Diazaphosphinanes as hydride, hydrogen atom, proton or electron donors under transition-metal-free conditions: thermodynamics, kinetics, and synthetic applications.
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ABSTRACT:
SUBMITTER: Zhang J
PROVIDER: S-EPMC8152589 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Although the mechanism for the transformation of carbon dioxide to formate with copper hydride is well understood, it is not clear how formic acid is ultimately released. Herein, we show how formic acid is formed in the decomposition of the copper formate clusters Cu(II)(HCOO)3 - and Cu(II)2 (HCOO)5 - . Infrared irradiation resonant with the antisymmetric C-O stretching mode activates the cluster, resulting in the release of formic acid and carbon dioxide. For the binary cluster, electronic structure calculations indicate that CO2 is eliminated first, through hydride transfer from formate to copper. Formic acid is released via proton-coupled electron transfer (PCET) to a second formate ligand, evidenced by close to zero partial charge and spin density at the hydrogen atom in the transition state. Concomitantly, the two copper centers are reduced from Cu(II) to Cu(I). Depending on the detailed situation, either PCET or hydrogen atom transfer (HAT) takes place.
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Project description:Redox events in which an electron and proton are exchanged in a concerted elementary step are commonly referred to as proton-coupled electron transfers (PCETs). PCETs are known to operate in numerous important biological redox processes, as well as recent inorganic technologies for small molecule activation. These studies suggest that PCET catalysis might also function as a general mode of substrate activation in organic synthesis. Over the past three years, our group has worked to advance this hypothesis and to demonstrate the synthetic utility of PCET through the development of novel catalytic radical chemistries. The central aim of these efforts has been to demonstrate the ability of PCET to homolytically activate a wide variety of common organic functional groups that are energetically inaccessible using known molecular H atom transfer catalysts. To do so, we made use of a simple formalism first introduced by Mayer and co-workers that allowed us to predict the thermodynamic capacity of any oxidant/base or reductant/acid pair to formally add or remove H· from a given substrate. With this insight, we were able to rationally select catalyst combinations thermodynamically competent to homolyze the extraordinarily strong E-H σ-bonds found in many common protic functional groups (BDFEs > 100 kcal/mol) or to form unusually weak bonds to hydrogen via the reductive action of common organic π-systems (BDFEs < 35 kcal/mol). These ideas were reduced to practice through the development of new catalyst systems for reductive PCET activations of ketones and oxidative PCET activation of amide N-H bonds to directly furnish reactive ketyl and amidyl radicals, respectively. In both systems, the reaction outcomes were found to be successfully predicted using the effective bond strength formalism, suggesting that these simple thermochemical considerations can provide useful and actionable insights into PCET reaction design. The ability of PCET catalysis to control enantioselectivity in free radical processes has also been established. Specifically, multisite PCET requires the formation of a pre-equilibrium hydrogen bond between the substrate and a proton donor/acceptor prior to charge transfer. We recognized that these H-bond interfaces persist following the PCET event, resulting in the formation of noncovalent complexes of the nascent radical intermediates. When chiral proton donors/acceptors are employed, this association can provide a basis for asymmetric induction in subsequent bond-forming steps. We discuss our efforts to capitalize on this understanding via the development of a catalytic protocol for enantioselective aza-pinacol cyclizations. Lastly, we highlight an alternative PCET mechanism that exploits the ability of redox-active metals to homolytically weaken the bonds in coordinated ligands, enabling nominally strong bonds (BDFEs ∼ 100 kcal) to be abstracted by weak H atom acceptors with concomitant oxidation of the metal center. This "soft homolysis" mechanism enables the generation of metalated intermediates from protic substrates under completely neutral conditions. The first example of this form of catalysis is presented in the context of a catalytic C-N bond forming reaction jointly mediated by bulky titanocene complexes and the stable nitroxyl radical TEMPO.
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Project description:In a possibly biomimetic fashion, formally copper(III)-oxygen complexes LCu(III)-OH (1) and LCu(III)-OOCm (2) (L2- = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Cm = α,α-dimethylbenzyl) have been shown to activate X-H bonds (X = C, O). Herein, we demonstrate similar X-H bond activation by a formally Cu(III) complex supported by the same dicarboxamido ligand, LCu(III)-O2CAr1 (3, Ar1 = meta-chlorophenyl), and we compare its reactivity to that of 1 and 2. Kinetic measurements revealed a second order reaction with distinct differences in the rates: 1 reacts the fastest in the presence of O-H or C-H based substrates, followed by 3, which is followed by (unreactive) 2. The difference in reactivity is attributed to both a varying oxidizing ability of the studied complexes and to a variation in X-H bond functionalization mechanisms, which in these cases are characterized as either a hydrogen-atom transfer (HAT) or a concerted proton-coupled electron transfer (cPCET). Select theoretical tools have been employed to distinguish these two cases, both of which generally focus on whether the electron (e-) and proton (H+) travel "together" as a true H atom, (HAT), or whether the H+ and e- are transferred in concert, but travel between different donor/acceptor centers (cPCET). In this work, we reveal that both mechanisms are active for X-H bond activation by 1-3, with interesting variations as a function of substrate and copper functionality.
