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[Ru2(OH)2(3,6-tBu2Q)2(btpyan)]2+ (tBu2Q, 3,6-di-tert-butyl-1,2-benzoquinone; btpyan, 1,8-bis(2,2′:6′,2′-terpyridyl)anthracene) is one of a handful of structurally well-defined homogeneous catalysts that can electrocatalytically oxidize water at room temperature. Unfortunately, the exact composition and the chemical properties of the redox intermediates leading to the catalytically competent species remains poorly resolved. On the basis of the UV-vis spectra the catalyst was previously speculated to lose two protons spontaneously to form an intermediate containing the key O-O bond in water. We evaluated this mechanistic scenario computationally and found that the associated pKa values are in the range of 21, much too high to justify spontaneous deprotonation under experimental conditions of pH = 4. In later work, the O-O bond formation was speculated to occur after removal of two protons and two electrons. Extensive exploration of the various oxidation and protonation states that the diruthenium complex may access during catalyst activation reveals surprisingly complex electronic structure patterns in several redox intermediates: the quinone and tpy ligands become redox noninnocent, i.e., they participate actively in the electron transfer processes by temporarily storing redox equivalents. On the basis of this new insight into the electronic structure we propose a novel alternative explanation of the spectroscopic observations reported previously and characterize the electronic structure of the key intermediates in detail. Finally, the redox potential for the first two-electron oxidation is evaluated based on our proposed intermediates and predicted to be 0.411 V, which compares well with the experimentally observed broad two-electron wave at ~0.32 V. [ABSTRACT FROM AUTHOR]

The resting state of the recently reported water oxidation catalyst [tpyRu(ll)-OH2]2L3+ (tpy = terpyricline; L = bipyridylpyrazolylic anion) ([2,2]>3+) must be activated by a series of proton-coupled oxidations in which four protons and four electrons are removed overall to afford the catalytically competent species [tpyRu(lV)=O]2L3+ ([4,4]3+) We have examined all of the plausible redox intermediates utilizing density functional theory coupled to a continuum solvation model. Our calculations reproduce well the first three redox potentials under pH = 1 conditions, and a reasonable correlation between theory and experiment is found for the fourth irreversible redox process that accompanies O2 generation. The computed oxidation potentials to access [5,4]4+ and [5,5]5+ 1.875 and 2.032 V vs NHE, respectively, exclude the otherwise plausible possibilities of the catalytically active species having a higher oxidation state. [4,4]3+ has an antiferromagnetically coupled ground state in which one ruthenium has two unpaired electrons antiparallel to those of the other ruthenium. As we found in our previous work, two radicaloid terminal oxygen moieties with different spin orientations that are induced by spin polarization from the electron-deficient Ru(IV) centers are found. Two mechanistic scenarios are relevant and interesting for the key O-O bond formation event: intramolecular oxo-oxo coupling and coupling between one terminal oxo and the oxygen atom of the incoming water substrate. The intramolecular oxo-oxo coupling is facile, with a low barrier of 13.9 kcal mol―1, yielding a peroxo intermediate. The necessary subsequent addition of water in an associative substitution mechanism to cleave one of the Ru-peroxo bonds, however, is found to be impractical at room temperature, with a barrier of ▵G* = 30.9 kcal mol―1. Thus, while plausible, the intramolecular oxo-oxo coupling is unproductive for generating molecular dioxygen. The intermolecular O-O coupling is associated with a high barrier (▵G* = 40.2 kcal mol―1) and requires the assistance of an additional proton, which lowers the barrier dramatically to 24.5 kcal mol―1. [ABSTRACT FROM AUTHOR]

One of the many hypotheses on the pathogenesis of Alzheimer’s disease is that the amyloid-β peptide (Aβ) binds CuII and can catalytically generate H2O2, leading to oxidative damage in brain tissues. For a molecular level understanding of such catalysis it is critical to know the structure of the Aβ–CuII complex precisely. Unfortunately, no high-resolution structure is available to date and there is considerable debate over the copper coordination environment with no clear consensus on which residues are directly bound to CuII. Considering all plausible isomers of the copper-bound Aβ42 and Aβ40 using a combination of density functional theory and classical molecular dynamics methods, we report an atomic resolution structure for each possible complex. We evaluated the relative energies of these isomeric structures and surprisingly found that Aβ42 and Aβ40 display very different binding modes, suggesting that shorter peptides that are truncated at the C-terminus may not be realistic models for understanding the chemistry of the most neurotoxic peptide, Aβ42. [ABSTRACT FROM AUTHOR]

