Phosphinite Complexes Research Articles

Advanced-designed Ru(II) complexes containing phosphinite ligands derived from chiral amino alcohols: Electrochemical behavior, DFT calculations, and biological activity

We report two phosphinite ligands derived from chiral amino alcohols, and their complexes with ruthenium(II). These compounds were characterized by spectroscopic methods. Then, antimicrobial, antioxidant, and DNA binding activities of the chiral Ru(II)-phosphinite complexes were tested. Complex (1R)-2-{benzyl[(1S)-1-(naphthalen-1-yl)ethyl]amino}-1-phenylethyl diphenylphos- phinito[dichloro(η6-p-cymene)ruthenium(II)], 7 showed both the highest radical scavenging activity (90.93 ± 0.98 %) and the highest metal chelating activity (65.45 ± 1.46 %) at 200.0 mg l-1 concentration. In addition, all of complexes had different rates of binding activity to calf thymus DNA (CT-DNA). Moreover, extensive theoretical and experimental investigations were conducted to gain a more profound understanding of the chemical descriptors and the diverse electronic transitions taking place within the ruthenium complexes, as well as their electrochemical characteristics.

Half‐sandwich ruthenium(II) and iridium(III) complexes of imidazole based phosphinite ligands: antioxidant and antibacterial activities as well as electrochemical properties

AbstractRuthenium(II) and iridium(III) complexes of phosphinites including imidazole moiety were synthesized and characterized by microanalysis, IR, MS, and NMR spectroscopies. Antibacterial activity against Gram‐positive and Gram‐negative bacterial strains was assessed in all complexes. The highest radical scavenging (72.2 %) was obtained for [3‐(3‐chloro‐2‐({[dichloro(η6‐benzene)ruthenium]diphenylphosphanyl}oxy)propyl)‐1‐butyl‐1H‐imidazol‐3‐ium chloride], 4 at 200 μg/mL concentration, while [3‐(3‐chloro‐2‐({[dichloro(η5‐pentamethylcyclopentadienyl)‐iridium]diphenylphosphanyl}oxy)pro‐pyl)‐1‐butyl‐1H‐imidazol‐3‐ium chloride], 6 demonstrated the highest antibacterial activity as 13 mm inhibition zone against E. hirea. Furthermore, optical and electrochemical featured of metal complexes containing imidazole phosphinite were investigated utilizing UV–vis absorption and cyclic voltammetry techniques. Consequently, all complexes can be proposed as metal‐based charge convertible phosphinite complexes which may be employed as new generation and synergistic Dye‐Sensitized Solar Cell (DSSC) materials.

Biological assays, electrochemical behavior, and theoretical DFT calculations of Ru(II) complexes of chiral phosphinite based based on β-amino alcohols: Transfer hyrogenation of ketones using a HCOOH/Et3N mixture

Synthesis of two phosphinite ligands based on β-amino alcohols, in high yields has been demonstrated. When we treated [Ru(arene)(μ-Cl)Cl]2 {arene:p-cymene,benzene} with chelating phosphinite ligands, we obtained neutral Ru(II)-complexes possessing the general formula [Ru(arene)phosphiniteCl2]. The structure of the ligands and complexes was confirmed using analytical and spectroscopic techniques. The quantum chemical calculations were carried out for the ruthenium complexes at the DFT/CAM-B3LYP level of theory in gas phase. The phosphinite complexes were subjected to cyclic voltammetry studies in order to determine the energies of HOMO and LUMO levels and to estimate their electrochemical and some electronic properties. Organic complex-based memory substrates were immobilized using TiO2-modified ITO electrodes, and the memory functions of phosphinite-based organic complexes were verified by chronoamperometry (CA) and open-circuit potential amperometry (OCPA). In the present study, the antioxidant potentials of ruthenium-based p-cymene and benzene complexes through DPPH radical scavenging, metal chelating, and reducing power activities were also determined. In addition, DNA binding abilities and antimicrobial activities of these complexes against pathogenic bacteria were studied. Finally, the ruthenium complex, (2S)-1-{[(2S)-2-[(diphenylphosphanyl)oxy]propyl][(1R)-1-phenylethyl]amino}propan-2-yldiphenyl phosphinitobis[dichloro(η6-benzene)ruthenium(II)] also catalyzed asymmetric transfer hydrogenation of acetophenone with high conversion (up to 99%) and good enantioselectivity (ee up to 89 %), in the existence of formic acid and triethylamine in dichloromethane medium under air atmosphere.

