Bond Adduct Research Articles

Polar X-H Bond (X=O, S, N) Activation at a Cage Silanide.

Low-valent silicon compounds such as neutral silylenes display versatile reactivity for the activation of small molecules. In contrast, their anionic congeners silanides ([R3 Si- ]) have primarily been investigated for their nucleophilic reactivity. Here we show that incorporating a silanide center in a bicyclic cage structure allows for formal oxidative addition of polar element-hydrogen bonds (RX-H, R=aromatic residue, X=O, S, NH). The resulting hydrosilicates were isolated and characterized structurally and spectroscopically. Density Functional Theory (DFT) calculations and experimental observations support an ionic mechanism for RX-H bond activation. Finally, the reactivity of the RS-H bond adduct was further investigated, revealing that it behaves as a Lewis pair upon facile heterolytic cleavage of the Si-S bond.

Polar X–H Bond (X = O, S, N) Activation at a Cage Silanide

Low‐valent silicon compounds such as neutral silylenes display versatile reactivity for the activation of small molecules. In contrast, their anionic congeners silanides ([R3Si–]) have primarily been investigated for their nucleophilic reactivity. Here we show that incorporating a silanide center in a bicyclic cage structure allows for formal oxidative addition of polar element‐hydrogen bonds (RX–H, R = aromatic residue, X = O, S, NH). The resulting hydrosilicates were isolated and characterized structurally and spectroscopically. Density Functional Theory (DFT) calculations and experimental observations support an ionic mechanism for RX–H bond activation. Finally, the reactivity of the RS–H bond adduct was further investigated, revealing that it behaves as a Lewis pair upon facile heterolytic cleavage of the Si–S bond.

Computationally rational design of metal-involving halogen bonds with π-covalency: Structures and bonding analysis.

Traditional π-covalent interactions have been proved in the non-metal halogen bond adducts formed by chloride and halogenated triphenylamine-based radical cations. In this study, we have rationally designed two metal-involving halogen bond adducts with π-covalency property, such as [L1-Pd···I-PTZ]+ (i.e., 1) and [L2-Pd···I-PTZ]+ (i.e., 2), in which the square-planar palladium complexes serve as halogen bond acceptor and 3,7-diiodo-10H-phenothiazine radical cation (i.e., [I-PTZ]•+ ) acts as halogen bond donor. Noncovalent interaction analysis and quantum theory of atoms in molecules analysis revealed that there are notable halogen bond interactions along the Pd···I direction without genuine chemical bond formed in both designed adducts. Energy decomposition analysis together with natural orbital for chemical valence calculations were performed to gain insight into their bonding nature, which demonstrated the presence of remarkable π-covalent interactions and σ-covalent interactions in both 1 and 2. We therefore proposed a new strategy for building the metal-involving halogen bonds with π-covalency property, which will help the further development of new types of halogen bonds.

Nature of halogen bond adducts of carbones with XCF3 (X = Cl, Br, I) species

Quantum chemical calculations have been utilized to study the covalent character of halogen bonding in addition complexes of trifluorohalomethanes XCF3 (X = Cl, Br, I) with well-studied electron-pair-donor divalent C(0) compounds C(NHC)2, NHC-C-CO, NHC-C-PH3 and PPh3-C-CO [NHC = N-heterocyclic carbene, (CHNH)2C]. The geometries of the complexes in which the dicoordinated carbon molecules bind as ligands to one and two XCF3 moieties, respectively, have been optimized to analyze the strength and nature of bond formation using energy decomposition analysis and natural bond orbital calculations. Addition of one XCF3 to the carbones occurs through the lone pair having σ orbital character in the plane of the L→C←Lʹ bond, while the addition of a subsequent XCF3 leads to the carbone acquiring a tetrahedral geometry. The bond dissociation energy (BDE) for removing ICF3 from one of the carbone adducts is higher than for ClCF3 or BrCF3, while the BDE for the second XCF3 is clearly less than for the first dissociation. Even though electrostatic interactions are known to dominate in halogen bonding, we find considerable contributions from orbital and dispersion interactions.

Photoexcited Pd(ii) auxiliaries enable light-induced control in C(sp3)-H bond functionalisation.

Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C–H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd(iii)/Pd(iv) redox couples. The Pd(ii) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp3)–H halogenation products rather than C–C bond adducts. Online photochemical ESI-MS analysis implicates participation of a mononuclear Pd(iii) species which promotes C–X bond formation via a distinct Pd(iii)/Pd(iv) pathway. To demonstrate the synthetic utility, we developed a general method for inert C(sp3)–H bond bromination, chlorination and iodination with alkyl halides. This new strategy in auxiliary-directed C–H activation provides predictable and controllable access to distinct reactivity pathways proceeding via Pd(iii)/Pd(iv) redox couples induced by visible light irradiation.

Optimizing the yield of transient mono-dimethyl disulfide adducts for elucidating double bond positions of long chain alkenones

Long chain alkenones (LCAs) are among the most successful biomarkers for paleotemperature reconstructions. However, fundamental questions regarding the biosynthesis and cellular functions of alkenones in haptophyte algae remain poorly understood. Recent discoveries of LCAs with double bond positions and chain lengths that differ from common structures further highlight the importance of continued research into structural variations of this important class of lipid biomarkers to improve LCA applications as temperature proxies. Double bond positions on alkyl chains can be effectively determined by preparing mono-double bond adducts with dimethyl disulfide (DMDS-1), and subsequent gas chromatography–mass spectrometry (GC–MS) analysis. However, previously published procedures for adduct preparation were originally designed for mono-unsaturated fatty acids, and generally produce low product yields when applied to alkenones. Here we demonstrate that the problem originates mainly from DMDS and alkenone overreaction at high temperatures for long time periods, and, secondarily, insufficient amount of iodine catalyst. The overreaction results in DMDS reacting with multiple double bonds, and the possible formation of intermolecular linkages, creating non-volatile products. These products are of little use for elucidating alkenone structures. We demonstrate that by reducing the reaction temperature and time, and by using an optimal amount of iodine, we can maximize the yield of transient DMDS-1 adducts for alkenone structure determination.

Catalyst-free aza-Michael addition of azole to β,γ-unsaturated-α-keto ester: an efficient access to C–N bond formation

An efficient aza-Michael addition of azoles to β,γ-unsaturated-α-keto esters under room temperature conditions has been developed. In this conjugate addition, no additional catalyst is employed. Azole reacts with β,γ-unsaturated-α-keto ester smoothly to afford new C–N bond adducts in good to excellent yields (up to 96%).

Catalyst-Free Efficient Aza-Michael Addition of Azoles to Nitroalkenes

An efficient aza-Michael addition of azole to nitroalkene has been developed. In this conjugate addition, no catalyst was employed and azole reacted with nitroolefin smoothly to afford new C-N bond adducts in good to excellent yields.

Adsorption mechanisms of isoxazole and oxazole on Si(100)-2 × 1 surface: Si–N dative bond addition vs. [4+2] cycloaddition

The surface reaction pathways of isoxazole and oxazole on Si(100)-2 × 1 surface were theoretically investigated. They both form a weakly bound Si-N dative bond adduct on Si(100)-2 × 1 surface. In the case of isoxazole, the barrierlessly formed Si-N adduct is the most important surface product, that cannot be easily converted into other species. On the other hand, a facile concerted [4+2](CC) cycloaddition without involving the initial Si-N dative bond adduct was also found in the case of oxazole adsorption. The existence of Diels-Alder reactions is attributed to the particular arrangement of the two heteroatoms of oxazole in such a way that the two Si-C σ-bonds can be formed in a [4+2] fashion. In short, the unique geometric arrangements and electronegativity of these similar heteroatomic molecules yielded distinctively different surface reaction characteristics.

