Retrospect of Se and Te Chemistry

Retrospect of organoselenium and tellurium chemistry for these 30 years is described focusing on our novel findings in this field: (1) telluroxide elimination leading to alkenes and allylic compounds, (2) Pd-catalyzed or –mediated carbodetelluration for a new C–C bond formation, (3) synthesis of chiral diferrocenyl dichalcogenides and their use as chiral auxiliaries, (4) asymmetric selenoxide elimination for making optically active allenes and alkenes, (5) meta chloroperbenzoic acid (MCPBA) oxidation of organic selenides and tellurides leading to a substitution of a PhSe or PhTe moiety, as well as (6) preparation of chalcogen-bridged diruthenium complexes and their catalytic use for propargylic substitution reactions.     

Clarification on the Reactivity of Diaryl Diselenides toward Hexacyclohexyldilead under Light

In this study, the reactivity of organochalcogen compounds toward a representative alkyl-lead bond compound under light was investigated in detail. Under light irradiation, the Cy-Pb bond of Cy₆Pb₂ (Cy = cyclohexyl) undergoes homolytic cleavage to generate a cyclohexyl radical (Cy•). This radical can be successfully captured by diphenyl diselenide, which exhibits excellent carbon-radical-capturing ability. In the case of (PhS)₂ and (PhTe)₂, the yields of the corresponding cyclohexyl sulfides and tellurides were lower than that of (PhSe)₂. This probably occurred due to the low carbon-radical-capturing ability of (PhS)₂ and the high photosensitivity of the cyclohexyl-tellurium bond.     

Mechanistic studies of the tellurium(II)/tellurium(IV) redox cycle in thiol peroxidase-like reactions of diorganotellurides in methanol

Di-n-hexyl telluride (2), di-p-methoxyphenyl telluride (3), and (S)-2-(1-N,N-dimethylaminoethyl)phenyl phenyl telluride (4) catalyzed the oxidation of PhSH to PhSSPh with H(2)O(2) in MeOH. Telluride 2 displayed greater rate acceleration than the diaryltellurides 3 and 4 as determined by the initial velocities, v(0), for the rate of appearance of PhSSPh determined at 305 nm by stopped-flow spectroscopy. Rate constants for the oxidation of tellurides 2-4 (k(ox)), rate constants for the introduction of PhSH as a ligand to the Te(IV) center (k(PhSH)) of oxidized tellurides 5-7, and thiol-independent (k(1)) and thiol-dependent (k(2)) rate constants for reductive elimination at Te(IV) in oxidized tellurides 5-7 were determined using stopped-flow spectroscopy. Oxidation of the Te atom of the electron-rich dialkyl telluride 2 was more rapid than oxidation of diaryl tellurides 3 and 4. The dimethylaminoethyl substituent of 4, which acts as a chelating ligand to Te(IV), did not affect k(ox). Values of k(PhSH) for the introduction of PhSH to oxidized dialkyl tellurane 5 and oxidized diaryl tellurane 6 were comparable in magnitude, while the chelating dimethylaminoethyl ligand of oxidized telluride 7 diminished k(PhSH) by a fator of 10(3). Reductive elimination by both first-order, thiol-independent (k(1)) and second-order, thiol-dependent (k(2)) pathways was slower from dialkyl Te(IV) species derived from 2 than from diaryl Te(IV) species derived from 3. The chelating dimethylaminoethyl ligand of Te(IV) species derived from 4 diminished k(1) by a factor of 50 and k(2) by a factor of 3 (relative to the 3-derived species).     

New synthesis of cis-3,4-diaryl-1-tosylpyrrolidines

Unsymmetrically substituted cis-3,4-diarylpyrrolidines are synthesized in nearly 25% overall yields starting from 4-aryl-1,2,5,6-tetrahydropyridines by iterative reactions using the combination of m-chloroperoxybenzoic acid (MCPBA) and boron trifluoride etherate (BF₃·OEt₂) followed by Grignard addition, elimination and hydrogenation sequence.     

