Solvent-Induced Reduction of Palladium-Aryls, a Potential Interference in Pd Catalysis
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Abstract:
The decomposition of the Pd-aryl complex (NBu4)2[Pd2(μ-Br)2Br2(C6F5)2] (1) to the reduction product C6F5H was checked in different solvents and conditions. 1 is not stable in N-alkyl amides (DMF, NMP, DMA), cyclohexanone, and diethers (1,4-dioxane, DME) at high temperatures (above 80 °C). Other solvents such as nitriles, THF, water, or toluene are safe, and no significant decomposition occurs. The solvent is the source of hydrogen, and the decomposition mechanisms have been identified by analyzing the reaction products coming from the solvent. β-H elimination involving the methyl group in a N-coordinated amide is the predominant pathway for amides. An O-coordinated diether undergoes β-H elimination and subsequent deprotonation of the resulting oxonium salt to give an enol ether. A palladium enolate from cyclohexanone leads to cyclohexenone, a reaction favored by the presence of a base. Oxygen strongly increases the extent of decomposition, and we propose this occurs by reoxidation of the Pd(0) species formed in the process and regeneration of active Pd(II) complexes.Keywords:
Silyl enol ether
Reductive elimination
Mesitylene
Transmetalation
Oxonium ion
Amide
Bonding and reactivity of [(RN4)Pd nCH3X]( n-2)+ complexes have been investigated at the M06/BS2//B3LYP/BS1 level. Feasible mechanisms for the unselective formation of ethane and methyl chloride from mono-methyl PdIII complexes and selective formation of ethane or methyl chloride from PdIV complexes are reported here. Density functional theory (DFT) results indicate that PdIV is more reactive than PdIII and Pd in different oxidation states that follow different mechanisms. PdIII complexes react in three steps: (i) conformational change, (ii) transmetalation, and (iii) reductive elimination. In the first step a five-coordinate PdIII intermediate is formed by the cleavage of one Pd-Nax bond, and in the second step one methyl group is transferred from the PdIII complex to the above intermediate via transmetalation, and subsequently a six-coordinate PdIV intermediate is formed by disproportion. In this step, transmetalation can occur on both singlet and triplet surfaces, and the singlet surface is lying lower. Transmetalation can also occur between the above intermediate and [(RN4)PdII(CH3)(CH3CN) ]+, but this not a feasible path. In the third step this PdIV intermediate undergoes reductive elimination of ethane and methyl chloride unselectively, and there are three possible routes for this step. Here axial-equatorial elimination is more facile than equatorial-equatorial elimination. PdIV complexes react in two steps, a conformational change followed by reductive elimination, selectively forming ethane or methyl chloride. Thus, PdIII complex reacts through a six-coordinate PdIV intermediate that has competing C-C and C-Cl bond formation, and PdIV complex reacts through a five-coordinate PdIV intermediate that has selective C-C and C-Cl bond formation. Free energy barriers indicate that iPr, in comparison to the methyl substituent in the RN4 ligand, activates the cleaving of the Pd-Nax bond through electronic and steric interactions. Overall, reductive elimination leading to C-C bond formation is easier than the formation of a C-Cl bond.
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Abstract A comprehensive DFT (M06‐L‐D3(SMD)/BS2//M06‐L/BS1 level) investigation has been carried out to explore in detail the mechanism of the transmetalation and reductive elimination reactions of abnormal N‐heterocyclic carbene (aNHC) palladium(IV) complexes within the framework of Suzuki–Miyaura cross‐coupling reactions. Emphasis was placed on the role of base and the effect of countercations on the critical transmetalation and reductive elimination events involving palladium(IV) complexes. Of the two competing roles of the base, the route involving boronate formation followed by halide exchange prevails over that of direct halide exchange for the intermediates [Pd IV (aNHC)(OMe) 2 Cl] − Na + (pathway A), [Pd IV (aNHC)(OMe)(Cl) 2 ] − Na + (pathway B), and [Pd IV (aNHC)Cl 3 ] − Na + (pathway C) emanating from the oxidative addition reaction. The results of the calculations are in accordance with our previous theoretical findings of favorable energetics for palladium intermediates incorporating two coordinated methoxy groups. The negative role played by the countercation in the transmetalation step is mainly due to the overstabilization of the pre‐transmetalation intermediate, which is in line with experimental kinetic results. The anionic complexes exhibit greater affinity for the transmetalation and reductive elimination reactions than the neutral variants.
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Oxidative addition
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The direct α-arylation of carbonyl compounds emerged over the last two decades as a straightforward method for the formation of C(sp3)–C(sp2) bonds. Mechanistic studies suggested a classical cross-coupling catalytic cycle. This consists of oxidative addition of the aryl halide (ArX) to the Pd(0)-catalyst, transmetallation of the Na- or K-enolate generated in situ, and subsequent reductive elimination. Even though the general reaction mechanism was thoroughly investigated, studies focusing on enantioselective variants of this transformation are rare. Here, the computational study of the [Pd(BINAP)]-catalyzed α-arylation of 2-methyltetralone with bromobenzene is reported. The whole reaction energy profile was computed and several mechanistic scenarios were investigated for the key steps of the reaction, which are the enolate transmetallation and the C–C bond-forming reductive elimination. Among the computed mechanisms, the reductive elimination from the C-bound enolate Pd complex was found to be the most favorable one, providing a good match with the stereoselectivity observed experimentally with different ligands and substrates. Detailed analysis of the stereodetermining transition structures allowed us to establish the origin of the reaction enantioselectivity.
