A computational approach towards predicting π-facial selectivity in sterically unbiased olefins: an evaluation of the relative importance of electrostatic and orbital effects

Computational studies are presented to show that electrostatic interactions significantly impact the stereochemical outcome in electrophilic addition to a number of sterically unbiased alkenes. Transition states have been located for the reaction of different electrophiles with all the sterically unbiased alkenes studied here and the calculations effectively include interactions involving the σ and σ^∗ orbitals of the newly formed bond. Electrostatic interaction between the substrates and electrophiles was modelled by removing the electrophiles from the transition state geometry and placing the calculated charge at a distance from a selected atom as observed in TS structures. Electrostatic interactions between the electrophiles and the substrate seem to effectively determine the face selectivities in the systems studied and our model calculations indicate that it may not be important to invoke Cieplak type orbital interactions to rationalize the observed face selectivities. The face selectivities predicted for these alkenes and electrophiles with DFT B3LYP/6-31G^∗ and ab initio MP2/6-31G^∗ levels are generally in good agreement.     

Di-t-butyl(ferrocenylmethyl)phosphine: air-stability, structural characterization, coordination chemistry, and application to palladium-catalyzed cross-coupling reactions

Di-t-butyl(ferrocenylmethyl)phosphine (1) has been isolated and structurally characterized. This ligand was found to be reasonably air stable as a solid and it has been shown to possess electron donating ability similar to that of tri-i-propylphosphine. A palladium catalyst bearing this ligand performed room temperature Suzuki-Miyaura coupling reactions with aryl bromides. Modest Heck coupling reactivity with aryl bromides was also observed at 100 °C. Complexation of 1 with Pd₂(dba)₃ led to formation of (1)₂Pd⁰. Addition of 4-bromoanisole to solutions containing both 1 and Pd₂(dba)₃ led to formation of an oxidative addition product when 1:Pd ratios were ?1. With a 2:1 ratio of 1:Pd, monophosphine complex formation and oxidative addition were significantly inhibited.     

Effect of CO on the Oxidative Addition of Arene C?H Bonds by Cationic Rhodium Complexes

Sequential addition of CO molecules to cationic aryl–hydrido Rh^III complexes of phosphine‐based (PCP) pincer ligands was found to lead first to C? H reductive elimination and then to C? H oxidative addition, thereby demonstrating a dual role of CO. DFT calculations indicate that the oxidative addition reaction is directly promoted by CO, in contrast to the commonly accepted view that CO hinders such reactions. This intriguing effect was traced to repulsive π interactions along the aryl‐Rh‐CO axis, which are augmented by the initially added CO ligand (due to antibonding interactions between occupied Rh d_π orbitals and occupied π orbitals of both CO and the arene moiety), but counteracted by the second CO ligand (due to significant π back‐donation). These repulsive interactions were themselves linked to significant weakening of the π‐acceptor character of CO in the positively charged rhodium complexes, which is concurrent with an enhanced σ‐donating capability. Replacement of the phosphine ligands by an analogous phosphinite‐based (POCOP) pincer ligand led to significant changes in reactivity, whereby addition of CO did not result in C? H reductive elimination, but yielded relatively stable mono‐ and dicarbonyl aryl–hydrido POCOP–Rh^III complexes. DFT calculations showed that the stability of these complexes arises from the higher electrophilicity of the POCOP ligand, relative to PCP, which leads to partial reduction of the excessive π‐electron density along the aryl‐Rh‐CO axis. Finally, comparison between the effects of CO and acetonitrile on C? H oxidative addition revealed that they exhibit similar reactivity, despite their markedly different electronic properties. However, DFT calculations indicate that the two ligands operate by different mechanisms.     