Chemical reasoning based on ligand-field theory suggests that homoleptic cyano complexes should exhibit low-spin configurations, particularly when the coordination sphere is nearly saturated. Recently, the well-known chromium hexacyano complex anion [Cr(CN)6]4- was shown to lose cyanide to afford [Cr(CN)5)3- in the absence of coordinating cations. Furthermore, (NEt4)3[Cr(CN)5] was found to be in a high-spin (S = 2) ground state, which challenges the common notion that cyanide is a strong field ligand and should always enforce low-spin configurations. Using density functional theory coupled to a continuum solvation model, we examined both the instability of the hexacyanochromate(II) anion and the relative energies of the different spin states of the pentacyanochromate(II) anion. By making direct comparisons to the analogous FeII complex, we found that cyanide electronically behaves as a strong-field ligand for both metals because the orbital interaction is energetically more favorable in the low-spin configuration than in the corresponding high-spin configuration. The Coulombic repulsion between the anionic cyanide ligands, however, dominates the overall energetics and ultimately gives preference to the high-spin complex, where the ligand-ligand separation is larger. Our calculations highlight that for a quantitative understanding of spin-state energetic ordering in a transition metal complex, ligand-ligand electrostatic interactions must be taken into account in addition to classical ligand-field arguments based on M-L orbital interaction energies. [ABSTRACT FROM AUTHOR]

The mechanism of the catalytic oxidation of water by cis,cis-[(bpy)2Ru(OH2)]2O4+ to give molecular dioxygen was investigated using Density Functional Theory (DFT). A series of four oxidation and four deprotonation events generate the catalytically competent species cis,cis-[(bpy)2RuvO]2O4+, which breaks the H-OH bond homolytically at the rate determining transition state to give a hydroperoxo intermediate. Our calculations predict a rate determining activation barrier of 25.9 kcal/mol in solution phase, which is in reasonable agreement with the previously reported experimental estimate of 18.7-23.3 kcal/mol. A number of plausible coupling schemes of the two metal sites including strong coupling, weak ferromagnetic and weak antiferromagnetic coupling have been considered. In addition, both high-spin and low-spin states at each of the Ru(V)-aβ centers were explored and we found that the high-spin states play an important mechanistic role. Our calculations suggest that cis,cis-[(bpy)2RuvO]2O4+ performs formally an intramolecular ligand-to-metal charge transfer when reacting with water to formally give a cis,cis-[(bpy)2RuIvO.]2O4+ complex. We propose that the key characteristic of the diruthenium catalyst that allows it to accomplish the most difficult first two oxidations of the overall four-electron redox reaction is directly associated with this in situ generation of two radicaloid oxo moieties that promote the water splitting reaction. A proton coupled metal-to-metal charge transfer follows to yield a Ru(V)/Ru(III) peroxo/aqua mixed valence complex, which performs the third redox reaction to give the superoxo/aqua complex. Finally, intersystem crossing to a ferromagnetically coupled Ru(IV)/Ru(lll) superoxo/aqua species is predicted, which will then promote the last redox event to release triplet dioxygen as the final product. A number of key features of the computed mechanism are explored in detail to derive a conceptual understanding of the catalytic mechanism. [ABSTRACT FROM AUTHOR]

We elucidate the hydroxylation of camphor by cytochrome P450 with the use of density functional and mixed quantum mechanics/ molecular mechanics methods. Our results reveal that the enzyme catalyzes the hydrogen-atom abstraction step with a remarkably low free-energy barrier. This result provides a satisfactory explanation for the experimental failure to trap the proposed catalytically competent high-valent heme re(IV) oxo (oxyferryl) species responsible for this hydroxylation chemistry. The primary and previously unappreciated contribution to stabilization of the transition state is the interaction of positively charged residues in the active-site cavity with carboxylate groups on the heme periphery. A similar stabilization found in dioxygen binding to hemerythrin, albeit with reversed polarity, suggests that this mechanism for controlling the relative energetics of redox-active intermediates and transition states in metalloproteins may be widespread in nature. [ABSTRACT FROM AUTHOR]

Focuses on the conversion of methane to methanol which is catalyzed at the active site of a metalloenzyme known as methane monooxygenase (MMO). Kinds of MMO systems in methanotrophic organisms; Hydroxylation step in the catalytic cycle; Evidence for cationic and radical intermediates.

