CO2 insertion into metal-carbon bonds: a computational study of Rh(I) pincer complexes

Catalytic carboxylation reactions that use CO(2) as a C1 building block are still among the 'dream reactions' of molecular catalysis. To obtain a deeper insight into the factors that control the fundamental steps of potential catalytic cycles, we performed a detailed computational study of the insertion reaction of CO(2) into rhodium-alkyl bonds. The minima and transition-state geometries for 38 pincer-type complexes were characterized and the according energies for the C-C bond-forming step were determined. The electronic properties of the Rh-alkyl bond were found to be more important for the magnitude of the activation barrier than the interaction between rhodium and CO(2). The charge of the alkyl-chain carbon atom, as well as agostic and orbital interactions with the rhodium, exhibit the most pronounced influence on the energy of the transition states for the CO(2) insertion reaction. By varying the backbone and the donor groups of the pincer ligand those properties can be tuned over a very broad range. Thus, it is possible to match the electronic and steric properties with the fundamental requirements of the CO(2) insertion into rhodium-alkyl bonds of the ligand framework.     

Organometallic complex compounds,part 6(1). Property-specific ligand control of stability and reactivity of (1,3-dimethyl-η3-allyl)methylnickel-ligand complexes. Separation of electronic and steric effects

Summary The property-specific ligand control of 28 ligands on the decomposition temperatures in solution, measured by d.t.a. of a four-coordinate nickel(II)-complex is reported. A quantitative separation of electronic and steric effects by a multilinear regression analysis (75% electronic and 25% steric influence for the chosen ligands) is presented. The controlling effect of the selectivity on the decomposition (fraction of the C-C-linked product) (25 P-ligands) leads to an electronic: steric ratio of the property-specific ligand control of 5545 for the chosen ligands. An increase in the relative acceptor character of the P-ligands relatively destabilizes the complexes and thereby favours formation of a C-C-bond. An increase in steric hindrance also favours C-C-bond formation. A method for revising the steric parameter of P-ligands is presented and is used to correct the -value of (PhCH₂)₃P is corrected to 135°. SCCC-MO-calculations for testing the chemical reasoning of the separated electronic and steric ligand property control are shown.     

Stereocontrol mechanism in CO/p-methylstyrene copolymerisation catalysed by aryl-alpha-diimine Pd(II) complexes

Structural analysis of the first steps of isotactic CO/p-methylstyrene copolymerisation, catalysed by aryl-alpha-diimine Pd(II) complexes, highlights the influence of steric effects on the stereoselectivity of olefin insertion.     

Copper-Phosphido Catalysis: Enantioselective Addition of Phosphines to Cyclopropenes**

We describe a copper catalyst that promotes the addition of phosphines to cyclopropenes at ambient temperature. A range of cyclopropylphosphines bearing different steric and electronic properties can now be accessed in high yields and enantioselectivities. Enrichment of phosphorus stereocenters is also demonstrated via a Dynamic Kinetic Asymmetric Transformation (DyKAT) process. A combined experimental and theoretical mechanistic study supports an elementary step featuring insertion of a Cu^I-phosphido into a carbon-carbon double bond. Density functional theory calculations reveal migratory insertion as the rate- and stereo-determining step, followed by a syn-protodemetalation.     

Molecular modeling of the regiochemistry of olefin insertion with single-site polymerization catalysts

The regiochemistry of propene insertion promoted by metallocene and postmetallocene Column 4 metal catalysts is investigated with quantum mechanics techniques. It is shown that steric effects dominate the regiochemistry of monomer insertion with classical metallocene-based catalysts, whereas both electronic and steric effects contribute to the regiochemistry of propene insertion with postmetallocene catalysts. The text was submitted by the authors in English.     

Insertion reactions of t-Bu(η5-C5H5)Fe(CO)2

The complex t-Bu(η⁵-C₅H₅)FE(CO)₂ has been treated with triphenylphosphine in refluxing THF to produce t-BuCO(η⁵-C₅H₅)Fe(CO)(PPh₃). The large steric bulk of the t-butyl group suggests that this reaction should be faster than the reaction involving the methyl group, and a kinetic investigation illustrates this to be the case. The same steric bulk predicts that the reaction with SO₂ should be slow, and indeed we have been unable to effect the related SO₂ insertion reaction. Attempts to prepare the corresponding t-Bu(η⁵-C₅H₅)W(CO)₃ led to formation of the related isobutyl complex.     