Application of Half‐Sandwich Metal‐Phosphinite Compounds to Biological Activities: Determine the energies of the HOMO and LUMO levels

AbstractMononuclear transition metal complexes 1‐(furan‐2‐yl)ethyldiphenyl[dichloro(η6‐p‐cymene)ruthenium(II)]phosphinite, (2), 1‐(furan‐2‐yl)ethyldiphenyl[dichloro(η6‐benzene) ruthenium(II)] phosphinite (3), 1‐(furan‐2‐yl)ethyldipheny[chloro(η4‐1,5‐cyclooctadiene)rhodium(I)]phosphinite (4), 1‐(furan‐2‐yl)ethyldiphenyl[dichloro (η5pentamethylcyclopentadienyl)iridium (III)] phosphinite (5) were synthesized and characterized by microanalysis, infrared, MS, and NMR spectroscopy. The biological activities of the complexes were also tested. Compounds 2 and 5 were the best complexes at DPPH radical scavenging and reducing power activity at 73.27 % and 0.41 at 200 μg/mL, respectively. The highest antimicrobial activity exhibited by complex 3 as 14 mm inhibition zone against S. aureus. All of the complexes have cleaved the DNA from the double‐strand and exhibited three bands on gel electrophoresis. Moreover, cyclic voltammetry studies of the phosphinite complexes were carried out to determine the energies of the HOMO and LUMO levels as well as to estimate their electrochemical and some electronic properties.

Imine containing C2-Symmetric chiral half sandwich η6-p-cymene-Ru(II)- phosphinite complexes: Investigation of their catalytic activity in the asymmetric transfer hydrogenation of ketones

New chiral C2-symmetric bis(phosphinite) ligands containing imine group were synthesized from 1-({[(1R,2R)-2-{[(2-hydroxynaphthalen-1-yl)methylidene] amino}cyclohexyl]- imino}methyl)- naphthalen-2-ol and two equivalents of Ph2PCl, (i-Pr)2PCl or (Cy)2PCl, in high yields. Binuclear C2-symmetric half sandwich η6-p-cymene-Ru(II) complexes of the chiral phosphinite ligands were synthesized by treating of [Ru(η6-p-cymene)(μ-Cl)Cl]2 with the phosphinites in 1:1 M ratio in CH2Cl2. Their catalytic activities in asymmetric transfer hydrogenation (ATH) were investigated for the reaction of acetophenone derivatives with isopropyl alcohol. The corresponding optically active secondary alcohols were obtained in excellent levels of conversion (96–99%) and moderate enantioselectivity (up to 60% ee). Among three complexes investigated, complex 4 was the most efficient one.

Synthesis of ionic liquid-based Ru(II)–phosphinite complexes and evaluation of their antioxidant, antibacterial, DNA-binding, and DNA cleavage activities

Two Ru(II) complexes were synthesized by reaction of phosphinite-functionalized imidazolium salts [(Ph2PO)C7H11N2Cl]Cl (1) and [(Cy2PO)C7H11N2Cl]Cl (2) with 1/2 equivalent of [Ru(η6-p-cymene)(µ-Cl)Cl]2 in anhydrous CH2Cl2 and under argon atmosphere. The complexes were then isolated as analytically pure substances and characterized using multinuclear NMR and infrared spectroscopies and elemental analysis. The Ru(II) compounds were used to study their biological assay. For this purpose, radical scavenging, reducing power, antibacterial activity, DNA binding, and DNA cleavage activity were fully studied. The maximum 1,1-diphenyl-2-picrylhydrazyl radicals (DPPH) scavenging (78.9%) and reducing power were obtained from compound 4 at the concentration of 200 µg/ml. The compounds were also tested against three Gram-positive and three Gram-negative bacteria, and they were found to be more effective against Gram-positive bacteria. In addition, both compounds showed excellent DNA binding and DNA cleavage activity.

Computational Study of Enantioselectivity in the Asymmetric Allylation of Aldehydes with Chiral Pt(II) Phosphinite Complexes.