Radicals Formed in N-Acetylproline by Electron Attachment: Electron Spin Resonance Spectroscopy and Computational Studies

In this study, the reactions of electrons with N-acetylproline are investigated by electron spin resonance (ESR) spectroscopy and density functional theory. Electrons are produced by γ irradiation or by photoionization of K(4)Fe(CN)(6) in neutral 7.5 M LiCl-D(2)O aqueous glasses at low temperatures with identical results. Electrons are found to add to both the peptide bond and the carboxyl group of the acetyl-proline moiety at 77 K. On annealing, both the electron adducts undergo fragmentation of the peptide bond between the nitrogen and the α carbon of the peptide structure. However, the peptide bond electron adduct radical reacts much more rapidly than the carboxyl group electron adduct radical. The DFT calculations predict that the carboxyl adduct is substantially more stable than the peptide bond adduct, with the activation barrier to N-Cα cleavage 3.7 kcal/mol for the amide electron adducts and 23 kcal/mol for the carboxyl electron adducts in inagreement with the relative reactivity found by experiment.

Ground state isomerism in betacarboline hydrogen bond complexes: The charge transfer nature of its large Stokes shifted emission

The hydrogen bonding and excited state proton transfer reactions between betacarboline, 9 H-pyrido[3,4- b]indole, BC, and 1,1,1,3,3,3-hexafluoropropan-2-ol, HFIP, have been studied in the aprotic solvents cyclohexane and toluene by absorption, steady state and time resolved fluorescence measurements. On the basis of these results and those of previous works (Refs. [A. Sánchez-Coronilla, C. Carmona, M.A. Muñoz, M. Balón, Chem. Phys., 327 (2006) 70] and [A. Sánchez-Coronilla, M. Balón, M.A. Muñoz, C. Carmona, Chem. Phys. 344 (2008) 72]) two main fundamental conclusions can be drawn on the photophysical behaviour of BC. Thus, it is shown, for the first time, that the non-cyclic double hydrogen bond complexes formed through both nitrogen atoms of BC, DHB, can suffer, in their ground state, an isomerisation process. These adducts acquire a quinoid structure in cyclohexane, but maintain a dipolar zwitterionic structure in toluene. Moreover, it is concluded that the observed large Stokes shifted emission, around 520 nm, is not due, as it has been so far generally accepted, to the emission of a BC zwitterionic phototautomer, but to the intramolecular charge transfer, ICT, excited state emissions of the DHB hydrogen bond adducts.

Influence of hydrogen bonding in the ground and the excited states of the isomers of the β-carboline anhydrobase (N 2-methyl-9H-pyrido[3,4-b]indole) in aprotic solvents

The ground and excited state hydrogen bonding interactions between N 2-methyl-9H-pyrido[3,4-b]indole, BCA, and 1,1,1,3,3,3-hexafluoropropan-2-ol, HFIP, are comparatively studied in the aprotic solvents cyclohexane and toluene by absorption, steady state and time resolved fluorescence measurements. The different photophysical behaviours of the BCA–HFIP hydrogen bond complexes in these solvents definitively confirm the existence of two ground state BCA isomers. As previously proposed [A. Sánchez-Coronilla, C. Carmona, M.A. Muñoz, M. Balón, Chem. Phys. 327 (2006) 70.] we assume quinoid, Q, and zwitterionic, Z, structures for these isomers. Upon excitation, the hydrogen bond adducts of each isomer give dual fluorescence emitting from their locally excited states, LE, and from their intramolecular charge transfer states, ICT. In the hydrogen bond adducts of the Q form, the ICT process is favoured while it is disfavoured for the corresponding adducts of the Z form. The implication that these results could have on the current mechanistic interpretation of the excited state intramolecular proton transfer and phototautomerism of the betacarbolines is discussed.

Comparative theoretical study of Mu addition to the C=O and C=S bonds

Conventional wisdom regarding the hyperfine properties of H addition products to the C=S bond is largely inferred from a knowledge base pertaining to C=O, few actual calculations or experiments having been carried out on C=S adducts. Here for the first time the energetics, optimum geometries, and hyperfine properties of radicals formed by hydrogen atom addition to several corresponding pairs of C=O and C=S bond-containing molecules are calculated using hybrid density functional methods. The results clarify the reasons for what appear to be persistent experimental misassignments in μSR studies of C=S bond adducts.

