Mechanism of the palladium-catalyzed metal-carbon bond formation. A dual pathway for the transmetalation step

Molecularly engineered hole-transport material for low-cost perovskite solar cells

Triphenylamine-N-phenyl-4-(phenyldiazenyl)aniline (TPA-AZO) is synthesized via a facile CuI-catalyzed reaction and used as a hole transport material (HTM) in perovskite solar cells (PSCs), as an alternative to the expensive spiro-type molecular materials, including commercial 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD). Experimental and computational investigations reveal that the highest occupied molecular orbital (HOMO) level of TPA-AZO is deeper than that of spiro-OMeTAD, and optimally matches with the conduction band of the perovskite light absorber. The use of TPA-AZO as a HTM results in PSC prototypes with a power conversion efficiency (PCE) approaching that of the spiro-OMeTAD-based reference device (17.86% vs. 19.07%). Moreover, the use of inexpensive starting reagents for the synthesis of TPA-AZO makes the latter a new affordable HTM for PSCs. In particular, the cost of 1 g of TPA-AZO ($22.76) is significantly lower compared to that of spiro-OMeTAD ($170-475). Overall, TPA-AZO-based HTMs are promising candidates for the implementation of viable PSCs in large-scale production.

Catalyzed M–C coupling reactions in the synthesis of σ-(pyridylethynyl)dicarbonylcyclopentadienyliron complexes

The reactions between terminal ethynylpyridines, (trimethylsilyl)ethynylpyridines and cyclopentadienyliron dicarbonyl iodide were studied under Pd/Cu-catalyzed conditions to develop a synthetic approach to the σ-alkynyl iron complexes Cp(CO)₂Fe–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C–R (R = ortho-, meta-, para-pyridyl). Depending on the catalyst and reagents used, the yields of the desired σ-pyridylethynyl complexes varied from 40 to 95%. In some cases the reactions with ortho-ethynylpyridine gave as byproduct the unexpected binuclear FePd μ-pyridylvinylidene complex [Cp(CO)Fe{μ₂-η¹(C_α):η¹(C_α)-κ¹(N)-C_α Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C_β(H)(o-C₅H₄N)}(μ-CO)PdI]. The conditions, catalysts, and reagents that provide the highest yields of the desired σ-pyridylethynyl iron compounds were determined. The methods developed allowed the synthesis of the corresponding σ-4-benzothiadiazolylethynyl complex Cp(CO)₂Fe–CC–(4-C₆H₃N₂S) as well. Eventually, synthetic approaches to σ-alkynyl iron complexes of the type Cp(CO)₂Fe–CC–R (R = ortho-, meta-, para-pyridyl, 4-benzothiadiazol-2,1,3-yl) based on the Pd/Cu-catalyzed cross-coupling reactions were elaborated.

Two approaches were developed for the synthesis of iron σ-pyridylethynyl complexes based on Pd/Cu- and Pd-catalyzed Fe–C coupling reactions.

Suzuki-Miyaura coupling of arylboronic acids to gold(iii)

Cyclometalated gold(iii) aryls are prepared through palladium catalysis. Mono- and diarylation are demonstrated. A wide range of functional groups is tolerated.

Gold(iii) is prominent in catalysis, but its organometallic chemistry continues to be restricted by synthesis. Metal–carbon bond formation often relies on organometallic complexes of electropositive elements, including lithium and magnesium. The redox potential of gold(iii) interferes with reactions of these classic reagents. Resort to toxic metals is common, including reagents based on mercury and thallium. We report that the palladium-catalyzed Suzuki–Miyaura coupling of arylboronic acids extends to cyclometalated gold(iii) chlorides. Both monoarylation and diarylation are achieved. We propose a mechanism where oxidative addition to palladium with rearrangement at gold(iii) fixes the stereochemistry of monoarylated intermediates. Singly arylated species form as thermodynamic isomers. These entities then go on to form diarylated complexes. Reactions proceed at room temperature, and the products are stable to air, moisture, and chromatography.

Quantitative kinetic investigation on transmetalation of ArZnX in a Pd-catalysed oxidative coupling

Transmetalation is the rate-limiting step. [R-Pd-Ar] was suggested to be the resting species from the kinetic studies. Quantitative measurement of the transmetalation of ArZnCl with [R-Pd-Ar] from a live Pd-catalysed oxidative coupling reaction was conducted and the corresponding activation enthalpy was determined as 12.3 kcal mol(-1).

