Synthesis and transformations of metallacycles 40. Catalytic cycloalumination in the synthesis of 3-substituted phospholanes

An efficient one-pot method was developed for the synthesis of 3-alkyland 3-benzyl-substituted phospholanes through the successive Cp₂ZrCl₂-catalyzed cycloalumination of α-olefins in the presence of AlEt₃ giving the corresponding aluminacyclopentanes followed by the in situ replacement of the aluminum atom in the latter compounds by a P atom by means of methyl(phenyl)dichlorophosphines. The oxidation of 3-alkyl- and 3-benzyl-1-methyl(phenyl)-phospholanes with hydrogen peroxide affords 3-alkyl- and 3-benzyl-1-methyl(phenyl)-phospholane 1-oxides, respectively.     

Synthesis and transformations of metallacycles 46. Catalytic cycloalumination reaction in the synthesis of bis(phospholanes)

An efficient one-pot synthesis of bis(phospholanes) was elaborated, which included a sequential Cp₂ZrCl₂-catalyzed cycloalumination of α,ω-diolefins with AlEt₃, giving the corresponding bis(aluminacyclopentanes), and their in situ reaction with dihalophosphines to furnish the target α,ω-bis(phospholanes). The reaction of these compounds with H₂O₂ or elementary sulfur gave the corresponding bis(phospholane 1-oxides) and bis(phospholane 1-sulfides).

     

New chiral monodentate phospholane ligands by highly stereoselective hydrophosphination

New chiral phospholanes 6 were prepared in both enantiomeric forms starting from l- and d-tartaric acid. The key step in the synthetic sequence is the double hydrophosphination of unsaturated chiral bis(lactone) 9 by NaPhPH(BH₃). This method was used for the first time in the formation of chiral phospholanes. The structure of phospholane 6a was confirmed by X-ray crystallography. σ-Donor properties of the phospholanes were estimated by measurement of ¹J(³¹P–⁷⁷Se) coupling constants in the corresponding phosphine selenides. The new phospholanes were tested as ligands in the Rh-catalyzed enantioselective hydrogenation of functionalized standard olefins (65–92% ee).     

Photochemistry of purine 3-oxides in hydroxylic solvents

UV irradiation of the potent oncogen hypoxanthine 3-oxide in aqueous solution induces elimination of and rearrangement of the nitrogen-bound oxygen. The extent of each reaction shows a complex variation over the pH range 0–7. The variations in quantum yield for product formation are shown to result from the presence in the neutral molecule of tautomeric species with differing photochemistries that ionize in the excited state (pK_a^* ～ 3.5) just above the protonation pK_a (1.2). The photochemical reactivity of each ionic and each tautomeric form was assigned by comparing the effect of pH changes between 0 and 11 on the quantum yields for formation of each photoproduct from hypoxanthine 3-oxide with those of two model compounds, 1-hydroxyhypoxanthine and 6-methoxypurine 3-oxide. Photoreduction of the 3-oxides occurs via the triplet state. This process has a relatively consistent low quantum yield (Φ = 0.005 to 0.04) for most ionic and tautomeric forms of both purine 1-oxides and purine 3-oxides. Photorearrangement is a much more efficient process for purine 3-oxides (Φ = 0.3) than for purine 1-oxides (Φ = 0.04).     

Synthesis of isatin-conjugated 3H-indole-N-oxides and their serendipitous conversion to spiroindolenines

Isatin-conjugated 3H-indole-N-oxides were synthesized from isatin-derived propargylic alcohols and nitrosobenzenes in moderate yields. Isatin-conjugated 3H-indole-N-oxides were converted to novel spiroindolenines under PPh₃-mediated deoxygenation reaction condition, serendipitously.     

