Tungsten(0)- and rhenium(I)-catalyzed tandem cyclization of acetylenic dienol silyl ethers based on geminal carbo-functionalization of alkynes

Tungsten(0)- and rhenium(I)-catalyzed reactions of acetylenic dienol silyl ethers based on the concept of geminal carbo-functionalization of alkynes are reported. Treatment of 3-siloxy-1,3-diene-7-ynes with catalytic amounts of [W(CO)(6)] or [ReCl(CO)(5)] under photoirradiation conditions gives synthetically useful bicyclo[3.3.0]octane derivatives in good yields. Extremely high catalytic activity is noted for the rhenium(I) complex. The reaction has been extended to substrates containing a nitrogen atom in their tethers. In this case, two kinds of synthetically useful heterocyclic compounds-the 2-azabicyclo[3.3.0]octane derivatives 9 and the monocyclic dihydropyrroles 10, with allenyl substituents-are obtained, and selective preparation of either product can be achieved through the use of an appropriate combination of the nitrogen substituent and the type of the rhenium(I) catalyst. The 2-azabicyclo[3.3.0]octane derivatives 9 are obtained selectively by carrying out treatment of N-Ns derivatives in the presence of [ReCl(CO)(4)(PPh(3))], whereas the dihydropyrrole derivatives 10 are obtained by treatment of N-Mbs derivatives with [ReCl(CO)(5)]/AgSbF(6) . Finally, we have applied this geminal carbo-functionalization to one-carbon-elongated substrates containing N-Ts moieties in their tethers. Selective 5-exo cyclization is achieved in the presence of gold(I) or rhenium(I) catalysts, whereas 6-endo cyclization is observed on use of [W(CO)(6)].     

Inhibition mechanism of flavin by deprenyl as an acetylenic irreversible inhibitor

Possible inhibition mechanisms of flavin (isoalloxazine) with (−)-deprenyl as an acetylenic irreversible inhibitor have been investigated in detail by ab initio methods with the 6-31G* basis set through the simplified model compounds 3-formyl-2-imino-1-hydropyrazine and propargylamine. The resulting compounds have been verified by calculations with the 3-21G basis set using flavin itself and the model of (−)-deprenyl for confirmation of the reactions through the simplified models. Two cyclic O4,N5- and C4a,N5-covalent adducts have been found. The latter was the most stable and was considered to be the final irreversible product. The intermediates in the reaction, the acyclic C4a- or N5-allenic compounds and their hydrogen-transferred cyanine-type compounds, are in agreement with the results of experimental photochemical reactions. In most of the reaction processes, hydrogen migration played an important role. Received: 26 June 1998 / Accepted: 28 August 1998 / Published online: 11 November 1998     

Electron-intramolecular-vibration interactions in positively charged phenanthrene-edge-type hydrocarbons

Electron-phonon interactions in positively charged phenanthrene-edge-type hydrocarbons such as phenanthrene, chrysene, and picene are studied. The C-C stretching modes around 1500 cm(-1) and the low-frequency modes around 500 cm(-1) strongly couple to the highest occupied molecular orbitals (HOMO) in phenanthrene-edge-type hydrocarbons. The total electron-phonon coupling constants for the monocations (lHOMO) of 0.251, 0.135, and 0.149 eV for phenanthrene, chrysene, and picene, respectively, are estimated to be larger than those of 0.130, 0.107, and 0.094 eV for anthracene, tetracene, and pentacene, respectively. The phase patterns difference between the HOMO localized on carbon atoms which are located at the molecular edge in acene-edge-type hydrocarbons and the delocalized HOMO in phenanthrene-edge-type hydrocarbons is the main reason for the result. Strengths of orbital interactions between two neighboring carbon atoms in the HOMO become weaker with an increase in molecular size because the electron density on each carbon atom in the HOMO becomes smaller with an increase in molecular size in phenanthrene-edge-type hydrocarbons. On the other hand, the frontier orbitals of acene-edge-type hydrocarbons have somewhat nonbonding characters and thus cannot strongly couple to the totally symmetric vibrational modes compared with the frontier orbitals of phenanthrene-edge-type hydrocarbons. This is the reason why the lHOMO value for phenanthrene-edge-type hydrocarbons decreases with an increase in molecular size more significantly than that for acene-edge-type hydrocarbons, and the reason why the lHOMO value for polyphenanthrene with C2v geometry (0.033 eV) is estimated to be similar to that for polyacene (0.036 eV). The reorganization energies between the neutral molecules and the corresponding monocations for phenanthrene-edge-type hydrocarbons with large molecular size are estimated to be larger than those for acene-edge-type hydrocarbons with large molecular size.     