Examines the electrochemical behavior of cyclooctatetraene (COT) and nitrobenzene using the density functional theory. Effect of ion-pair formation on the relative energies of redox species; Reductive electrochemistry of COT; Structural change of the tub-shaped neutral molecules.

Epoxide and CO2 coupling reactions catalyzed by (salen)CrIIICl have been modeled computationally to contrast a monometallic vs. a bimetallic mechanism. A low-energy CO2 insertion step into the metal-alkoxide bond was located. [ABSTRACT FROM AUTHOR]

The computational analysis of the rhodiuth-catalyzed Pauson-Khand reaction indicates that the key transition state is highly charge-polarized, wherein different diastereoisomers have distinctively different charge polariza- tion patterns. Experimental studies demonstrate that chioro- enynes provide the optimal a-electron-withdrawing group to promote polarization and thereby reduce the activation barrier to provide a highly diastereoselective reaction at room tem- perature. [ABSTRACT FROM AUTHOR]

The article reports that the blue Rubidium (Ru) dimer is the most extensively studied. Unfortunately, its instability during catalytic turnover prevents technological utilization. Rational approaches to increasing the robustness of the catalyst while enhancing its activity are difficult to conceive, in part because the catalytic mechanism of water oxidation is not known. A few reasonable proposals have been made, but a consensus mechanism has not emerged to date. Computational studies would be helpful and allow for quantification of the energetic requirements of possible reaction pathways. However, high-level calculations of the Ru dimer are difficult because the bipyridine ligands cannot be represented adequately by a smaller model ligand, thus making computer models expensive and hard to analyze. The overall four-electron redox process is coupled to loss of protons requiring the protonation state at the various redox steps to be explored.

The diabatic couplings between the metal centers in M(Cp)[sub 2, sup 0/+] (M = Fe, Co; Cp = C[sub 5]H[sub 5]) were evaluated using ab initio methods at the Hartree-Fock level of theory. Excellent agreement with the experimental estimate is found for the Fe(Cp)[sub 2, sup 0/+] couple. For Co(Cp)[sub 2, sup 0/+], the calculated numbers are substantially higher than the experimental estimates, suggesting that the latter may represent a considerable underestimate (a possibility mentioned in the experimental publication). [ABSTRACT FROM AUTHOR]

In the structure of dl-proline, C5H9NO2, the mol­ecules are connected via classical inter­molecular N—H⋯O hydrogen bonds involving the amine and carbox­yl groups [N⋯O = 2.7129 (15) and 2.8392 (16) Å], and form chains along the b-axis direction and parallel to ( 01). The chains are linked into sheets via weak non-classical hydrogen bonds. The conformation of the mol­ecule and its packing are notably different from the monohydrated dl-proline form. [ABSTRACT FROM AUTHOR]

The rhodium(I) catalyzed [2 + 2 + 1] carbocyclization of tethered diene-enes to afford substituted hexahydropentalenones with high levels of diastereoselectivity was modeled using density functional theory. Previously, this transformation was observed to be facile, whereas the analogous bis-ene substrate could not be cyclized under any reasonable conditions. To establish a conceptual understanding of the carbocyclization mechanism and to identify the functional role of the diene fragment we analyzed the simulated reaction mechanisms using the two parent systems. We discovered a thus far unrecognized, but intuitively plausible, role of the CO ligand for controlling the electron density at the metal center, which affects the feasibility of oxidative addition and reductive elimination steps that are key components of the mechanism. Our calculations suggest that the diene moiety is unique and required because of its ability to undergo a η2→ η4 reorganization allowing for the thermoneutral expulsion of one CO ligand, which in turn generates an electron-rich, coordinatively saturated Rh(I) center that can efficiently promote the oxidative addition with a low barrier. A number of functionalization strategies were considered explicitly to derive a rational plan for optimizing the catalysis and to expose the roles of the different components of the reactant—catalyst complex. [ABSTRACT FROM AUTHOR]

