Xiufang Xu

Ligand Effects on Rates and Regioselectivities of Rh(I)-Catalyzed (5 + 2) Cycloadditions: A Computational Study of Cyclooctadiene and Dinaphthocyclooctatetraene as Ligands

The first theoretical study on the effects of ligands on the mechanism, reactivities, and regioselectivities of Rh(I)-catalyzed (5 + 2) cycloadditions of vinylcyclopropanes (VCPs) and alkynes has been performed using density functional theory (DFT) calculations. Highly efficient and selective intermolecular (5 + 2) cycloadditions of VCPs and alkynes have been achieved recently using two novel rhodium catalysts, [Rh(dnCOT)](+)SbF(6)(-) and [Rh(COD)](+)SbF(6)(-), which provide superior reactivities and regioselectivities relative to that of the previously reported [Rh(CO)(2)Cl](2) catalyst. Computationally, the high reactivities of the dnCOT and COD ligands are attributed to the steric repulsions that destabilize the Rh-product complex, the catalyst resting state in the catalytic cycle. The regioselectivities of reactions with various alkynes and different Rh catalysts are investigated, and a predictive model is provided that describes substrate-substrate and ligand-substrate steric repulsions, electronic effects, and noncovalent π/π and C-H/π interactions. In the reactions with dnCOT or COD ligands, the first new C-C bond is formed proximal to the bulky substituent on the alkyne to avoid ligand-substrate steric repulsions. This regioselectivity is reversed either by employing the smaller [Rh(CO)(2)Cl](2) catalyst to diminish the ligand-substrate repulsions or by using aryl alkynes, for which the ligand-substrate interactions become stabilizing due to π/π and C-H/π dispersion interactions. Electron-withdrawing groups on the alkyne prefer to be proximal to the first new C-C bond to maximize metal-substrate back-bonding interactions. These steric, electronic, and dispersion effects can all be utilized in designing new ligands to provide regiochemical control over product formation with high selectivities. The computational studies reveal the potential of employing the dnCOT family of ligands to achieve unique regiochemical control due to the steric influences and dispersion interactions associated with the rigid aryl substituents on the ligand.

Generation of Rhodium(I) Carbenes from Ynamides and Their Reactions with Alkynes and Alkenes

Rh(I) carbenes were conveniently generated from readily available ynamides. These metal carbene intermediates could undergo metathesis with electron-rich or neutral alkynes to afford 2-oxopyrrolidines or be trapped by tethered alkenes to yield 3-azabicyclo[3.1.0]hexanes, a common skeleton in numerous bioactive pharmaceuticals. Although the scope of the former is limited, the latter reaction tolerates various substituted alkenes.

Rh-Catalyzed (5+2) Cycloadditions of 3-Acyloxy-1,4-enynes and Alkynes: Computational Study of Mechanism, Reactivity, and Regioselectivity

The mechanism of Rh-catalyzed (5+2) cycloadditions of 3-acyloxy-1,4-enyne (ACE) and alkynes is investigated using density functional theory calculations. The catalytic cycle involves 1,2-acyloxy migration, alkyne insertion, and reductive elimination to form the cycloheptatriene product. In contrast to the (5+2) cycloadditions with vinylcyclopropanes (VCPs), in which alkyne inserts into a rhodium-allyl bond, alkyne insertion into a Rh-C(sp(2)) bond is preferred. The 1,2-acyloxy migration is found to be the rate-determining step of the catalytic cycle. The electron-rich p-dimethylaminobenzoate substrate promotes 1,2-acyloxy migration and significantly increases the reactivity. In the regioselectivity-determining alkyne insertion step, the alkyne substituent prefers to be distal to the forming C-C bond and thus distal to the OAc group in the product.

Z-Selective Ethenolysis with a Ruthenium Metathesis Catalyst: Experiment and Theory

The Z-selective ethenolysis activity of chelated ruthenium metathesis catalysts was investigated with experiment and theory. A five-membered chelated catalyst that was successfully employed in Z-selective cross metathesis reactions has now been found to be highly active for Z-selective ethenolysis at low ethylene pressures, while tolerating a wide variety of functional groups. This phenomenon also affects its activity in cross metathesis reactions and prohibits crossover reactions of internal olefins via trisubstituted ruthenacyclobutane intermediates. In contrast, a related catalyst containing a six-membered chelated architecture is not active for ethenolysis and seems to react through different pathways more reminiscent of previous generations of ruthenium catalysts. Computational investigations of the effects of substitution on relevant transition states and ruthenacyclobutane intermediates revealed that the differences of activities are attributed to the steric repulsions of the anionic ligand with the chelating groups.

Nickel-Catalyzed Hydroacylation of Styrenes with Simple Aldehydes: Reaction Development and Mechanistic Insights.

The first nickel-catalyzed intermolecular hydroacylation reaction of alkenes with simple aldehydes has been developed. This reaction offers a new approach to the selective preparation of branched ketones in high yields (up to 99%) and branched selectivities (up to 99:1). Experimental data provide evidence for reversible formation of acyl-nickel-alkyl intermediate, and DFT calculations show that the aldehyde C-H bond transfer to a coordinated alkene without oxidative addition is involved. The origin of the reactivity and regioselectivity of this reaction was also investigated computationally, which are consistent with experimental observations.

The anion recognition properties of hydrazone derivatives containing anthracene.

