Β-hydride Elimination Process Research Articles

Kinetically-Controlled Ni-Catalyzed Direct Carboxylation of Unactivated Secondary Alkyl Bromides without Chain Walking.

Herein, we report the direct carboxylation of unactivated secondary alkyl bromides enabled by the merger of photoredox and nickel catalysis, a previously inaccessible endeavor in the carboxylation arena. Site-selectivity is dictated by a kinetically controlled insertion of CO2 at the initial C(sp3)-Br site by the rapid formation of Ni(I)-alkyl species, thus avoiding undesired β-hydride elimination and chain-walking processes. Preliminary mechanistic experiments reveal the subtleties of stereoelectronic effects for guiding the reactivity and site-selectivity.

Single-Site Heterogeneous Organometallic Ir Catalysts Embedded on Graphdiyne: Structural Manipulation Beyond the Carbon Support.

Accurate control over the coordination circumstances of single-atom catalysts (SACs) is decisive to their intrinsic activity. Here, two single-site heterogeneous organometallic catalysts (SHOCs), Cp*Ir-L/GDY (L = OH- and Cl- ; Cp* = pentamethylcyclopentadienyl), with the fine-tuned local coordination and electronic structure of Ir sites, are constructed by anchoring Cp*Ir complexes on graphdiyne (GDY) matrix via a one-pot procedure. The spectroscopic studies and theoretical calculations indicate that the Ir atoms in Cp*Ir-Cl/GDY and Cp*Ir-OH/GDY have a much higher oxidation state than Ir in the SAC Ir/GDY. As a proof-of-principle demonstration, the GDY-supported SHOCs are used for formic acid dehydrogenation, which display a fivefold enhancement of catalytic activity compared with SAC Ir/GDY. The kinetic isotope effect and in situ Fourier-transform infrared studies reveal that the rate-limiting step is the β-hydride elimination process, and Cp* on the Ir site accelerates the β-hydride elimination reaction. The GDY-supported SHOCs integrate the merits of both SACs and molecular catalysts, wherein the isolated Ir anchored on GDY echoes with SACs' behavior, and the Cp* ligand enables precise structural and electronic regulation like molecular catalysts. The scheme of SHOCs adds a degree of freedom in accurate regulation of the local structure, the electronic property, and therefore the catalytic performance of single-atom catalysts.

DFT Probe into the Mechanism of Formic Acid Dehydrogenation Catalyzed by Cp*Co, Cp*Rh, and Cp*Ir Catalysts with 4,4'-Amino-/Alkylamino-Functionalized 2,2'-Bipyridine Ligands.

The mechanistic landscape of H2 generation from formic acid catalyzed by Cp*M(III) complexes (M = Co or Rh or Ir) with diamino-/dialkylamino-substituted 2,2'-bipyridine ligand architectures have been unveiled computationally. The calculations indicate that the β-hydride elimination process is the rate-determining step for all the investigated catalysts. The dialkylamino moieties on the 2,2'-bipyridine ligand were found to reduce the activation free energy required for the rate-limiting β-hydride elimination step and increase the hydridic nature of the Ir-hydride bond, which accounts for the experimentally observed enhanced catalytic activity. Furthermore, the protonation by H3O+ ion was found to be the kinetically most favorable route than the conventional protonation by formic acid. The origin for this preference lies in the increased electrophilicity of the proton from hydronium ion which facilitates easy protonation of the metal-hydride with low activation energy barrier. The Co and Rh analogues of the chosen iridium catalyst were computationally designed and were estimated to possess a rate-determining activation barrier of 16.9 and 14.5 kcal/mol, respectively. This illustrates that these catalysts are potential candidates for FAD. The insights derived in this work might serve as a vital knowledge that could be capitalized upon for designing cost-effective catalyst for FAD in future.

Deciphering the Mechanistic Details of Manganese-Catalyzed Formic Acid Dehydrogenation: Insights from DFT Calculations.

