Arnold Zweig

Chemical shifts in donor substituted polycyclic aromatics

Abstract The proton chemical shifts for 29 methoxy, dimethylamino and methylthio substituted polycylic aromatics are reported. A strong deshielding effect on α-protons due to peri substitution is always found. This effect appears to increase in going from the nitrogen to the oxygen derivatives and is largest for the sulfur compounds. In 9,10-bis-(methylthio)anthracene where the S-Me is constrained to lie out of the plane of the rings, the peri effect is largest. A combination of charge distribution, ring current and peri effects reproduces the measured aromatic proton chemical shifts with reasonable accuracy.

Pre-annihilation electrochemiluminescence☆

Abstract Electrochemiluminescence generated by alternating electrolysis of solutions of fluorescent aromatic hydrocarbons is attributed primarily to radical-anion—cation annihilation. However, light can also be detected while oxidizing the anion or reducing the cation of certain molecules at potentials insufficient to generate the oppositely charged radical. The mechanism of the pre-annihilation ecl has been studied for two aromatic hydrocarbons. The light pulse produced while oxidizing the rubrene anion in a double-potential-step experiment was found to be singlet emission (spectrum identical to fluorescence) even though the overpotentials applied were considerably less than the 2·3 eV singlet energy of this molecule. Possible energy doubling mechanisms are discussed. Most of the pre-annihilation light generated from the rubrene anion is shown to result from the oxidation of an anion decomposition product, followed by a homogeneous chemiluminescent reaction. Phenanthrene, unlike most molecules, exhibits pre-annihilation luminescence which is not the same colour as its fluorescence (violet). Oxidation of the anion at overvoltages of about 2·6 V produces a green emission with a maximum between 4900 and 5000 A. This corresponds closely to the published room temperature solution phosphorescence. The emission is quenched by appropriate triplet quenchers. This is believed to be the first positive evidence for electrochemically generated triplets.

Asymmetric Synthesis: Synthesis of (S) 2,3,5,6-Tetrahydro-6-Phenylimidazo[2,1-b]Thiazole (Levamisole)

Reduction of 1-(2-methoxyethyl)-3acyl-4-phenyl-4-imidazolin- 2-ones 7 (a-f) with (+) DIOP-Rh(COD)Cl as catalyst and hydrogen provide the corresponding optically active 1-(2-methoxyethyl)-3- acyl-4-phenyl-2-imidazolidones 8 (a-f) in greater than 90 percent yield and in up to 33 percent enantiomeric excess. The latter were hydrolyzed to l-(2-methoxyethyl)-4-phenyl-2-imidazolidone 5b. Further reaction of 5b with phosphorus pentasulfide followed by cyclization with hydrochloric acid provides the same degree of enantiomeric excess of the anthelmintic (S) 2,3,5,6-tetrahydro-6- phenylimidazo[2,1-b]thiazole (levamisole).

Photochemical generation of imines from azasuccinic anhydrides

Abstract Triarylazasuccinic anhydrides have been prepared and found to efficiently decompose photochemically by two pathways to give either the corresponding triarylimine, carbon dioxide and carbon monoxide or a diarylcarbenoid intermediate, carbon dioxide and an arylisocyanate. The type of photocleavage appears to be dependent on the position in the molecule of the chromophore with the lowest excited singlet state.

Electronic and ESR spectra of 1, 4-bis(methylthio)naphthalene cation-radical

Abstract The ESR spectrum of the radical cation of 1,4-bis(methylthio)naphthalene is reported together with the efficiency of its formation in sulfuric acid. The concentration of radical species, obtained from the ESR measurements is used to determine the extinction coefficients of the electronic spectrum in the ultraviolet and visible regions.

Preparation and photolysis of diphenyldiazasuccinic anhydride

The hitherto inaccessible diphenyldiazasuccinic anhydride has been prepared and found to photodecompose to cis-azobenzene together with carbon monoxide and carbon dioxide via a new type of photoelimination reaction.