IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene] on a 5‐mmol scale in 30 min, with the reactants loaded in air.     

Electronegativity Governs Enantioselectivity: Alkyl‐Aryl Cross‐Coupling with Fenchol‐Based Palladium‐Phosphorus Halide Catalysts

A series of bulky, modular, monodentate, fenchol‐based phosphites has been employed in an intramolecular palladium‐catalyzed alkyl‐aryl cross‐coupling reaction. This enantioselective α‐arylation of N‐(2‐bromophenyl)‐N‐methyl‐2‐phenylpropanamide is accomplished with [Pd(C₃H₅)(BIFOP‐X)(Cl)] as precatalysts, which are based on biphenyl‐2,2′‐bisfenchol phosphites (BIFOP‐X, X=F, Cl, Br, etc.). The phosphorus fluoride BIFOP‐F gives the highest enantioselectivity and good yields (64% ee, 88%). Lower selectivities and yields are found for BIFOP halides with heavier halogens (Cl: 74%, 47% ee, Br: 63%, 20% ee). NMR studies on catalyst complexes reveal two equilibrating diastereomeric complexes in equal proportions. In all cases, the phosphorus‐halogen moiety remains intact, pointing to its remarkable stability, even in the presence of nucleophiles. The increasing enantioselectivity of the catalysts with the phosphorus halide ligands correlates with the rising electronegativity of the halide (bromine<chlorine<fluorine), as can be rationalized from structural parameters and DFT computations.     

Palladium‐Catalyzed Carbonylation of Diols to Cyclic Carbonates

The catalytic alkoxycarbonylation of 1,2‐diols by (neocuproine)palladium(II) acetate (neocuproine=2,9‐dimethyl‐1,10‐phenanthroline) or palladium(II) acetate/(−)‐sparteine using N‐chlorosuccinimide as the oxidant affords cyclic carbonates. The oxidative carbonylation of diols proceeds under mild conditions, requiring only 1 atm of carbon monoxide, and produces cyclic carbonates in moderate to good yields. Both 1,2‐ and 1,3‐diols can be carbonylated using (neocuproine)Pd(OAc)₂ and sodium dichloroisocyanuric acid, which serves as a competent oxidant and base for this system, to yield 5‐ and 6‐membered cyclic carbonates.     

Calixarene‐Derived Mono‐Iminophosphoranes: Highly Efficient Ligands for Palladium‐ and Nickel‐Catalysed Cross‐Coupling

The first mono‐iminophosphoranes based on a calix[4]arene skeleton have been synthesised and tested in the arylation of aryl bromides and aryl chlorides. Combining these ligands with [Pd(OAc)₂] or [Ni(cod)₂] resulted in highly active Suzuki–Miyaura and Kumada–Tamao–Corriu cross‐coupling catalysts, respectively. TOFs up to ca. 4×10⁵ mol(ArBr)⋅mol(M)⁻¹⋅h⁻¹ were obtained in each case. The remarkable activities observed probably arise from the ligands’ ability to form complexes with cavity‐entrapped “MArX” moieties (endo‐complexes), their highly crowded metal environment favouring formation of mono‐ligated intermediates over that of less reactive bis‐ligated ones. Possible supramolecular interactions within the cavity involving the receptor wall and the aromatic substrate may also significantly influence the reaction rates, notably by increasing the proportion of endo‐complexes.     

Palladium‐Catalyzed Heck‐Type Reactions of Allylic Esters with Arylboronic Acids or Potassium Aryltrifluoroborates

A selective and general route to (E)‐1,3‐diaryl‐prop‐1‐enes and (E)‐3‐arylallyl acetates has been developed by palladium‐catalyzed Heck‐type reactions of allylic esters with arylboronic acids or potassium aryltrifluoroborates. The present method selectively proceeds including β‐OAc elimination or β‐H elimination on the basis of the boronic acids. Whereas a variety of allylic esters were reacted with arylboronic acids, palladium(II) acetate [Pd(OAc)₂], tetra(n‐butyl)ammonium chloride [(n‐Bu)₄NCl] and postassium dihydrogen phosphate (KH₂PO₄) to afford the corresponding diarylation products in moderate to good yields, treatment of allylic esters with potassium aryltrifluoroborates furnished the corresponding monoarylation products.