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Reductive elimination
Bromobenzene
Oxidative addition
Aryl halide
Catalytic cycle
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A novel pathway for the homocoupling reaction has been achieved using a similar protocol as the cross-coupling reaction. Ethyl bromoacetate is chosen to initiate the coupling reaction through oxidative addition to a Pd(0) species, and an PdBr(enolate) intermediate is formed. This intermediate can undergo double transmetalation with an alkynyl copper reagent, and reductive elimination produces a variety of diynes in high yields.
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Ethyl bromoacetate
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Coupling reaction
Stille reaction
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This Article describes the development of a decarbonylative Pd-catalyzed aryl–fluoroalkyl bond-forming reaction that couples fluoroalkylcarboxylic acid-derived electrophiles [RFC(O)X] with aryl organometallics (Ar–M′). This reaction was optimized by interrogating the individual steps of the catalytic cycle (oxidative addition, carbonyl de-insertion, transmetalation, and reductive elimination) to identify a compatible pair of coupling partners and an appropriate Pd catalyst. These stoichiometric organometallic studies revealed several critical elements for reaction design. First, uncatalyzed background reactions between RFC(O)X and Ar–M′ can be avoided by using M′ = boronate ester. Second, carbonyl de-insertion and Ar–RF reductive elimination are the two slowest steps of the catalytic cycle when RF = CF3. Both steps are dramatically accelerated upon changing to RF = CHF2. Computational studies reveal that a favorable F2C–H---X interaction contributes to accelerating carbonyl de-insertion in this system. Finally, transmetalation is slow with X = difluoroacetate but fast with X = F. Ultimately, these studies enabled the development of an (SPhos)Pd-catalyzed decarbonylative difluoromethylation of aryl neopentylglycol boronate esters with difluoroacetyl fluoride.
Transmetalation
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Oxidative addition
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Organometallic Chemistry
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Ni-catalyzed selective C-O bond activation opens a door for the cross-coupling of aryl esters. The present study reports a thorough theoretical analysis of Ni-catalyzed cross-coupling between aryl esters and arylboronic acids, with an emphasis on explaining the cause for the surprising selectivity in C-O activation. The overall catalytic cycle is found to include three basic steps: oxidative addition, transmetalation, and reductive elimination. Oxidative addition of Ar-OAc to Ni(0) in the presence of PCy(3) ligand proceeds through the monophosphine pathway (instead of the alternative two-phosphine pathway) with a relatively low barrier of +22.9 kcal/mol. Transmetalation proceeds via a base-assisted mechanism with a barrier of +31.2 kcal/mol. Reductive elimination is the most facile step in the whole catalytic cycle. Comparatively, oxidative addition of ArO-Ac to Ni(0) is a more facile process (barrier = +14.2 kcal/mol) than oxidative addition of Ar-OAc to Ni(0). However, the former process is associated with a fairly low reverse barrier, and its product does not transmetalate easily (barrier = +33.1 kcal/mol). By comparison, the latter process is an irreversible reaction, and its product transmetalates more readily. These results explain why only the cross-coupling products from the Ar-OAc activation (but not from the ArO-Ac activation) were observed in experiments.
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This communication describes the synthesis of an organometallic NiIV complex bearing a labile trifluoroacetate (OTFA) ligand via the oxidation of a NiII precursor with PhI(OTFA)2. Intramolecular C(sp2)–O bond-forming reductive elimination from this NiIV complex is relatively slow, requiring 6 h at 70 °C to reach completion. In contrast, transmetalation with TMSCF3 occurs within just 1 h at room temperature to generate a NiIV–CF3 complex. These studies show that intermolecular reactions such as transmetalation can be competitive with intramolecular reductive elimination processes at NiIV centers.
Transmetalation
Reductive elimination
Oxidative addition
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Transmetalation
Oxidative addition
Catalytic cycle
Reductive elimination
Chemoselectivity
Amide
Decarbonylation
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Three-coordinate bipyridyl complexes of gold, [(κ2-bipy)Au(η2-C2H4)][NTf2], are readily accessed by direct reaction of 2,2′-bipyridine (bipy), or its derivatives, with the homoleptic gold ethylene complex [Au(C2H4)3][NTf2]. The cheap and readily available bipyridyl ligands facilitate oxidative addition of aryl iodides to the Au(I) center to give [(κ2-bipy)Au(Ar)I][NTf2], which undergo first aryl-zinc transmetalation and second C–C reductive elimination to produce biaryl products. The products of each distinct step have been characterized. Computational techniques are used to probe the mechanism of the oxidative addition step, offering insight into both the origin of the reversibility of this process and the observation that electron-rich aryl iodides add faster than electron-poor substrates. Thus, for the first time, all steps that are characteristic of a conventional intermolecular Pd(0)-catalyzed biaryl synthesis are demonstrated from a common monometallic Au complex and in the absence of directing groups.
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Reductive elimination
Oxidative addition
Homoleptic
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This paper presents an experimental and theoretical investigation of the Pd-catalyzed Negishi coupling reaction and reveals a novel second transmetalation reaction between an Ar1−Pd−Ar2 species and the organozinc reagent Ar2−ZnX. Understanding of this second step reveals how homocoupling and dehalogenation products are formed. Thus, the second transmetalation generates Ar2PdAr2 and Ar1ZnCl, which upon reductive elimination and hydrolysis, respectively, give the homocoupling product Ar2−Ar2 and the dehalogenation product Ar1H. The ratio of the cross-coupling product Ar1−Ar2 and the homocoupling product Ar2−Ar2 is determined by competition between the second transmetalation and reductive elimination steps. This mechanism is further supported by density functional theoretical calculations. Calculations on a series of reactions suggest a strategy in controlling the selectivity of cross-coupling and homocoupling pathways, which we have experimentally verified.
Transmetalation
Negishi coupling
Reductive elimination
Metalation
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