Polymers with electron-deficient olefin moieties by oxidation of precursors containing “push–pull” olefin (β-alkylmercaptoenoate) moieties in the main chain

Oxidation of a polymer (1) having β-alkylmercaptoenoate moieties in the main chain by m-chloroperbenzoic acid (MCPBA) is described. Oxidation of 1 proceeded selectively to give polymers having vinyl sulfoxide or vinyl sulfone moieties in the main chain by using an appropriate amount of MCPBA. In the case of 1.1 equiv of MCPBA (relative to the sulfur atom), a polymer (4) having vinyl sulfoxide moieties selectively in the main chain was obtained in quantitative yield without side reactions. Likewise, a polymer (5) having vinyl sulfone moieties was obtained in quantitative yield by using 2.6 equiv of MCPBA. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2589–2592, 1998     

Selectivity in epoxidation: acyclic diastereo- and enantio-selection with peroxide mediated by titanium(iv) and with peracid

New diastereoselective epoxidation on the olefin 1 with TBHP mediated by Ti(IV) is described to give the epoxide 2 which has reverse orientation of the one, 3 with MCPBA. Ti-mediation was ineffective on the olefin 4, on which MCPBA worked in high selectivity. Example of kinetic resolution is also described on 1b under Katsuki-Sharpless condition.     

Two Exceptional Homoleptic Iron(IV) Tetraalkyl Complexes

The formation of the high‐valent iron complex [Fe(cyclohexyl)₄] from Fe^II under reducing conditions is best explained by disproportionation of a transient organoiron intermediate which is driven by dispersive forces between the cyclohexyl ligands and the formation of short and strong Fe−C bonds. The (meta)stability of this diamagnetic complex (S=0) is striking if one considers that it has empty d‐orbitals at its disposal and contains, at the same time, no less than twenty H‐atoms available for either α‐ or β‐hydride elimination.     

Computational study on the mechanism and selectivity of C-H bond activation and dehydrogenative functionalization in the synthesis of rhazinilam

The key platinum mediated C-H bond activation and functionalization steps in the synthesis of (-)-rhazinilam (Johnson, J. A.; Li, N.; Sames, D. J. Am. Chem. Soc. 2002, 124, 6900) were investigated using the M06 and B3LYP density functional approximation methods. This computational study reveals that ethyl group dehydrogenation begins with activation of a primary C-H bond in preference to a secondary C-H bond in an insertion/methane elimination pathway. The C-H activation step is found to be reversible while the methane elimination (reductive elimination) transition state controls rate and diastereoselectivity. The chiral oxazolinyl ligand induces ethyl group selectivity through stabilizing weak interactions between its phenyl group (or cyclohexyl group) and the carboxylate group. After C-H activation and methane elimination steps, Pt-C bond functionalization occurs through β-hydride elimination to give the alkene platinum hydride complex.     

A materials approach to site-isolation of Grubbs catalysts from incompatible solvents and m-chloroperoxybenzoic acid

The development of a method for site-isolation of Grubbs second-generation catalyst from MCPBA is described. In these reactions, Grubbs catalyst was dissolved in a solvent consisting of a mixture (1:1 v/v) of 1-butyl-3-methylimidazolium hexafluorophosphate and methylene chloride and completely encapsulated within a thimble fabricated from polydimethylsiloxane (PDMS). A series of molecules that react by cross metathesis or ring-closing metathesis were added to the interior of the thimble and allowed to react. In the last step, m-chloroperoxybenzoic acid (MCPBA) dissolved in MeOH/H(2)O (1:1 v/v) was added to the exterior of the PDMS thimble. Small organic molecules diffused through the PDMS to react with MCPBA to form epoxides, but the Grubbs catalyst remained encapsulated. This result is important because Grubbs catalyst catalytically decomposes MCPBA at ratios of MCPBA to Grubbs of 3000 to 1. The yields for this two-step cascade sequence ranged from 67 to 83 %. The concept behind this sequence is that small organic molecules have high flux through PDMS but large molecules--such as Grubbs catalyst--and ionic reagents--such as MCPBA--have much lower flux through PDMS. Small molecules can thus react both outside and inside PDMS thimbles, whereas incompatible catalysts and reagents remain site-isolated from each other. This method does not require alteration of structures of the catalysts or reagents, so it may be applied to a wide range of homogeneous catalysts and reagents. To demonstrate further that the catalyst was encapsulated, the Grubbs catalyst was successfully recycled within the cascade sequence.     