A comprehensive density functional theory investigation has been carried out to unravel the complete mechanistic landscape of aqueous-phase formic acid dehydrogenation (FAD) catalyzed by a pyridyl-imidazoline-based Mn(I) catalyst [Mn(PY-NHIM)(CO)3Br], which was recently reported by Beller and co-workers. The computed free energy profiles show that for the production of a Mn-formate intermediate [Mn(HCO2-)], a stepwise mechanism is both kinetically and thermodynamically favorable compared to the concerted mechanism. This stepwise mechanism involves the dissociation of a Br- ion from a Mn-bromide complex [Mn(Br)] to create a vacant site and coordination of water solvent to this vacant site, followed by the dissociative exchange of the aqua ligand with the formate ion to form Mn(HCO2-). Non-covalent interaction analysis revealed that the steric hindrance at the transition state is the cardinal reason for the preference to a stepwise mechanism. The β-hydride elimination process was estimated to be the rate-determining step with a barrier of 19.0 kcal/mol. This confirms the experimental observation. The generation of a dihydrogen-bound complex was found to occur through the protonation of Mn-hydride by a hydronium ion instead of formic acid. The mechanistic details and insights presented in this work would promote future catalytic designing and exploration of earth-abundant Mn-based catalytic systems for potential applications toward FAD.

Computational Analysis of Two-State Reactivity in β-Hydride Elimination Mechanisms of Fe(II)– and Co(II)–Alkyl Complexes Supported by β-Diketiminate Ligand

β-Hydride elimination in Fe(II)– and Co(II)–alkyl complexes is known to occur through two different spin multiplicity states and is thus a good example of two-state reactivity. In this study, the automated reaction path search method combined with the mixed-spin effective Hamiltonian approach has been applied to understand the detailed reaction mechanisms including the characterization of the spin-inversion points between the high-spin and low-spin potential energy surfaces for the Fe(II)–C2H5 and Co(II)–C2H5 complexes supported by a β-diketiminate ligand. Density functional theory (DFT) with different exchange-correlation functionals has been used in the reaction path search calculations. We found that the β-hydride elimination process for these complexes consists of multiple steps including two spin-inversion points. We have also investigated the substituent effect in the β-diketiminate ligand to understand the steric and electronic effects on the spin-inversion process. The efficiency of the spin-inver...

An intramolecular Heck reaction of enol ethers involving β-alkoxyl elimination followed by the β-hydride elimination process: access to (Z)-ortho-formyl/keto-cinnamates.

A highly regio- and stereoselective intramolecular Heck reaction of enol ethers involving β-alkoxyl elimination followed by the β-hydride elimination process has been developed. Various o-iodobenzyl enol ethers derived from allenoates or alkynoates are found to be efficient substrates for the syntheses of (Z)-ortho-formyl/keto-cinnamates. It is the first attempt to realize the β-alkoxy elimination of enol ethers in a palladium-catalyzed Heck reaction.

Pd-Catalyzed Umpolung of π–Allylpalladium Intermediates: Assembly of All-Carbon α-Vinyl Quaternary Aldehydes through C(sp3)–C(sp3) Coupling

Construction of sterically congested all-carbon quaternary centers represents a formidable challenge in synthetic chemistry. The method described herein provides direct and facile access to a series of structurally diverse and synthetically useful aliphatic aldehydes, bearing an all-carbon α-vinyl quaternary center and a 1,5-diene functionality, through Pd-catalyzed umpolung of vinylethylene carbonates (VECs). The reaction features electrophilic-to-nucleophilic reactivity reversal of the VEC-derived π-allyl-palladium intermediate via an unusual β-hydride elimination process, and the resultant enolate is chemoselectively coupled with allylic acetate to form an α-vinyl aldehyde embedded with an all-carbon quaternary center.

Ligand-Controlled Chemoselective C(acyl)-O Bond vs C(aryl)-C Bond Activation of Aromatic Esters in Nickel Catalyzed C(sp2)-C(sp3) Cross-Couplings.

A ligand-controlled and site-selective nickel catalyzed Suzuki-Miyaura cross-coupling reaction with aromatic esters and alkyl organoboron reagents as coupling partners was developed. This methodology provides a facile route for C(sp2)-C(sp3) bond formation in a straightforward fashion by successful suppression of the undesired β-hydride elimination process. By simply switching the phosphorus ligand, the ester substrates are converted into the alkylated arenes and ketone products, respectively. The utility of this newly developed protocol was demonstrated by its wide substrate scope, broad functional group tolerance and application in the synthesis of key intermediates for the synthesis of bioactive compounds. DFT studies on the oxidative addition step helped rationalizing this intriguing reaction chemoselectivity: whereas nickel complexes with bidentate ligands favor the C(aryl)-C bond cleavage in the oxidative addition step leading to the alkylated product via a decarbonylative process, nickel complexes with monodentate phosphorus ligands favor activation of the C(acyl)-O bond, which later generates the ketone product.

Alkene-alkyl interconversion: an experimental and computational study of the olefin insertion and β-hydride elimination processes.