Computational investigations on the enantioselectivity observed in the allylation of cinnamaldehyde have been carried out using four closely related chiral platinum catalysts with ascorbic acid-based bisphosphinite ligands. Enantioselectivity depends on the substitution of hydrogen on the hydroxyl group by benzyl group(s) in the ascorbic acid framework. The key intermediate is assumed to be one, where both the aldehyde (via oxygen) and the η1 allyl group are coordinated to the metal center, and C-C bond formation between the terminal allylic carbon and carbonyl carbon is the rate-determining step. A screening process has been adopted to select potentially relevant reactant adducts from a total of several hundred theoretically possible conformers by molecular mechanics (MM) calculations. Only 50 transition state structures (TSs) lead to the product based on a two-layer QM/MM study employing B3LYP and UFF methods. Out of these 50 TSs, around 10 structures were selected for the final step QM calculation at B3LYP/LANL2TZ(f)(Pt),6-311G(d,p)//B3LYP/LANL2DZ(Pt),6-31G(d,p) level of theory. This computational approach predicts chirality induction correctly in most of the seven catalysts examined. The successful prediction of enantioselectivity carried out using the MM, QM/MM, and QM approaches suggests that the described simplified procedure could be useful in other catalyst systems also.

Chiral C2-symmetric η6-p-cymene-Ru(II)-phosphinite complexes: Synthesis and catalytic activity in asymmetric reduction of aromatic, methyl alkyl and alkyl/aryl ketones

Abstract Chiral C2-symmetric bis(phosphinite) ligands and their binuclear ruthenium(II) complexes have been synthesized and used as catalysts in the ruthenium-catalyzed asymmetric transfer hydrogenation of aromatic, methyl alkyl and alkyl/aryl ketones using 2-propanol as both the hydrogen source and solvent in the presence of KOH. Under optimized conditions, all complexes showed high catalytic activity as catalysts in the reduction of various ketones to corresponding chiral secondary alcohols. Products were obtained with high conversions (99%) and moderate to good enantioselectivities (82% ee). Furthermore, C2-symmetric bis(phosphinite) ligands and their binuclear ruthenium(II) complexes were characterized by multinuclear NMR spectroscopy, FT-IR spectroscopy, LC/MS-MS and elemental analysis.

Ferrocene based chiral binuclear η6‐benzene‐Ru(II)‐phosphinite complexes: Synthesis, characterization and catalytic activity in asymmetric reduction of ketones

In the present study, a series of chiral C2‐symmetric ferrocenyl based binuclear η6‐benzene‐Ru(II) complexes bearing diphenylphosphinite and diisopropylphosphinite moieties have been synthesised. The new binuclear η6‐benzene‐Ru(II)‐phosphinite complexes were characterised based on nuclear magnetic resonance (1H, 13C, 31P–NMR), FT‐IR spectroscopy and elemental analysis. Then, these complexes have been screened as catalytic precursors in the transfer hydrogenation of acetophenone with 2‐propanol as both the hydrogen source and solvent in the presence of KOH. The corresponding optically active secondary alcohols were obtained in excellent conversion rates between 96 and 99% and moderate to good enantioselectivities (up to 78% ee). The complex 5 was the most efficient catalyst among the four new complexes investigated herein.

Transfer hydrogenation reaction using novel ionic liquid based Rh(I) and Ir(III)-phosphinite complexes as catalyst

Hydrogen transfer reduction methods are attracting increasing interest from synthetic chemists in view of their operational simplicity. Thus, interaction of [Rh(μ-Cl)(cod)]2 and Ir(η5-C5Me5)(μ-Cl)Cl]2 with phosphinite ligand [(Ph2PO)-C7H11N2Cl]Cl, 1 gave new monodendate (1-chloro-3-(3-methylimidazolidin-1-yl)propan-2-yl diphenylphosphinite chloride) (chloro ɳ4-1,5-cyclooctadiene rhodium(I))], 2 and (1-chloro-3-(3-methylimidazolidin-1-yl)propan-2-yl diphenylphosphinite chloride) (dichloro ɳ5-pentamethylcyclopentadienyl iridium(III))], 3 complexes, which were characterized by a combination of multinuclear NMR spectroscopy, IR spectroscopy, and elemental analysis. 1H-{31P} NMR, 1H-13C HETCOR or 1H-1H COSY correlation experiments were used to confirm the spectral assignments. The novel catalysts were applied to transfer hydrogenation of acetophenone derivatives using 2-propanol as a hydrogen source. The results showed that the corresponding alcohols could be obtained with high activity (up to 99%) under mild conditions. Notably, (1-chloro-3-(3-methylimidazolidin-1-yl)propan-2-yl diphenylphosphinite chloride) (chloro ɳ4-1,5-cyclooctadiene rhodium(I))], 2 complex is much more active than the other analogous complex, 3 in the transfer hydrogenation. Furthermore, compound, 2 acts as excellent catalysts, giving the corresponding alcohols in 97–99% conversions in 5 min (TOF ≤ 1176 h−1).

