Remote functional group directed C-H activation by an Ir(iii) phenanthroline complex.

The regioselectivity of the C-H activation of 1-butanol and 1-methoxybutane by an iridium(iii) phenanthroline complex was studied in the gas phase and revealed activation at gamma and delta carbons. In the ether, nearly exclusive gamma activation was observed. DFT calculations were used to explore the origin of this substrate-driven selectivity. The data show that the iridium(iii) complex is a potent and potentially highly selective remote C-H activation agent.

An Oxy-anion Accelerated [1,5]-o-Quinone Methide Shift During the Nucleophilic Epoxidation of Salicylfulvene.

A new facet of nucleophilic fulvene epoxidations has been uncovered. 6-Arylfulvenes containing an ortho or para hydroxyl group react with basic hydrogen peroxide in an unusual manner; the epoxidation of the fulvene exocyclic double bond is followed by a phenoxide ion initiated epoxide ring opening to form an o-quinone methide (o-QM) intermediate. The resulting cyclopentadienolate undergoes an unusual oxy-anion accelerated [1,5]-sigmatropic o-QM shift. Computational studies reveal that the activation energy for the [1,5]-QM-shift in the cyclopentadienolate intermediate is quite low, signifying the acceleration caused by the oxy-anion group. Placement of a second hydroxyl group in the 6-aryl ring at C5 epoxidation via electron donation to the o-QM carbon; instead, an intramolecular oxa-6-π-electrocyclization of the o-QM intermediate onto the cyclopentadiene is observed.

Singlet oxygenation of triquinacene, barrelene, and homobarrelene.

The singlet oxygenation of three polycyclic hydrocarbons, triquinacene, barrelene and homobarrelene was studied. Triquinacene reacted by way of a perepoxide intermediate, transferring an oxygen atom to another triquinacene molecule to give exclusively the mono epoxide. Barrelene, on the other hand, underwent a rare homo-Diels-Alder reaction with 1O2 to give the decomposition product from the initial tetracyclic 1,2-dioxolane leading to benzofuran. The latter reacted with 1O2 in a [2+2] cycloaddition to give an unstable 1,2-dioxetane which collapsed to 2-formylphenyl formate. The latter was independently synthesize via singlet oxygenation of authentic benzofuran. Homobarrelene reacted in a similar fashion to give a homoDiels product, decomposition of which led to a keto aldehyde which was characterized spectroscopically. Computational work confirms the barrelene and homobarrelene reactions with 1O2 as concerted [π2s+π2s+π2s] cycloadditions.

Gas-Phase Dehydrogenation of Alkanes: C-H Activation by a Graphene-Supported Nickel Single-Atom Catalyst Model.

A gas-phase anionic nickel(0) fluorenyl complex is shown to effect the dehydrogenation of linear, branched, and cyclic alkanes via C-H activation. It performs dehydrogenations via a C-H insertion followed by ß-hydride elimination. When given energy via collision-induced dissociation, the system is capable of second and third dehydrogenations to form dienes and aromatics such as benzene. Kinetic isotope effects and DFT calculations completed at the M06/6-311+G** level support the proposed mechanism. The metal complex can act as an experimental model for graphene-supported nickel single-atom catalysts and suggests that these catalysts are capable of alkane dehydrogenation via C-H activation.

Substitution Reactions on Iodine and Bromine: Mechanisms for Facile Halogenations of Heterocycles.

Gas-phase techniques were used to examine the halogenation of deprotonated heterocycles by perfluoroaryl and perfluoroalkyl halides. The results indicate that SN2@Br and SN2@I reactions can be very facile and are effective means of halogenating heterocycles. 2-Iodoheptafluoropropane is exceptionally selective for SN2@I reactions with yields upward of 90%. The results also provide evidence counter to the recent suggestion that t-butoxide-induced halogenations of heterocycles proceed via a radical mechanism.

Imidazolidin-4-ones via (3+2) cycloadditions of aza-oxyallyl cations onto (E)-Narylideneanilines.

In the course of our studies on the chemistry of oxyallyl species we uncovered a new (3+2) cycloaddition of aza-oxyallyl systems, generated in situ from N-benzyloxy-2-chloroamides in the presence of NEt3, onto N-arylimines yielding imidazolidin-4-ones in moderate to good yields. The cycloadditions are regioselective. Computational modelling using DFT at the M062×/6-311+G** level is in support the observed regioselectivities. Although the path to the trans imidazolin-4-one is favored, the cis product is preferred by almost 8 kcal/mol and could be formed by base-catalyzed epimerization. All products were isolated by chromatography and characterized by means of their FTIR, NMR and HRMS data.

