Li Dang

A Facile Route to Aryl Boronates: Room-Temperature, Copper-Catalyzed Borylation of Aryl Halides with Alkoxy Diboron Reagents

A simple but effective copper-catalyzed borylation of aryl halides, including electron-rich and sterically hindered aryl bromides, with alkoxy diboron reagents occurs under mild conditions (see scheme). Preliminary DFT studies of the mechanism suggest that sigma-bond metathesis between a copper-boryl intermediate and the aryl halide generates the aryl boronate product.

Synthesis, Structure, and Reactivity of Anionic sp(2) -sp(3) Diboron Compounds: Readily Accessible Boryl Nucleophiles.

Lewis base adducts of tetra-alkoxy diboron compounds, in particular bis(pinacolato)diboron (B2 pin2 ), have been proposed as the active source of nucleophilic boryl species in metal-free borylation reactions. We report the isolation and detailed structural characterization (by solid-state and solution NMR spectroscopy and X-ray crystallography) of a series of anionic adducts of B2 pin2 with hard Lewis bases, such as alkoxides and fluoride. The study was extended to alternative Lewis bases, such as acetate, and other diboron reagents. The B(sp(2) )-B(sp(3) ) adducts exhibit two distinct boron environments in the solid-state and solution NMR spectra, except for [(4-tBuC6 H4 O)B2 pin2 ](-) , which shows rapid site exchange in solution. DFT calculations were performed to analyze the stability of the adducts with respect to dissociation. Stoichiometric reaction of the isolated adducts with two representative series of organic electrophiles-namely, aryl halides and diazonium salts-demonstrate the relative reactivities of the anionic diboron compounds as nucleophilic boryl anion sources.

Hydrogen-mediated metal-carbon to metal-boron bond conversion in metal-carboranyl complexes.

A hydrogen-mediated Ru-C to Ru-B bond conversion was observed experimentally and supported by the theoretical calculations. Treatment of [eta(5):sigma(C)-Me(2)C(C(5)H(4))(C(2)B(10)H(10))]Ru(COD) (1) bearing a Ru-C(cage) sigma bond with PR(3) in the presence of H(2) gave Ru-B(cage) bonded complexes [eta(5):sigma(B)-Me(2)C(C(5)H(4))(C(2)B(10)H(10))]RuH(2)(PR(3)) (R = Cy (2), Ph (3)) (sigma(C): Ru-C(cage) sigma bond; sigma(B): Ru-B(cage) sigma bond). Complex 3 was converted to [eta(5):sigma(B)-Me(2)C(C(5)H(4))(C(2)B(10)H(10))]Ru(L(2)) in the presence of L(2) (L(2) = dppe (4), PPh(3)/P(OEt)(3) (5), PPh(3)/pyridine (6)) via liberation of H(2) upon heating. These complexes were fully characterized by various spectroscopic techniques, elemental analyses, and single-crystal X-ray diffraction studies. DFT calculations show that this conversion process is both kinetically and thermodynamically favorable and requires involvement of a hydride ligand.

The mechanism of alkene addition to a nickel bis(dithiolene) complex: the role of the reduced metal complex.

The binding of an alkene by Ni(tfd)(2) [tfd = S(2)C(2)(CF(3))(2)] is one of the most intriguing ligand-based reactions. In the presence of the anionic, reduced metal complex, the primary product is an interligand adduct, while in the absence of the anion, dihydrodithiins and metal complex decomposition products are preferred. New kinetic (global analysis) and computational (DFT) data explain the crucial role of the anion in suppressing decomposition and catalyzing the formation of the interligand product through a dimetallic complex that appears to catalyze alkene addition across the Ni-S bond, leading to a lower barrier for the interligand adduct.

