ISSN:0947-6539    

DOI:10.1002/chem.19970030603

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe oxiranyl anion is a very unstable nucleophilic epoxide. This previously uncommon nucleophile has now been used for the synthesis of polycyclic ethers. Alkylation of an oxiranyl anion, a three‐carbon building block, with a triflate derived from tetrahydropyran‐methanol followed by acid‐catalyzed 6‐endocyclization provides atrans‐fused bicyclic compound. Reiterative application of this procedure enables the rapid construction oftrans‐fused polytetra

ISSN:0947-6539    

DOI:10.1002/chem.19970030604

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractTetracarbonyl‐π2‐alkyne–carbene chromium(0) complexes3a–cstabilized by a rigid C2‐arene unit bridging the alkyne and the carbene ligands were synthesized from their pentacarbonyl precursors2a–cby low‐temperature photodecarbonylation. The chelates3a–care the first stable alkyne–carbene complexes containing a Group VIB metal carbonyl unit, and can be regarded as arrested intermediates in the chromium‐mediated benzannulation reaction. Comparative X‐ray and13C NMR studies indicate only weak coordination of the alkyne, both in the solid state and in solution. Decomposition of chelates3a–cas well as the thermal decarbonylation of their pentacarbonyl precursors2a–cresults in dimerization of the alkynylcarbene ligands to give densely substituted oxygenated chrysene derivatives. A modification of the substitution pattern in the central chrysene rings results in a gradual distortion of the arene system across

ISSN:0947-6539    

DOI:10.1002/chem.19970030605

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe bis(ethene) complex [Tp*Ir‐(C2H4)2] (1*) (Tp*= tris(3,5‐dimethyl‐1‐pyrazol‐1‐yl)hydroborato) undergoes thermal rearrangement to the hydrido–allyl complex [Tp*IrH(π3‐C3H4Me)] (6*), through the intermediacy of the hydrido–vinyl complex [Tp*IrH(C2H3)(C2H4)] (2*). The overall conversion of1*into6*corresponds formally to the dimerisation of ethene by an unprecedented pathway that involves sequential C–H bond activation of a coordinated olefin molecule and C–C bond formation by coupling of the resulting vinyl and ethene moieties. Similar transformations have been observed for monosubstituted olefins like propene and 1‐butene, while the internal alkenecis−2‐butene experiences allylic activation of an sp3C–H bond, which provides an alternative route to6*. The extension of these investigations to the analogous complexes of the unsubstituted tris(pyrazolyl)hydroborato ligand Tp is also reported. Mechanistic studies on the formation of the C–C bond by coupling of the vinyl and the olefin ligands suggest the participation of a vinylidene complex (formed by α‐H abstraction from the vinyl group), which then rearranges to an allene species. Evidence for the involvement of these and other key

ISSN:0947-6539    

DOI:10.1002/chem.19970030606

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractDodecamethyl‐2,3,5,6,7,8‐hexa‐sila‐lλ3,4λ3‐diphosphabicyclo[2.2.2]oc‐tane (1) and its arsenic analogue2are readily accessible in 69 and 73% yield, respectively, by the cyclocondensation reaction of 1,2‐dichloro‐1,1,2,2‐tetrame‐thyldisilane (5) with the lithium pnictides [LiEH2(dme)] (E = P (6), As(7); dme = 1,2‐dimethoxyethane). The reactions proceed via 1,4‐diphosphaoctamethyltetrasi‐lacyclohexane (8) and its arsenic analogue9, respectively, which were isolated and structurally characterized by X‐ray crystallography. The molecular structures of1and2, which are isotypic, were also established by single‐crystal X‐ray analysis: they possessD3point symmetry with the expected Si–E bond lengths (E = P, As) but unusually long Si–Si bonds. The latter are 0.02–0.03 Å longer than those in8and9, mainly due to through‐bond interactions (TB) between donating n orbitals of the E atoms and the σ*acceptor orbitals of the Si–Si bond. The first expanded analogues of1, namely,12and14, with hexamethyltrisilane and dodecamethyl‐hexasilane chains bridging the two phosphorus atoms, were synthesized in a onepot cyclocondensation reaction of the corresponding 1,3‐ and 1,6‐dichloro‐oligosilanes11and13, respectively, with [LiPH2(dme)]6. Ab initio calculations on the parent compounds1a, 12a, and the second‐row analogue 1,4‐diazabicyclo‐[2.2.2]octane (B) were carried out in order to analyze the different coupling constants and magnitudes of intramolecular interactions (through‐space/through‐bond coupling). TS and TB coupling inBwere found to be about two times stronger than in the congener1a, due to the compactness of the N2C6skeleton and the greater extent of s, p hybridization at nitrogen. Evidence for TB interactions in1was obtained by photoelectron spectroscopy and by comparison of the two first vertical ionization potentials with calculated values for1a. The best agreement with experimental data was achieved when1awas calculated at the MP2 level. Compound1apreferentially adoptsD3point symmetry; the higher‐symmetryD3hform possesses one imaginary frequency and is slightly less stable (0.46 kcal mol−1at HF/6–31 G*//HF/6–31 G*and 1.58 kcal mol−1at MP2/ 6–31 G*//HF/6–31 G*level), suggesting that this structure corresponds to a transition state on the potential energy surface. The structures corresponding to the global minimum ofBand12ahaveD3handC3hsymmetry, respectively. At the HF/6–31 G*//HF