The bifunctional binding of the anticancer drug cisplatin to two adjacent nucleobases in DNA is modeled using density functional theory. Previous experimental studies revealed that cisplatin binding to adjacent guanine and adenine is sensitive to nucleobase sequence. Whereas AG 1,2-intrastrand cross- links are commonly observed, the analogous GA adducts are not known. This study focuses on understanding this directional preference by constructing a full reaction profile using quantum chemical simulation methods. Monofunctional and bifunctional cisplatin adducts were generated, and the transition states that connect them were located for the dinucleotides d(pApG) and d(pGpA), assuming that initial platination takes place at the guanine site. Our computer simulations reveal a significant kinetic preference for formation of the AG over the GA adduct. The activation free energies of ∼23 kcal/mol for AG and ∼32 kcal/mol for GA suggest that bifunctional closure is ∼6 orders of magnitude faster for AG than for GA. A strong hydrogen bond between one of the ammine ligands of cisplatin and the 5' phosphate group of the DNA backbone is responsible for the stabilization of the transition state that affords the AG adduct. This interaction is absent in the transition state that leads to the GA adduct because the right-handed helix of the DNA backbone places the phosphate out of reach for the ammine ligand. We found only an insignificant thermodynamic difference between AG and GA adducts and conclude that the preference of AG over GA binding is largely under kinetic control. The puckering of the deoxyribose ring plays an important role in determining the energetics of the bifunctional platination products. Whereas the 3'-nucleoside remains in the native C2'-endo/C3'-exo form of B-DNA, the deoxyribose of the 5'-nucleoside always adopts the C2'-exo/C3'-endo puckering in our simulations. A detailed analysis of the energies and structures of the bifunctional adducts revealed that the observed sugar puckering patterns are necessary for platinum to bind in a relaxed coordination geometry. [ABSTRACT FROM AUTHOR]

Despite steady progress in catalytic methods for the borylation of hydrocarbons, methane has not yet been subject to this transformation. Here we report the iridium-catalyzed borylation of methane using bis(pinacolborane) in cyclohexane solvent. Initially, trace amounts of borylated products were detected with phenanthroline-coordinated Ir complexes. A combination of experimental high-pressure and high-throughput screening, and computational mechanism discovery techniques helped to rationalize the foundation of the catalysis and identify improved phosphine-coordinated catalytic complexes. Optimized conditions of 150°C and 3500- kilopascal pressure led to yields as high as ~52%, turnover numbers of 100, and improved chemoselectivity for monoborylated versus diborylated methane. [ABSTRACT FROM AUTHOR]

Isomeric alkylidene complexes syn- and anti-(PNP)Tiᆖ[CtBu(C6F5)](F) (1) and (PNP)Tiᆖ[CtBu(C7F7)](F) (2) have been generated from C–F bond addition of hexafluorobenzene (C6F6) and octafluorotoluene (C7F8) across the alkylidyne ligand of transient (PNP)Ti≡CtBu (A) (PNP−ᆖN[2-P(CHMe2)2-4-methylphenyl]2), which was generated from the precursor (PNP)TiᆖCHtBu(CH2tBu). Two mechanistic scenarios for the activation of the C–F bond by A are considered: 1,2-CF addition and [2 + 2]-cycloaddition/β-fluoride elimination. Upon formation of the alkylidenes 1 and 2, the kinetic and thermodynamic alkylidene product is the syn isomer, which gradually isomerizes to the corresponding anti isomer to ultimately establish an equilibrium mixture (when using 1, 65/35) if the solution is heated in benzene to 105 °C for 1 h. Single crystal X-Ray crystallographic data obtained for the two isomers of 2 (and syn isomer of 1) are in good agreement with computed DFT-optimized models. Our calculations suggest convincingly that the isomerization process proceeds via a concerted rotation involving a heterolytic bond cleavage about the alkylidene bond. The two rotamers are thermodynamically very close in energy and interconvert with an estimated barrier of ∼26 kcal/mol. The electronic reason for this unexpectedly low barrier is investigated. [Copyright &y& Elsevier]