The preparation and characterization of various alkyl, allyl or alkene Rh(iii) and Ir(iii) complexes as well as studies on the intramolecular reactions leading to transformation of one into another are reported. The silyl-hydrido-Rh(iii) complex {Rh(H)[SiMe(o-C6H4SMe)2](PPh3)}[BArF4], with a vacant coordination site, reacts with 1,5-cyclooctadiene (cod) leading to olefin insertion into the Rh-H bond and rearrangement to yield the 16e cyclooctenyl-Rh(iii) complex {Rh(η3-cyclooctenyl)[SiMe(o-C6H4SMe)2]}[BArF4] (1). This compound can be also synthesized by reaction of the 18e chloride precursor {Rh(η3-cyclooctenyl)[SiMe(o-C6H4SMe)2]Cl} with NaBArF4. The reaction of the thioether-silane SiMeH(o-C6H4SMe)2 with [Rh(nbd)Cl]2 (nbd = norbornadiene) leads to {Rh(σ-ntyl)[SiMe(o-C6H4SMe)2]Cl} (ntyl = nortricyclyl) (2). The abstraction of chloride from this neutral 16e ntyl-Rh(iii) complex with NaBArF4 results in the unusual isomerization of σ-nortricyclyl into σ,π-norbornenyl forming the 16e and cationic {Rh(σ,π-nbyl)[SiMe(o-C6H4SMe)2][BArF4] (nbyl = norbornenyl)} compound 3. Coordinatively saturated {Ir(η3-cyclooctenyl)[SiMe(o-C6H4SMe)2]Cl} (4) has been synthesized by the reaction of [Ir(cod)Cl]2 with SiMeH(o-C6H4SMe)2. The reaction of 4 with NaBArF4 led to the formation of the unsaturated and cationic Ir(iii) compound {Ir(η3-cyclooctenyl)[SiMe(o-C6H4SMe)2]}[BArF4] (5). Compound 5 shows low stability in solution and undergoes successive β-hydride elimination and olefin insertion steps, which were elucidated by DFT calculations, to form 18e {Ir(H)[SiMe(o-C6H4SMe)2](η4-cod)}[BArF4] (6).

Palladium-Catalyzed Intermolecular Oxidative Cyclization of Allyltosylamides with AcOH: Assembly of 3-Pyrrolin-2-ones.

The first example of Pd-catalyzed oxidative cyclization of allyltosylamides with acetic acid is reported. This transformation involved C-N/C-C bond formation and provided 3-pyrrolin-2-ones in a one-pot manner with easy-operation, excellent atom economy and good yields. Mechanistic studies indicate that the reaction proceeds through intermolecular aminopalladation, migratory insertion, reinsertion and β-hydride elimination processes.

Palladium-Catalyzed Oxidative Cross-Coupling of Conjugated Enynones with Organoboronic Acids.

A palladium-catalyzed oxidative cross-coupling reaction of conjugated enynones with organoboronic acids is developed. This reaction provides an efficient methodology for the synthesis of functionalized furan derivatives, including 2-alkenylfurans and furan-substituted 1,3-dienes. Palladium-carbene migratory insertion is proposed as the key step in these transformations. Notably, the β-hydride elimination process occurs in a stereoselective manner, resulting in the formation of double bonds with high (E)-selectivity.

Asymmetric Synthesis of Fluorinated Isoindolinones through Palladium-Catalyzed Carbonylative Amination of Enantioenriched Benzylic Carbamates.

The asymmetric synthesis of fluorinated isoindolinones has been achieved by a palladium-catalyzed aminocarbonylation reaction of the corresponding α-fluoroalkyl o-iodobenzylamines. A base-mediated anti β-hydride elimination process was suggested to explain the partial erosion of the optical purity observed in some cases. This mechanistic rationale enabled the minimization of this partial racemization by fine-tuning the pKa of the base.

Mechanistic insights into iron catalyzed dehydrogenation of formic acid: β-hydride elimination vs. direct hydride transfer

Density functional theory calculations reveal a complete reaction mechanism with detailed energy profiles and transition state structures for the dehydrogenation of formic acid catalyzed by an iron complex, [P(CH2CH2PPh2)3FeH](+). In the cationic reaction pathway, a β-hydride elimination process is confirmed to be the rate-determining step in this catalytic reaction. A potential reaction pathway starting with a direct hydride transfer from HCOO(-) to Fe is found to be possible, but slightly less favorable than the catalytic cycle with a β-hydride elimination step.