Copper(I)-Phosphinite Complexes in Click Cycloadditions: Three-Component Reactions and Preparation of 5-Iodotriazoles.

The remarkable activity displayed by copper(I)–phosphinite complexes of general formula [CuBr(L)] in two challenging cycloadditions is reported: a) the one‐pot azidonation/cycloaddition of boronic acids, NaN3, and terminal alkynes; b) the cycloaddition of azides and iodoalkynes. These air‐stable catalysts led to very good results in both cases and the expected triazoles could be isolated in pure form under ‘Click‐suitable’ conditions.

Structure of a Novel Rhodium Phosphinite Compound: Agostic Interactions as a Model for an Oxidative Addition Intermediate

Pincer ligand metalation is presumed to proceed via initial coordination to the phosphorus atoms followed by C–H oxidative addition. Few isolated intermediates in this process are known. A rhodium phosphinite complex has been isolated and structurally characterized that exhibits a strong agostic interaction with a C–H bond in the ligand backbone. The [(tBuPOCOP)Rh(CO)H]+ system exhibits greater acidity and reactivity than the analogous iridium species (tBuPOCOP = κ3-C6H3-1,3-[OP(tBu)2]2).

ChemInform Abstract: Ruthenium(II) Complexes Derived from C2‐Symmetric Ferrocene‐Based Chiral Bis(phosphinite) Ligands: Synthesis and Catalytic Activity Towards the Asymmetric Reduction of Acetophenones.

Chiral secondary alcohols are very important building blocks and valuable synthetic intermediates both in organic synthesis and in the pharmaceutical industry for producing biologically active complex molecules. A series of new chiral Ru–phosphinite complexes (1, 2, 3, 4, 5, 6, 7, 8) were prepared from chiral C2-symmetric ferrocenyl phosphinites and corresponding chloro complex, [Ru(η6-p-cymene)(μ-Cl)Cl]2. The complexes were characterized using conventional spectroscopic methods. The binuclear complexes were tested as pre-catalysts and were found to be good pre-catalysts for the asymmetric transfer hydrogenation of substituted acetophenones in basic 2-propanol at 82°C, providing the corresponding optically active alcohols with almost quantitative conversion and modest to high enantioselectivities (46–97%). Amongst the all complexes, complex 6 gave the highest ee of 97% in the reduction of 2-methoxyacetophenone to (S)-1-(2-methoxyphenyl)ethanol at 82°C. Copyright © 2015 John Wiley & Sons, Ltd.

ChemInform Abstract: Catalysis Involving Phosphinite‐Based Metallacycles

AbstractReview: 143 refs.

Catalysis Involving Phosphinite-Based Metallacycles

Phosphinite-based metallacycles are important intermediates, catalyst precursors, or catalysts for a variety of chemical transformations. Facile, reversible formation of P–O bonds allows phosphinites to act as catalytic directing groups that lead to more efficient and selective metal-catalyzed processes. Relatively low costs and convenient syntheses make metallacyclic phosphinite complexes attractive compounds for catalytic studies. Given the high interest in developing the related phosphine- and phosphite-based catalysts, this Perspective focuses specifically on the comparisons between these different metallacyclic complexes in catalytic reactions. In a number of examples, phosphinite-based metallacycles are more efficient catalysts due to better-matched ligand properties for rate-limiting steps or faster conversion of the complexes to catalytically active species.

