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21.
Mohammad Shaharuzzaman James ChickosCheok N. Tam Timothy A. Keiderling 《Tetrahedron: Asymmetry》1995,6(12):2929-2932
The synthesis of (2R,3R) and (2S,3S) dideuteriosuccinic acid in 63 ± 10% enantiomeric excess by reduction of the half acid ester of ethyl fumarate from (R) and (S) BINAP ruthenium (II) diacetate complex, respectively, is reported. (2R,3R) and (2S,3S)-Dideuteriosuccinic acid has also been prepared from (R)-BINAP ruthenium (II) dicarboxylate complex directly in similar optical purity by reversing the sequence of introduction of the isotopic label. 相似文献
22.
Bajor G Veszprémi T Riague el H Guillemin JC 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(15):3649-3656
Vinyl, allyl, and homoallyl selenols were easily prepared by a chemoselective reduction of the corresponding selenocyanates with aluminum hydrides. Two stable vinyl and five stable allyl conformers of both series were predicted on the potential-energy surface. The interaction of SeH or SeCN groups with the vinyl group has been investigated with UV photoelectron spectroscopy and quantum chemical calculations, using the MP2/cc-pVTZ and B3LYP/cc-pVTZ levels. In the vinyl derivatives, a surprisingly strong direct conjugation of the selenium lone electron pair and the C=C double bond was observed. On the other hand, in allyl position the selenium lone pair is independent on the C=C double bond, and the hyperconjugation between the Se-C bond and the double bond is the ruling effect. Thus is clarified the type and extent of the interaction between the SeH or SeCN group and the unsaturated moiety. 相似文献
23.
Peptides with C-terminal α-carboxamides were synthesized from a multi-detachable benzhydrylamine-resin containing a Boc-(4-acetoxy)benzhydryl-amine handle of unambiguous origin. The peptides bound to the new resin are stable to trifluoroacetic acid, but are cleavable by hydrogen fluoride, base and nucleophiles to give unprotected or protected peptide fragments. 相似文献
24.
György Schultz Tamás Nagy Gustavo Portalone Fabio Ramondo István Hargittai Aldo Domenicano 《Structural chemistry》1993,4(3):183-190
The molecular structure and benzene ring distortions of ethynylbenzene have been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF/6-31G* and 6-3G** levels. Least-squares refinement of a model withC
2v, symmetry, with constraints from the MO calculations, yielded the following important bond distances and angles:r
g(C
i
-C
o
)=1.407±0.003 Å,r
g(C
o
-C
m
)=1.397±0.003 Å,r
g(C
m
-C
p
)=1.400±0.003 Å,r
g(Cr
i
-CCH)=1.436 ±0.004 Å,r
g(C=C)=1.205±0.005 Å, C
o
-C
i
-C
o
=119.8±0.4°. The deformation of the benzene ring of ethynylbenzene given by the MO calculations, including o-Ci-Co=119.4°, is insensitive to the basis set used and agrees with that obtained by low-temperature X-ray crystallography for the phenylethynyl fragment, C6H5-CC-, in two different crystal environments. The partial substitution structure of ethynylbenzene from microwave spectroscopy is shown to be inaccurate in the ipso region of the benzene ring. 相似文献
25.
Tamás Veszprémi Yoshiya Harada Koichi Ohno Hideki Mutoh 《Journal of organometallic chemistry》1983,252(2):121-125
Penning ionization electron spectroscopy and CNDO/S calculations have been successfully applied to the analyses of the UV photoelectron spectra of 2- and 2,5-bistrimethylsilylthiophene and 2-t-butylthiophene. The relative intensities of the π type bands are greatly enhanced in the Penning spectra compared with those of the UV photoelectron spectra. 相似文献
26.
Tam Nga Chiu Fletcher Stephen P. Vogels Christopher M. Westcott Stephen A. Decken Andreas 《Transition Metal Chemistry》2003,28(1):103-109
Dioxomolybdenum(VI) complexes derived from ethyl maltol (2-ethyl-3-hydroxy-4-pyrone) and 1-alkyl-2-ethyl-3-hydroxy-4-pyridin-4(1H)-ones have been prepared and characterized using physical methods including 1H- and 13C-n.m.r. spectroscopy, i.r., elemental analysis, and X-ray diffraction for the pyrone and 3-methylpyridine pyridinone derivatives. The octahedral complexes have the general formula cis-MoO2L2 where L = the deprotonated pyrone or pyridinone ligand. 相似文献
27.
Wang X Andrews L Tam S DeRose ME Fajardo ME 《Journal of the American Chemical Society》2003,125(30):9218-9228
The reaction of laser-ablated Al atoms and normal-H(2) during co-deposition at 3.5 K produces AlH, AlH(2), and AlH(3) based on infrared spectra and the results of isotopic substitution (D(2), H(2) + D(2) mixtures, HD). Four new bands are assigned to Al(2)H(4) from annealing, photochemistry, and agreement with frequencies calculated using density functional theory. Ultraviolet photolysis markedly increases the yield of AlH(3) and seven new absorptions for Al(2)H(6) in the infrared spectrum of the solid hydrogen sample. These frequencies include terminal Al-H(2) and bridge Al-H-Al stretching and AlH(2) bending modes, which are accurately predicted by quantum chemical calculations for dibridged Al(2)H(6), a molecule isostructural with diborane. Annealing these samples to remove the H(2) matrix decreases the sharp AlH(3) and Al(2)H(6) absorptions and forms broad 1720 +/- 20 and 720 +/- 20 cm(-1) bands, which are due to solid (AlH(3))(n). Complementary experiments with thermal Al atoms and para-H(2) at 2.4 K give similar spectra and most product frequencies within 2 cm(-1). Although many volatile binary boron hydride compounds are known, binary aluminum hydride chemistry is limited to the polymeric (AlH(3))( solid. Our experimental characterization of the dibridged Al(2)H(6) molecule provides an important link between the chemistries of boron and aluminum. 相似文献
28.
