Symmetry breaking in the cyclic C(3)C(2)H radical

We have employed high-level coupled cluster methods including connected triple excitations to study the possibility of symmetry-breaking in the (2)B(2) ground state of the c-C(3)C(2)H radical. Specifically, we find that spin-restricted open-shell Hartree-Fock (ROHF) reference orbitals yield a C(2v) structure, whereas spin-unrestricted Hartree-Fock (UHF) and Brueckner orbitals lead to a symmetry-broken C(s) minimum-energy geometry. Equation-of-motion coupled cluster singles and doubles method for ionized states yields a C(s) structure with a double-zeta basis set, but not with a triple-zeta basis set. Through a detailed analysis of the orbital instability/near-instability behavior of each type of Hartree-Fock reference, we have determined that the UHF reference wave function is more reliable than the ROHF reference in this case and that the Born-Oppenheimer potential surface for c-C(3)C(2)H exhibits a symmetry broken C(s) global minimum. This result is supported by excited-state computations, which indicate that a second-order (pseudo) Jahn-Teller interaction is responsible for the symmetry-breaking.     

Theoretical study of M(PH3)2 complexes of C60, corannulene (C20H10), and sumanene (C21H12) (M = Pd or Pt). Unexpectedly large binding energy of M(PH3)2(C60)

DFT and MP2 to MP4(SDQ) methods were applied to M(PH3)2(C60), Pt(PH3)2(C20H10), and Pt(PH3)2(C21H12) (M = Pd or Pt, C20H10 = corannulene, and C21H12 = sumanene). The binding energy considerably fluctuates around MP2 and MP3 levels but much less upon going from MP3 to MP4(SDQ) in Pt(PH3)2(C2H4), Pt(PH3)2(C20H10), and Pt(PH3)2(C21H12). Also, the MP4(SDQ) method presents a binding energy similar to that of the CCSD(T) method in Pt(PH3)2(C2H4). Thus, it is likely that the MP4(SDQ) method is useful to evaluate binding energies of these complexes. The binding energies of Pt(PH3)2(C20H10) and Pt(PH3)2(C21H12) are evaluated to be 24.9 and 26.1 kcal/mol, respectively, by the MP4(SDQ) method and only +5.8 and -2.6 kcal/mol, respectively, by the DFT(B3LYP) method. These MP4(SDQ)-calculated binding energies of Pt(PH3)2(C20H10) and Pt(PH3)2(C21H12) are similar to that of Pt(PH3)2(C2H4), which strongly suggests that these complexes can be successfully synthesized. The binding energy of Pt(PH3)2(C60) is evaluated to be 44.8 and 45.5 kcal/mol with the ONIOM(MP4(SDQ):UFF) and ONIOM(MP4(SDQ):B3LYP) methods, respectively, and that of the Pd analogue is evaluated to be 39.9 kcal/mol with the ONIOM(MP4(SDQ):UFF) method, whereas the DFT(B3LYP), DFT(BVP86), and DFT(BPW91) methods provide much smaller binding energies. It is noted that these binding energies are much larger than those of the ethylene, corannulene, and sumanene analogues. This difference is reasonably interpreted in terms that the LUMO of C60 is at much lower energy than those of ethylene, corannulene, and sumanene. We investigated also how to separate the high level and the low level regions in the ONIOM calculation of M(PH3)2(C60) and proposed here the reasonable way to evaluate the binding energy of transition-metal complexes of C60.     

Rhodium(I)‐Catalyzed Sequential C(sp)C(sp3) and C(sp3)C(sp3) Bond Formation through Migratory Carbene Insertion

A Rh^I‐catalyzed three‐component reaction of tert‐propargyl alcohol, diazoester, and alkyl halide has been developed. This reaction can be considered as a carbene‐involving sequential alkyl and alkynyl coupling, in which C(sp)? C(sp³) and C(sp³)? C(sp³) bonds are built successively on the carbenic carbon atom. The Rh^I‐carbene migratory insertion of an alkynyl moiety and subsequent alkylation are proposed to account for the two separate C? C bond formations. This reaction provides an efficient and tunable method for the construction of all‐carbon quaternary center.     