Ruthenium(II) complexes derived from C2‐symmetric ferrocene‐based chiral bis(phosphinite) ligands: synthesis and catalytic activity towards the asymmetric reduction of acetophenones

Chiral secondary alcohols are very important building blocks and valuable synthetic intermediates both in organic synthesis and in the pharmaceutical industry for producing biologically active complex molecules. A series of new chiral Ru–phosphinite complexes (1, 2, 3, 4, 5, 6, 7, 8) were prepared from chiral C2‐symmetric ferrocenyl phosphinites and corresponding chloro complex, [Ru(η6‐p‐cymene)(μ‐Cl)Cl]2. The complexes were characterized using conventional spectroscopic methods. The binuclear complexes were tested as pre‐catalysts and were found to be good pre‐catalysts for the asymmetric transfer hydrogenation of substituted acetophenones in basic 2‐propanol at 82°C, providing the corresponding optically active alcohols with almost quantitative conversion and modest to high enantioselectivities (46–97%). Amongst the all complexes, complex 6 gave the highest ee of 97% in the reduction of 2‐methoxyacetophenone to (S)‐1‐(2‐methoxyphenyl)ethanol at 82°C. Copyright © 2015 John Wiley & Sons, Ltd.

Synthesis and unexpected reactivity of [Ru(η 6-cymene)Cl2(PPh2Cl)], leading to [Ru(η 6-cymene)Cl2(PPh2H)] and [Ru(η 6-cymene)Cl 2 (PPh2OH)] complexes

The reaction of [Ru(η 6-cymene)Cl2]2 and PPh2Cl in the ratio 1:2 gives a stable [Ru(η 6-cymene) Cl2(PPh2Cl)] complex. Attempts to make the cationic [Ru(η 6-cymene)Cl(PPh2Cl)2]Cl with excess PPh2Cl and higher temperatures led to adventitious hydrolysis and formation of [Ru(η 6-cymene)Cl2(PPh2OH)]. Attempts to make a phosphinite complex by reacting [Ru(η 6-cymene)Cl2]2 with PPh2Cl in the presence of an alcohol results in the reduction of PPh2Cl to give [Ru(η 6-cymene)Cl2(PPh2H)] and the expected phosphinite. The yield of the hydride complex is highest when the alcohol is 1-phenyl-ethane-1,2-diol. All three half-sandwich complexes are characterized by X-ray crystallography. Surprisingly, the conversion of chlorodiphenylphosphine to diphenylphosphine is mediated by 1-phenyl-ethane-1,2-diol even in the absence of the ruthenium half-sandwich precursor. Synthesis and unusual reactivity of some ruthenium half-sandwich complexes including an unusual organocatalysed conversion of PPh2Cl to PPh2H are reported.

ChemInform Abstract: Cross‐Coupling Reactions in Water Using Ionic Liquid‐Based Palladium(II)‐Phosphinite Complexes as Outstanding Catalysts.

Two new phosphinite ligands based on ionic liquids [(Ph2PO)C7H14N2Cl]Cl (1) and [(Cy2PO)C7H14N2Cl]Cl (2) were synthesized by reaction of 1-(3-chloro-2-hydoxypropyl)-3-methylimidazolium chloride, [C7H15N2OCl]Cl, with one equivalent of chlorodiphenylphosphine or chlorodicyclohexylphosphine, respectively, in anhydrous CH2Cl2 and under argon atmosphere. The reactions of 1 and 2 with MCl2(cod) (M=Pd, Pt; cod=1,5-cyclooctadiene) yield complexes cis-[M([(Ph2PO)C7H14N2Cl]Cl)2Cl2 ]a ndcis-[M(Cy2PO) C7H14N2Cl]Cl)2Cl2], respectively. All complexes were isolated as analytically pure substances and characterized using multinuclear NMR and infrared spectroscopies and elemental analysis. The catalytic activity of palladium complexes based on ionic liquid phosphinite ligands 1 and 2 was investigated in Suzuki cross-coupling. They showoutstanding catalytic activity in coupling of a series of aryl bromides or aryl iodides with phenylboronic acid under the optimized reaction conditions in water. The complexes provide turnover frequencies of 57 600 and 232 800h 1 in Suzuki coupling reactions of phenylboronic acid with p-bromoacetophenone or p-iodoacetophenone, respectively, which are the highest values ever reported among similar complexes for Suzuki coupling reactions in water as sole solvent in homogeneous catalysis. Furthermore, the palladium complexes were also found to be highly active catalysts in the Heck reaction affordingtrans-stilbenes. Copyright © 2014 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web-site.