Role of metal co-cations in improving CuY zeolite performance for DMC synthesis: A theoretical study

First principle calculations based on density functional theory are conducted to investigate the influence of metal cations including Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, La (OH)²⁺ and Ce (OH)²⁺ in the small cage of zeolite on the electronic environment of adjacent active center, Cu⁺ in CuY zeolite as well as the process of CO insertion into CH₃O to form CH₃OCO for oxidative carbonylation of methanol. The study explains the theoretical reasons for the effects of metal cations on the catalytic activity of zeolites. It was found that, the presence of co-cations in the small cage can affect the electronic properties and also the catalytic activity in two ways. Firstly, the presence of co-cations, viz., Ca²⁺, Sr²⁺, Mg²⁺, Ba²⁺ and La species in small cage hinders the migration of active Cu⁺ cations from the super cage to small cage. Secondly, the co-cations greatly affect the charge transfer from zeolite framework to Cu⁺ present in the adjacent super cage, leading to the increase of the net charge and binding energy of Cu⁺. The findings can improve the CO adsorption and insertion efficiencies, and the stability of transition states, which results in the enhanced catalytic activity of corresponding zeolites.     

Iron mediated diene activation: Facile nucleophile addition to a coordinated diene: The crystal structure of [(η5-C5Me5)Fe(methylhexa-2,4-dienoate)CO][BF4]

The site of attack on the cation [(η⁵-C₅Me₅)Fe(methylhexa-2,4-dienoate)CO]⁺ by borohydride or methoxide is determined by the electronic effect of the carbomethoxy group and by the steric interaction between the olefinic dienemethyl and one of the cyclopentadienylmethyls.     

Rate-structure correlations for migratory CO insertion. Steric acceleration for the octahedral iron(II) case

The kinetics of the migratory insertion reaction of cis,cis-[(diars)- Fe(CO)₂L(Me)]⁺, (Ia–Ig), (L = C₆H₁₁O₃P (ETPB), P(OMe)3, PhP(OMe)₂, PMe₃, PhPMe₂ Ph₂OMe) or Ph₂PMe) in the presence of excess L′ (L′ = P(OMe)₃ or i-C₃H₇NC) in methylene chlorine-d₂ to form [(diars)- Fe(CO)LL′(C(O)Me)]⁺, (IIa–IIg), follow the rate law: Rate = k_insert [(I)]. The first order rate constant, k_insert, depends primarily on steric factors and shows an overall increase of a factor of 270 (at 290 K) as L varies from ETPB (cone angle 101°) to Ph₂PMe (cone angle 136°). The pronounced steric acceleration of insertion rate is in accord with a unimolecular rate determining insertion step which requires neither inter- nor intramolecular nucleophilic participation.     

Propellanes—XLII: Secondary-orbital-overlap control during certain diels-alder reactions

We show additional cases of Diels-Alder reactions between propellane imides. When bulky substituents are attached to the imide-nitrogen, their steric effect partly offsets the electronic influence of the CO groups in overlapping with the dienophile nitrogen lone pairs.     

金属羰基化合物的氧原子转移反应——取代亚碘酰苯存在下Cr(CO)6的CO取代反应动力学研究

在取代亚碘酰苯存在下,研究了Cr(CO)₆与外来配体L(L=CH₃CN、PPh₃)间的羰基取代反应动力学.结果表明,该试剂的空间效应加快了反应速度,其活性顺序为:邻位取代>间位取代>对位取代>未取代;不同基团的对位取代亚碘酰苯的电子效应与氧原子转移速度之间呈良好的线性关系.吸电子基促进反应,在位阻相近的情况下,取代亚碘酰苯的活性随I-O键强减小而增大.     

The synthesis of [(η-FluH)Mn(CO)3]PF6, and its deprotonation to yield a novel coordination mode of fluorene

The synthesis of [(η⁶-Flu^∗H)Mn(CO)₃]PF₆ is reported. The complex may be deprotonated to give a neutral zwitterionic complex. Instead of deprotonation at the 9-position, as seen in the non-methylated complex, deprotonation of a ring methyl is observed. This yields a previously unknown coordination mode of fluorene, and may be explained on both electronic and steric grounds.     