Stereoselective Introduction of Steroid Side Chains at C (17) and C (20)

A simple and efficient new method for the highly stereoselective (at C (17) and C (20)) introduction of steroid side chains which are suitably functionalized for further elaboration is presented. The ene reaction of (17 Z)-ethylidene steroids, which are readily obtained from 17-keto steroids via a Wittig reaction, with various enophiles such as formaldehyde and acrylate esters leads to useful intermediates which contain the natural steroid configuration at C (20). Catalytic hydrogenation of the Δ¹⁶-double bond occurs from the α-face to stereospecifically generate the correct configuration at C (17). An additional chiral center at C (23) is also introduced stereoselectively by the use of methyl 2-chloroacrylate as the enophile.     

Modeling of configurations and third-order nonlinear optical properties of C(36) and C(34)X(2) (X=B,N)

Using the ab initio method, the geometrical structures of C(36) and the X (B,N)-doped isomers C(34)X(2) have been optimized. On the basis of the optimized structures, then, the third-order nonlinear optical polarizabilities gamma in the different optical processes of the third-harmonic generation, electric-field induced second-harmonic generation and degenerate four-wave mixing, and two-photon absorption (TPA) cross sections delta are calculated by using TDB3LYP method coupled with the sum-over-states method. The calculated results show that the one-photon allowed excitation process dominate the two-photon excitation process for C(36)-D(6h), whereas the two-photon allowed excitation process dominate the one-photon excitation process for C(36)-D(2d) and C(34)X(2) (B,N). It is found that the largest resonant TPA peaks of dopant fullerenes have a blueshift and the TPA cross sections have an enhancement compared with those of the parent fullerenes of isomers C(36)-D(6h) and C(36)-D(2d).     

Enantiospecific access to various C(9),C(10)-disubstituted camphors: scope and limitations

The valuable chiral sources C(9),C(10)-disubstituted camphors can be enantiospecifically obtained from the corresponding C(9)-substituted camphors by a general and straightforward synthetic method. This method involves the electrophilic treatment of a key 2-methylenenorbornan-1-ol intermediate, followed by a controlled tandem carbon-carbon double-bond addition-Wagner-Meerwein rearrangement of the norbornane framework. Discussion of the results presented suggests possible extensions and limitations of the methodology used. The feasibility of this method has been exemplified by the highly efficient enantiospecific preparation of several interesting C(9)-halogen-, C(10)-halogen, O-, S-, or Se-substituted camphors.     

Electronic excitations of C(60) aggregates

Excitation properties of the isolated C(60) and (C(60))(N) model clusters (N = 2, 3, 4, 6 and 13) are studied using an a priori parameterized and self-consistent Hamiltonian, the Complete Neglect of Differential Overlap considering the l azimuthal quantum number method. This method properly describes electron excitations of the isolated C(60) after the configuration interaction of singles (CIS) procedure, when those are compared with experimental data in n-hexane solution and in a molecular beam. Geometry models of (C(60))(N) clusters to model the effect of aggregation were obtained from the fullerene fcc crystal. Some peaks in the low energy edge of the absorption spectrum appear corresponding to clustering effects, as well as small increases of bandwidths in the strong bands at the UV region. An analysis of the theoretical absorption spectrum for dimer models has been carried out, taking into account the influence of the distance between fullerene centers. The density of states of CIS for fullerene clusters in the range from 2.0 to 6.5 eV shows the possibility of electron transitions as functions of the size of the clusters.     

Rollover‐Assisted C(sp2)C(sp3) Bond Formation

Rollover cyclometalation involves bidentate heterocyclic donors, unusually acting as cyclometalated ligands. The resulting products, possessing a free donor atom, react differently from the classical cyclometalated complexes. Taking advantage of a “rollover”/“retro‐rollover” reaction sequence, a succession of oxidative addition and reductive elimination in a series of platinum(II) complexes [Pt(N,C)(Me)(PR₃)] resulted in a rare C(sp²)?C(sp³) bond formation to give the bidentate nitrogen ligands 3‐methyl‐2,2′‐bipyridine, 3,6‐dimethyl‐2,2′‐bipyridine, and 3‐methyl‐2‐(2′‐pyridyl)‐quinoline, which were isolated and characterized. The nature of the phosphane PR₃ is essential to the outcome of the reaction. This route constitutes a new method for the activation and functionalization of C?H bond in the C(3) position of bidentate heterocyclic compounds, a position usually difficult to functionalize.     