Photocatalytic deoxygenation of N–O bonds with rhenium complexes: from the reduction of nitrous oxide to pyridine N-oxides

The accumulation of nitrogen oxides in the environment calls for new pathways to interconvert the various oxidation states of nitrogen, and especially their reduction. However, the large spectrum of reduction potentials covered by nitrogen oxides makes it difficult to find general systems capable of efficiently reducing various N-oxides. Here, photocatalysis unlocks high energy species able both to circumvent the inherent low reactivity of the greenhouse gas and oxidant N₂O (E⁰(N₂O/N₂) = +1.77 V vs. SHE), and to reduce pyridine N-oxides (E_1/2(pyridine N-oxide/pyridine) = −1.04 V vs. SHE). The rhenium complex [Re(4,4′-tBu-bpy)(CO)₃Cl] proved to be efficient in performing both reactions under ambient conditions, enabling the deoxygenation of N₂O as well as synthetically relevant and functionalized pyridine N-oxides.

A rhenium-based photocatalyst enables the deoxygenation of several compounds containing N–O bonds, such as N₂O and pyridine N-oxides.     

Reactions of 1,1,1-trihalo-3-nitrobut-2-enes with enamines derived from cycloalkanones. Rearrangement of trifluoromethylated 1,2-oxazine N-oxide into 1-pyrroline N-oxides and stereochemistry of the products

1,2-Oxazine N-oxides derived from (E)-1,1,1-trifluoro-3-nitrobut-2-ene and cyclohexanone enamines underwent spontaneous rearrangement with ring contraction to give 1-pyrroline N-oxides. Reactions of (E)-1,1,1-trifluoro(trichloro)-3-nitrobut-2-enes with N-cyclopent-enylmorpholine resulted in a series of novel CX₃-containing nitroalkyl enamines and g-nitro ketones; the stereochemistry of the synthesized compounds were studied by NMR spectroscopy and X-ray diffraction.     

Synthesis and transformations of metallacycles 44. Cycloalumination of methylenecyclobutane terpene derivatives with Et3Al catalyzed by Cp<Subscript>2</Subscript>ZrCl<Subscript>2</Subscript>

Cycloalumination of methylenecyclobutane terpene derivatives (d(+)-camphor, (+)-camphene, β-pinene, and l(–)-menthol) with Et₃Al catalyzed by Cp₂ZrCl₂ produced novel spirofused organoaluminum compounds in high yields. The synthesized compounds were in situ transformed into the corresponding spiro tetrahydroselenophenes and spiro phospholanes.     

Synthesis of 4<Emphasis Type="Italic">H</Emphasis>-imidazole-5-carbaldoxime 3-oxides and 4<Emphasis Type="Italic">H</Emphasis>-imidazole-5-carbonitrile 3-oxides

The condensation of 3-hydroxyamino-3-methylbutan-2-one or 3-ethyl-3-hydroxyamino-pentan-2-one with aldehydes and ammonia afforded a series of new 1-hydroxy-4-methyl-2,5-dihydroimidazoles, whose oxidation gave rise to the corresponding 5-methyl-4H-imidazole 3-oxides. The latter, like 1-hydroxy-4-methyl-2,5-dihydroimidazoles, react with PrⁱONO in the presence of bases to form 4H-imidazole-5-carbaldoxime 3-oxides, which are transformed into 4H-imidazole-5-carbonitrile 3-oxides in the reaction with TsCl in the presence of Et₃N. The by-products produced in different steps of the synthesis were isolated and characterized. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1487–1503, July, 2008.     

Direct access to 1,4-benzothiazine 4,4-dioxides and 4-oxides via a domino reaction

The domino reactions of 2-fluoro benzensulfonyl acetonitrile and α-chloro oximes in the presence of Cs₂CO₃ in aprotic high boiling point solvents have been achieved to provide isoxazole?fused 4H-1,4-benzothiazine-4,4-dioxides via an unprecedented transition metal-free one-pot addition/cyclization process. The tunable synthesis of either isoxozolo-1,4-benzothiazin-4-oxides or their precursor 5-aminoisoxazoles can be controlled depending on the solvent selection. The observed products were characterized by means of (IR, ¹H, ¹³C NMR and HRMS) and physical methods.     