| S-EPMC7197025 | biostudies-literature
Project description:Solar energy can be converted into chemical energy by photocatalytic water splitting to produce molecular hydrogen. Details of the photo-induced reaction mechanism occurring on the surface of a semiconductor are not fully understood, however. Herein, we employ a model photocatalytic system consisting of single atoms deposited on quantum dots that are anchored on to a primary photocatalyst to explore fundamental aspects of photolytic hydrogen generation. Single platinum atoms (Pt1) are anchored onto carbon nitride quantum dots (CNQDs), which are loaded onto graphitic carbon nitride nanosheets (CNS), forming a Pt1@CNQDs/CNS composite. Pt1@CNQDs/CNS provides a well-defined photocatalytic system in which the electron and proton transfer processes that lead to the formation of hydrogen gas can be investigated. Results suggest that hydrogen bonding between hydrophilic surface groups of the CNQDs and interfacial water molecules facilitates both proton-assisted electron transfer and sorption/desorption pathways. Surface bound hydrogen atoms appear to diffuse from CNQDs surface sites to the deposited Pt1 catalytic sites leading to higher hydrogen-atom fugacity surrounding each isolated Pt1 site. We identify a pathway that allows for hydrogen-atom recombination into molecular hydrogen and eventually to hydrogen bubble evolution.
| S-EPMC7505813 | biostudies-literature
Project description:The factors that control the reactivities of aryl radicals toward hydrogen-atom donors were studied by using a dual-cell Fourier-transform ion cyclotron resonance mass spectrometer. Hydrogen-atom abstraction reaction efficiencies for two substrates, cyclohexane and isopropyl alcohol, were measured for 23 structurally different, positively charged aryl radicals, which included dehydrobenzenes, dehydronaphthalenes, dehydropyridines, and dehydro(iso)quinolines. A logarithmic correlation was found between the hydrogen-atom abstraction reaction efficiencies and the (calculated) vertical electron affinities (EA) of the aryl radicals. Transition state energies calculated for the reaction of three of the aryl radicals with isopropyl alcohol were found to correlate linearly with their (calculated) EAs. No correlation was found between the hydrogen-atom abstraction reaction efficiencies and the (calculated) enthalpy changes for the reactions. Measurement of the reaction efficiencies for the reactions of 15 different hydrogen-atom donors with two selected aryl radicals revealed a logarithmic correlation between the hydrogen-atom abstraction reaction efficiencies and the vertical ionization energies (IE) of the hydrogen-atom donors, but not the lowest homolytic X-H (X = heavy atom) bond dissociation energies of the hydrogen-atom donors. Examination of the hydrogen-atom abstraction reactions of 29 different aryl radicals and 18 different hydrogen-atom donors showed that the reaction efficiency increases (logarithmically) as the difference between the IE of the hydrogen-atom donor and the EA of the aryl radical decreases. This dependence is likely to result from the increasing polarization, and concomitant stabilization, of the transition state. Thus, the hydrogen-atom abstraction reaction efficiency for an aryl radical can be "tuned" by structural changes that influence either the vertical EA of the aryl radical or the vertical IE of the hydrogen atom donor.
| S-EPMC2682590 | biostudies-literature
Project description:The rate constants for typical concerted proton-coupled electron transfer (PCET) reactions depend on the vibronic coupling between the diabatic reactant and product states. The form of the vibronic coupling is different for electronically adiabatic and nonadiabatic reactions, which are associated with hydrogen atom transfer (HAT) and electron-proton transfer (EPT) mechanisms, respectively. Most PCET rate constant expressions rely on the Condon approximation, which assumes that the vibronic coupling is independent of the nuclear coordinates of the solute and the solvent or protein. Herein we test the Condon approximation for PCET vibronic couplings. The dependence of the vibronic coupling on molecular geometry is investigated for an open and a stacked transition state geometry of the phenoxyl-phenol self-exchange reaction. The calculations indicate that the open geometry is electronically nonadiabatic, corresponding to an EPT mechanism that involves significant electronic charge redistribution, while the stacked geometry is predominantly electronically adiabatic, corresponding primarily to an HAT mechanism. Consequently, a single molecular system can exhibit both HAT and EPT character. The dependence of the vibronic coupling on the solvent or protein configuration is examined for the soybean lipoxygenase enzyme. The calculations indicate that this PCET reaction is electronically nonadiabatic with a vibronic coupling that does not depend significantly on the protein environment. Thus, the Condon approximation is shown to be valid for the solvent and protein nuclear coordinates but invalid for the solute nuclear coordinates in certain PCET systems. These results have significant implications for the calculation of rate constants, as well as mechanistic interpretations, of PCET reactions.