Pd‐Catalyzed Borylative Polycyclization of Enediynes to Alkylboronates

Pd-catalyzed bicyclization of 1-ene-6,11-diynes with bis(pinacolato)diboron smoothly affords synthetically useful homoallylic alkylboronates under mild conditions, avoiding the use of highly nucleophilic or basic reagents. One C-B bond and two new C-C bonds are created, and two new stereogenic centers are stereospecifically formed in a single tandem operation. The obtained products suggest a cascade reaction in which the last step is the transmetalation of bis(pinacolato)diboron, this being faster than a possible β-hydride elimination process. The reaction is general and has been applied to enediynes with different substituents on the alkene unit, the bridging groups, and the alkyne moiety. The bicyclized alkylboronates thus obtained can be functionalized by oxidation to alcohols, formation of trifluoroborate salts, or by Suzuki coupling reactions.

Why Did Incorporation of Acrylonitrile to a Linear Polyethylene Become Possible? Comparison of Phosphine−Sulfonate Ligand with Diphosphine and Imine−Phenolate Ligands in the Pd-Catalyzed Ethylene/Acrylonitrile Copolymerization

Palladium-catalyzed coordination-insertion copolymerization of ethylene with acrylonitrile (AN) proceeded only by using phosphine-sulfonate (P-SO(3)) as a ligand among the neutral and anionic ligands we examined, those are phosphine-sulfonate (P-SO(3)), diphosphine (P-P), and imine-phenolate (N-O). In order to answer a question that is unique for P-SO(3), theoretical and experimental studies were carried out for the three catalyst systems. By comparing P-SO(3) and P-P, it was elucidated that (i) the π-acrylonitrile complex [(L-L')PdPr(π-AN)] is less stable than the corresponding σ-complex [(L-L')PdPr(σ-AN)] in both the phosphine-sulfonato complex (L-L' = P-SO(3)) and the diphosphine complex (L-L' = P-P) and (ii) the energetic difference between the π-complex and the σ-complex is smaller in the P-SO(3) complexes than in the P-P complexes. Thus, the energies of the transition states for both AN insertion and its subsequent ethylene insertion relative to the most stable species [(L-L')PdPr(σ-AN)] are lower for P-SO(3) than for P-P. The results nicely explain the difference between these two types of ligands. That is, ethylene insertion subsequent to AN insertion was detected for P-SO(3), while aggregate formation was reported for cationic [(L-L)Pd(CHCNCH(2)CH(3))] complex. Aggregate formation with the cationic complex can be considered as a result of the retarded ethylene insertion to [(L-L)Pd(CHCNCH(2)CH(3))]. In contrast, theoretical comparison between P-SO(3) and N-O did not show a significant energetic difference in both AN insertion and its subsequent ethylene insertion, implying that ethylene/AN copolymerization might be possible. However, our experiment using [(N-O)PdMe(lutidine)] complex revealed that β-hydride elimination terminated the ethylene oligomerization and, more importantly, that the resulting Pd-H species lead to formation of free N-OH and Pd(0) particles. The β-hydride elimination process was further studied theoretically to clarify the difference between the two anionic ligands, P-SO(3) and N-O.

DFT studies of reductive elimination, C–H activation and β-hydride elimination in alkyl and aryl palladium amine complexes

The factors affecting C–N bond formation via reductive elimination from Pd(I t Bu)(neopentyl)(morpholide) (I t Bu = 1,3-Di-tert-butyl-imidazol-2-ylidene) are studied computationally. DFT calculations indicate that choosing an alkyl group without β hydrides, such as neopentyl, has a detrimental effect on the possibility of C–N reductive elimination. In the absence of β hydride elimination, a pathway of lower energy than reductive elimination is found, namely morpholide-promoted C–H activation of neopentyl t Bu has a significantly lower activation energy than reductive elimination. Changing the ancillary ligand from I t Bu to tricyclopentylphosphine (P(Cyp)3) has little impact. By contrast, replacing neopentyl by phenyl leads to a c. 50% reduction in activation energy. Study of Pd(I t Bu)(2-methylpropyl)(morpholide) permits comparison of the potential energy surfaces for three possible processes; (1) reductive elimination (2) morpholide-promoted C–H activation and (3) β-hydride elimination and reveals that the activation energies for these processes increase in the order of (3) < (2) < (1). Reductive elimination, C–H activation and β-hydride elimination processes in alkyl and aryl palladium amine complexes are studied computationally, in order to establish why reductive elimination in the alkyl complexes is so elusive.