Synthesis and reactions of a monosubstituted dithiirane 1-oxide, 3-(9-triptycyl)dithiirane 1-oxide

A 3-monosubstituted dithiirane 1-oxide, 3-(9-triptycyl)dithiirane 1-oxide, was prepared for the first time, by the reaction of (9-triptycyl)diazomethane and S₈O. The dithiirane 1-oxide was obtained as cis- and trans-isomers, and the structure of the trans-isomer was verified by X-ray crystallography. The cis-isomer isomerized gradually to the trans-isomer in solution. The divalent sulfur atom of the cis- and trans-dithiirane 1-oxides were removed on treatment with triphenylphosphine to give the corresponding Z- and E-sulfines, respectively. The reaction of the trans-dithiirane 1-oxide with (Ph₃P)₂Pt(C₂H₄) provided the (sulfenato-thiolato)Pt^II complex, and that with Lawesson's reagent yielded the 1,3,4,2-trithiaphospholane and 1,2,4,5,3-tetrathiaphosphorinane derivatives.     

New description of the substituent effect on electronic spectra by means of substituent constants—VI. Utraviolet spectra of 4-substituted pyridine N-oxides and blue shifted iodine bands of their EDA complexes with iodine

Electronic spectra of 4-substituted pyridine N-oxides and their EDA complexes with iodine were studied. The substituent effect on the near u.v.¹A₁ intramolecular CT bands of the N-oxides and on the blue shifted iodine bands caused by CT complex formation are discussed in terms of a general equation, theoretically derived in order to describe the substituent effect on electronic spectra by means of substituent constants. The results are quite successful and supported by semi-empirical SCFMO-CI calculations. Based on the results mentioned above, the character of n-σ type N-oxide—iodine CT complexes is also examined. The complex formation constants (log K) and pK_a values of the N-oxides correlate especially well, indicating that the CT interaction mechanism cannot be neglected in proton addition reactions such as hydrogen bonding and pK_a values.     

XPS study on valence band structures of transition-metal trisulfides,TiS3, NbS3, and TaS3

Transition-metal trisulfides, TiS₃, NbS₃, and TaS₃, with a quasi-one-dimensional structure are investigated by X-ray photoelectron spectroscopic (XPS) measurements to obtain information on the valence band structures. The band structures at the Fermi level of these compounds correspond well to their transport properties. A shoulder is observed at the top of the valence band in NbS₃ and TaS₃, suggesting that this band is made up of the metal d_z² electrons. The d_z² band is occupied in NbS₃ and TaS₂ and empty in TiS₃. The characteristic features at the top of the valence band in NbS₃ imply the occurrence of d_z² band separation, which leads to a semiconducting nature.     

Synthesis of 1,2,3,4-tetrazine 1,3-dioxides annulated with 1,2,3-triazoles and 1,2,3-triazole 1-oxides

1,2,3,4-Tetrazine 1,3-dioxides annulated with 1,2,3-triazoles and 1,2,3-triazole 1-oxides have been synthesized by the reaction of 4-amino-5-(tert-butyl-NNO-azoxy)-2-R-2H-1,2,3-triazoles (R=Me, i-Pr, t-Bu) and their 1-oxides (R=H, Me, Et, i-Pr) with the HNO₃/H₂SO₄/Ac₂O system. Their thermal stability, spectroscopic, and X-ray properties have been studied.     

A MALDI mass spectrometry investigation of the compositions of the products of the partial acidolysis of MeSi(OMe)3

The molecular-mass compositions of the products of partial acidohydrolytic polycondensation that are formed during the interaction of MeSi(OMe)₃ with CH₃COOH have been studied via MALDI mass spectrometry. It has been shown that, depending on the molar ratio of MeSi(OMe)₃ and CH₃COOH (m/n), a wide range of oligomethylmethoxysiloxanes with the average composition [MeSiO _n/m (OMe)_{(3 ? 2n/m)}] _p are formed. The analysis of molecular masses and ²⁹Si NMR spectra of the products has revealed various types of molecular structures, which change from linear, branched, and cyclic to polycyclic clusters with condensed cycles after a change in the degree of polycondensation α = 100 × [2n/3m] from 66.67 to 83.33%. At a degree of polycondensation of α = 86.67% or higher, the polycyclic clusters form a spatially crosslinked structure (gel).     