Diverse modes of reactivity of 6-(chloromethyl)-6-methylfulvene.

The title compound exhibits a number of reactivities toward nucleophiles/bases owing to the presence of several electrophilic and potentially nucleophilic sites in the molecule. We explored the reactions of 6-(chloromethyl)-6-methylfulvene with oxygen- and nitrogen nucleophiles and bases as well as a carbon-based nucleophile (an enamine) and realized all possible reactivity modes predicted on the basis of electrophilic and nucleophilic positions in this compound.

Cyclopentadienones via a tandem C-cyclopropylnitrone cyclization-cycloreversion sequence.

Aldonitrones derived from spiro[2.4]hepta-4,6-diene-1-carbaldehyde and its benzo analog undergo a tandem uncatalyzed intramolecular cyclopropane-nitrone cyclization-5,6-dihydro-1,2-oxazine cycloreversion to give cyclopentadienones. Similarly, the NH-nitrone generated in situ from spiro[cyclopropane-1,1'-indene]carbaldehyde oxime leads to benzocyclopentadienone (1H-inden-1-one) by the same mechanism. DFT calculations are in favor of a concerted yet highly asynchronous pathway for the cyclizations. Control experiments with the dihydro and tetrahydro derivatives show that the spirocyclopentadiene unit is essential for the success of the reaction, invoking spiroconjugative effects for increased cyclopropane reactivity.

A Robust Analytical Approach for the Identification of Specific Protein Carbonylation Sites: Metal-Catalyzed Oxidations of Human Serum Albumin.

The formation of protein carbonyls in the metal-catalyzed oxidation of human serum albumin (HSA) is characterized using a new analytical approach that involves tagging the modification site with multiple hydrazide reagents. Protein carbonyl formation at lysine and arginine residues was catalyzed with copper and iron ions, and the resulting oxidation patterns in HSA are contrasted. A total of 18 modification sites were identified with iron ion catalysis and 14 with copper ion catalysis. However, with the more stringent requirement of identification with at least two tagging reagents, the number of validated modification sites drops to 10 for iron and 9 for copper. Of the 14 total validated sites, there were only five in common for the two metal ions. The results illustrate the value of using multiple tagging agents and highlight the selective and specific nature of metal-catalyzed protein oxidations.

Effect of Ring Size and Migratory Groups on [1,n] Suprafacial Shift Reactions. Confirmation of Aromatic and Antiaromatic Transition-State Character by Ring-Current Analysis.

Suprafacial sigmatropic shift reactions of 5-substituted cyclopentadienes, 3-substituted cyclopropenes, and 7-substituted cycloheptatrienes have been studied computationally at the MP2/6-31+G* level for structures and energetics and with the ipsocentric method at the CHF/6-31G** level to calculate current-density maps. The hydrogen shifts in cyclopentadienes have a diatropic ring current indicating aromatic, cyclopentadienide anion character. This result stands in contrast to the fluorine shift in 5-fluorocyclopentadiene which requires much more energy and has a paratropic ring current in the TS pointing to antiaromatic, cyclopentadienyl cation character. [1,3] hydrogen shifts in cyclopropenes are very difficult, passing through transition states that have an extended C-C bond. For 3-fluorocyclopropene, the [1,3] fluorine shift is much easier than the hydrogen shift. For 7-fluorocycloheptatriene, the [1,7] hydrogen shift is predicted but requires very high energy and has a paratropic ring current and antiaromatic character. The [1,7] suprafacial fluorine shift is relatively easy, having a TS with cycloheptatrienyl cation character. Patterns of currents, and the reversal for H and F migration, are rationalized by orbital analysis based on the ipsocentric method. Calculated charges and structural features for reactants and transition states support these conclusions.

Unusual hydroxyl effect on fulvene endoperoxide decompositions.

The thermal decomposition of fulvene endoperoxides ordinarily proceeds via an allene oxide intermediate affording oxepin-2(3H)-one derivatives. We have now uncovered new, unusual pathways in these decompositions where the presence of a hydroxyl group on the alkyl or aryl attached to the fulvene exocyclic double bond has a profound effect on the fate of the reactive intermediates derived from the unstable endoperoxides. Computational work supports the proposed mechanistic pathways.

A cleavable biotin tagging reagent that enables the enrichment and identification of carbonylation sites in proteins.