Bimetallic catalysis using transition and Group 11 metals: an emerging tool for C-C coupling and other reactions

Bimetallic catalysis refers to homogeneous processes in which either two transition metals (TM), or one TM and one Group 11 (G11) element (occasionally Hg also), cooperate in a synthetic process (often a C-C coupling) and their actions are connected by a transmetalation step. This is an emerging research area that differs from the isolated or tandem applications of the now classic processes (Stille, Negishi, Suzuki, Hiyama, Heck). Most of the reactions used so far combine Pd with a second metal, often Cu or Au, but syntheses involving very different TM couples (e.g., Cr/Ni in the catalyzed vinylation of aldehydes) have also been developed. Further development of the topic will soon demand a good knowledge of the mechanisms involved in bimetallic catalysis, but this knowledge is very limited for catalytic processes. However, there is much information available, dispersed in the literature, coming from basic research on exchange reactions occurring out of any catalytic cycle, in polynuclear complexes. These are essentially the same processes expected to operate in the heart of the catalytic process. This Review gathers together these two usually isolated topics in order to stimulate synergy between the bimetallic research coming from more basic organometallic studies and the more synthetic organic approaches to this chemistry.

Synthesis of Aryliron Complexes [CpFe(CO)2Ar] by Palladium-Catalyzed Reactions of [CpFe(CO)2I] with Arylzinc, -Boron, or -Indium Reagents

Transmetalation between [CpFe(CO)₂I] and arylzinc iodide-lithium chloride complexes proceeds in the presence of catalytic amounts of palladium acetate and N,N,N’,N’-tetramethyl-1,2-cyclohexanediamine to yield the corresponding aryliron complexes [CpFe(CO)₂Ar]. Phenylation of [CpFe(CO)₂I] also takes place when triphenylindium is used under similar conditions. Arylboronic acids undergo arylation in the presence of cesium carbonate and a palladium-N-heterocyclic carbene complex, PEPPSI.  The present methods are useful for the facile synthesis of various functionalized [CpFe(CO)₂Ar]. The products [CpFe(CO)₂Ar] represent an interesting class of aryl metals that undergo several transformation.

Observation of a hidden intermediate in the Stille reaction. Study of the reversal of the transmetalation step

A study of the reaction of cis-[PdRf2(AsPh3)2] (Rf = 3,5-C6Cl2F3) with ISnBu3 (that is the reversal of the natural Stille reaction of [PdRfI(AsPh3)2] with RfSnBu3) allows for the observation of cis-[PdRf2(AsPh3)(ISnBu3)], the expected intermediate from a cyclic transmetalation in the direct Stille reaction, thus providing experimental support to the operation of cyclic transmetalation pathways.

The Mechanism of the Stille Reaction Investigated by Electrospray Ionization Mass Spectrometry

On-line monitoring of Stille reactions was performed via direct infusion electrospray ionization mass spectrometry (ESI-MS) and its tandem version (ESI-MS/MS). When operated in the positive ion mode, ESI(+)-MS was able to transfer, directly from solution to the gas phase, the species involved in all main steps of a Stille reaction, that is, the catalytically active palladium species Pd(PPh3)2, in its molecular ion form as well as the key cationic Pd(II) intermediates, including cyclic IPd-(CH2CH)Sn species. When searching for anionic species, ESI(-)-MS monitoring showed I- as the only anion detectable in the reaction medium. A detailed catalytic cycle for a Stille reaction was elaborated in which reaction intermediates and the previously elusive catalytically active Pd(0) species are shown in association with the respective ionic species intercepted by ESI-MS and further characterized by ESI-MS/MS.