Hapalosin was initially synthesized by macrolactonization, and a second synthesis was achieved by cycloamidation. In both syntheses, three of the five stereocenters in hapalosin were established by two Brown allylboration reactions. The synthesis of the non-N-Me analog of hapalosin involved chelation-controlled reduction of a gamma-amino-beta-keto ester and cycloamidation. In CDCl(3) at 25 degrees C, synthetic hapalosin exists as a 2.3:1 mixture of conformers, while its non-N-Me analog exists only as a single conformer. (1)H,(1)H-NOESY and computation reveal that the configuration of the amide bond is responsible for the conformations of the two compounds. The major conformer of hapalosin is found to be an s-cis amide, the minor conformer an s-trans amide, and the non-N-Me analog an s-trans amide. Applying distance constraints to protons that exhibit NOESY correlations, computation shows that the major conformer of hapalosin and the non-N-Me analog have very different conformations. By contrast, the minor conformer of hapalosin and the non-N-Me analog have very similar conformations. 相似文献
29.
Ring strain energies (RSEs) are predicted using homodesmotic reactions at the B3LYP/6-31G* level of theory. Substituents are conserved in the acyclic reference and any difference in energy between the ring and the acyclic reference corresponds exclusively to RSE. Small rings are stabilized by alkyl substituents and this stabilization decreases as the size of the ring increases. There is a destabilization of medium sized rings. Greater stabilization is found upon alkyl substitution at a double bond in an unsaturated ring and this stabilization decreases as ring size increases. The effects of cis-1,2-disubstitution on RSEs have been evaluated and indicate stabilization for both small and medium sized rings. RSEs of saturated and unsaturated polycyclic systems agree well with the RSEs derived from experimental thermochemical data. RSEs are reported for substituted norbornanes, norbornenes, and norbornadienes to complement experimental studies. 相似文献
30.
Correia I Costa Pessoa J Duarte MT Henriques RT Piedade MF Veiros LF Jakusch T Kiss T Dörnyei A Castro MM Geraldes CF Avecilla F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(9):2301-2317
The Schiff base N,N'-ethylenebis(pyridoxylideneiminato) (H(2)pyr(2)en, 1) was synthesized by reaction of pyridoxal with ethylenediamine; reduction of H(2)pyr(2)en with NaBH(4) yielded the reduced Schiff base N,N'-ethylenebis(pyridoxylaminato) (H(2)Rpyr(2)en, 2); their crystal structures were determined by X-ray diffraction. The totally protonated forms of 1 and 2 correspond to H(6)L(4+), and all protonation constants were determined by pH-potentiometric and (1)H NMR titrations. Several vanadium(IV) and vanadium(V) complexes of these and other related ligands were prepared and characterized in solution and in the solid state. The X-ray crystal structure of [V(V)O(2)(HRpyr(2)en)] shows the metal in a distorted octahedral geometry, with the ligand coordinated through the N-amine and O-phenolato moieties, with one of the pyridine-N atoms protonated. Crystals of [(V(V)O(2))(2)(pyren)(2)].2 H(2)O were obtained from solutions containing H(2)pyr(2)en and oxovanadium(IV), where Hpyren is the "half" Schiff base of pyridoxal and ethylenediamine. The complexation of V(IV)O(2+) and V(V)O(2) (+) with H(2)pyr(2)en, H(2)Rpyr(2)en and pyridoxamine in aqueous solution were studied by pH-potentiometry, UV/Vis absorption spectrophotometry, as well as by EPR spectroscopy for the V(IV)O systems and (1)H and (51)V NMR spectroscopy for the V(V)O(2) systems. Very significant differences in the metal-binding abilities of the ligands were found. Both 1 and 2 act as tetradentate ligands. H(2)Rpyr(2)en is stable to hydrolysis and several isomers form in solution, namely cis-trans type complexes with V(IV)O, and alpha-cis- and beta-cis-type complexes with V(V)O(2). The pyridinium-N atoms of the pyridoxal rings do not take part in the coordination but are involved in acid-base reactions that affect the number, type, and relative amount of the isomers of the V(IV)O-H(2)Rpyr(2)en and V(V)O(2)-H(2)Rpyr(2)en complexes present in solution. DFT calculations were carried out and support the formation and identification of the isomers detected by EPR or NMR spectroscopy, and the strong equatorial and axial binding of the O-phenolato in V(IV)O and V(V)O(2) complexes. Moreover, the DFT calculations done for the [V(IV)O(H(2)Rpyr(2)en)] system indicate that for almost all complexes the presence of a sixth equatorial or axial H(2)O ligand leads to much more stable compounds. 相似文献