Rhodium catalyzed hydroformylation of olefins

DFT and CCSD(T) methods were used to examine 61 different rhodium catalysts for the hydroformylation of ethylene. The carbon monoxide (CO) stretching frequency was a key electronic parameter to understand the π-accepting nature of the ligand. Normally, π-accepting ligands lead to increased CO stretching frequencies and a reduction in CO dissociation energy. There was no relationship between CO dissociation energy and CO stretching frequency. However, a clear relationship exists between the ethylene insertion barrier (from the rhodium dicarbonyl hydride resting state) and the CO stretching frequency as stronger π-accepting ligands systematically led to a reduction in the barrier. Due to the multistep nature of the rate-limiting step, the overall barrier can be divided into the CO/ethylene equilibrium and an intrinsic ethylene insertion barrier and both are systematically reduced as the π-accepting nature of the ligand is increased. A comparison of the carbonylation transition state (TS) to the ethylene insertion TS allowed us to understand reversibility of olefin insertion. While the ethylene insertion TS systematically decreases with increasing CO stretching frequency, the carbonylation TS is relatively flat. The lines cross at 2156 cm⁻¹ implying a change in the rate-limiting step in this region given a standard set of process conditions. © 2018 Wiley Periodicals, Inc.     

A theoretical study of steric and electronic effects in the rhodium-catalyzed carbonylation reactions.

We present a QM and QM/MM study of steric and electronic effects in the main steps of Rh-catalyzed carbonylation reactions. All the considered systems adopt a square-planar geometry prior to CH(3)I oxidative addition. As regards the octahedral complexes after CH(3)I oxidative addition, a comparison between the various models indicates that the energy gain due to the CH(3)I oxidative addition is reduced by the steric pressure of the substituents on the ligand. The substantially similar results obtained with the QM/MM and QM models indicate that electronic effects are not particularly relevant in determining the energetic of oxidative addition. As regards the P,P-Ph octahedral complex, the geometries in which the CO group is trans to the added CH(3) group, or trans to one of the P atoms, are of similar energy. A comparison between the various models indicates that the energy barrier of the CO insertion reaction is lowered by the presence of substituents on the chelating ligands. This effect is related to a relief of the steric pressure on the complex as the systems move from a six-coordinated octahedral geometry toward a five-coordinated square-pyramidal geometry. The energy barrier calculated for the P,S-Ph system is in rather good agreement with the experimental value, whereas that of the P,P-Ph system is somewhat underestimated. Inclusion of solvent effects with a continuum model leads to a slightly better agreement. The thermodynamic products adopt a square-pyramidal geometry with the COCH(3) group in the apical position.     

Regiochemistry of propene insertion with group 4 polymerization catalysts from a theoretical perspective

The regiochemistry of monomer insertion in propene polymerization promoted by group 4 metal catalysts has been investigated by using DFT methods. We calculated primary and secondary propene insertion on metallocenes, constrained geometry catalysts and post-metallocene systems analyzing the effects of ligand framework, growing chain and metal. Our study supports the concept that for metallocene-based catalysts the regiochemistry of propene is mainly originated by steric effects. Instead, for octahedral systems a delicate balance between steric and electronic effects is found. This allows to play with the electronic properties of the ligand framework to tune finely the regiochemistry of polymerization.     

Intermolecular migratory insertion of unactivated olefins into palladium-nitrogen bonds. Steric and electronic effects on the rate of migratory insertion

We report a detailed examination of the effect of the steric and electronic properties of the ancillary ligand and the alkene reactant on the rate of migratory insertion of unactivated alkenes into the palladium-nitrogen bond of isolated palladium amido complexes. A series of THF-bound and THF-free amidopalladium complexes ligated by cyclometalated benzylphosphine ligands possessing varied steric and electronic properties were synthesized. The THF-free complexes react with ethylene at -50 °C to form olefin-amido complexes that were observed directly and that undergo migratory insertion, followed by β-hydride elimination to generate enamine products. The effect of the steric properties of the ancillary ligand on the binding of the alkene and the rate of migratory insertion were evaluated individually. The relative binding affinity of ethylene vs THF is larger for the less sterically hindered complex than for the more hindered complex, but the less hindered complex undergoes the insertion of ethylene more slowly than does the more hindered complex. These two changes in relative equilibrium and rate constants cause the rates of reaction of ethylene with the two THF-ligated species having different steric properties to be similar to each other. Reactions of the complexes containing electronically varied ancillary ligands showed that the more electron-poor complexes underwent the migratory insertion step faster than the more electron-rich complexes. Reactions of a THF-ligated palladium-amide with substituted vinylarenes showed that electron-poor vinylarenes reacted with the amido complex slightly faster than electron-rich vinylarenes. Separation of the energetics of binding and insertion indicate that the complex of an electron-rich vinylarene is more stable in this system than the complex of a more electron-poor vinylarene but that the insertion step of the bound, electron-rich vinylarene is slower than the insertion step with the bound, electron-poor vinylarene.     

