Estudo anatômico e palinológico de Antônia ovata Pohl (Loganiaceae)

Nesta comunicação o autor considera a anatomia do caule, pecíolo, lâmina foliar e madeira, além dos aspectos morfológicos externo e palinológico, de espécimes de Antonia ovata, ocorrentes na floresta da região do rio Jarí (Estado do Pará) e nos cerrados da Amazônia e do Brasil Central; nomeia os espécimes da mata como sendo uma variedade nova para a ciência: Antonia ovata Pohl var. excelsa Paula.In this paper the author studies extern morphological, palinological and anatomical aspects, aiming to put an end to the doubts in the taxonomic studies of the specimens of Antonia ovata Pohl (or aiming make clear the taxonomy of the specimens of Antonia ovata. Specimens of Antonia ovata from the woods of the region of Jarí river (Amazônia) are considered by the author as a new variety. With its description, the number of varieties of Antonia ovata rose to three: pilosa, ovata and excelsa (new variety). The extern morphological aspect is found among the individuals from three habitats: "cerrados" of Amazônia, Brasil Central and forest of the region Jarí river. The identification of the three varieties is based on the following characteristic. Presence or lack of hairs on the leaves and branches; microscopic structure of wood (see comparative table); height and diameter of the specimens; and finally the habitat. Pollen grains of these two varieties excelsa and ovata present polymorphism. The leaf of that species has structure of a higrophyllous plants. The stem is rich in mucilaginous cells; vascular bundles are bicollateral; the leafknot is bilacunar, and the trace is formed by two vascular bundles

Tuning the Regioselectivity of Palladium-Catalyzed Direct Arylation of Azoles by Metal Coordination

Cleavage of C–H bonds of free and Cu^I-bound <i>N</i>-methylimidazole and oxazole via a concerted metalation–deprotonation (CMD) pathway was evaluated, and the distortion–interaction analysis was performed to quantify the various contributions to the CMD transition states. Metal binding to the N3 atom for these azoles imparts an increase of C–H bond acidities and, thus, enhances CMD reactivity for all C–H bonds, leading to a reliable C2 > C5 > C4 reactivity compared to a bias for C5 > C2 arylation for noncoordinated azoles. This type of substrate activation and tuning of regioselectivity by metal coordination to heteroarenes can be used for many other classes of substrates for direct arylation reactions

Reactivity and Regioselectivity of Palladium-Catalyzed Direct Arylation in Noncooperative and Cooperative Processes

Recent discovery (J. Am. Chem. Soc 2012, 134, 3683) of the involvement of the cyclometalated [Pd (^tBu₂PCMe₂CH₂)­(OAc)]₂ complex in direct arylation of pyridine <i>N</i>-oxide suggested that the mechanism of this reaction may involve a process in which C–H activation occurs at one Pd center and the aryl group undergoes coupling with another aryl group at a second Pd center (a cooperative catalysis). In this work, cleavage of arene C–H bonds of different (hetero)­arenes via a concerted metalation–deprotonation (CMD) pathway was evaluated for both noncooperative and cooperative processes so that the two processes could be compared in terms of reactivity and regioselectivity. The distortion–interaction analysis was performed to quantify the various contributions to the CMD transition states. Calculated barriers of the C–H bond cleavage in the two processes indicate that the cooperative and noncooperative processes lead to the same regioselectivity of arylation. Differences in contributions to the activation barriers between the two processes are fairly minor. This allows us to use the existing data about (hetero)­arene C–H reactivity and regioselectivity in the noncooperative arylation and apply it to predict reactivity and regioselectivity of arylation in the cooperative process

Complexes with a Single Metal–Metal Bond as a Sensitive Probe of Quality of Exchange-Correlation Functionals

The electronic structure of the vanadium dimer complex [V­(C₅H₅)]₂Pn with a single metal–metal bond was characterized, and the energies of higher spin states were evaluated. To simplify evaluation of orbital contributions to bonding between atoms and fragments, occupancy-perturbed bond orders were introduced. The structure and experimentally determined singlet–triplet gap in this complex can be used to test the quality of modern exchange-correlation functionals. Most generalized gradient approximation (GGA) functionals were determined to be quite suitable to reproduce the metal–metal distance and the single–triplet energy gap in [V­(C₅H₅)]₂Pn. Further accuracy improvement can be achieved by using empirical dispersion corrections. Hybrid exchange-correlation functionals, including the B3LYP functional, performed poorly for both structural and energy predictions. The hybrid functionals significantly overestimate the stability of the singlet state with the antiferromagnetically coupled high-spin metal ions relative to the lowest-energy triplet state and the singlet state with stronger metal–metal interactions. Thus, these XC functionals are not quite suitable for computational studies of multinuclear 3d transition metal complexes with weak-to-intermediate metal–metal bonding

Rhodium(III)-Catalyzed Heterocycle Synthesis Using an Internal Oxidant: Improved Reactivity and Mechanistic Studies

Directing groups that can act as internal oxidants have recently been shown to be beneficial in metal-catalyzed heterocycle syntheses that undergo C−H functionalization. Pursuant to the rhodium(III)-catalyzed redox-neutral isoquinolone synthesis that we recently reported, we present in this article the development of a more reactive internal oxidant/directing group that can promote the formation of a wide variety of isoquinolones at room temperature while employing low catalyst loadings (0.5 mol %). In contrast to previously reported oxidative rhodium(III)-catalyzed heterocycle syntheses, the new conditions allow for the first time the use of terminal alkynes. Also, it is shown that the use of alkenes, including ethylene, instead of alkynes leads to the room temperature formation of 3,4-dihydroisoquinolones. Mechanistic investigations of this new system point to a change in the turnover limiting step of the catalytic cycle relative to the previously reported conditions. Concerted metalation−deprotonation (CMD) is now proposed to be the turnover limiting step. In addition, DFT calculations conducted on this system agree with a stepwise C−N bond reductive elimination/N−O bond oxidative addition mechanism to afford the desired heterocycle. Concepts highlighted by the calculations were found to be consistent with experimental results

Analysis of the Concerted Metalation-Deprotonation Mechanism in Palladium-Catalyzed Direct Arylation Across a Broad Range of Aromatic Substrates

Analysis of the Concerted Metalation-Deprotonation Mechanism in Palladium-Catalyzed Direct Arylation Across a Broad Range of Aromatic Substrate

High-Yielding Palladium-Catalyzed Intramolecular Alkane Arylation: Reaction Development and Mechanistic Studies

Palladium-catalyzed alkane arylation reactions with aryl halides are described for the preparation of 2,2-dialkyl-dihydrobenzofuran substrates. These reactions occur in excellent yield and very high selectivity for the formation of one sole product arising from a reaction at nearby methyl groups. Mechanistic and computational studies point to the involvement of a concerted, inner-sphere palladation−deprotonation pathway that is enabled by the presence of three-center agostic interactions at the transition state. This mechanism accurately predicts the experimentally observed kinetic isotope effect as well as the site selectivity and should be useful in the design of new reactions and catalysts

Analysis of the Palladium-Catalyzed (Aromatic)C–H Bond Metalation–Deprotonation Mechanism Spanning the Entire Spectrum of Arenes

A comprehensive understanding of the C–H bond cleavage step by the concerted metalation–deprotonation (CMD) pathway is important in further development of cross-coupling reactions using different catalysts. Distortion–interaction analysis of the C–H bond cleavage over a wide range of (hetero)­aromatics has been performed in an attempt to quantify the various contributions to the CMD transition state (TS). The (hetero)­aromatics evaluated were divided in different categories to allow an easier understanding of their reactivity and to quantify activation characteristics of different arene substituents. The CMD pathway to the C–H bond cleavage for different classes of arenes is also presented, including the formation of pre-CMD intermediates and the analysis of bonding interactions in TS structures. The effects of remote C2 substituents on the reactivity of thiophenes were evaluated computationally and were corroborated experimentally with competition studies. We show that nucleophilicity of thiophenes, evaluated by Hammett σ_p parameters, correlates with each of the distortion–interaction parameters. In the final part of this manuscript, we set the initial equations that can assist in the development of predictive guidelines for the functionalization of C–H bonds catalyzed by transition metal catalysts

Catalytic H/D Exchange of Unactivated Aliphatic C–H Bonds

Complex Cp­(iPr3P)­RuH3 (1) catalyzes H/D exchange between a variety of organic substrates and C6D6 or D2O, including aliphatic groups. Experimental data show the preference in activation of methyl groups versus benzylic positions. Methylene positions react only if the substrate contains a functional group (arene, halogen, O- or N-based groups). DFT calculations were used for the model complex Cp­(Me3P)­RuH3 (2) to study the mechanism of C–H bond activation

Mechanism of N₂O Reduction by the μ₄-S Tetranuclear Cu_Z Cluster of Nitrous Oxide Reductase

Reaction thermodynamics and potential energy surfaces are calculated using density functional theory to investigate the mechanism of the reductive cleavage of the N−O bond by the μ4-sulfide-bridged tetranuclear CuZ site of nitrous oxide reductase. The CuZ cluster provides an exogenous ligand-binding site, and, in its fully reduced 4CuI state, the cluster turns off binding of stronger donor ligands while enabling the formation of the CuZ−N2O complex through enhanced CuZ → N2O back-donation. The two copper atoms (CuI and CuIV) at the ligand-binding site of the cluster play a crucial role in the enzymatic function, as these atoms are directly involved in bridged N2O binding, bending the ligand to a configuration that resembles the transition state (TS) and contributing the two electrons for N2O reduction. The other atoms of the CuZ cluster are required for extensive back-bonding with minimal σ ligand-to-metal donation for the N2O activation. The low reaction barrier (18 kcal mol-1) of the direct cleavage of the N−O bond in the CuZ−N2O complex is due to the stabilization of the TS by a strong CuIV2+−O- bond. Due to the charge transfer from the CuZ cluster to the N2O ligand, noncovalent interactions with the protein environment stabilize the polar TS and reduce the activation energy to an extent dependent on the strength of proton donor. After the N−O bond cleavage, the catalytic cycle consists of a sequence of alternating protonation/one-electron reduction steps which return the CuZ cluster to the fully reduced (4CuI) state for future turnover