ESR and electrical conduction in pyrolyzed polyimides

Electrical conductivity and electron spin resonance (ESR) of pyrolyzed polyimides change drastically in air after passage of a characteristic time, which depends on the temperature and time of pyrolysis. Samples must be kept from air throughout the measurements. ESR and resistivity measurements in vacuum show that there are two types of magnetic species: localized and delocalized. A. variable-range hopping model is proposed as a possible mechanism for conduction.     

MO calculations of episulfonium ion intermediates and regiospecificity in the additions of Cl−

The calculations were carried out for the ethylene, propylene, iso-butylene and butadiene episulfonium ion intermediates by CNDO/2 method to optimize the geometric parameters and investigate the electronic states of the intermediates, resulting that the p-form intermediates were unexpectedly most stable. It was shown that the differences in extension of the lowest unoccupied molecular orbitals on ethylenic carbon atoms in the intermediates have an important role on the regiospecificity of the ring-opening additions of Cl^? to the intermediates.     

Midgap levels of photoexcited conductive polymers. II. Detailed analysis of trans-polyacetylene

We have studied in more detail aspects of the soliton-antisoliton pair in a trans-polyacetylene chain based on our previous study of the midgap levels associated with the photogenerated oppositely charged soliton-antisoliton pair in conductive polymers employing the concept of the molecular orbital interaction (Part I of this study, this issue). The intersoliton distance has been estimated to be about 10 Å from the Pariser–Parr–Pople method. We have found that the energy gap between the midgap levels is estimated to be 0.45 eV, being significantly related to an additional photoinduced absorption.     

Quantum mechanical studies of environmental effects on biomolecules

SCFab initio computations have been performed on the structure, molecular potential and hydration scheme of thiourea in view of a comparison with urea and more generally as a model of the conjugated S=CNH-group as compared to O=CNH-. In contrast to the carbonyl oxygen, both σ and π acceptor, the sulfur atom of the thiocarbonyl is a σ donor but a π acceptor and this results in an enhancement of the double-bond character of thiourea. The CN bond is less attractive for a proton than urea. The hydration scheme indicates a maximum number of four water molecules directly bound to thiourea.     

High-Resolution Laser Spectroscopy of the AΠ1←XΣ System of IBr with a Titanium:Sapphire Ring Laser

The vibrotational absorption spectra of the A←X electronic transition of I^79/81Br were measured in the 11 330- to 13 220-cm⁻¹ region using a Ti:sapphire ring laser. The P-, Q-, and R-branch lines of the rotational states from J=10 to 100 belonging to the v′←v″=(3∼20)′,←(1∼6)″ bands were assigned. The P- and R-branch lines, unlike the Q-branch lines, were split into the doublet by the nuclear quadrupole coupling effect of the I atom. The quadrupole coupling constants of eQq₀ and eQq₂ in the A state were estimated to be −0.030±0.018 and −0.062±0.018 cm⁻¹, respectively, by using the first order perturbation theory. The unperturbed line positions for the rotational lines higher than J=20 were determined. The Dunham coefficients of the X state were determined by the least squares fitting method using the pseudo vibrotational transition wavenumbers obtained by calculating the combination differences between the electronic spectral lines assigned and the far infrared vibrotational lines reported by Nelander et al. (7). The spectroscopic constants of Tv′, Bv′, Dv′, and Hv′ of the A state were determined suitable for the vibrational states from v′=3 to 20 by using a least squares fitting procedure.