A new regioselective synthesis and ambient light photochemistry of quinazolin-1-oxides

Quinazolin-1-oxides were prepared by the oxidation of tetrahydroquinazolines with H₂O₂-tungstate and their ambient light photochemistry was investigated. Substituent effects on their photochemical cyclization and the reactions of the products 1aH-[1,2]oxazireno[2,3-a]quinazolines under photochemical and thermal conditions are reported. The cyclization of quinazolin-1-oxides and the reactions of 1aH-[1,2]oxazireno[2,3-a]quinazolines show pronounced solvent isotope and solvent effects.     

Spin transition in the molecular heterospin complex of Cu(hfac)2 with 4,4,5,5-tetramethyl-2-(1-methylpyrazol-5-yl)-4,5-dihydroimidazole-1-oxyl 3-oxide

The synthesis, structure, and magnetic properties of the products of the reaction for Cu(hfac)₂ (hfac is hexafluoroacetylacetonate) with spin-labeled nitronyl nitroxides 4,4,5,5-tetramethyl-2-(1-R-1H-pyrazol-5-yl)-3-imidazoline-1-oxyl 3-oxides L^5/R (R = Me, Et, Pr, Bu), viz., binuclear complex [Cu(hfac)₂L^5/Me]₂ and chain polymer complexes [Cu(hfac)₂L^5/R]_n, are described. The polymer heterospin chains are built according to “head-to-head” (R = Me, Et, Pr, Bu) and “head-to-tail” (R = Pr, Bu) motifs. Compound [Cu(hfac)₂L^5/Me]₂ is characterized by the ability to reveal the reversible effect of thermally induced spin transition at a temperature about 75 K (without hysteresis). In the set of heterospin Cu^II compounds with spin-labeled pyrazoles, this is the earlier unknown example of a molecular complex exhibiting a similar magnetic anomaly.     

Novel and efficient synthesis of substituted quinoline-1-oxides and the complex compounds SnL2Cl2 (L=2-aminoquinoline-1-oxides) with the aid of stannous chloride

A novel and efficient approach to the synthesis of substituted quinoline-1-oxides and the complex compounds SnL₂Cl₂ (L=2-aminoquinoline-1-oxides) was developed. The reaction has fancy selectivity when 3-(2-nitrophenyl)acrylonitriles were treated with the aid of SnCl₂ reagent under the same conditions. When R is –CN or –COOR′ the complex compounds SnL₂Cl₂ (L=2-aminoquinoline-1-oxides) were obtained. Whereas, when R is H or aryl another series of substituted quinoline-1-oxides were formed. The products have been screened for their anticancer activities.     

‘Reductive ozonolysis’ via a new fragmentation of carbonyl oxides

This account describes the development of methodologies for ‘reductive’ ozonolysis, the direct ozonolytic conversion of alkenes into carbonyl groups without the intermediacy of 1,2,4-trioxolanes (ozonides). Ozonolysis of alkenes in the presence of DMSO produces a mixture of aldehyde and ozonide. The combination of DMSO and Et₃N results in improved yields of carbonyls but still leaves unacceptable levels of residual ozonides; similar results are obtained using secondary or tertiary amines in the absence of DMSO. The influence of amines is believed to result from conversion to the corresponding N-oxides; ozonolysis in the presence of amine N-oxides efficiently suppresses ozonide formation, generating high yields of aldehydes. The reactions with amine oxides are hypothesized to involve an unprecedented trapping of carbonyl oxides to generate a zwitterionic adduct, which fragments to produce the desired carbonyl group, an amine, and ¹O₂.     

Protonation of 1,2,4-triazine 4-oxides

The basicity of some 1,2,4-triazine 4-oxides was estimated on a quantitative level, and their probable protonation patterns were ascertained. The dissociation constants of mono- and diprotonated 3-R-6-phenyl-1,2,4-triazine 4-oxides in aqueous buffer solutions and aqueous sulfuric acid solutions were determined (6-phenyl-1,2,4-triazine 4-oxide: $pK_{BH^ + } = 1.1$ , $pK_{BH^{2 + } } = - 6.02$ ). According to the results of DFT calculations (B3LYP/6-311**) and spectral data, first protonation of 1,2,4-triazine 4-oxides involves the N¹ nitrogen atom, and the second proton adds to the N-oxide oxygen atom.