Organische tellurverbindungen : III. Photoreaktionen von tellurolestern in lösung

Six new telluroesters Ia—If of aromatic carboxylic acids and the photoreactions of this class of compounds are described for the first time. These telluroesters undergo photoinduced α-cleavage and, in three cases, telluro-photo-Fries-reactions with the formation of free radicals II, III and IV; these free radicals yield the aldehydes V, the tellurides VII or the ditellurides VI and VIII either via hydrogen abstraction, dimerization and elimination of tellurium or via dimerization. Two methylthio as well as aryltelluro substituted benzophenone derivatives (VIId and VIIId) exhibit new types of aromatic photosubstitution reactions, yielding thioxanthone IX, via a cyclization reaction, which also occurs under electron-impact conditions.     

Effect of kosmotrope and chaotrope anions on rate and equilibria processes for the α-cyclodextrin catalysed reaction of 3-chloroperbenzoic acid with iodide

Solutes, ranging from small hydrophobic molecules to macromolecular structures such as proteins, are heavily influenced by the presence of salts, particularly the anionic component. It is well known that anions possessing a high charge density, such as sulfate, are able to reduce the solubility of these solutes, with this effect likely to be entropically driven and based on the influence the anions have on the structure of water. Other anions with a lower charge density, such as perchlorate, can have the opposite effect, improving solubility. The current consensus for the latter effect is that rather than being as a result of changes to water structure, it is due to specific binding of the anions to solute molecules, acting to reduce unfavourable interactions with the solvent. In this paper we report on how both sulfate and perchlorate influence binding and catalysis for the α-cyclodextrin mediated reaction of iodide with 3-chloroperbenzoic acid (MCPBA) for which the cyclodextrin–MCPBA complex is the catalytic species. Kinetic runs have been carried out over the temperature range 15–35 °C in dilute nitric acid and with added sulfate and perchlorate, allowing the calculation of enthalpies and entropies for the rate and equilibria processes involved in this system. We show that both sulfate and perchlorate demonstrate the expected Hofmeister behaviours, with sulfate acting to exclude the MCPBA from solution into the cyclodextrin cavity, thus increasing the amount of the catalytic species, whereas perchlorate acts to solubilise the MCPBA. Perchlorate has a complex range of effects, since as well as forming an association complex with cyclodextrin, the thermodynamic parameters indicate that it may also associate with MCPBA and the MCPBA–cyclodextrin complex. These associations may explain the catalysis that is observed with increasing perchlorate concentration. The effects observed in this study may be relevant to other binding processes in chemistry and biology.     

Synthesis and Characterization of Thiophenetriptyeene-8-sulfenic Acid

Abstract:

Reaction of the trilithium salt, prepared by treatment of 1,1,1-tris(2-bromo-5-methyl-3-thienyl)ethane, with O,O′-diethyl thiocarbonate gave thiophenetriptycene-8-thiol in 42% yield. The thiol was not oxidized with m-chloroperoxybenzoic acid (MCPBA). However, treatment of the thiol with NaH in THF followed by oxidation with MCPBA gave thiophenetriptycene-8-sulfenic acid in 70% yield. The structure of the sulfenic acid was determined by spectroscopic means and X-ray single-crystal analysis.     

Novel cross-coupling reactions between organotellurides and Grignard reagents employing a MnCl2/CuI catalytic system

We present a general protocol for the cross-coupling reaction of Grignard reagents and organic tellurides. Aryl Grignard reagents react stereospecifically with vinyl tellurides in the presence of a catalytic amount of manganese (II) chloride and copper (I) iodide to produce good yields of the corresponding cross-coupling products.     