A series of artificial receptors, hydrazone derivatives containing anthracene, have been designed and synthesized. The interaction of these receptors with biologically important anions was determined by UV-vis, fluorescence and 1H NMR titration experiments and theoretical investigation. Results indicate that the receptor (1) without NO2 shows no binding ability for various anions. The other receptors (2 and 3) show the highest binding ability for acetate (AcO-) among studied anions (fluoride (F-), dihydrogen phosphate (H2PO4-), chloride (Cl-), bromide (Br-), iodide (I-)); and the binding ability for AcO- is not interfered by the existence of other anions. The additions of AcO-, F- and H2PO4- can arouse different degrees of fluorescence quenching. 1H NMR titration shows that the interaction between the receptor 2 and F- firstly depends on the hydrogen-bond formation; later the interacted site NH is deprotonated and the added F- forms hydrogen bond with the near CH in Schiff base. Moreover, visual color changes accompany guest binding, enabling this system to act as colorimetric anion sensors.

The relationship between adsorption energies of methyl on metals and the metallic electronic properties: A first-principles DFT study

A theoretical study of CH3 adsorbed on the (111) surface of some transition and noble metal surfaces M (M = Cu, Ni, Rh, Pt, Pd, Ag, Au) and on the Fe(100) is presented. We find that the hollow site is preferred more than the top one for Fe, Ni, Rh, and Cu, but it is the other way for Pt, Pd, Au, and Ag. In addition, a good linear relationship was observed between the chemisorption energy and d‐band center for Group VIII metals or the square of the coupling matrix element for Group IB metals at the hollow site. Interestingly, with a detailed comparison of the adsorption energies at the top and hollow sites, we find that the adsorption energies among each group are very similar on the top site, which supports the theoretical model of Hammer and Nørskov that the coupling between the HOMO of adsorbate and sp states of the metal is dominant and almost equal, and that the second coupling to the d‐band contributes less but reflects the change of the adsorption energy. It confirms that the coupling to the d band comprises two opposite factors, that is, the d‐band center was attractive and the square of the coupling matrix element was repulsive, such that the contributions from the two factors can counteract each other at the top site.

Systematic investigation of the molecular behaviors of heterofullerenes C48X2 (X=B, N)

Abstract A systematic investigation on all possible substituted fullerene isomers of C 48 B 2 and C 48 N 2 has been performed using the semiempirical methods AM1 and MNDO. The equilibrium geometrical structures, heats of formation, strain, aromaticity, HOMO–LUMO energy gaps, ionization potentials, electronic affinities, the absolute hardness and electronegativity have been studied. The results indicate that the isomer-78, which corresponds to 1,4-substitution in the six-membered ring located on the equator, is the most stable isomer for both C 48 B 2 and C 48 N 2 . The driving force governing the stabilities of the present studied C 48 X 2 (X=B, N) isomers is the strain being inherent in the C 50 cage. The contribution of the conjugation effect to the stabilization is not able to compete with that of the strain. From an application of the HSAB principle, the absolute hardness of the more stable isomers of both C 48 B 2 and C 48 N 2 are larger than that of C 50 , and the direction of electron flow for forming a complex among them may be C 48 N 2 →C 50 →C 48 B 2 according to the calculated absolute electronegativity. The more stable C 48 X 2 isomers have larger ionization potentials and smaller electronic affinities compared with C 50 , which suggests that it is more difficult to oxidize and reduce C 48 X 2 , i.e., the redox characteristics of C 50 can be weakened by doping. The vibrational spectra and electronic absorption spectra of these substituted fullerenes have been calculated, which could serve as a framework to interpret future experimental results. The computed nucleus independent chemical shifts (NICS) values also provide a basis for the possible characterization of these C 48 X 2 isomers.

Fuzzy Symmetry Characteristics of Propadine Molecule

Abstract The fuzzy symmetry characteristics for the internal-rotation of propadine were analyzed using the fuzzy symmetry theory for molecule and molecular orbital (MO). In the process of rotation, three different symmetry point groups D2h, D2d, and D2 were considered. Using the D4h point group, which is the minimal point group including all symmetry elements of D2h, D2d, and D2, we can analyze the fuzzy symmetry for this process. The elements included in D4h point group can be classified to four subsets: (i) G0—it includes all the elements in D2 point group, also belongs to all the above three point groups of D2h, D2d, and D2; (ii) G1—it includes the elements in D2h point group, but not in D2d point group; (iii) G2—it includes the elements in D2d point group, but not in D2h point group; (iv) G3—it includes the elements in D4h point group, but not in D2h or D2d point group. On the basis of the above four subsets, we analyzed the membership functions and the regularity of variation in MOs for the internal-rotation of propadine.

A systematic investigation on the molecular behaviors of substituted fullerenes C34X2 (X=N, B)

Abstract A systematic investigation on the molecular behaviors of all the possible isomers of C 34 B 2 and C 34 N 2 formed from the initial C 36 fullerene with C 6 v and C 6 h symmetries have been performed using the semi-empirical AM1 and MNDO methods. The equilibrium geometrical structures, heats of formation, HOMO–LUMO gap energies, heats of atomization, ionization potentials (IP) and affinity potentials (AP), conjugate effect and deformation energies of C 34 X 2 (X=N, B) have been studied. The calculation results show that the heterofullerenes are less stable than C 36 , and the C 34 N 2 should be more stable than the boron analog C 34 B 2 . Both empirical methods in this work indicate that the most stable isomer of C 34 X 2 (X=N, B) corresponds to 1,4-substitution in the cyclohexatriene unit which locating at the equatorial belt of the C 36 cage. Generally speaking, the C 34 X 2 (X=N, B) isomers with the doped atoms near the equatorial belt are more stable that the rest. The heterofullerenes C 34 X 2 have bigger IP and smaller AP compared with C 36 , thus the redox activity of C 36 can be reduced by doping. The results of π-orbital axis vector analysis show that replacements of carbon atoms with either nitrogen or boron can notably release the strains in local part of the cage. Both C 34 N 2 and C 34 B 2 are expected to have significantly different chemical and physical properties from those of the fullerenes.