Oxidative addition of aryl chlorides to palladium N-heterocyclic carbene complexes and their role in catalytic arylamination

Density functional theory has been used to investigate various solvated species that may be formed from palladium bis N-heterocyclic carbene complexes, [Pd(cyclo-C{NRCH}₂)₂], (PdL₂) in benzene solution. Formation of an η²-arene complex is shown to stabilise a monocarbene species, PdL(η²-C₆H₅X), where the arene is either the solvent or a reacting aryl halide. Oxidative addition of an aryl chloride has been modelled, and the most likely transition state has been established as a PdL(arylchloride) species, with just one carbene ligand coordinated to the palladium. The catalytic cycle for aryl amination has been investigated and the oxidative addition of the aryl halide shown to be the rate determining step. Reductive elimination of the aryl amine has a lower activation energy. Oxidative addition of alkyl halides has been shown to be less favourable because of the absence of an unsaturated group, such as the aryl ring, to bond to the palladium.     

Mechanisms in the Reaction of Palladium(II)–π‐Allyl Complexes with Aryl Halides: Evidence for NHC Exchange Between Two Palladium Complexes

A detailed experimental and DFT study (PBE level) of the reaction of [Pd(η³‐C₃H₅)(tmiy)(PR₃)]BF₄ (tmiy=tetramethylimidazolin‐2‐ylidene, PR₃=phosphane), precursors to monoligated Pd⁰ species, with aryl electrophiles yielding 2‐arylimidazolium salt is reported. Experiments establish that an autocatalytic ligand transfer mechanism is preferred over Pd^IV and σ‐bond metathesis pathways, and that transmetalation is the rate‐determining step. Calculations indicate that the key step involves the concerted exchange of NHC and iodo ligands between two different Pd^II complexes. This is corroborated by experimental results showing the slower reaction of complexes containing the bulkier dipdmiy (dipdmiy = diisopropyldimethylimidazolin‐2‐ylidene).     

Mechanistic Insights into Palladium‐Catalyzed Silylation of Aryl Iodides with Hydrosilanes through a DFT Study

The catalytic cycles of palladium‐catalyzed silylation of aryl iodides, which are initiated by oxidative addition of hydrosilane or aryl iodide through three different mechanisms characterized by intermediates R₃Si−Pd^II−H (Cycle A), Ar−Pd^II−I (Cycle B), and Pd^IV (Cycle C), have been explored in detail by hybrid DFT. Calculations suggest that the chemical selectivity and reactivity of the reaction depend on the ligation state of the catalyst and specific reaction conditions, including feeding order of substrates and the presence of base. For less bulky biligated catalyst, Cycle C is energetically favored over Cycle A, through which the silylation process is slightly favored over the reduction process. Interestingly, for bulky monoligated catalyst, Cycle B is energetically more favored over generally accepted Cycle A, in which the silylation channel is slightly disfavored in comparison to that of the reduction channel. Moreover, the inclusion of base in this channel allows the silylated product become dominant. These findings offer a good explanation for the complex experimental observations. Designing a reaction process that allows the oxidative addition of palladium(0) complex to aryl iodide to occur prior to that with hydrosilane is thus suggested to improve the reactivity and chemoselectivity for the silylated product by encouraging the catalytic cycle to proceed through Cycles B (monoligated Pd⁰ catalyst) or C (biligated Pd⁰ catalyst), instead of Cycle A.     

Microscopic Reactivity of Phenylferrate Ions toward Organyl Halides

Despite its practical importance, organoiron chemistry remains poorly understood due to its mechanistic complexity. Here, we focus on the oxidative addition of organyl halides to phenylferrate anions in the gas phase. By mass-selecting individual phenylferrate anions, we can determine the effect of the oxidation state, the ligation, and the nuclearity of the iron complex on its reactions with a series of organyl halides RX. We find that Ph₂Fe(I)⁻ and other low-valent ferrates are more reactive than Ph₃Fe(II)⁻; Ph₄Fe(III)⁻ is inert. The coordination of a PPh₃ ligand or the presence of a second iron center lower the reactivity. Besides direct cross-coupling reactions resulting in the formation of RPh, we also observe the abstraction of halogen atoms. This reaction channel shows the readiness of organoiron species to undergo radical-type processes. Complementary DFT calculations afford further insight and rationalize the high reactivity of the Ph₂Fe(I)⁻ complex by the exothermicity of the oxidative addition and the low barriers associated with this reaction step. At the same time, they point to the importance of changes of the spin state in the reactions of Ph₃Fe(II)⁻.     

Formation of XPhos-Ligated Palladium(0) Complexes and Reactivity in Oxidative Additions

Understanding the nature of the intermediate species operating within a palladium catalytic cycle is crucial for developing efficient cross-coupling reactions. Even though the XPhos/Pd(OAc)₂ catalytic system has found numerous applications, the nature of the active catalytic species remains elusive. A Pd⁰ complex ligated to XPhos has been detected and characterized in situ for the first time using cyclic voltammetry and NMR techniques. In the presence of XPhos, Pd(OAc)₂ initially associates with the ligand to form a complex in solution, which has been characterized as Pd^II(OAc)₂(XPhos). This Pd^II center is then reduced to the Pd⁰(XPhos)₂ species by an intramolecular process. This study also sheds light on the formation of Pd^I–Pd^I dimers. Finally, a kinetic study probes a dissociative mechanism for the oxidative addition with aryl halides involving Pd⁰(XPhos) as the reactive species in equilibrium with the unreactive Pd⁰(XPhos)₂. Remarkably, the reportedly poorly reactive PhCl reacts at room temperature in the oxidative addition, which confirms the crucial role of the XPhos ligand in the activation of aryl chlorides.     

Ligand Size Effect on PdLn Oxidative Addition with Aryl Bromide: A DFT Study

The process and mechanism of the ligand volume controlled Pd(PR₃)₂ (PR₃=PH₃, PMe₃, and PtBu₃) oxidative addition with aryl bromide were investigated, using density functional theory method with the conductor-like screening model. Association pathway and dissocia-tion pathway were investigated by the comparison of several energies. The cleavage energy of Pd(PR₃)₂ complex was calculated, as well as the oxidative addition reaction barrier energy of Pd(PR₃)_n (n=1,2) with aryl bromide in N,N-dimethylformamide solvent. This study proved that the ligands volume possessed a great impact on the mechanism of oxidative addition: less bulky ligand palladium associated with aryl bromide via two donor ligands,but larger bulky ligand palladium coordinated via monoligand.     

Tiazenido complexes with M = Mo or W and R = aryl OR alkyl

The preparation, structure and some properties of compounds

(M = Mo, W; R = aryl and R′ = aryl or alkyl) are reported. Temperature dependent ¹³C NMR spectra show that for R ≠ R′ = aryl the complexes are fluxional, but not when one of the R groups is an alkyl group. The fluxional mechanism involves an interchange of the two CO groups via Berry type pseudorotations.Mass spectrometric measurements show the formation of nitrene type species [(η⁵-C₅H₅)(CO)_nM=NR]⁺ with n = 0, 1, 2.     

Theoretical investigation of C–HM interactions in organometallic complexes: A natural bond orbital (NBO) study

The nature of C–HM agostic interactions in model metal complexes [M²⁺(CH₂CH₃)(PH₃)_nCl] (where M = Sc, Ti, V, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn; n = 1, 2, 3, 4) was studied with the natural bond orbital analysis (NBO) approach using density functional theory (DFT) optimized geometries at the B3LYP/6-31G(d,p) level of theory. The effect of nature of metal, coordination number, oxidation state and ligand field effects on the agostic interaction is examined. A set of 20 crystal structures of organometallic complexes taken from the Cambridge Structural Database (CSD) was studied computationally employing AIM theory and NBO analysis, and the applicability of these methods was critically accessed in demarcating the two types of interaction.     

