Photoelektronen‐Spektren und Moleküleigenschaften, 95. 1,2‐Dithiolan – Bindungsmodell für α‐Liponsäure

Abstract

Die biochemische Wirkgruppe von a-Liponsaure (1) -ein gesattigter Funfring mit zwei benachbarten Schwefelzentren -findet sich in 1,a-Dithiolan (2) wieder, welches ungewohnliche Molekuleigenschaften besitzt: Entsprechend dem geringen Torsionswinkel von etwa 30" zwischen den Schwefel-Elektronenpaaren ns sind die beiden ns-Ionisierungen um 1.72 eV aufgespalten. Infolge der niedrigen ersten Ionisierungsenergie laRt sich -wie auch aus a-Liponsaure selbst -durch selektive Oxidation mit AICI, in H,CCI, ein bestandiges Radikalkation (HZC),Sz' @ erzeugen, dessen temperaturabhangiges ESR-Spektrum eine Einebnung der Disulfid-Bindung sowie ein Umklappen der abgewinkelten Methylen-Briicke innerhalb der ESR-Zeitskala anzeigt. Geometrieoptimierte closed-und open shell-SCF-Berechnungen stutzen die Spektrenzuordnung und geben Einblick in die Ladungsverteilung von Funfring-Disulfiden. Photoelectron Spectra and Molecular Properties, 95 1,ZDithiolane -Bonding Model for UrLipoic AcidThe biochemically active group of a-lipoic acid (1) -a saturated five-membered ring containing two adjacent sulfur centers -is retained in 1,2-dithiolane (2), which displays unusual molecular properties: in accordance with the small dihedral angle of ca. 30" between the sulfur lone pairs ns, the two ns ionizations are splitted by 1.72 eV. Due to the low first ionization energy, selective oxidation using AICI, in H,CC12 yields -also from a-lipoic acid itself -a stable radical cation (H,C),S,' @, the temperature-dependent ESR spectrum of which evidences the planarization of the disulfide bond and a flickering of the twisted methylene bridge within the ESR time scale. Geometry-optimized closed and open shell SCF calculations support the assignment of the spectra and provide insight into the charge balance of five-membered ring disulfides. a-Liponsaure (1) spielt als Coenzym eine wichtige Rolle, z. B. bei der oxidativen Decarboxylierung von Brenztra~bensaure~). Hierbei wird die SS-Bindung unter gleichzeitigem ,,At~m-Transfer"~) reduktiv gedffnet und oxidativ geschlossen. Stets erfolgt die reduktive und/oder nucleophile offnung der SS-Brucke in gesattigten Funfring-Disul-

Oxidative ring contraction of 2-phenyl-1,3-dithiane in ZSM-5: restricted mobility of 1,2-dithiolane radical cations in zeolite channels

“…The average g value, g avg = 2.0172, is in reasonable agreement with the isotropic g value, g iso = 2.0183, reported for the radical cation of 1,2-dithiolane, 2 •+ , in solution. 12,13 Likewise, the average hfc observed in the zeolite, a avg = 9.7 G, agrees well with the isotropic coupling constant of 9.75 G reported for 2 •+ in solution. 12,13 The identity of the species was confirmed by an essentially identical EPR spectrum obtained upon incorporation of an independently synthesized 14 sample of 2 into ZSM-5.…”

Catalyst Leaching as an Efficient Tool for Constructing New Catalytic Reactions: Application to the Synthesis of Cyclic Vinyl Sulfides and Vinyl Selenides

“…The degree of deviation of the ring from planarity can be evaluated from the distance between the C4 atom and the mean plane of the other four ring atoms (0.678 Å). Considering the geometrical dimension of the five-membered ring it can be seen that the two C-Se bond lengths [1.971 (2) An interesting correlation between a decrease of the torsion angle around the disulfide bridge and an increase of the S-S bond length has been reported by Bock et al [21] It is now well established that the major contribution to the enhanced reactivity of 1,2-dithiolanes arises from the small dihedral angle around the S-S bond associated with the reduced size of the ring and by the resulting unfavorable interaction between the adjacent sulfur lone pairs. [22] Accordingly, the S-S bond in 1,2-dithiolanes is longer and weaker than that found in open-chain disulfides, whose dihedral angle is close to 90°.…”

Geminal conjugation in disulfides

“…An intense short-wave peak A corresponds to the transition from the HOMO+ ns-o& into the K vacancy of the ~ 'As mentioned previously, there is the competitive n, -n, conjugation. But n , + n, and n, -n, levels do not resolve in SK, spectra because of the small difference between energies (0.2-0.6 eV [57]) and a rather large width of partial lines in the SK, spectra (21.2 eV).…”

“…It should be noted that the n, -M, interaction does not lead to sufficient integral redistribution of the electron density and changes of bond lengths and leads only to the ns level splitting (AZP,,). n, interaction efficiency which increases with the growth of the z value deviation from 90" [50,57] AIP,,, eV = 0.13 + 1.78 cos z (4)…”