The catalytic mechanism of the hydrogen-mediated coupling of acetylene to carbonyl compounds and imines has been examined using three techniques: (a) ESI-MS and ESI-CAD-MS analyses, (b) computational modeling, and (c) experiments wherein putative reactive intermediates are diverted to alternate reaction products. ESI-MS analysis of reaction mixtures from the hydrogen-mediated reductive coupling of acetylene to α-ketoesters or N-benzenesulfonyl aldimines corroborate a catalytic mechanism involving C=X (X = O, NSO2Ph) insertion into a cationic rhodacyclopentadiene obtained by way of acetylene oxidative dimerization with subsequent Brønsted acid cocatalyzed hydrogenolysis of the resulting oxa- or azarhodacycloheptadiene. Hydrogenation of 1,6-diynes in the presence of α-ketoesters provides analogous coupling products. ESI mass spectrometric analysis again corroborates a catalytic mechanism involving carbonyl insertion into a cationic rhodacyclopentadiene. For all ESI-MS experiments, the structural assignments of ions are supported by multistage collisional activated dissociation (CAD) analyses. Further support for the proposed catalytic mechanism derives from experiments aimed at the interception of putative reactive intermediates and their diversion to alternate reaction products. For example, rhodium-catalyzed coupling of acetylene to an aldehyde in the absence of hydrogen or Brønsted acid cocatalyst provides the corresponding (Z)-butadienyl ketone, which arises from β-hydride elimination of the proposed oxarhodacycloheptadiene intermediate, as corroborated by isotopic labeling. Additionally, the putative rhodacyclopentadiene intermediate obtained from the oxidative coupling of acetylene is diverted to the product of reductive [2 + 2 + 2] cycloaddition when N-p-toluenesulfonyl-dehydroalanine ethyl ester is used as the coupling partner. The mechanism of this transformation also is corroborated by isotopic labeling. Computer model studies based on density functional theory (DFT) support the proposed mechanism and identify Brønsted acid cocatalyst assisted hydrogenolysis to be the most difficult step. The collective studies provide new insight into the reactivity of cationic rhodacyclopentadienes, which should facilitate the design of related rhodium-catalyzed C-C couplings. [ABSTRACT FROM AUTHOR]

The electronic structures of the low and high-spin states of the cationic complex [Ni(H)(OH)]+ that was previously found to be highly reactive toward OH4 and O2 were examined. Earlier computational work suggested that the low-spin doublet state D0 of the NiIII-d7 system is significantly lower in energy than its high-spin quartet analogue Q1. Recent DFT-studies indicated, however, that Q1 is the reactive species requiring Q1 to have a sufficiently long lifetime for undergoing thermal reactions with the small molecule reactants under single collision conditions in the gas phase. These observations raise the question as to why Q1 does not spontaneously undergo intersystem crossing. Our work based on DFT, coupled-cluster and MCSCF calculations suggests that the hydroxyl ligand behaves as a redox noninnocent ligand and becomes oxidized to formally afford an electronic structure that is consistent with a NiII-(OH)• species. As a result, the doublet and quartet ground states are not related by a single electron spin flip and the intersystem crossing becomes inhibited, as indicated by unexpectedly small spin-orbit coupling constants. After extensive sampling of the potential energy surfaces, we concluded that there is no direct way of converting Q1 to the ground state doublet D0. Alternative multistep pathways for the Q1 - D0 decay involving doublet excited states were also evaluated and found to be energetically not accessible under the experimental conditions. [ABSTRACT FROM AUTHOR]