ISSN:0947-6539    

DOI:10.1002/chem.19970030607

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe structures, energetics, and dynamics of various isomers of [Fe‐(PR3)4H3]+(R = H, Me) were studied by density functional theory. The conformations considered were derived from a square planar (P), tetrahedral (T) and C2v‐butterfly (C) arrangement of the phosphine ligands. For PH3, the stability ranking PC>T was obtained, whereas in the PMe3case the P isomer was least stable, T>C>P. Mechanisms for the hydrogen exchange in the C and T isomers are discussed. For the latter, this process might be described as a tetrahedral jump, but could also be viewed as a dodecahedral distortion of a cubic arrangement. The theoretical findings are compared with the results of experimental studies of [Fe(PR3)4H3]+(R = Me,

ISSN:0947-6539    

DOI:10.1002/chem.19970030608

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractCationic iridium(I) complexes of chiral phosphanodihydrooxazoles were used as catalysts for the enantioselective hydrogenation of prochiralN‐alkyl andN‐aryl imines. The complexes are air‐stable crystalline solids that can be readily prepared and are easy to handle. The structures of two complexes were determined by X‐ray analysis. ForN‐alkyl imines of acetophenone, enantiomeric excesses of up to 79% were obtained. Dialkyl ketimines and cyclic imines showed lower reactivity and selectivity. A remarkable dilution effect was observed for the hydrogenation of theN‐phenyl imine of acetophenone: decreasing the substrate and catalyst concentration led to a significant improvement of the enantioselectivity. Thus, up to 89%eecould be achieved using 0.1 mol% of catalyst. The highest enantioselectivities were obtained in weakly coordinating solvents such as CH2Cl2. Additives such as halides, imides, or amines were found to poison the catalyst. Hydrogen pressures of 100 bar were usually employed, but in some cases identical results were achieved with on

ISSN:0947-6539    

DOI:10.1002/chem.19970030609

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe possible configurations of two molecules of tetrolic acid in a solvent box containing 226 carbon tetrachloride molecules are studied by molecular dynamics with the GROMOS package and force field over periods of up to 2000 picoseconds. The cyclic hydrogen‐bonded dimer was the most persistent configuration, but events leading to the cleavage of one hydrogen bond and thus the formation of a precursor to the crystal catemer motif were found to occupy up to 10% of the simulation times. The experimental bond‐breaking enthalpy was correctly reproduced. Two different crystal structure generation procedures were employed to reproduce the two observed polymorphic crystal structures and to predict other possible polymorphs; in all cases, some unobserved structures had more cohesive packing energies than the observed ones. The possible application of molecular dynamics in the study of the preliminary steps in crystal nucleation is discus

ISSN:0947-6539    

DOI:10.1002/chem.19970030610

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractOligosilsesquioxanes (OSS) may be considered as molecular sections of silica. The incompletely condensed (cy‐clopentyl)7T7(OH)3(T = SiO3/2) has been used to generate a phosphine‐functionalized T8‐OSS system by reaction with 2‐(trichlorosilyl)diphenylphosphinoethane. The resulting model molecule (cyclopentyl)7T8(CH2)2PPh2(1) is well suited to coordinate to transition metal clusters via the phosphorous atom. We selected the tetrahedral bimetallic cluster [HFeCo3‐(CO)12]. Depending on the stoichiometry, one or two phosphine ligands could be added to the cobalt atoms, with substitution of CO molecules. Owing to the cyclopentyl substituents, the resulting complexes [HFeCo3(CO)11{Ph2P(CH2)2T8‐(cyclopentyl)7}] (2) and [HFeCo3‐(CO)10{Ph2P(CH2)2T8(cyclopentyl)7}2] (3) are soluble in numerous organic solvents. Therefore, they could be completely characterized by1H,13C,29Si, and31P NMR

ISSN:0947-6539    

DOI:10.1002/chem.19970030611

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractWe present idealised geometrical models for the α‐Ga and β‐Ga structures based on two differently distorted, corrugated 36nets. We investigated the structural stability of two sets of two‐dimensional and three‐dimensional model structures consisting of these corrugated nets as a function of the net puckering. We used the simple tight‐binding (TB) Hückel model with the structural energy difference theorem and the advanced full‐potential linear muffin‐tin orbital (FP‐LMTO) method in the framework of local‐density functional theory. Both methods show the existence of an optimum puckering angle that corresponds well to the situation in the experimental α‐Ga structure. The geometrical model of the α‐Ga structure follows the building principle of terminally coordinated deltahedral clusters, extended to two‐dimensional structures. The chemical bonding in α‐Ga is interpreted in terms of multicentre bonding within the corrugated 36nets and two‐electron–two

ISSN:0947-6539    

DOI:10.1002/chem.19970030612

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY