Takahiro Kanda

Catalytic Chain-Breaking Pyridinol Antioxidants

The synthesis of 3-pyridinols carrying alkyltelluro, alkylseleno, and alkylthio groups is described together with a detailed kinetic, thermodynamic, and mechanistic study of their antioxidant activity. When assayed for their capacity to inhibit azo-initiated peroxidation of linoleic acid in a water/chlorobenzene two-phase system, tellurium-containing 3-pyridinols were readily regenerable by N-acetylcysteine contained in the aqueous phase. The best inhibitors quenched peroxyl radicals more efficiently than alpha-tocopherol, and the duration of inhibition was limited only by the availability of the thiol reducing agent. In homogeneous phase, inhibition of styrene autoxidation absolute rate constants k(inh) for quenching of peroxyl radical were as large as 1 x 10(7) M(-1) s(-1), thus outperforming the best phenolic antioxidants including alpha-tocopherol. Tellurium-containing 3-pyridinols could be quantitatively regenerated in homogeneous phase by N-tert-butoxycarbonyl cysteine methyl ester, a lipid-soluble analogue of N-acetylcysteine. In the presence of an excess of the thiol, a catalytic mode of action was observed, similar to the one in the two-phase system. Overall, compounds bearing the alkyltelluro moiety ortho to the OH group were much more effective antioxidants than the corresponding para isomers. The origin of the high reactivity of these compounds was explored using pulse-radiolysis thermodynamic measurements, and a mechanism for their unusual antioxidant activity was proposed. The tellurium-containing 3-pyridinols were also found to catalyze reduction of hydrogen peroxide in the presence of thiol reducing agents, thereby acting as multifunctional (preventive and chain-breaking) catalytic antioxidants.

A New Preparative Method of Organoalkali and Organoalkaline-Earth Metals Using Metal-Tellurium Exchange Reactions

Abstract Diorganyl tellurides (RTeR′) undergoes exchange reaction by the treatment with organometallic reagents of alkali and organoalkaline-earth metals (M[dbnd]Li, Na, K, MgX and CaX) in a fashion that thermodynamically more stable carbanions are formed, which then react with carbonyl compounds to give corresponding addition products in good yields. The scope and limitation of the exchange reactions are also discussed.

Catalytic Chain-breaking Pyridinol Antioxidants

When assayed for their capacity to inhibit azo-initiated peroxidation of linoleic acid in a water/chlorobenzene two-phase system, tellurium-containing 3-pyridinols were readily regenerable by N-acetylcysteine contained in the aqueous phase. The best inhibitors quenched peroxyl radicals more efficiently than alpha-tocopherol, and the duration of inhibition was limited only by the availability of the thiol reducing agent. The compounds were also found to catalyze reduction of hydrogen peroxide in the presence of thiol reducing agent.

Generation of allyl- and benzyllithiums from the corresponding halides by the aid of lithium-tellurium exchange reactions

Abstract A variety of allyl- and benzyllithiums were prepared by lithium-tellurium exchange reactions of allylic and benzylic tellurides generated in situ from the corresponding halides. The produced organolithiums were trapped successfully with electrophiles such as aldehydes, ketones, and trimethylchlorosilane. Benzyllithiums having an alkyl, alkoxy, fluoro, chloro, or cyano substituent(s) on their aromatic ring were generated efficiently in THF. Benzylic tellurides bearing a bromo or iodo substituent afforded a mixture of products under similar conditions arising from the competing lithium-halogen exchange and/or the displacement of the halogen atom with organolithiums used, but they were converted selectively to benzyllithiums in ether without affecting halogen substituents on the benzene ring. Several allyllithiums including dilithioisobutene were generated from allylic halides in a similar way via allylic tellurides. Wurtz-type coupling was negligible in any reactions examined.

Crystalline sodium seleno- and telluro-carboxylates: Localization of negative charge on electropositive chalcogeno atoms

Abstract Crystalline sodium seleno- and telluro-carboxylates were prepared from the corresponding bis(acyl)selenide and tellurides with sodium ethoxide. The salts have the unsymmetrical ionic structure where the negative charge is localized on the selenium or tellurium atom.

Stereoselective generation and trapping of lithium eneselenolates leading to ketene selenothioacetals and selenothioesters

Abstract The reaction of lithium alkyneselenolates generated from terminal acetylenes with thiols gave rise to lithium eneselenolates with high stereoselectivity. The trapping with alkyl halides afforded ketene selenothioacetals, whereas the trapping with allylic bromides yielded γ,δ-unsaturated selenothioesters via seleno-Claisen rearrangement.

REACTION OF LITHIUM 1-ALKOXYENESELENOLATES WITH PROPARGYLIC BROMIDES : ONE-POT CONVERSION TO POLY-SUBSTITUTED SELENOPHENES

Abstract Lithium 1-alkoxyeneselenolates 2 in tetrahydrofuran were characterized by 1 H NMR for the first time. Their reaction with propargyl bromide was found to proceed through a propargylic rearrangement to generate allenic selenoic acid O -esters 3 , which led to poly-substituted selenophenes in moderate yields under reflux.

Highly efficient one-pot synthesis of α,α-disubstituted selenoamides and their first reduction to amines

α,α-Disubstituted selenoamides are synthesized from terminal acetylenes, selenium, amines and allylic bromides in good to high yields; their reduction with LiAlH4 affords alkenylamines quantitatively.

Te-alkyl tellurocarboxylates ― isolation and characterization

Abstract A series of simple alkyl tellurocarboxylates were synthesized from the sodium tellurocarboxylates and alkyl iodides, and spectroscopically characterized.

Reductive cyclization of γ,δ-unsaturated selenothioic acid S-esters leading to tetrahydroselenophenes

Reductive cyclization of γ,δ-unsaturated selenothioic acid S-esters with NaBH4 or LiAlH4 proceeds viaδ,Iµ-unsaturated selenols to afford tetrahydroselenophenes in good to high yields.