A Joint Experimental and Theoretical Study of the Palladium-Catalyzed Electrophilic Allylation of Aldehydes

Palladium-catalyzed electrophilic allylation of aldehydes with allylstannanes has been proposed in the literature as a model reaction illustrating the potential of nucleophilic eta(1)-allyl palladium pincer complexes to promote new catalytic processes. This reaction was studied by a joint experimental and theoretical approach. It was shown that pincer palladium complexes featuring a S approximately P approximately S and a S approximately C approximately S tridentate ligand are efficient catalysts for this reaction. The full mechanism of this transformation was studied in detail by means of DFT calculations. Two pathways were explored: the commonly proposed mechanism involving eta(1)-allyl palladium intermediates and a Lewis acid promoted mechanism. Both of these mechanisms were compared to the direct transformation that was shown experimentally to occur under mild conditions. The mechanism involving an eta(1)-allyl palladium intermediate has been discarded on energetic grounds, the nucleophilic attack and the transmetalation step being more energetically demanding than the direct reaction between allyltin and the aldehyde. On the other hand, a mechanism where the palladium acts as a Lewis acid proved to be fully consistent with all experimental and theoretical results. This mechanism involves (L approximately X approximately L)Pd(+) species which activate the aldehyde moiety toward nucleophilic attack.

Insights into the mechanism of the site-selective sequential palladium-catalyzed cross-coupling reactions of dibromothiophenes/dibromothiazoles and arylboronic acids. Synthesis of PPARbeta/delta agonists

A reactivity study, aided by NMR spectroscopy, allowed a mechanistic rationale to be postulated for the palladium-catalyzed regioselective coupling of arylboronic acid (and arylstannane where feasible) at the position next to the sulfur atom in functionalized dibromothiophenes and dibromothiazoles. The analysis of the NMR spectra (using 19F from the boronic acid CF3 group and 31P from the phosphine of the catalyst as probes) of the entire reaction starting from the dibromoheterocycles allowed the qualitative proposal that the transmetalation is the rate-limiting step for both sequential substitution processes. The extremely facile oxidative addition at the C-Br bond next to the sulfur atom of the heterocycle instead determines the positional selectivity. An additional Stille reaction then replaced the second halogen, providing the trisubstituted heterocyclic scaffolds of PPAR ligands, which displayed PPARbeta/delta agonist activity, as revealed by reporter assays in living cells.

Mechanistic study on the coupling reaction of aryl bromides with arylboronic acids catalyzed by (iminophosphine)palladium(0) complexes. Detection of a palladium(ii) intermediate with a coordinated boron anion

The complexes [Pd(eta2-dmfu)(P-N)] [P-N = 2-(PPh2)C6H4-1-CH=NR, R = C(6)H(4)OMe-4; CHMe2; C6H3Me2-2,6; C6H3(CHMe2)-2,6] react with an excess of BrC6H4R1-4 (R1= CF3; Me) yielding the oxidative addition products [PdBr(C6H4R1-4)(P-N)] at different rates depending on R [C6H4OMe-4 > C6H3(CHMe2)-2,6 > CHMe2 approximately C6H3Me2-2,6] and R1 (CF3>> Me). In the presence of K2CO3 and activated olefins (ol = dmfu, fn), the latter compounds react with an excess of 4-R2C6H4B(OH)2 (R2= H, Me, OMe, Cl) to give [Pd(eta2-ol)(P-N)] and the corresponding biaryl through transmetallation and fast reductive elimination. The transmetallation proceeds via a palladium(II) intermediate with an O-bonded boron anion, the formation of which is markedly retarded by increasing the bulkiness of R. The intermediate was isolated for R = CHMe2, R1 = CF3 and R2= H. The boron anion is formulated as a diphenylborinate anion associated with phenylboronic acid and/or as a phenylboronate anion associated with diphenylborinic acid. In general, the oxidative addition proceeds at a lower rate than transmetallation and represents the rate-determining-step in the coupling reaction of aryl bromides with arylboronic acids catalyzed by [Pd(eta2-dmfu)(P-N)].

Computational Characterization of a Complete Palladium-Catalyzed Cross-Coupling Process: The Associative Transmetalation in the Stille Reaction

[reaction: see text] The species presumably involved in the associative ligand substitution mechanism for the Stille cross coupling of vinyl bromide and trimethylvinyl stannane with Pd(PMe(3))(2)/PMe(3) as catalysts in DMF (as ligand and solvent) have been structurally and energetically characterized. The cyclic four-coordinate transition state for the rate-determining transmetalation step explains the retention of configuration in the Stille coupling of chiral nonracemic alkyl stannanes.