Synthesis, Characterization, and Interconversion of the Rhenium Polyhydrides [ReH3(η4-NP3)] and [ReH4(η4-NP3)]+ {NP3 = tris[2-(diphenylphosphanyl)ethyl]amine}

The rhenium(III) dichloride complex [ReCl2(η4-NP3)]Cl (1) was prepared from [ReCl3(CH3CN)(PPh3)2] by treatment with the tripodal tetradentate ligand N(CH2CH2PPh2)3 (NP3) in ethanol. The reaction of 1 with LiAlH4 in THF gave the rhenium(III) trihydride [ReH3(η4-NP3)] (2), which was converted into the rhenium(V) tetrahydride [ReH4(η4-NP3)]BPh4 (3) by protonation in CH2Cl2 with HBF4·OMe2, followed by a metathetical reaction with NaBPh4. The classical polyhydride nature of 2 and 3, as well as the overall molecular structures in solution, were determined by NMR spectroscopy, 1H NMR relaxation, and IR spectroscopy. The polyhydride complexes 2 and 3 are stereochemically nonrigid in solution, and the thermodynamic parameters associated with the fluxional processes were determined by variable-temperature NMR studies. A single-crystal X-ray analysis of 3 has shown the complex cation [ReH4(η4-NP3)]+ to be eight-coordinated by the four donor atoms of NP3 and by four terminal hydride ligands in a distorted dodecahedral geometry. An in situ IR study in CH2Cl2 has shown that the protonation of 3 occurs regioselectively at the metal center with no formation of a dihydrogen complex. Kinetic hydrogen bond products of the formula [(η4-NP3)H3Re···HOR] (ROH = C2H5OH, CFH2CH2OH, CF3CH2OH) were intercepted by IR spectroscopy at low temperature. The thermodynamic parameters associated with the formation of the hydrogen bond adducts were determined by either IR spectroscopy applying the Iogansen equation or van’t Hoff plots of the formation constant vs. temperature.

Synthesis, Characterization, and Interconversion of the Rhenium Polyhydrides [ReH3(η4-NP3)] and [ReH4(η4-NP3)]+ {NP3 = tris[2-(diphenylphosphanyl)ethyl]amine}

The rhenium(III) dichloride complex [ReCl2(η4-NP3)]Cl (1) was prepared from [ReCl3(CH3CN)(PPh3)2] by treatment with the tripodal tetradentate ligand N(CH2CH2PPh2)3 (NP3) in ethanol. The reaction of 1 with LiAIH4 in THF gave the rhenium(III) trihydride [ReH3(η4-NP3)] (2), which was converted into the rhenium(V) tetrahydride [ReH4(η4-NP3)]BPha4 (3) by protonation in CH2Cl2 with HBF4·OMe2, followed by a metathetical reaction with NaBPh4. The classical polyhydride nature of 2 and 3, as well as the overall molecular structures in solution, were determined by NMR spectroscopy, 1H NMR relaxation, and IR spectroscopy. The polyhydride complexes 2 and 3 are stereochemically nonrigid in solution, and the thermodynamic parameters associated with the fluxional processes were determined by variable-temperature NMR studies. A single-crystal X-ray analysis of 3 has shown the complex cation [ReH4 (qη4-NP3)]+ to be eight- coordinated by the four donor atoms of NP3 and by four terminal hydride ligands in a distorted dodecahedral geometry. An in situ IR study in CH2Cl2 has shown that the protonation of 3 occurs regioselectively at the metal center with no formation of a dihydrogen complex. Kinetic hydrogen bond products of the formula [(qη4NP3)H3Re··· HOR] (ROH = C2H5OH, CFH2CH2OH, CF3CH2OH) were intercepted by IR spectroscopy at low temperature. The thermodynamic parameters associated with the formation of the hydrogen bond adducts were determined by either IR spectroscopy applying the logansen equation or van't Hoff plots of the formation constant vs. temperature.

The CH ⋯ O hydrogen bond adduct of two trinitromethanes to dioxane

Recrystallization of (O2N)3CH from dioxane yields the triple aggregate (O2N)3CH ⋯ O(CH2CH2)2O ⋯ HC(NO2)3 with the shortest C(H)⋯ O distances reported so far of 293.7(1) pm.