Stereoselective synthesis of C(1)–C(9) and C(11)–C(17)fragments of protomycinolide iv based on asymmetric pinacol-typerearrangement

Two chiral intermediates, C(1)–C(9) and C(11)–C(17) portionsof protomycinolide IV, were synthesized both from (S)-ethyl lactatevia asymmetric pinacol-type rearrangement followed bydiastereoselective reactions on α-methyl-β,γ-unsaturatedcarbonyl compounds.     

Towards Stereoselective Synthesis of the C(31)–C(39) and C(20)–C(27) Fragments of Phorboxazole A

The stereoselective synthesis of the C(31)–C(39) and C(20)–C(27) fragments of phorboxazole A ( 1 ) was achieved from commercially available and inexpensive D ‐mannitol. Crimmins aldol reaction and a decarboxylative Claisen‐type reaction are the key steps for the C(31)–C(39) fragment, and L ‐proline‐catalyzed aldol reaction, Sharpless asymmetric epoxidation, and epoxide ring opening reaction with Gilman's reagent are the key steps for the C(20)–C(27) fragment of phorboxazole.     

Probing adsorption sites of silica-supported platinum with (13)C(16)O + (12)C(16)O and (13)C(18)O + (12)C(16)O mixtures: a comparative fourier transform infrared investigation

A comparative investigation of the adsorption of (13)C(18)O + (12)C(16)O and (13)C(16)O + (12)C(16)O mixtures on silica-supported Pt has been conducted. It is advantageous to use (13)C(18)O + (12)C(16)O mixtures rather than (13)C(16)O + (12)C(16)O to probe the adsorption sites and electronic state of supported Group VIII metals because the vibrational bands of the adsorbed (13)C(18)O and (12)C(16)O isotopic molecules do not overlap. In addition, while an intensity redistribution suppresses the lower-frequency band with adsorbed (13)C(16)O and (12)C(16)O with vibrational frequencies differing by 50 cm(-1), the intensity redistribution is less pronounced with the adsorbed (13)C(18)O and (12)C(16)O in which the frequency difference is 100 cm(-1). Moreover, the small intensity redistribution that does occur between the bands of adsorbed (13)C(18)O and (12)C(16)O still allows the detection of the vibrational band of adsorbed (13)C(18)O at (13)C(18)O gas-phase concentrations as low as 3%. At such low concentrations, the dipole-dipole interaction between adsorbed (13)C(18)O molecules is negligible, and, hence, both the singleton frequency and the dipole-dipole shift for adsorbed CO may be obtained in a single experiment. Two types of strongly bound and one type of weakly bound linear CO-Pt adsorption complexes have been identified and characterized by their singleton frequencies and dipole-dipole coupling shifts. The origin of these CO adsorption modes is discussed.     

Highly selective and simple synthesis of C(2)(m)--X--C(2)(n) fullerene dimers

Energetic-radiation-induced dimerization reaction of fullerenes was found to be a simple and highly selective method for synthesis of C2m-X-C2n (m = n or m not equal n) type molecules without formation of other products. Utilizing the new method, C70-C-C70, C60-C-C70, C60-C-C60, and C70-O-C70 were prepared and characterized. The method is capable of synthesizing new C2m-X-C2n molecules by introducing X (different atoms) into the reaction system. Energetic radiation created reactive sites for covalently bonded bridges between fullerene molecules originally only weakly bound by van der Waals force. This observation may open a new subject and practicable approach for polymer sciences of fullerenes.     