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Project description:Many photoactive metal complexes can act as electron donors or acceptors upon photoexcitation, but hydrogen atom transfer (HAT) reactivity is rare. We discovered that a typical representative of a widely used class of iridium hydride complexes acts as an H-atom donor to unactivated olefins upon irradiation at 470 nm in the presence of tertiary alkyl amines as sacrificial electron and proton sources. The catalytic hydrogenation of simple olefins served as a test ground to establish this new photo-reactivity of iridium hydrides. Substrates that are very difficult to activate by photoinduced electron transfer were readily hydrogenated, and structure-reactivity relationships established with 12 different olefins are in line with typical HAT reactivity, reflecting the relative stabilities of radical intermediates formed by HAT. Radical clock, H/D isotope labeling, and transient absorption experiments provide further mechanistic insight and corroborate the interpretation of the overall reactivity in terms of photo-triggered hydrogen atom transfer (photo-HAT). The catalytically active species is identified as an Ir(ii) hydride with an IrII-H bond dissociation free energy around 44 kcal mol-1, which is formed after reductive 3MLCT excited-state quenching of the corresponding Ir(iii) hydride, i.e. the actual HAT step occurs on the ground-state potential energy surface. The photo-HAT reactivity presented here represents a conceptually novel approach to photocatalysis with metal complexes, which is fundamentally different from the many prior studies relying on photoinduced electron transfer.
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Project description:Hydropersulfides (RSSH) are highly reactive as nucleophiles and hydrogen atom transfer reagents. These chemical properties are believed to be key for them to act as antioxidants in cells. The reaction involving the radical species and the disulfide bond (S-S) in RSSH, a known redox-active group, however, has been scarcely studied, resulting in an incomplete understanding of the chemical nature of RSSH. We have performed a high-level theoretical investigation on the reactions of the hydroxyl radical (?OH) toward a set of RSSH (R = -H, -CH3, -NH2, -C(O)OH, -CN, and -NO2). The results show that S-S cleavage and H-atom abstraction are the two competing channels. The electron inductive effect of R induces selective ?OH substitution at one sulfur atom upon S-S cleavage, forming RSOH and ?SH for the electron donating groups (EDGs), whereas producing HSOH and ?SR for the electron withdrawing groups (EWGs). The H-Atom abstraction by ?OH follows a classical hydrogen atom transfer (hat) mechanism, producing RSS? and H2O. Surprisingly, a proton-coupled electron transfer (pcet) process also occurs for R being an EDG. Although for RSSH having EWGs hat is the leading channel, S-S cleavage can be competitive or even dominant for the EDGs. The overall reactivity of RSSH toward ?OH attack is greatly enhanced with the presence of an EDG, with CH3SSH being the most reactive species found in this study (overall rate constant: 4.55 × 1012 M-1 s-1). Our results highlight the complexity in RSSH reaction chemistry, the extent of which is closely modulated by the inductive effect of the substituents in the case of the oxidation by hydroxyl radicals.
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Project description:Hydrogen atom abstraction from propargyl C-H sites of alkynes plays a critical role in determining the reactivity of alkyne molecules and understanding the formation of soot precursors. This work reports a systematic theoretical study on the reaction mechanisms and rate constants for hydrogen abstraction reactions by hydrogen and hydroxy radicals from a series of alkyne molecules with different structural propargyl C-H atoms. Geometry optimizations and frequency calculations for all species are performed at M06-2X/cc-pVTZ level of theory and the hindered internal rotations are also treated at this level. The high-level W1BD and CCSD(T)/CBS theoretical calculations are used as a benchmark for a series of DFT calculations toward the selection of accurate DFT functionals for large reaction systems in this work. Based on the quantum chemistry calculations, rate constants are computed using the canonical transition state theory with tunneling correction and the treatment of internal rotations. The effects of the structure and reaction site on the energy barriers and rate constants are examined systematically. To the best of our knowledge, this work provides the first systematic study for one of the key initiation abstraction reactions for compounds containing propargyl hydrogen atoms.
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Project description:Kinetic analysis of the host guest complexation of a large, open molecular basket and a highly complementary adamantoid guest reveals that for these types of systems a dissociative mechanism is in operation. Hence, the resident adamantyl guest must completely vacate the cavity before another guest molecule can move in to replace it. As a result of the rigid nature of the host, the energy barrier to this process is relatively high, about 16 kcal mol(-1) at room temperature. Modifying the cavity of the host by dangling either a methyl group or a hydroxyl group from the portal rim alters the thermodynamic binding profile of these hosts. (1)H NMR shift data analysis also reveals that these functional groups can adjust the orientation that monosubstituted guests adopt within the cavity. Additionally, (1)H NMR studies of the binding of (E)1,4-dibromoadamantane allow the observation of two energetically similar diastereomeric complexes. An examination of this guest binding to the three hosts reveals that the interchange between the isomers is much faster than the entry and egression rates, and that the functional groups at the rim of each cavity influence both the rates of reorientation and the equilibrium relating the isomers.
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