Oxidation of polyunsaturated fatty acids and lipids through thiyl and sulfonyl radicals: Reaction kinetics, and influence of oxygen and structure of thiyl radicals

Thiyl free radicals have been shown to react with polyunsaturated fatty acids via abstraction of bisallylic hydrogen, forming pentadienyl radicals, and via addition to the double bonds. In the absence of oxygen, the latter pathway leads to regeneration of thiyl radicals through β-elimination or “repair” of the adduct radicals by thiols. In the presence of oxygen, fixation of thiyl-induced damage occurs through reaction of O 2 with the pentadienyl radical (yielding conjugated dienyl peroxyl radicals) and also with the thiyl-to-double bond adduct radical. A quantitative reaction scheme evaluated from these data considers abstraction, addition, rearrangement, and repair reactions, and the evaluation of rate constants for the individual steps. Absolute rate constants have been measured, in particular, for reactions of thiyl free radicals from glutathione, cysteine, homocysteine, N-acetylcysteine, cysteine ethyl ester, penicillamine, captopril, mercaptoethanol, and dithiothreitol with polyunsaturated fatty acids (PUFAs) ranging from 18:2 to 22:6, and the lipids trilinolein and trilinolenin. The rate constants for hydrogen abstraction were found to be typically of the order of 10 7 mol −1 dm 3 s −1 and to increase with increasing lipophilicity of the attacking thiyl radical. Thioperoxyl radicals, RSOO., were found to be rather unreactive toward PUFAs, in contrast to the isomer sulfonyl radicals, RSO 2., which not only abstract hydrogen from the bisallylic methylene groups of the PUFAs (although only at relatively small yield) but also readily add to the PUFA double bonds (major pathway). Specific information was obtained on the optical properties of the thiyl radical derived from the ACE inhibitor captopril, CpS. (λ max = 340 nm, ϵ = 460 ± 50 mol −1 dm 3 cm −1), and its conjugate disulfide radical anion (CpS∵SCp) − (λ max = 420 nm).

The interaction of dihydrofurylsilanes with dichlorocarbene under phase-transfer conditions

Dichlorocarbene generated in a two-phase catalytic system reacts with 3-trimethylsilyl-2,5-dihydrofuran to give both possible products of mono-insertion into the CH bond in positions 2 and 5 of the ring, together with a CC bond adduct. Reaction of 2-trimethylsilyl-4,5-dihydrofuran with dichlorocarbene leads to 2,3-dichloro-2-trimethylsilyl-5,6-dihydro-2 H-pyran, which is formed via isomerization of the primary bicyclic adduct.

Hindered internal rotation in carbamates: An NMR study of the conformations of alkyl and arylN‐(alkylsulfonylmethyl)‐N‐methylcarbamates and arylN‐(arylsulfonylmethyl)‐N‐methylcarbamates

AbstractDifferences in intramolecular shielding of the ester alkyl protons between rotamers in a series of alkylN‐(arylsulfonylmethyl)‐N‐methylcarbamates were used as an unambiguous tool for rotamer signal assignment. On the basis of these results, aromatic solvent‐induced shifts (Δv) and trifluoroacetic acid (TFA)‐induced shifts (Δv) were found to be applicable as criteria for signal assignment of carbamate rotamers. Using these criteria, the conformations of previously unreported alkyl and arylN‐(alkylsulfonylmethyl)‐N‐methylcarbamates and arylN‐(arylsulfonylmethyl)‐N‐methylcarbamates have been determined. The rotamer population (E/Zratio) is discussed in terms of the structures of these carbamates. The smaller Δvvalues of carbamates as compared with carboxamides are primarily the result of the lower ground state basicity of the carbamate function and of the presence of more than one site of comparable acceptor strength. This conclusion is supported by an IR spectroscopic study of hydrogen bond adduct formation between phenol and a series ofN, N‐disubstituted carbamates.

Sulfonation d'alcynes siliciés et synthèse d'acides sulfoniques α-acetylèniques

The sulfonation of α-silylalkynes R′-CCSiR 3 under standard conditions yields different unsaturated sulfonic compounds, depending on the starting materials: SO 3 triple bond adducts or silyl α-acetylenesulfonates R′CCSO 3SiR 3. Hydrolysis of these latter compounds gives, when R′ is a trialkylsilyl or tert-butyl group, the corresponding α-acetylenesulfonic acids, which as far as we know, have not yet been reported. A mechanism of these sulfonation reactions is proposed.