Silylation as a protective method in acetylene chemistry. : Syntheses in the polyeneyne series,R3Si(CC)n(CHCH)4(CC)nSiR3 and H(CC)n(CHCH)4(CC)nH (n = 1, 2)

The Grignard reagents R₃Si(CC)_nMgBr (R = Me, n = 1; R = Et, n = 1,2) couple with cyclooctatetraene dibromide 1 in THF to give, as major products, the silyl-stabilised E, Z, Z, E-polyeneynes, Me₃SiCC(CHCH)₄CCSiMe₃3a, Et₃SiCC(CHCH)₄CCSiEt₃4a and Et₃Si(CC)₂(CHCH)₄(CC)₂SiEt₃6a together with minor proportions of configurational isomers Z, E, Z, Z 3c, all -E 3b, 4b, 6b and compounds in which a bicyclo-octadiene structure 2, 5 and 7 is retained. Irradiation converts the cis(Z)-rich isomers e.g. 3c into the all-trans(E) products. Treatment of the bissilyl compounds 3, 4 and 6 with aqueous base liberates the respective parent polyeneynes, H(CC)_n(CHCH)₄(CC)_nH, in each case.     

A simple synthesis route and characterisation of Co(3)Mo(3)C

A simple, one-step thermal decomposition method for the preparation of Co(3)Mo(3)C is reported in this paper. In this novel synthesis route, a mixed-salt precursor, containing Co(CH(3)COO)(2) x 4H(2)O, (HMT)(2)(NH(4))(4)Mo(7)O(24) x 2H(2)O (HMT = hexamethylenetetramine), and excess HMT is directly decomposed to the bimetallic carbide under flowing argon at 1023 K. The role of HMT in the preparation process has been investigated and a detailed reaction mechanism is proposed based on the experimental results. The bimetallic carbide is characterised by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, BET surface area measurement and X-ray photoelectron spectroscopy. Furthermore, the activity of the as-prepared Co(3)Mo(3)C is evaluated by a 3-methylpyridine hydrodenitrogenation (HDN) reaction. The catalyst produced from this method provides better reactivity compared to the Co(3)Mo(3)C catalyst prepared by the conventional temperature-programmed reduction method.     

Synthesis of the C(18)-C(34) fragment of amphidinolide C and the C(18)-C(29) fragment of amphidinolide F

A convergent synthesis of the C(18)-C(34) fragment of amphidinolide C and the C(18)-C(29) fragment of amphidinolide F is reported. The approach involves the synthesis of the common intermediate tetrahydrofuranyl-β-ketophosphonate via cross metathesis, Pd(0)-catalyzed cyclization, and hydroboration-oxidation. The β-ketophosphonate was coupled to three side chain aldehydes using a Horner-Wadsworth-Emmons (HWE) olefination reaction to give dienones, which were reduced with l-selectride to give the fragments of amphidinolide C and F.     

Structures and electronic properties of C(56)Cl(8) and C(56)Cl(10) fullerene compounds.

Stimulated by the recent observation of the first C(56)Cl(10) chlorofullerene (Science, 2004, 304, 699), we performed a systematic density functional study of the structures and properties of C(56)Cl(10) and related compounds. The fullerene derivatives C(56)Cl(8) and C(56)Cl(10) based on the parent fullerene C(56)(C(2v):011), rather than those from the most stable C(56) isomer with D(2) symmetry, are predicted to possess the lowest energies, and they are highly aromatic. Further investigations show that the heats of formation of the C(56)Cl(8) and C(56)Cl(10) fullerene derivatives are highly exothermic, that is, -48.59 and -48.89 kcal mol(-1) per Cl(2) (approaching that of C(50)Cl(10)), suggesting that adding eight (or ten) Cl atoms releases much of the strain of pure C(56)(C(2v):011) fullerene and leads to highly stable derivatives. In addition, C(56)Cl(8) and C(56)Cl(10) possess large vertical electron affinities, especially for C(56)Cl(8) with value of 3.20 eV, which is even larger than that (3.04 eV) of C(50)Cl(10), indicating that they are potential good electron acceptors with possible photonic/photovoltaic applications. Finally, the (13)C NMR chemical shifts and infrared spectra of C(56)Cl(8) and C(56)Cl(10) are simulated to facilitate future experimental identification.     