Formation of diaryl telluroxides and tellurones by photosensitized oxygenation of diaryl tellurides

Aerobic oxygenation of diaryl tellurides under photosensitized conditions is investigated. Unlike Ph(2)S and Ph(2)Se, reaction of diaryl tellurides with singlet oxygen proceeds smoothly to yield diaryl telluroxides and the corresponding tellurones. The product distribution is largely affected by the substrate and the reaction conditions. In particular, the photooxygenation of bulky diaryl tellurides principally produces tellurones. The results of a series of trapping experiments suggest that the diaryl telluroxides can capture transient intermediates such as Me(2)S(+)OO(-) and Ar(2)Te(+)OO(-), generated in the singlet oxygen oxidation of chalcogenides, to yield diaryl tellurones, and therefore it may be the most potent precursors of the tellurones.     

Selective preparation of glycosyl sulfone or glycal by treatment of phenyl thioglycoside of N-acetylneuraminic acid with m-chloroperbenzoic acid

Treatment of acetylated phenyl thioglycoside of N-acetylneuraminic acid with m-chloroperbenzoic acid (MCPBA) in CH₂Cl₂ affords quantitatively mixtures of the respective sulfone and glycal free from the sulfoxide. The outcome of the reaction does not depend on the anomeric configuration of the starting thioglycoside. The sulfone can be selectively prepared (yield 100%) by oxidation with an excess of MCPBA and NaHCO₃. In the presence of pyridine (2 equiv.) and MCPBA (2 equiv.), the major product is glycal (yields 81—88%). This version of the reaction can be regarded as a new method for the preparation of sialic acid glycals.     

Synthesis and Reaction of Vinylic Tellurides

Vinylic tellurides were synthesized by the addition of diorganyl tellurides to acetylenes in the presence of a radical initiator or by visible light irradiation. Vinylic tellurides underwent transmetalation by the reaction with triethylaluminium or diethylzinc to form the corresponding vinylmetal reagents with retention of the double bond configuration. Oxidation of vinylic tellurides formed by carbotelluration reactions followed by pyrolysis gave internal acetylenes in good yields.     

Molecular modelling of some para-substituted aryl methyl telluride and diaryl telluride antioxidants

Quantum mechanical calculations using the 3-21G(d) basis-set were performed on some p-substituted diaryl tellurides and aryl methyl tellurides, and the corresponding cationic radicals of these compounds. Calculated relative radical stabilization energies (RSE:s) were shown to correlate with experimentally determined peak oxidation potentials (R=0.93) and ¹²⁵Te-NMR chemical shifts (R=0.91). A good correlation was also observed between the RSE:s and the Mulliken charge at the tellurium atoms (R=0.97). The results showed that Hartree–Fock calculations using the 3-21G(d) basis set was sufficiently accurate for estimating the impact of p-substituents in aryl tellurides on experimentally determined properties such as peak oxidation potentials and ¹²⁵Te-NMR chemical shifts.     

Stereoselective Synthesis of 1,3-Enynyl Tellurides by Palladium-Catalyzed Cross-Coupling Reaction

The coupling reaction of (E)-β-bromovinyl tellurides with terminal alkynes by using catalyst of PdCl₂(PPh₃)₂-CuI in the presence of Et₃N as a base give the corresponding 1, 3-enynyl tellurides     

Conversion of hydrazones to alkyl chlorides under Swern oxidation conditions

The unsubstituted hydrazones derived from aromatic ketones and aldehydes were converted in high yield to the corresponding alkyl chlorides under Swern oxidation conditions. In this unusual oxidation/reduction sequence the substrate undergoes a net reduction under the well established Swern oxidation conditions. Unsubstituted hydrazones derived from cyclohexyl ketones returned elimination products.     

A FACILE STEREOSELECTIVE SYNTHESIS OF (E,E)-CONJUGATED DIENYL TELLURIDES BY PALLADIUM-CATALYZED CROSS-COUPLING REACTION

A cross-coupling reaction of (E)-β-bromovinyl tellurides with alkenes in the presence of catalytic amount of Pd(OAc)₂ and PPh₃, AgCO₃ as a base carried out in acetonitrile to produce (E,E)-conjugated dienyl tellurides.