Ortho Derivatization of Phenols through C–H Nickelation: Synthesis, Characterization, and Reactivities of Ortho-Nickelated Phosphinite Complexes

Reported here are the synthesis and characterization of ortho-nickelated complexes derived from phosphinite ligands and investigated as model compounds in the development of C–H functionalization strategies for arenol substrates. Reaction of i-Pr2POPh with 0.6 equiv of [(i-PrCN)NiBr2]n and 0.8 equiv of NEt3 in toluene (100 °C, 36 h) gave the yellow, monomeric cyclometalated complex trans-{κ2P,C-C6H4OP(i-Pr)2}Ni(i-Pr2POPh)Br (3a) in 93% yield. The closely related yellow-orange dimeric species [{κ2P,C-C6H4OP(i-Pr)2}Ni(μ-Br)]2 (4a) was obtained in 70% yield when i-Pr2POPh was treated with 2 equiv each of the Ni precursor and NEt3. These complexes have been characterized fully and shown to interconvert in the presence of excess ligand (4a → 3a) or excess Ni precursor (3a → 4a). Treatment of 3a or 4a with benzyl bromide at 90 °C over extended periods led to benzylation of the Ni–aryl moiety in these complexes. Examination of the cyclometalation pathway for i-Pr2POPh has shown that the first species formed from its ambient-temperature reaction with [(i-PrCN)NiBr2]n is trans-(i-Pr2POPh)2NiBr2 (2a). NMR studies showed that 2a undergoes a rapid ligand exchange at room temperature, which can be slowed down at −68 °C; this fluxional process shifts in the presence of NEt3, implying the partial formation of an amine adduct. Heating toluene mixtures of 2a and NEt3 at 90 °C for 38 h led to the formation of 3a via C–H nickelation. That phosphinite dissociation from 2a precedes the C–H nickelation step is implied by the observation that the formation of 3a is hindered in the presence of excess i-Pr2POPh. The impact of phenol ring substituents on the C–H nickelation rate was probed by preparing substituted derivatives of 2a, trans-(4-R-C6H4OP(i-Pr2)}2NiBr2 (R = OMe (2b), Me (2c), COOMe (2d)), and measuring their relative rates of C–H nickelation. These studies showed that the formation of cyclonickelated products is favored in the order COOMe < Me < OMe, which is consistent with an electrophilic nickelation mechanism. Studying the C–H nickelation of 3-F-C6H4OP(i-Pr2) allowed us to establish that metalation is favored at the para position with respect to F (85:15).

Cross‐coupling reactions in water using ionic liquid‐based palladium(II)–phosphinite complexes as outstanding catalysts

Two new phosphinite ligands based on ionic liquids [(Ph2PO)C7H14N2Cl]Cl (1) and [(Cy2PO)C7H14N2Cl]Cl (2) were synthesized by reaction of 1‐(3‐chloro‐2‐hydoxypropyl)‐3‐methylimidazolium chloride, [C7H15N2OCl]Cl, with one equivalent of chlorodiphenylphosphine or chlorodicyclohexylphosphine, respectively, in anhydrous CH2Cl2 and under argon atmosphere. The reactions of 1 and 2 with MCl2(cod) (M = Pd, Pt; cod = 1,5‐cyclooctadiene) yield complexes cis‐[M([(Ph2PO)C7H14N2Cl]Cl)2Cl2] and cis‐[M(Cy2PO)C7H14N2Cl]Cl)2Cl2], respectively. All complexes were isolated as analytically pure substances and characterized using multi‐nuclear NMR and infrared spectroscopies and elemental analysis. The catalytic activity of palladium complexes based on ionic liquid phosphinite ligands 1 and 2 was investigated in Suzuki cross‐coupling. They show outstanding catalytic activity in coupling of a series of aryl bromides or aryl iodides with phenylboronic acid under the optimized reaction conditions in water. The complexes provide turnover frequencies of 57 600 and 232 800 h−1 in Suzuki coupling reactions of phenylboronic acid with p‐bromoacetophenone or p‐iodoacetophenone, respectively, which are the highest values ever reported among similar complexes for Suzuki coupling reactions in water as sole solvent in homogeneous catalysis. Furthermore, the palladium complexes were also found to be highly active catalysts in the Heck reaction affording trans‐stilbenes. Copyright © 2014 John Wiley &amp; Sons, Ltd.