Appropriately designed chemical architectures can fold to adopt well-defined secondary structures without the need for structural motifs of biological origin. We have designed tris(N-salicylideneaniline)- based hyperbranched molecules that spontaneously collapse to compact three-blade propeller geometry of either (F)- or (A'-handedness. For a homologous series of compounds, a direct correlation was established between the absolute screw sense, either (F)- or (M)-, of this helical folding and the absolute configuration, either (R)- or (S)-, of the chiral alcohol groups introducing local asymmetric bias to the conformationally restricted molecular backbone. [sup1]H NMR and CD spectroscopic studies provided significant insights into structural folding and unfolding of these chiral molecules in solution, which proceed via reversible assembly and disassembly of the C[sub3]-symmetric hydrogen-bonding network. Notably, solvents profoundly influenced this dynamic process. A strong correlation between the solvent donor number (DN) or solvent basicity (SB) parameters and the change in the Cotton effects pointed toward specific O-H"solvent interactions that drive structural unfolding and eventual refolding to apparently opposite helicity. This unusual chirality inversion process could also be induced by installation of chemical protecting groups that simulate specific solvent-solute interactions. Removal of this covalent mimic of the solvent shell restored the original screw sense of the parent molecule, thus establishing the feasibility of covalently triggered helicity inversion as a new mode of operation for chiroptical molecular switches. [ABSTRACT FROM AUTHOR]

The new complex, [Zr(pda)2]n (1, pda2- = N,N′-bis(neo-pentyl)-ortho-phenylenediamide, n = 1 or 2), prepared by the reaction of 2 equiv of pdaLi2 with ZrCl4, reacts rapidly with halogen oxidants to afford the new product ZrX2(disq)2 (3, X = Cl, Br, I; disq- = N,N′-bis(neo-pentyl)-ortho-diiminosemiquinonate) in which each redox-active ligand has been oxidized by one electron. The oxidation products 3a-c have been structurally characterized and display an unusual parallel stacked arrangement of the disq- ligands in the solid state, with a separation of ~3 Å. Density functional calculations show a bonding-type interaction between the SOMOs of the disq- ligands to form a unique HOMO while the antibonding linear combination forms a unique LUMO. This orbital configuration leads to a closed-shell-singlet ground-state electron configuration (S = 0). Temperature-dependent magnetism measurements indicate a low-lying triplet excited state at ~750 cm-1. In solution, 3a-c show strong disq-based absorption bands that are invariant across the halide series. Taken together these spectroscopic measurements provide experimental values for the one- and two-electron energies that characterize the π-stacked bonding interaction between the two disq- ligands. [ABSTRACT FROM AUTHOR]

The neopentylidene-neopentyl complex (PNP)TiCHtBu(CH2tBu) (2; PNP- = N[2-P(CHMe2)2-4-methylphenyl]2), prepared from the precursor (PNP)TiCHtBu(OTf) (1) and LiCH2tBu, extrudes neopentane in neat benzene under mild conditions (25 °C) to generate the transient titanium alkylidyne, (PNP)TiCtBu (A), which subsequently undergoes 1,2-CH bond addition of benzene across the TiC linkage to generate (PNP)TiCHtBu(C6H5) (3). Kinetic, mechanistic, and theoretical studies suggest the CH activation process to obey pseudo-first-order in titanium, the α-hydrogen abstraction to be the rate-determining step (KIE for 2/2-d3 conversion to 3/3-d3 = 3.9(5) at 40 °C) with activation parameters ΔH‡ = 24(7) kcal/mol and ΔS‡ = -2(3) cal/mol·K, and the post-rate-determining step to be C-H bond activation of benzene (primary KIE = 1.03(7) at 25 °C for the intermolecular C-H activation reaction in C6H6 vs C6D6). A KIE of 1.33(3) at 25 °C arose when the intramolecular C-H activation reaction was monitored with 1,3,5-C6H3D3. For the activation of aromatic C-H bonds, however, the formation of the σ-complex becomes rate-determining via a hypothetical intermediate (PNP)TiCtBu(C6H5), and C-H bond rupture is promoted in a heterolytic fashion by applying standard Lewis acid/base chemistry. Thermolysis of 3 in C6D6 at 95 °C over 48 h generates 3-d6, thereby implying that 3 can slowly equilibrate with A under elevated temperatures with k = 1.2(2) × 10-5 s-1, and with activation parameters ΔH‡ = 31(16) kcal/mol and ΔS‡ = 3(9) cal/mol·K. At 95 °C for one week, the EIE for the 2 → 3 reaction in 1,3,5-C6H3D3 was found to be 1.36(7). When 1 is alkylated with LiCH2SiMe3 and KCH2Ph, the complexes (PNP)TiCHtBu(CH2SiMe3) (4) and (PNP)TiCHtBu(CH2Ph) (6) are formed, respectively, along with their corresponding tautomers (PNP)TiCHSiMe3(CH2tBu) (5) and (PNP)TiCHPh(CH2tBu) (7). By means of similar alkylations of (PNP)TiCIHSiMe3(OTf) (8), the degenerate complex (PNP)TiCHSiMe3(CH2SiMe3) (9) or the non-degenerate alkylidene-alkyl complex (PNP)TiCHPh(CH2SiMe3) (11) can also be obtained, the latter of which results from a tautomerization process. Compounds 4/5 and 9, or 6/7 and 11, also activate benzene to afford (PNP)TiCHR(C6H5) (R = SiMe3 (10), Ph (12)). Substrates such as FC6H5, 1,2-F2C6H4, and 1,4-F2C6H4 react at the aryl C-H bond with intermediate A, in some cases regioselectively, to form the neopentylidene-aryl derivatives (PNP)TiCHtBu(aryl).… [ABSTRACT FROM AUTHOR]