Computational Studies on Ethylene Addition to Nickel Bis(dithiolene)

The density functionals B3LYP, B3PW91, BMK, HSE06, LC-ωPBE, M05, M06, O3LYP, TPSS, ω-B97X, and ω-B97XD are used to optimize key transition states and intermediates for ethylene addition to Ni(edt)(2) (edt = S(2)C(2)H(2)). The efficacy of the basis sets 6-31G**, 6-31++G**, cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ is also examined. The geometric parameters optimized with different basis sets and density functionals are similar and agree well with experimental values. The ω-B97XD functional gives relative energies closest to those from CCSD, while M06 and HSE06 yield results close to those from CCSD(T). CASSCF and CASSCF-PT2 calculation results are also given. Variation of the relative energies from different density functionals appears to arise, in part, from the multireference character of this system, as confirmed by the T1 diagnostic and CASSCF calculations.

Apparent anti-Woodward-Hoffmann addition to a nickel bis(dithiolene) complex: the reaction mechanism involves reduced, dimetallic intermediates.

Nickel dithiolene complexes have been proposed as electrocatalysts for alkene purification. Recent studies of the ligand-based reactions of Ni(tfd)2 (tfd = S2C2(CF3)2) and its anion [Ni(tfd)2](-) with alkenes (ethylene and 1-hexene) showed that in the absence of the anion, the reaction proceeds most rapidly to form the intraligand adduct, which decomposes by releasing a substituted dihydrodithiin. However, the presence of the anion increases the rate of formation of the stable cis-interligand adduct, and decreases the rate of dihydrodithiin formation and decomposition. In spite of both computational and experimental studies, the mechanism, especially the role of the anion, remained somewhat elusive. We are now providing a combined experimental and computational study that addresses the mechanism and explains the role of the anion. A kinetic study (global analysis) for the reaction of 1-hexene is reported, which supports the following mechanism: (1) reversible intraligand addition, (2) oxidation of the intraligand addition product prior to decomposition, and (3) interligand adduct formation catalyzed by Ni(tfd)2(-). Density functional theory (DFT) calculations were performed on the Ni(tfd)2/Ni(tfd)2(-)/ethylene system to shed light on the selectivity of adduct formation in the absence of anion and on the mechanism in which Ni(tfd)2(-) shifts the reaction from intraligand addition to interligand addition. Computational results show that in the neutral system the free energy of activation for intraligand addition is lower than that for interligand addition, in agreement with the experimental results. The computations predict that the anion enhances the rate of the cis-interligand adduct formation by forming a dimetallic complex with the neutral complex. The [(Ni(tfd)2)2](-) dimetallic complex then coordinates ethylene and isomerizes to form a Ni,S-bound ethylene complex, which then rapidly isomerizes to the stable interligand adduct but not to the intraligand adduct. Thus, the anion catalyzes the formation of the interligand adduct. Significant experimental evidence for dimetallic species derived from nickel bis(dithiolene) complexes has been found. ESI-MS data indicate the presence of a [(Ni(tfd)2)2](-) dimetallic complex as the acetonitrile adduct. A charge-neutral association complex of Ni(tfd)2 with the ethylene adduct of Ni(tfd)2 has been crystallographically characterized. Despite the small driving force for the reversible association, very major structural reorganization (square-planar → octahedral) occurs.

Perchloric Acid Catalyzed Homogeneous and Heterogeneous Addition of β-Dicarbonyl Compounds to Alcohols and Alkenes and Investigation of the Mechanism

The direct addition of various beta-dicarbonyl compounds to a series of secondary alcohols and alkenes has been achieved using 1 mol % perchloric acid (HClO(4)) as the catalyst. The HClO(4)-catalyzed reactions could be conveniently conducted in commercial solvent and gave moderate to excellent yields. Moreover, the silica gel-supported HClO(4) could also catalyze the heterogeneous addition for a series of substrates with similar or even higher yields in comparison with the homogeneous ones. The supported catalyst could be readily recovered and reused for four runs. Furthermore, the mechanism of the HClO(4)-catalyzed addition of the beta-diketone to alcohol was investigated, and an S(N)1 mechanism was proved unambiguously for the first time through a series of experiments. The discrimination of catalytic abilities among different Brønsted acids was also rationalized by DFT calculations.