The utility of a new, cleavable tag for identifying and enriching protein carbonyls is examined. Using a model system, human serum albumin modified with acrolein, the EZ-Link alkoxyamine-PEG4-SS-PEG4-biotin affinity tag, was tested for its ability to label protein carbonyls in proteomic analyses of protein carbonylation. The efficiency of the labeling was assayed and compared to standard biotin hydrazide reagents. The label was also tested in liquid chromatography-tandem mass spectrometry (LC/MS/MS) experiments. The quality of the fragmentation spectra was assessed and the relative detection efficiency of various modification sites was compared to standard biotin hydrazide reagents. Finally, the viability of using the label with streptavidin bead enrichment protocols in a standard proteomics workflow was probed.

Development and Evaluation of a Variable-Temperature Quadrupole Ion Trap Mass Spectrometer.

A new, variable-temperature mass spectrometer system is described. By applying polyimide heating tape to the end-cap electrodes of a Bruker (Bremen, Germany) Esquire ion trap, it is possible to vary the effective temperature of the system between 40 and 100°C. The modification does not impact the operation of the ion trap and the heater can be used for extended periods without degradation of the system. The accuracy of the ion trap temperatures was assessed by examining two gas-phase equilibrium processes with known thermochemistry. In each case, the variable-temperature ion trap provided data that were in good accord with literature data, indicating the effective temperature in the ion trap environment was being successfully modulated by the changes in the set-point temperatures on the end-cap electrodes. The new design offers a convenient and effective way to convert commercial ion trap mass spectrometers into variable-temperature instruments.

A mass spectrometric method for rapidly assaying the chiral selectivities of the copper(I) complexes of C2-symmetric ligands.

A gas-phase method for rapidly assaying the enantioselectivity of metal-centered catalysts is presented. It relies on gas-phase equilibrium measurements in a quadrupole ion trap mass spectrometer. A group of well-established C2-symmetric bis-oxazoline copper(I) complexes was used to identify chiral probe reagents that are capable of profiling the quality of the asymmetric environment provided by the metal complex. The chiral probes were then applied to a wide variety of copper(I) bis-di-imine complexes. Complexes based on a BINAM backbone exhibited selectivities that were comparable to the bis-oxazolines. Taking advantage of the mass selectivity capabilities of the ion trap mass spectrometer, the method was also applied to an un-purified mix of copper(I) complexes derived from a combinatorial synthesis of bis-di-imine ligands. This approach holds promise as a rapid screening tool for potential chiral catalysts.

The Competition between Elimination Pathways in the Reactions of a Wide Variety of Bases with 2-Fluoro- and 2-Chlorobutane in the Gas Phase.

Ab initio methods are used to examine the regio- and stereoselectivities of the elimination reactions of 2-fluorobutane and 2-chlorobutane with a series of nucleophiles (F(-), HO(-), CH3O(-), (CH3)3CO(-), NH2(-), CH3(-), H(-), Cl(-), HS(-), and PH2(-)). The data suggest that regiochemistry is most closely related to the nature of the transition state on the E2 spectrum with E1cb-like reactions favoring the least-substituted alkene product and E1-like reactions favoring the most-substituted alkene product. There appears to be no correlation between the extent of π-bond formation (as measured by the Cα-Cß distance) and the preference for forming the more highly substituted alkene. The stereochemistry (E vs Z) is less sensitive to the nucleophile and is relatively constant with the exception of a few systems that appear to have long-range interactions that reduce the bias against the Z product. Comparisons with experimental results in solution show, with a few exceptions, similar reactivity trends in solution and the gas phase.

Are copper(I) carbenes capable intermediates for cyclopropanations? The case for ylide intermediates.

A novel approach is used to synthesize a stable, ligated copper(I) carbene in the gas phase that is capable of typical metal carbenoid chemistry. However, it is shown that copper(I) carbenes generally undergo rapid unimolecular rearrangements including insertions into copper-ligand bonds and Wolff rearrangements. The results indicate that most copper(I) carbenes are inherently unstable and would not be viable intermediates in condensed-phase applications; an alternative intermediate that is less prone to rearrangements is required. Computational data suggest that ylides formed by the complexation of the carbene with solvent or other weak nucleophiles are viable intermediates in the reactions of copper(I) carbenes.

Intermolecular C-H Bond Activation by a Cationic Iridium(III) Dichloride Phenanthroline Complex.

It is demonstrated that a cationic iridium(III) dichloride phenanthroline complex is capable of C-H activation and H/D exchange. It can cleave benzylic and unactivated secondary C-H bonds, but exhibits unique selectivity when compared to similar systems that have been studied in the condensed phase. Gas-phase rate constants and kinetic isotope effects are reported for a variety of substrates and the analysis is supported by DFT calculations at the M06/QZVP level.

