Ketene-acetylene [2 + 2] cycloadditions: cyclobutenone and/or oxete formation?

The [2 + 2] cycloaddition of monosubstituted acetylenes to ketene has been studied by ab initio(G2(MP2,SVP) and DFT (B3LYP/6-31Gd)) methods. The activation barrier decreases with increasing electron-donating ability of the acetylene substituent, and it can be roughly correlated with the energy of the acetylene HOMO. The addition to the C[double bond, length as m-dash]C bond of ketene (giving cyclobutenones) is preferred for the less electron-rich acetylenes, but for the most electron rich ones (X = NH(2) and NMe(2)) the addition to the C[double bond, length as m-dash]O bond (giving oxetes) becomes competitive, with activation barriers as low as ca. 45 (30) kJ mol(-1) for the two computational methods used. The cyclobutenones and oxetes can undergo ring opening to vinylketenes and acylallenes, respectively. Furthermore, the latter two compounds can interconvert by a 1,3-shift of the substituent X. The acylallenes become thermodynamically more stable than the vinylketenes for [small pi]-(lone pair) donating substituents X, and the 1,3-shift barrier also decreases, to ca. 130 kJ mol(-1) for X = NMe(2). In contrast, the 1,3-shifts of CH(3) and H have very high barriers.     

13C NMR study of halogen bonding of haloarenes: measurements of solvent effects and theoretical analysis

Solvent effects on the NMR spectra of symmetrical (X = F (1), X = Cl (2), X = Br (3), X = I (4), X = NO2 (5), X = CN (6)) and unsymmetrical (X = I, Y = MeO (7), Y = PhO (8)) para-disubstituted acetophenone azines X-C6H4-CMe=N-N=CMe-C6H4-Y and of models X-C6H4-CMe=N-Z (X = I, Z = H (9), Z = NH2 (10)), 4-iodoacetophenone (11), and iodobenzene (12) were measured in CDCl(3), DMSO, THF, pyridine, and benzene to address one intramolecular and one intermolecular issue. Solvent effects on the (13)C NMR spectra are generally small, and this finding firmly establishes that the azine bridge indeed functions as a "conjugation stopper," an important design concept in our polar materials research. Since intermolecular halogen bonding of haloarenes do occur in polar organic crystalline materials, the NMR solution data pose the question as to whether the absence of solvent shifts indicates the absence of strong halogen bonding in solution. This question was studied by the theoretical analysis of the DMSO complexes of iodoarenes 4, 9-12, and of iodoacetylene. DFT and MP2 computations show iodine bonding, and characteristic structural and electronic features are described. The nonrelativistic complexation shifts and the change in the spin-orbit induced heavy atom effect of iodine compensate each other, and iodine bonding thus has no apparent effect on Ci in the iodoarenes. For iodides, complexation by DMSO occurs and may or may not manifest itself in the NMR spectra. The absence of complexation shifts in the NMR spectra of halides does not exclude the occurrence of halogen bonding in solution.     

Synthesis of 6-Hydroxy Derivatives of Steroidal Hormones by SeO 2 Mediated Oxidation

Selenium dioxide oxidation of molecules with cyclopentanoperhydrophenanthrene skeleton and allylic moieties, such as the well known human steroidal hormones progesterone and testosterone enables the syntheses of potential active 6-hydroxysteriods.     

Polyglycerol as a high-loading support for boronic acids with application in solution-phase Suzuki cross-couplings

In this paper, we describe the usage of a soluble high-loading polyglycerol support for functionalized boronic acids without further linker design. The quantitatively formed polyglycerol boron esters were subsequently employed in homogeneous Suzuki cross-coupling reactions to give high yields (84-91%) of functional biaryls with minimal amounts of the Pd catalyst (0.2 mol %). In situ precipitation and ultrafiltration were used as simple and effective purification protocols. Furthermore, the reaction conditions were optimized by the choice of the solvent and the catalyst.     

Computational studies on 3-aza-Cope rearrangements: protonation-induced switch of mechanism in the reaction of vinylpropargylamine

The 3-aza-Cope rearrangements of 3-azoniahexa-1,5-diene (1), 3-azoniahex-1-ene-5-yne (3), and 3-azahex-1-ene-5-yne (5) were investigated up to the coupled-cluster level, CCSD(T), by using a valence triple-zeta basis set. Activation barriers and geometrical parameters of the transition states are provided. Conformational studies were performed for all reactants and products of the reactions. Solvent effects were estimated from self-consistent reaction field calculations. In contrast to the other two species, the Cope rearrangement of 5 was found to proceed by a stepwise mechanism.     

The azine bridge as a conjugation stopper: an NMR spectroscopic study of electron delocalization in acetophenone azines

Dipole parallel-alignment of organic molecular crystals of azines has been achieved with a design that was based on the hypothesis that the azine bridge is a conjugation stopper. This hypothesis has now been tested in detail, and (1)H and (13)C NMR spectroscopic data of symmetric and asymmetric acetophenone azines are presented in support of this design concept. Previous structural, ab initio, and electrochemical studies have shown that the azine bridge largely inhibits through-conjugation in molecules with the general structure DPhC(Me) [double bond] N [bond] N [double bond] C(Me)PhA, where D is a donor group and A is an acceptor group. NMR spectroscopy is an excellent tool to probe the degree of conjugation through the azine bridge. The NMR results reported here for nine symmetrical and 18 asymmetrical azines show in a compelling fashion that the hypothesis holds. Varying the donor group does not change the chemical shifts of the aromatic hydrogen and carbon atoms on the acceptor-substituted phenyl ring. Likewise, varying the acceptor group does not change the chemical shifts of the atoms in the donor-substituted phenyl ring.     

[(α‐Imino)enamino]phosphonium salts from propyne iminium salts and a phosphorane imine

Propyne iminium triflates R¹C(N⁺R³R⁴)CCR² CF₃SO readily react with Ph₃PNPh to form 1:1 adducts which formally result from a metathetical addition of the phosphorane imine across the triple bond of the alkyne. These adducts are best described as enamino‐phosphonium salts or iminio‐substituted phosphorus ylides. The configuration of these salts has been determined from NMR data and an X‐ray crystal structure analysis of salt 3h . The base‐induced elimination of the PPh₃ substituent from enamino‐phosphonium salt 3a was studied. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:437–446, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20131     

A Spin-Statistics Theorem for Quantum Fields¶on Curved Spacetime Manifolds¶in a Generally Covariant Framework

A model-independent, locally generally covariant formulation of quantum field theory over four-dimensional, globally hyperbolic spacetimes will be given which generalizes similar, previous approaches. Here, a generally covariant quantum field theory is an assignment of quantum fields to globally hyperbolic spacetimes with spin-structure where each quantum field propagates on the spacetime to which it is assigned. Imposing very natural conditions such as local general covariance, existence of a causal dynamical law, fixed spinor- or tensor type for all quantum fields of the theory, and that the quantum field on Minkowski spacetime satisfies the usual conditions, it will be shown that a spin-statistics theorem holds: If for some of the spacetimes the corresponding quantum field obeys the “wrong” connection between spin and statistics, then all quantum fields of the theory, on each spacetime, are trivial. Received: 1 March 2001 / Accepted: 28 May 2001