Uptake of one and two molecules of 1,3-butadiene by platinum bis(dithiolene): a theoretical study.

Platinum bis(dithiolene) complexes have reactivity toward alkenes like nickel bis(dithiolene) complexes. We examined the uptake of 1,3-butadiene by platinum bis(dithiolene) [Pt(tfd)2] (tfd = S2C2(CF3)2) via a density functional theory study; both 1,2- and 1,4-additions of 1,3-butadiene to the ligands of Pt(tfd)2 to form both interligand and intraligand adducts were studied. For single 1,3-butadiene addition, direct 1,4-addition on interligand S-S, 1,2-addition on intraligand S-S, and 1,4-addition on intraligand S-C are all feasible at room temperature and are controlled by the symmetry of the highest occupied molecular orbital of 1,3-butadiene and the lowest unoccupied molecular orbital of Pt(tfd)2. However, the formation of the interligand S-S adduct through 1,4-addition of one molecule of cis-1,3-butadiene is the most favorable route, with a reaction barrier of 9.3 kcal/mol. The other two addition processes cannot compete with this one due to both higher reaction barriers and unstable adducts. Other possible pathways, such as formation of cis-interligand S-S adduct from 1,2-addition of one molecule of 1,3-butadiene via a twisted trans-interligand S-S adduct, have higher barriers. Our calculated results show that 1,4-addition of a single molecule of 1,3-butadiene on the interligand S-S gives the kinetically stable product by a one-step pathway. But of at least equal importance is the apofacial 1,4-addition of two molecules of 1,3-butadiene on the intraligand S-C of the same ligand on Pt(tfd)2, which yields the thermodynamically stable product, obtained via a short lifetime intermediate, the 1:1 intraligand S-C adduct, being formed through several pathways. The calculated results in this study well explain the experimental observation that 1:1 interligand S-S adduct was formed in a short time, and the intraligand S-C adduct from two molecules of cis-1,3-butadiene was accumulated in 20 h at 50° and characterized by X-ray crystallography.

Density Functional Theory Study of CsCn− (n = 1−10) Clusters

In this paper, we report the design of numerous models of CsC(n)(-) (n = 1-10). By means of B3LYP density functional method, we carried out geometry optimization and calculation on the vibrational frequency. We found that the CsC(n)(-) (n = 4-10) clusters with Cs lightly embraced by C(n) are ground-state isomers. The structures are composed of C(n)(2-) and Cs(+) with the former being electronically stabilized by the latter. When n is even, the C(n) (n = 4-10) chain is polyacetylene-like. The CsC(n)(-) (n = 1-10) with even n are found to be more stable than those with odd n, and the result is in accord with the relative intensities of CsC(n)(-) (n = 1-10) observed in mass spectrometric studies. In this paper, we provide explanations for such trend of even/odd alternation based on concepts of the highest vibrational frequency, incremental binding energy, electron affinity, and dissociation channels.

A density functional study on nitrogen-doped carbon clusters CnN3- (n=1-8)

Using molecular graphics software, we designed numerous models of CnN3- (n=1-8). Geometry optimization and calculation on vibration frequency were carried out by the B3LYP density functional method. After comparison of structure stability, we found that the structures of ground-state CN3- and C2N3- are bent chains with a nitrogen atom at either end, whereas when n=3-8, the ground-state clusters show three branches, each with a nitrogen atom located at the end. When n=5-8, the longest branch of CnN3- is polyacetylenelike. When n=5 or 7, the longest branch is connected to the central sp2 carbon in a nonlinear manner. The CnN3- (n=1-8) with an even number of carbon atoms are more stable than those with odd numbers, matching the peak pattern observed in laser-induced mass spectra of CnN3-. The trend of such odd/even alternation is explained based on concepts of bonding characteristics, electron affinities, and incremental binding energies.