Ligand Effects on the Mechanisms of Thermal Bond Activation in the Gas‐Phase Reactions NiX+/CH4→Ni(CH3)+/HX (X=H,CH3, OH,F). Short Communication

The thermal ion‐molecule reactions NiX⁺+CH₄→Ni(CH₃)⁺+HX (X=H, CH₃, OH, F) have been studied by mass spectrometric methods, and the experimental data are complemented by density functional theory (DFT)‐based computations. With regard to mechanistic aspects, a rather coherent picture emerges such that, for none of the systems studied, oxidative addition/reductive elimination pathways are involved. Rather, the energetically most favored variant corresponds to a σ‐complex‐assisted metathesis (σ‐CAM). For X=H and CH₃, the ligand exchange follows a ‘two‐state reactivity (TSR)’ scenario such that, in the course of the thermal reaction, a twofold spin inversion, i.e., triplet→singlet→triplet, is involved. This TSR feature bypasses the energetically high‐lying transition state of the adiabatic ground‐state triplet surface. In contrast, for X=F, the exothermic ligand exchange proceeds adiabatically on the triplet ground state, and some arguments are proposed to account for the different behavior of NiX⁺/Ni(CH₃)⁺ (X=H, CH₃) vs. NiF⁺. While the couple Ni(OH)⁺/CH₄ does not undergo a thermal ligand switch, the DFT computations suggest a potential‐energy surface that is mechanistically comparable to the NiF⁺/CH₄ system. Obviously, the ligands X act as a mechanistic distributor to switch between single vs. two‐state reactivity patterns.     

<Emphasis Type="Italic">Ab initio</Emphasis> calculation of nitrogen oxide dimer structure and its anion-radical

A comparative quantum chemical analysis has been made for the most stable dimer of nitrogen oxide with the structure cis-ONNO in a singlet state ¹A₁ by ab initio method of SCF MO LCAO, allowing for electron correlation according to Meller-Plesset perturbation theory of the second order (MP2), and density functional technique (DFT). The computations by MP2 method show anion-radical (ONNO)^? to have a strong bond between nitrogen atoms (N-N 1.44 Å) in contrast to molecular weakly bound cis-dimer with equilibrium distance N-N 2.23 Å. Molecular orbital structure of the dimer and its anions was examined that made it possible to suggest a reason of preferable stabilization of nitrogen oxide dimer in the cis-form. Calculated high affinity to electron (E_a = ?1.55-?1.69 eV) for the molecular dimer ONNO (¹A₁) explains an intense strengthening of N-N bond in anion-radical and confirms the experimental data on a possibility of surface anion-π-radical formation on electron donor centers. The DFT computations indicate that this technique poorly reproduces the experimental geometry and electron structure of the cis-dimer ONNO having predicted a triplet ground state with the equilibrium distance N-N ≈2 Å instead of a singlet one with N-N 2.26 Å. The comparison between MP2 and DFT calculations for complex dimer ONNO with copper cation reveals the energy state of the complex (Cu-O₂N₂)⁺ corresponding to stabilization of anion-π-radical (N₂O₂)^? {term-³A₂, Cu(d)⁹-(ONNO)^?1} to be highly overestimated by DFT.     

Abnormal g tensor and multiconfigurational ground state of

The shift of g_lj = 2.0094 in

(geometry D_3d, state ^2A_2u) from 2.0023 is too large to be rationalized with theories of the g tensor based on a single-configurational ground state. This abnormality can be explained with a multi-configurational ground state representing spin polarization.     

In situ formation of palladium(0) from a P,C-palladacycle

In DMF at 80 °C, a Pd⁰ complex is generated in situ from the dimeric P,C-palladacycle (1) in the absence of any reducing agents, presumably via a reductive elimination. The Pd⁰ complex formed in an endergonic equilibrium has been trapped and stabilized by an additional P(o-Tol)₃ and has been detected in cyclic voltammetry by its oxidation peak. Its formation is favored by acetate anions (often used as base in Heck reactions) via the formation of a monomeric anionic P,C-palladacycle ligated by acetate ions. As postulated, P,C-palladacycles are a reservoir of monophosphine-Pd⁰ complexes active in oxidative additions with aryl halides.     