A series of FLn (n = 1–5) ligands, where FLn is a fluorescein modified with a functionalized 8-aminoquinoline group as a copper-binding moiety, were synthesized, and the chemical and photophysical properties of the free ligands and their copper complexes were investigated. UV-visible spectroscopy revealed a 1:1 binding stoichiometry for the Cu(II) complexes of FL1, FL3, and FL5 in pH 7.0 buffered aqueous solutions. The reactions of FL2 or FL4 with CuCl2, however, appear to produce a mixture of 1:1 and 1:2 complexes, as suggested by Job's plots. These binding modes were modeled by the synthesis and X-ray crystal structure determination of Cu(II) complexes of 2-[(quinolin-8-ylamino)methyl]phenol (modL), employed as a surrogate of the FLn ligand family. Two kinds of crystals, [Cu(modL)2](BF4)2 and [Cu2(modL′)2(CH3OH)](BF4)2 (modL′ = 2-[(quinolin-8-ylamino)methyl]phenolate), were obtained. The structures suggest that one oxygen and two nitrogen atoms of the FLn ligands most likely bind to Cu(II). Introduction of nitric oxide (NO) to pH 7.0 buffered aqueous solutions of Cu(FLn) (1 μM CuCl2 and 1 μM FLn) at 37 °C induces an increase in fluorescence. The fluorescence response of Cu(FLn) to NO is direct and specific, which is a significant improvement over commercially available small molecule-based probes that are capable of detecting NO only indirectly. The NO-triggered fluorescence increase of Cu(FL5) occurs by reduction of Cu(II) to Cu(I) with concomitant dissociation of the N-nitrosated fluorophore ligand from copper. Spectroscopic and product analyses of the reaction of the FL5 copper complex with NO indicated that the N-nitrosated fluorescein ligand (FL5-NO) is the species responsible for fluorescence turn-on. Density functional theory (DFT) calculations of FL5 versus FL5-NO reveal how N-nitrosation of the fluorophore ligand brings about the fluorescence increase. The copper-based probes described in the present work form the basis for real-time detection of nitric oxide production in living cells. [ABSTRACT FROM AUTHOR]

Carbolithiation has been studied with alkyllithium reagents in a series of six-through nine-membered 3-methylene-1,4-cycloalkadienes, efficiently producing the corresponding cyclic pentadienyl carbanions. These pentadienyl anions display unique reactivity, depending on ring size. Cyclooctadienyl anions readily undergo disrotatory electrocyclization to cis-bicyclo[3.3.0]octenyl systems, which are trapped with a variety of electrophiles to stereoselectively provide functionalized cis-bicyclo[3.3.0]octenes. The carbolithiation and electrocyclization processes are examined using low-temperature 1H NMR experiments. An expedient synthesis of a linear triquinane illustrates this methodology. Electrocyclization of the corresponding cyclononadienyl anion requires unusually high temperatures (120 °C), and computational studies provide insights into this change in reactivity. Cycloheptadienyl and cyclohexadienyl anions, generated via carbolithiation, provide functionalized cycloheptadienes and cyclohexadienes upon electrophilic capture. Trapping experiments reveal that the cycloheptadienyl anions are transformed to heptatrienyl anions. A series of experiments have been designed to explore evidence for the feasibility of equilibration of open and closed anionic systems, and these studies report the first isolation of a cis-bicyclo[3.1.0]hexene derived from electrocyclization of a cyclohexadienyl carbanion. [ABSTRACT FROM AUTHOR]

