This paper describes the unique catalytic activity of bimetallic Au/Pd alloy nanoclusters (NCs) for Ullmann coupling of chloroarenes in aqueous media at low temperature. The corresponding reaction cannot be achieved by monometallic Au and Pd NCs as well as their physical mixtures. On the basis of quantum chemical calculation, it was found that the crucial step to govern the unusual catalytic activity of Au/Pd is the dissociative chemisorption of ArCl, which is unlikely in the monometallic Au and Pd NCs.

Bowl-shaped aromatic compounds, namely buckybowls constitute a family of curved polycyclic aromatic carbons along with fullerenes and carbon nanotubes. Doping of heteroatoms to the carbon frameworks of such aromatic compounds drastically modulates their physical and chemical properties. In contrast to nitrogen-doped azafullerenes or carbon nanotubes, synthesis of azabuckybowls, nitrogen-doped buckybowls, remains an unsolved challenging task. Here we report the first enantioselective synthesis of a chiral azabuckybowl, triazasumanene. X-ray crystallographic analysis confirmed that the doping of nitrogen induces a more curved and deeper bowl structure than in all-carbon buckybowls. As a result of the deeper bowl structure, the activation energy for the bowl inversion (thermal flipping of the bowl structure) reaches an extraordinarily high value (42.2 kcal per mol). As the bowl inversion corresponds to the racemization process for chiral buckybowls, this high bowl inversion energy leads to very stable chirality of triazasumanene.

The aerobic oxidation of methanol to formic acid catalyzed by Au(20)(-) has been investigated quantum chemically using density functional theory with the M06 functional. Possible reaction pathways are examined taking account of full structure relaxation of the Au(20)(-) cluster. The proposed reaction mechanism consists of three elementary steps: (1) formation of formaldehyde from methoxy species activated by a superoxo-like anion on the gold cluster; (2) nucleophilic addition by the hydroxyl group of a hydroperoxyl-like complex to formaldehyde resulting in a hemiacetal intermediate; and (3) formation of formic acid by hydrogen transfer from the hemiacetal intermediate to atomic oxygen attached to the gold cluster. A comparison of the computed energetics of various elementary steps indicates that C-H bond dissociation of the methoxy species leading to formation of formaldehyde is the rate-determining step. A possible reaction pathway involving single-step hydrogen abstraction, a concerted mechanism, is also discussed. The stabilities of reactants, intermediates and transition state structures are governed by the coordination number of the gold atoms, charge distribution, cooperative effect and structural distortion, which are the key parameters for understanding the relationship between the structure of the gold cluster and catalytic activity in the aerobic oxidation of alcohols.

The mechanism of the aerobic oxidation of methanol to formic acid catalyzed by Au8− has been systematically investigated using density functional theory with the M06 functional. The reaction pathways were examined by taking into account the full structural relaxation of the Au8−. Stepwise and concerted reaction mechanisms are proposed. The stepwise mechanism is initiated by the hydrogen abstraction of a methoxy species by a superoxo‐like anion on the gold cluster, resulting in the formation of formaldehyde. Subsequently, the formaldehyde is activated by the hydroxyl group of a hydroperoxyl‐like complex, leading to the formation of a hemiacetal intermediate. The formation of formic acid in the final step is achieved by hydrogen abstraction of the hemiacetal intermediate by atomic oxygen attached to the gold cluster. Our calculations indicate that the first step of the stepwise mechanism, that is, hydrogen abstraction of the methoxy species, is the rate‐determining step. Another possible reaction pathway involving a single‐step hydrogen abstraction, a concerted mechanism, is also discussed. This mechanism may also be responsible for the reasonable catalytic activity of aerobic oxidation of methanol on Au8− because of the low activation energy barrier. © 2012 Wiley Periodicals, Inc.

The mechanism of the gold nanocluster-catalyzed aerobic homocoupling of arylboronic acids has been elucidated by means of DFT calculations with Au20(-) as a model cluster for the Au:[poly(N-vinylpyrrolidin-2-one)] catalyst. We found that oxygen affects the adsorption of phenylboronic acid and, by lowering the energy barrier, a water molecule enhances dissociation of the C-B bond, which is probably the rate-determining step. The key role of oxygen is in activating the surface of the gold cluster by generating Lewis acidic sites for adsorption and activation of the phenylboronic acid, leading to the formation of biphenyl through a superoxo-like species. Moreover, the oxygen adsorbed on the Au nanocluster can act as an oxidant for phenylboronic acid, giving phenol as a byproduct. As shown by NBO analysis, the basic aqueous reaction medium facilitates the reductive elimination process by weakening the Au-C bond, thereby enhancing the formation of biphenyl. The coupling of phenyl and reductive elimination of biphenyl occur at the top or facet site with low-energy-barrier through spillover of phenyl group on Au NC. The present findings are useful for the interpretation or design of other coupling reactions with Au NC.