Mass spectrometric studies of the path of carbon in photosynthesis: positional isotopic analysis of (13)C-labelled C(4) to C(7) sugar phosphates

The (EIMS) electron ionization mass spectrometric fragmentation patterns of the methoxime- and ethoxime-trimethylsilyl (TMS) derivatives of C(4) to C(7) sugars involved as phosphates in the Calvin pathway of photosynthesis in plants were analysed by gas chromatography/EIMS using specifically labelled (13)C analogs. In general, most but not all of the major ions in the mass spectra arise from single carbon-carbon bond cleavages of the straight-chain derivatives. The results confirm that GC/MS of the alkoxime-TMS derivatives is a viable method for measuring (13)C incorporations at individual carbon atoms in each of the sugar phosphates during photosynthetic experiments with (13)CO(2).     

Dirhodium(II)-Catalyzed C(sp2)−H Azidation of Benzaldehydes

Multiple steps are needed to achieve the C−H functional of aromatic aldehyde, since the C−H functional reaction usually occurs preferentially at the aldehydic C−H bond over the aryl C−H bond. We report an efficient azidation method mediated by dirhodium(II) catalysts to achieve the direct aryl azidation of aromatic aldehydes avoiding the simultaneous use of protected aldehydes and prefunctionalized arenes. The regioselectivity of this method is similar to those of typical aromatic electrophilic substitution reactions. The resulting azidobenzaldehyde products are versatile building blocks or precursors for the synthesis of many biologically active compounds. The mechanism studies indicate that the one-electron oxidative intermediate Rh₂^(II,III)N₃ is responsible for the azide transfer.     

CCSD calculations on C(14), C(18), and C(22) carbon clusters

The structure and energetics of the ring isomers of C(4n+2) (n=3-5) carbon clusters were studied by using coupled-cluster singles and doubles excitation theory to overcome the vast differences existing in the literature. The results obtained in the present study clearly indicate that C(14), C(18), and C(22) carbon rings have bond-length and bond-angle alternated acetylenic minimum energy structures. Contrarily, density functional theory calculations were unable to predict these acetylenic-type structures and they ended up with the cumulenic structures. It is found from the coupled-cluster studies that the lowest-energy ring isomer for the first two members of C(4n+2) series is a bond-angle alternated cumulenic D((2n+1)h) symmetry structure while the same for the remaining members is a bond-length and bond-angle alternated C((2n+1)h) symmetry structure. In C(4n+2) carbon rings, Peierls-type distortion, transformation from bond-angle alternated to bond-length alternated minimum energy structures, occurs at C(14) carbon ring.     

Iron Sulfido Derivatives of the Fullerenes C(60) and C(70)

Toluene solutions of C(60) react upon UV irradiation with Fe(2)S(2)(CO)(6) to give C(60)[S(2)Fe(2)(CO)(6)](n)() where n = 1-6. C(60)[S(2)Fe(2)(CO)(6)](n)() where n = 1-3 have been isolated and characterized. Crystallographic studies of C(60)S(2)Fe(2)(CO)(6) show that the S-S bond of the Fe(2) reagent is cleaved to give a dithiolate with idealized C(2)(v)() symmetry. The addition occurred at a 6,6 fusion, and the metrical details show that the Fe(2) portion of the molecule resembles C(2)H(4)S(2)Fe(2)(CO)(6). IR spectroscopic measurements indicate that the Fe(2)(CO)(6) subunits in the multiple-addition species (n > 1) interact only weakly. UV-vis spectra of the adducts show a shift to shorter wavelength with addition of each S(2)Fe(2)(CO)(6) unit. Photoaddition of the phosphine complex Fe(2)S(2)(CO)(5)(PPh(3)) to C(60) gave C(60)[S(2)Fe(2)(CO)(5)(PPh(3))](n)(), where n = 1-3. (31)P{(1)H} NMR studies show that the double adduct consists of multiple isomers. Photoaddition of Fe(2)S(2)(CO)(6) to C(70) gave a series of adducts C(70)[S(2)Fe(2)(CO)(6)](n)() where n = 1-4. HPLC analyses show one, four, and three isomers for the adducts, respectively.