Ion mobility measurements, combined with molecular mechanics simulations, are used to study enantiopure and racemic proline clusters formed by electrospray ionization. Broad distributions of cluster sizes and charge states are observed, ranging from clusters containing only a few proline units to clusters that contain more than 100 proline units (i.e., protonated clusters of the form [xPro + nH]n+ with x = 1 to > 100 and n = 1–7). As the sizes of clusters increase, there is direct evidence for nanometer scale, chirally induced organization into specific structures. For n = 4 and 5, enantiopura clusters of ∼50 to 100 prolines assemble into structures that are more elongated than the most compact structure that is observed from the racemic proline clusters. A molecular analogue, cis-4-hydroxy-proline, displays significantly different behavior, indicating that in addition to the rigidity of the side chain ring, intermolecular interactions are important in the formation of chirally directed clusters. This is the first case in which assemblies of chirally selective elongated structures are observed in this size range of amino acid clusters. Relationships between enantiopurity, cluster shape, and overall energetics are discussed. [ABSTRACT FROM AUTHOR]

The rates of H/D exchange have been measured between (a) the activated olefins methyl methacrylate-d5 and styrene-d8, and (b) the Cr hydrides (η5-C5Ph5)Cr(CO)3H (2a), (η5-C5Me5)Cr(CO)3H (2b), and (η5-C5H5)Cr(CO)3H (2c). With a large excess of the deuterated olefin the first exchange goes to completion before subsequent exchanges begin, at a rate first order in olefin and in hydride. (Hydrogenation is insignificant except with styrene and CpCr(CO)3H; in most cases, the radicals arising from the first Ho transfer are too hindered to abstract another Ho.) Statistical corrections give the rate constants kreinit for H• transfer to the olefin from the hydride. With MMA, kreinit decreases substantially as the steric bulk of the hydride increases; with styrene, the steric bulk of the hydride has little effect. At longer times, the reaction of MMA or styrene with 2a gives the corresponding metalloradical 1a as termination depletes the concentration of the methyl isobutyryl radical 3 or the α-methylbenzyl radical 4; computer simulation of [1a] as f(t) gives an estimate of ktr, the rate constant for Ho transfer from 3 or 4 back to Cr. These rate constants imply a ΔG (50 °C) of +11 kcal/mol for H• transfer from 2a to MMA, and a ΔG (50 °C) of +10 kcal/mol for H• transfer from 2a to styrene. The CH3CN pKa of 2a, 11.7, implies a BDE for its Cr-H bond of 59.6 kcal/mol, and DFT calculations give 58.2 kcal/mol for the Cr-H bond in 2c. In combination the kinetic ΔG values, the experimental BDE for 2a, and the calculated ΔS values for H• transfer imply a C-H BDE of 45.6 kcal/mol for the methyl isobutyryl radical 3 (close to the DFT-calculated 49.5 kcal/mol), and a C-H BDE of 47.9 kcal/mol for the α-methylbenzyl radical 4 (close to the DFT-calculated 49.9 kcal/mol). A solvent cage model suggests 46.1 kcal/mol as the C-H BDE for the chain-carrying radical in MMA polymerization. [ABSTRACT FROM AUTHOR]

The article presents a study on whether Al(CH3)3 could stabilize a highly polarized ScCH2 ligand, and the preparation of such a ligand, its isolation, and use as a Brønsted base. It states that the nycleophilic nature of the methylidene carbon, coupled with the large ionic radius of the Sc(III) iron, permits for this rare motif to coordinate two Al(CH3)3 ligands, one of which seems to be dissociating to allow reactivity.

The article cites a study that reports on the facile and nonreductive C—N activation and cleavage of pyridine by a transient titanium alkylidyne. Activation of the C—N bond in pyridine involves a unique process, a ring-opening metathesis step invoking the cycloadition of the aromatic C=N bond across a reactive TiΞC'Bu linkage. The combination of synthesis, isotopic labeling and theoretical studies allowed for understanding this rare transformation.