The correlation between the bowl-inversion energy and the bowl depth for sumanenes monosubstituted with an iodo, formyl, or nitro group was investigated experimentally and by theoretical calculations. The bowl-inversion energies of the substituted sumanenes were determined experimentally by two-dimensional NMR exchange spectroscopy measurements. Various density functional theory methods were examined for the calculation of the structure and the bowl-inversion energy of sumanene, and it was found that PBE0, ωB97XD, and M06-2X gave better fits of the experimental value than did B3LYP. The experimental value was well reproduced at these levels of theory. The bowl structures and bowl-inversion energies of monosubstituted sumanenes were therefore calculated at the ωB97XD/6-311+G(d,p) level of theory. In both the experiments and the calculations, the correlation followed the equation ΔE = acos 4 θ, where a is a coefficient, ΔE is the bowl-inversion energy, and cos θ is the normalized bowl depth, indicating that the bowl inversion follows a double-well potential energy diagram.

Asymmetric total syntheses and structure revisions of dihydroisocoumarin‐type natural products, eurotiumide A and eurotiumide B have been described. The key features of these total syntheses are the asymmetric Shi epoxidation, regio‐ and stereoselective epoxide opening, C1 insertion/lactonization cascade reaction for constructing the 4‐methoxyisochroman‐1‐one skeleton. We confirmed the structures and configurations of eurotiumide A and B on the basis of X‐ray crystallographic analysis of the key intermediate and revealed that eurotiumide A and B have cis and trans configurations at the H3/H4 positions, which indicates the opposite relationship of the stereochemistry with respect to the previous report. Their absolute configurations were also determined. These natural products are highly fluorescent in several solvents with large Stokes shifts involving the excited‐state intramolecular proton transfer mechanism, which is supported by time‐dependent density functional theory. Eurotiumide A also displays fluorescence in Bacillus cereus.

Electrophilic substitution reactions of sumanene were studied. Mono-, di-, and trisubstituted sumanenes were selectively prepared with the separation of all regioisomers. The regioselectivity was predicted well by the HOMO density values determined by DFT calculations.The regioselective synthesis of aromatic compounds, including sumanene derivatives, is very important in order to explore their physical properties.1 For example, two sumanene derivatives trisubstituted at the benzene rings, C 3 symmetric and unsymmetrical, are possible if ortho-substituted derivatives are excluded. Previously, we developed the functionalization of C 3 symmetric derivatives from precursor A (Scheme 1).2 However, direct preparation of both isomers from sumanene (1) 3 is also beneficial if these isomers can be easily separated or one isomer can be selectively obtained. A more complicated situation is encountered in the case of disubstituted sumanene synthesis; three possible isomers may be produced (Table 1).Electrophilic aromatic substitution (S E Ar) is one of the most reliable methods for the direct functionalization of aromatic compounds. However, due to the lack of regioselectivity and the difficulty in separation of the regioisomers, the S E Ar route has rarely been applied to sumanene derivatization. 4 The purpose of this study involves the development of regioselective electrophilic substitution reactions of sumanene, and prediction of their regioselectivity by DFT calculations.Selective syntheses of monosubstituted sumanenes were first investigated as summarized in Scheme 2.5 Iodosumanene (2) was selectively prepared in 75% yield by AuCl 3 -catalyzed iodination 6 with N-iodosuccinimide (NIS) with recovery of 1. Nitrosumanene (3) was obtained in 65% yield by nitration using trifluoroacetyl nitrate, generated in situ from trifluoroacetic anhydride and concd nitric acid.7 1 was completely consumed and dinitrosumanenes were not formed. However, some amount of degradation was observed due to the harsh conditions. Formylation was achieved using triflic anhydride and DMF 8 under microwave-assisted heating conditions to afford formylsumanene (4) in 60% yield. Because of the low reactivity of the reagent, high temperature (130°C) was required to complete the reaction. Although diformylsumanenes were also formed in 20% yield with 4, these compounds were easily separated from 4 by preparative thin layer chromatography (PTLC). Acetylation was achieved by adopting similar conditions as the formylation in the presence of DMA to afford acetylsumanene (5) in 64% yield and diacetylsumanenes in 10% yield. Benzoylsumanene (6) was also prepared in 68% yield using triflic acid and PhCOCl with complete consumption of 1.9 They were easily purified by PTLC as well.The thus-employed reaction conditions were further applied to the syntheses of disubstituted sumanenes (Table 1).10 Preparation of diiodosumanenes 7, dinitrosumanenes 8, diformylsumanenes 9, diacetylsumanenes 10, and dibenzoylsumanenes 11 were achieved by increasing the amount of reagent...