HREELS study of the adsorption and evolution of diethylamine (DEA) on Si(1 0 0) surfaces

The adsorption of diethylamine (DEA) on Si(1 0 0) at 100 K was investigated using high-resolution electron energy loss spectroscopy (HREELS) and electron stimulated desorption (ESD). The thermal evolution of DEA on Si(1 0 0) was studied using temperature programmed desorption (TPD). Our results demonstrate DEA bonds datively to the Si(1 0 0) surface with no dissociation at 100 K. Thermal desorption of DEA takes place via a β-hydride elimination process leaving virtually no carbon behind. Electronic processing of DEA/Si(1 0 0) at 100 K results in desorption of ethyl groups; however, carbon and nitrogen are deposited on the surface as a result of electron irradiation. Thermal removal of carbon and nitrogen was not possible, indicating the formation of silicon carbide and silicon nitride.

Evidence of an electron-induced channel for desorption of ethylene from diethylsilane-covered Si(100)

The effects of electron irradiation on the diethylsilane (DES) dosed Si(100) surface were studied using temperature programmed desorption and high-resolution electron energy loss spectroscopy (HREELS). Previous research has shown that without electron irradiation, carbon is thermally removed from the DES/Si(100) surface via a β-hydride elimination process that is characterized by desorption of ethylene at 725 K followed by desorption of hydrogen at 810 K. After irradiating the surface with electrons, HREELS data showed that the electrons dissociated ethyl groups and deposited CH x groups on the surface. Furthermore, electron irradiation of DES/Si(100) resulted in thermal desorption of ethylene at 810 K, with only trace amounts desorbing at 725 K. Hydrogen desorbs at 810 K but also exhibits a high temperature shoulder due to desorption of hydrogen from surface carbon deposited by electrons. This is indicative of an unexplored electron-induced channel for desorption of ethylene from DES/Si(100).

Surface reactions of 2-iodopropane on GaAs(1 0 0)

The surface reactions of 2-iodopropane ((CH 3) 2CHI) on gallium-rich GaAs(1 0 0)-(4 × 1), was studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). CH 3CHICH 3 adsorbs molecularly at 120 K but dissociates below room temperature to form chemisorbed 2-propyl ((CH 3) 2CH ) and iodide (I ) species. Thermal activation causes desorption of the molecular species at 240 K, and this occurs in competition with the further reactions of the (CH 3) 2CH and I chemisorbed species. Self-coupling of the (CH 3) 2CH results in the formation of 2,3-dimethylbutane ((CH 3) 2CH–CH(CH 3) 2) at 290 K. β-Hydride elimination in (CH 3) 2CH yields gaseous propene (CH 3CH CH 2) at 550 K while reductive elimination reactions of (CH 3) 2CH with surface hydrogen yields propane (CH 3CH 2CH 3) at 560 K. Recombinative desorption of the adsorbed hydrogen as H 2 also occurs at 560 K. We observe that the activation barrier to carbon–carbon bond formation with 2-propyls on GaAs(1 0 0) is much lower than that in our previous investigations involving ethyl and 1,1,1-trifluoroethyl species where the β-elimination process was more facile. The difference in the surface chemistry in the case of 2-propyl species is attributable to its rigid structure resulting from the bonding to the surface via the second carbon atom, which causes the methyl groups to be further away from the surface than in the case of linear ethyl and 1,1,1-trifluoroethyl species. The β-hydride and reductive elimination processes in the adsorbed 2-propyl species thus occurs at higher temperatures, and a consequence of this is that GaI desorption, which is expected to occur in the temperature range 550–560 K becomes suppressed, and the chemisorbed iodine leaves the surface as atomic iodine.

Adsorption and decomposition of triethylsilane on Si(1 0 0)

The adsorption and decomposition of triethylsilane (TES) on Si(1 0 0) were studied using temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption (ESD), and X-ray photoelectron spectroscopy (XPS). TPD and HREELS data indicate that carbon is thermally removed from the TES-dosed Si(1 0 0) surface via a β-hydride elimination process. At high exposures, TPD data shows the presence of physisorbed TES on the surface. These species are characterized by desorption of TES fragments at 160 K. Non-thermal decomposition of TES was studied at 100 K by irradiating the surface with 600 eV electrons. ESD of mass 27 strongly suggests that a β-hydride elimination process is a channel for non-thermal desorption of ethylene. TPD data indicated that electron irradiation of physisorbed TES species resulted in decomposition of the parent molecule and deposition of methyl groups on the surface that desorbed thermally at about 900 K. Without electron irradiation, mass 15 was not detected in the TPD spectra, indicating that the production of methyl groups in the TPD spectra was a direct result of electron irradiation. XPS data also showed that following electron irradiation of TES adsorbed on Si(1 0 0), carbon was deposited on the surface and could not be removed thermally.