Kinetics of the reaction of α-amino acids with phenalene-1,2,3-trione hydrate

The rate of the title reaction was studied as a function of pH of the reaction medium, temperature, basicities and steric environment of amino groups. Kinetic constants were calculated from a linear free-energy equation permits calculation of separate polar and steric parameters associated with the amino acids which influence rates. The mechanism deduced from the study of the pH-rate profile involving the attack of the unprotonated amino group as a basic amine nucleophile at the CO carbon. The activation parameters of the reaction were determined and ΔS^≠ was found to be a linear function of ΔH^≠.     

Aromatic heterocycles XII. Semiempirical PM3 study of Diels-Alder cycloaddition reaction of substituted phosphabenzenes

We report the results of a semiempirical PM3 study of the 1,4 cycloaddition reaction of substituted λ³-phosphabenzenes with alkynes. The influence of the nature, position and steric hindrance of substituents on the reaction energy is studied. Except for some values, the results are in reasonable agreement with experimental observations and electronic effects of substituents.     

Steric and electronic effects on the reactivity of rh and ir complexes containing P-S,P-P,and P-O ligands. Implications for the effects of chelate ligands in catalysis

Kinetic studies of the reactions of [M(CO)(L-L)I] [M = Rh, Ir; L-L = Ph(2)PCH(2)P(S)Ph(2) (dppms), Ph(2)PCH(2)CH(2)PPh(2) (dppe), and Ph(2)PCH(2)P(O)Ph(2) (dppmo)] with methyl iodide have been undertaken. All the chelate ligands promote oxidative addition of methyl iodide to the square planar M(I) centers, by factors of between 30 and 50 compared to the respective [M(CO)(2)I(2)](-) complexes, due to their good donor properties. Migratory CO insertion in [Rh(CO)(L-L)I(2)Me] leads to acetyl complexes [Rh(L-L)I(2)(COMe)] for which X-ray crystal structures were obtained for L-L = dppms (3a) and dppe (3b). Against the expectations of simple bonding arguments, methyl migration is faster by a factor of ca. 1500 for [Rh(CO)(dppms)I(2)Me] (2a) than for [Rh(CO)(dppe)I(2)Me] (2b). For M = Ir, alkyl iodide oxidative addition gives stable alkyl complexes [Ir(CO)(L-L)I(2)R]. Migratory insertion (induced at high temperature by CO pressure) was faster for [Ir(CO)(dppms)I(2)Me] (5a) than for its dppe analogue (5b). Reaction of methyl triflate with [Ir(CO)(dppms)I] (4a) yielded the dimer [[Ir(CO)(dppms)(mu-I)Me](2)](2+) (7), which was characterized crystallographically along with 5a and [Ir(CO)(dppms)I(2)Et] (6). Analysis of the X-ray crystal structures showed that the dppms ligand adopts a conformation which creates a sterically crowded pocket around the alkyl ligands of 5a, 6, and 7. It is proposed that this steric strain can be relieved by migratory insertion, to give a five-coordinate acetyl product in which the sterically crowded quadrants flank a vacant coordination site, exemplified by the crystal structure of 3a. Conformational analysis indicates similarity between M(dppms) and M(2)(mu-dppm) chelate structures, which have less flexibility than M(dppe) systems and therefore generate greater steric strain with the "axial" ligands in octahedral complexes. Ab initio calculations suggest an additional electronic contribution to the migratory insertion barrier, whereby a sulfur atom trans to CO stabilizes the transition state compared to systems with phosphorus trans to CO. The results represent a rare example of the quantification of ligand effects on individual steps from catalytic cycles, and are discussed in the context of catalytic methanol carbonylation. Implications for other catalytic reactions utilizing chelating diphosphines (e.g., CO/alkene copolymerization and alkene hydroformylation) are considered.     

Selectivity control in alkylidene carbene-mediated C-H insertion and allene formation

Regioselectivity of alkylidene carbene-mediated C-H insertion was explored utilizing electronic, conformational, steric, and stereoelectronic effects. Relying on these factors, highly regio- and chemoselective carbene insertion reaction of C-H bonds in different environments could be obtained. The observed selectivity clearly indicates that an electronic effect plays a more important role than steric effect. In general, C-H bonds in conformationally rigid cyclic environments are less reactive than those in acyclic systems toward carbene insertion, and in this situation, a competing intermolecular reaction between alkylidene carbene and trimethylsilyldiazomethane led to the formation of allenylsilanes. The formation of allenylsilane becomes more favorable as the concentration of reaction becomes higher, as well as the C-H bonds undergoing insertion becomes electronically and conformationally deactivated.