New Efficient Approach to 2,2-Diaryl-1,1-difluoro-1-alkenes and 1,1-Difluoro-2-aryl-1,3-dienes via Suzuki Coupling of α-Halo-β,β-difluorostyrenes

[reaction: see text] Alpha-halo-beta,beta-difluorostyrenes [ArCX = CF2; X = Br, I; Ar = aryl, heteroaryl; synthesized by the Pd(0)-catalyzed coupling reaction of the corresponding alpha-halo-beta,beta-difluoroethenylzinc reagents (CF2=CXZnCl, X = Br, I) with aryl iodides] were functionalized at the halogen site with arylboronic acids under Pd(0)-catalyzed Suzuki-Miyaura coupling reaction conditions to obtain 2,2-diaryl-1,1-difluoro-1-alkenes (ArAr'C=CF2, Ar' = aryl, heteroaryl) in 51-91% isolated yield. The corresponding reaction with alkenylboronic acids produced 1,1-difluoro-2-aryl-1,3-dienes in 53-80% isolated yield. Alternatively, 2,2-disubstituted-1,1-difluoro-1-alkenes were synthesized in moderate yield by a zinc-insertion reaction at the halogen site of the alpha-halo-beta,beta-difluorostyrenes, followed by Pd(0)-catalyzed cross-coupling of the zinc reagent with aryl or alkenyl iodides.

Synthesis, structure, theoretical studies, and Ligand exchange reactions of monomeric, T-shaped arylpalladium(II) halide complexes with an additional, weak agostic interaction

A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L = 1-AdPtBu2, PtBu3, or Ph5FcPtBu2 (Q-phos); X = Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or to Pd(dba)2 (dba = dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition of phenyl triflate to PdL2 (L = 1-AdPtBu2, PtBu3, or Q-phos) also did not form arylpalladium(II) complexes. The reaction of silver triflate with (1-AdPtBu2)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative addition to Pd(1-AdPtBu2)2 or Pd(PtBu3)2. Several of the arylpalladium complexes were characterized by X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which has the largest trans influence, is located trans to the open coordination site. The complexes appear to be stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-coordination site of the palladium center. The strength of the Pd.H bond, as assessed by tools of density functional theory, depended upon the donating properties of the ancillary ligands on palladium.

Regioselectivity in the Stille Coupling Reactions of 3,5-Dibromo-2-pyrone

The Stille couplings of 3,5-dibromo-2-pyrone normally take place regioselectively at C3, lower in electron density than C5, thus oxidative addition proceeds faster . When the reactions are carried out with Cu(I) in DMF or other polar aprotic solvent, however, the couplings occur predominantly at C5. The observed regiochemical reversal is attributed to the preferred formation of 5-pallado-2-pyrone intermediate which, in addition, turned out to be more reactive than 3-pallado-2-pyrone intermediate.

Mechanism of the Stille reaction catalyzed by palladium ligated to arsine ligand: PhPdI(AsPh3)(DMF) is the species reacting with vinylstannane in DMF

The kinetics of the reaction of PhPdI(AsPh(3))(2) (formed via the fast oxidative addition of PhI with Pd(0)(AsPh(3))(2)) with a vinyl stannane CH(2)[double bond]CH[bond]Sn(n-Bu)(3) has been investigated in DMF. This reaction (usually called transmetalation step) is the prototype of the rate determining second step of the catalytic cycle of Stille reactions. It is established here that the transmetalation proceeds through PhPdI(AsPh(3))(DMF), generated by the dissociation of one ligand AsPh(3) from PhPdI(AsPh(3))(2). PhPdI(AsPh(3))(DMF) is the reactive species, which leads to styrene through its reaction with CH(2)[double bond]CH[bond]SnBu(3). Consequently, in DMF, the overall nucleophilic attack mainly proceeds via a mechanism involving PhPdI(AsPh(3))(DMF) as the central reactive complex and not PhPdI(AsPh(3))(2). The dimer [Ph(2)Pd(2)(mu(2)-I)(2)(AsPh(3))(2)] has been independently synthesized and characterized by its X-ray structure. In DMF, this dimer dissociates quantitatively into PhPdI(AsPh(3))(DMF), which reacts with CH(2)[double bond]CH[bond]SnBu(3). The rate constant for the reaction of PhPdI(AsPh(3))(DMF) with CH(2)[double bond]CH[bond]SnBu(3) has been determined in DMF for each situation and was found to be comparable.