Selectfluor-mediated mild oxidative halogenation and thiocyanation of 1-aryl-allenes with TMSX (X = Cl,Br, I,NCS) and NH4SCN

Presence of TMSX (X = Cl, Br, I) unleashes the oxidative character of Selectfluor and provides a mild dihalogenation method for 1-arylallenes. Preference for 2,3-addition was observed with TMSCl in MeCN irrespective of the nature of the substituent on the aryl moiety, whereas 1,2-addition was preferred in [BMIM][BF₄]. With TMSBr and TMSI only products corresponding to 2,3-addition were observed. Reactions carried out with TMSBr in IL solvents gave the corresponding monobromoalkenes as a major product along with the isomeric dibromo-alkenes. Reaction with NH₄SCN provided convenient access to dithiocyanate derivatives. The same products were formed via TMS-NCS/Selectfluor. Formation of common products via TMSNCS and NH₄SCN points to the formation and interplay of SCN⁺/NCS⁺ as incipient electrophiles.     

Lithium aminoborohydrides 17. Palladium catalyzed borylation of aryl iodides, bromides, and triflates with diisopropylaminoborane prepared from lithium diisopropylaminoborohydride

The Alcaraz-Vaultier borylation of aryl halides and triflates is reported utilizing diisopropylaminoborane (BH₂N(ⁱPr)₂) prepared from the corresponding lithium aminoborohydride (LAB reagent). BH₂N(ⁱPr)₂, prepared by reacting lithium diisopropylaminoborohydride with trimethylsilyl chloride, provided the most consistent isolated yields from this reaction. Catalytic amounts of palladium dichloride produced the highest yields from aryl iodides, while catalytic tris(dibenzylideneacetone)dipalladium(chloroform) provided the best yields for aryl bromides and triflates. This route to boronic acids is mild enough to tolerate various functionalities and for the first time employs aryl triflates as substrates for the Alcaraz-Vaultier borylation. In addition, it was found that both boronic acid and ester compounds could be isolated from the reaction mixture utilizing simple work-up procedures. Treatment of the reaction intermediate with an acid/base work-up provided the corresponding boronic acid, while treating the same intermediate with a diol, such as neopentyl glycol, afforded the corresponding boronic ester.     

Monomer insertion mechanism of ring-opening polymerization of -caprolactone with yttrium alkoxide intermediate: A DFT study

The mechanism of -caprolactone (CL) insertion into a Y–OCH₃ bond was investigated using density functional theory (DFT) calculations. The optimized geometries and corresponding Gibbs-free energies of the intermediates were obtained, which confirmed a four-step coordination-insertion mechanism. The coordination of CL onto yttrium center led to a nucleophilic addition of the carbonyl group of CL, followed by an intramolecular alkoxide ligand exchange. A monomer insertion was completed by the CL ring opening via acyl–oxygen bond cleavage. The formation of the five-coordinated yttrium intermediate, 3, was found to be the rate-determining step. This study could be applicable to ring-opening polymerisation (ROP) of CL initiated by lanthanide metal complexes.     

Platinum-alkyl-B(C6F5)3 (or BF3) ‘in situ’ systems as tin(II) halide-free enantioselective hydroformylation catalysts

The dialkyl/diaryl-platinum complexes (Pt(CH₃)₂(bdpp); PtPh₂(bdpp) and Pt(2-Thioph)₂(bdpp), where bdpp stands for (2S,4S)-2,4-bis(diphenylphosphino)pentane) were reacted either with B(C₆F₅)₃, BPh₃ or BF₃. In the presence of PPh₃ or carbon monoxide cationic species with a general formulae [PtR(L)(bdpp)]⁺ (L = PPh₃, CO) were formed exclusively. The ability of boron additives to provide vacant coordination site at the platinum made these systems suitable as hydroformylation catalysts. Enantioselective hydroformylation of styrene was carried out in the presence of in situ catalysts formed from Pt(alkyl/aryl)₂(bdpp) and B(C₆F₅)₃ or BF₃. Moderate e.e-s depending strongly on the structure of the catalytic precursor have been obtained. DFT/PCM calculations reveal an S_N2-type reaction mechanism for the alkyl/aryl ligand abstraction with a notably lower activation barrier for BF₃.