Pyrrolidine catalyzed reactions of cyclopentadiene with α,ß-unsaturated carbonyl compounds: 1,2- versus 1,4-additions.

A systematic study of the reactions of cyclopentadiene with α,ß-unsaturated carbonyl compounds in the presence of catalytic pyrrolidine-H2O revealed that the reactions can either proceed with a Michael attack at the ß-carbon of enone, or 1,2-addition to the carbonyl, leadingeither to 4-cyclopentadienyl-2-butanones or 6-vinylfulvenes. The former can be isolated and/or converted to the corresponding 1,2-dihydropentalenes with base (or in one-pot at longer reaction times). Substitution pattern on the enones on the competing pathways have been studied and consistent mechanisms are proposed.

Protonation energies of 1-5-ring polycyclic aromatic nitrogen heterocyclics: comparing experiment and theory.

Polycyclic nitrogen heterocyclic compounds (PANHs) can be protonated in the gas phase in mass spectrometry, in solution in acidic and biological environments, and if present, in interstellar clouds. Intrinsic molecular effects on PANH basicities can be observed by their gas phase protonation thermochemistry. We determined the gas phase basicities/proton affinities (GBs/PAs) of prototype one-nitrogen, 3-5-ring PANH compounds of increasing sizes and polarizabilities by kinetic bracketing, using proton transfer reactions to reference bases. The experimental proton affinities increase from 1-ring (pyridine, 222.2); to 2-ring (quinoline, 227.8); to 3-5-ring compounds, 227-234 kcal mol(-1). We also calculated the GB/PA values at the M06-2X/6-311+G**//B3LYP/6-31g* level. The computed PAs agree, within the experimental uncertainty, with the experimental values anchored to the upper range of the NIST GB/PA database. Specifically, the computed PAs are smaller than the experimental values by 1.4 ± 0.9 kcal/mol for nonaromatic nitrogen reference bases and for 1-5-ring PANHs, independently of the number of rings, aromaticity, and molecular size. Therefore, a useful method to calculate proton affinities of PANH compounds can use M06-2X/6-311+G**//B3LYP/6-31g* computational PAs + 1.4 ± 0.9 kcal mol(-1). The agreement with experiment supports the NIST database within this accuracy, in the upper range up to 235 kcal mol(-1), even though there are no direct absolute experimental anchor points in this range. For astrochemical applications, the measured PAs allow calculating the energies of the (PANH)(+•) + H2 → (PANH)H(+) + H(•) reactions that may convert the radical ions to less reactive 11-electron ions. The reactions are endothermic or nearly thermoneutral for the 3-5-ring ions and would be very slow at low temperatures, allowing reactive (PANH)(+•) radical ions to persist in interstellar clouds.

Protonated polycyclic aromatic nitrogen heterocyclics: proton affinities, polarizabilities, and atomic and ring charges of 1-5-ring ions.

Calculated proton affinities, polarizabilities, and some ionization energies and atomic and ring NBO charges are reported for 31 polycyclic aromatic nitrogen heterocyclics (PANHs) with 1-5 rings, calculated on the on the M06-2X/6-311+g**//B3LYP/6-31g* level of theory. The calculated proton affinities from 226 to 241 kcal mol(-1) for 3-5-ring compounds, predict well the relative experimental values. The proton affinities increase with increasing molecular size and show a linear correlation with polarizabilities. Linear geometry and nitrogen located in the central ring also favor increased proton affinity. These trends estimate a PA > 241 kcal mol(-1) for an infinite linear chain, end-ring-N PANH molecule, and >261 kcal mol(-1) for an edge-N-doped graphene sheet, making it a superbase. NBO analysis shows that from pyridineH(+) to large 5-ring ions, the N-H nitrogen carries a constant q(N) = -0.46 ± 0.1 charge, and the N-H hydrogen a constant q(H) = 0.43 ± 0.01 positive charge, similar to the q(H) in NH4(+). Overall, the NH group is nearly electrically neutral, and a nearly full positive charge is distributed on the aromatic hydrocarbon rings of the ions. When the nitrogen is in a central ring, that ring is negative, and the positive ionic charge is delocalized toward the end rings. When the nitrogen is in an end ring, the ionic charge is distributed more evenly. Increasing proton affinities with increasing polarizability result not from increasing charge transfer from the proton to the aromatic rings, but from increasing delocalization of the transferred charge in the aromatic hydrocarbon rings of the ions. In two-nitrogen compounds, interactions between the ring nitrogens decrease the proton affinities, but this effect decreases in larger ions.