1,3-Bis(N,N-dialkylamino)imidazolin-2-ylidenes: synthesis and reactivity of a new family of stable N-heterocyclic carbenes

The synthesis of stable 1,3-bis(N,N-dialkylamino)imidazolin-2-ylidenes was accomplished from readily available chiral bis-hydrazones after reduction or addition of PhMgCl, cyclization to imidazolinium salts, and treatment with KN(SiMe3)2. This strategy allows the obtention of free imidazolin-2-ylidenes and their Rh(COD)Cl complexes in enantiomerically pure form. The sigma-donor ability of dialkylamino-substituted diaminocarbenes was found to be slightly higher than that of alkyl(aryl) analogues.     

Synthesis and structure of a new stable radical,di-(4-dimethylneopentylmethylphenyl)nitroxide

Russian Chemical Bulletin -     

Structure and reactivity of phosphorus-selenium heterocycles with peri-substituted naphthalene backbones

Reaction of naphthalene-1.8-bis(dichlorophosphine) with sodium selenide and selenium gives naphthalene-1,8-diyl)-1,3,2,4-diselenoxodiphosphetane 2,4-diselenide (27), a molecule with its four-membered PSePSe ring spanned by a naphthalene-1,8-diyl backbone. 27 was fully characterised, including by X-ray diffraction. Unusually, the reaction yielded not only 27, but also a polymeric or oligomeric material that was selenium deficient, containing significant amount of species with phosphorus atoms not fully oxidised to P(V). Reactions of the selenium deficient material with simple alcohols afforded two 1,2-diphosphaacenaphthenes, which were structurally characterised.     

Generation and reactivity of N,N-dimethylaminobenzotriazolylcarbene a new nucleophilic carbene

N,N-Dimethylaminobenzotriazolylcarbene ( 5 ) reacted with phenyl isocyanate in a [1+2+2] cycloaddition and then with nucleophiles to generate various hydantoins 10 in a one-pot procedure. It was also found that this novel carbene reacted with trans-dibenzoylethylene ( 11 ) in a [1+4] cycloaddition, generating 2-dimethylamino-3-benzoyl-5-phenylfuran ( 13 ) and 2-phenyl-3-[benzotriazol-1-yl]-4-benzoylfuran ( 14 ) whose structures were confirmed by ¹H-¹³C long range correlations as well as the structure of furan 14 being confirmed by X-ray crystallography.     

N,N'-diarylureas: a new family of atropisomers exhibiting highly diastereoselective reactivity

2,6-disubstituted N-aryl ureas rotate slowly about their Ar-N bonds and can exist as separable atropisomers. They also react remarkably diastereoselectively, with the urea axis controlling new stereogenic centers with high fidelity in a variety of nucleophilic and electrophilic addition reactions. The sense of diastereoselectivity in lateral lithiation-electrophilic quench reactions is electrophile-dependent and appears to be the result of stereospecific reaction with one of two interconvertible diastereoisomeric organolithiums.     

Increasing the persistency of stable free-radicals: synthesis and characterization of a nitroxide based [1]rotaxane

A persistent paramagnetic [1]rotaxane based on beta-cyclodextrin showing an increased persistency under reductive conditions has been prepared and characterized for the first time.     

Electrospray mass spectrometry of stable iminyl nitroxide and nitronyl nitroxide free radicals

Electrospray ionization (ESI) mass spectra have been recorded for a range of substituted nitronyl nitroxide and iminyl nitroxide monoradicals and biradicals. Secondary species formed in the ESI source were observed as the dominant ions in both the iminyl nitroxide and nitronyl nitroxide spectra. Daughter ion spectrometry was used to establish fragmentation mechanisms for the nitronyl nitroxide and iminyl nitroxide moieties as well as the secondary species under ESI conditions.     

A new method of synthesis of N,N-dibenzyl-DL-phenylalanine and N,N-dibenzyl-DL-alanine

Russian Chemical Bulletin -     

Unsymmetrically N,N'-substituted saturated carbenes: synthesis, reactivity and preparation of a rhodium(I) carbene complex

A versatile synthesis of unsymmetrically N,N'-substituted saturated carbenes is described. The novel racemic imidazolidin-2-ylidenes rac-5 have been synthesized by reductive desulfurization of the corresponding imidazolidin-2-thiones rac-4. The thiones were prepared in two reaction steps from aldimines and secondary amines. Three different substituents at N1, N3 and C4 of the five-membered N-heterocyclic ring can be introduced by choice of suitable aldimines and secondary amines. The dimerization behaviour (diaminocarbene/enetetramine equilibrium) for the unsymmetrically N,N'-substituted imidazolidin-2-ylidenes has been investigated by NMR spectroscopy. Unsymmetrically N-iPr and N-iBu substituted N-heterocyclic carbenes undergo a slow dimerization, whereas N-tBu substituted derivatives are stable as monomeric carbenes indefinitely. The carbene ligand rac-5d has been coordinated to rhodium(I) to give the square-planar rhodium carbene complex [Cl(cod)Rh(rac-5d)]rac-6d which has been characterized by an X-ray diffraction analysis.     

Cimicifugadine from Cimicifuga foetida, a new class of triterpene alklaoids with novel reactivity

[structure: see text] An unprecedented triterpene alkaloid glycoside, designated cimicifugadine (1), with a pyridine ring incorporated to a cycloartane triterpenoid nucleus, was isolated from the roots of Cimicifuga foetida. Its structure was established on the basis of extensive spectroscopic measurements and chemical transformation with the absolute configuration at C-24 determined to be S by a modified Mosher method. It demonstrated a novel reactivity in mild acidic media whereby the cyclopropane ring is opened followed by the formation of two isomeric conjugated trienes.     

Tetrahedranyllithium: synthesis, characterization, and reactivity

The first representative of stable tetrahedranyl anion, tris(trimethylsilyl)tetrahedranyllithium (3), has been synthesized by the reaction of tetrakis(trimethylsilyl)tetrahedrane (2) with methyllithium in tetrahydrofuran. The structural characterization of the tetrahedranyllithium has been achieved by X-ray crystallography, showing that the structure of 3.(TMEDA)1.5 represents a stretched tetrahedron. The endocyclic C(Li)-C(SiMe3) bond lengths range from 1.5408(15) to 1.5441(15) A (av 1.5425(15) A), and are longer than the endocyclic C(SiMe3)-C(SiMe3) bond lengths, which range from 1.4961(15) to 1.5009(15) A (av 1.4986(15) A). Methyl- and hydrogen-substituted tetrahedranes have also been prepared by the reaction of 3 with dimethyl sulfate and cyclopentadiene, respectively.     

Detection of a stable nitroxide during chemical depolymerization of heparin

The use of low-molecular-weight heparins (LMWHs) has become common, since compared to unfractionated heparin (UFH), they have a much longer plasma half-life and lower incidence of side effects. LMWHs are derived from the depolymerization of UFH, obtained either chemically, physically or enzymatically. We employed electron spin resonance (ESR) spectroscopy to study the depolymerization of UFH by copper in the presence of hydrogen peroxide. A stable nitroxide radical was detected. This could be generated by the hydroxyl radical attack either to the N-SO⁻₃ group or to free amino groups present in the UFH preparation.     

Chemically-derivatized nickel surfaces: Synthesis of a new class of stable electrode interfaces

The interactions of protonic acids (HClO₄ and CF₃SO₃H) and of alkylating agents (CF₃SO₃Et and Et₃O⁺BF₄^?) with cyclic ethers and 1,3-dioxacycloalkane s (DCA) in CH₂Cl₂ have been studied by dc polarography (DCP) and differential pulse polarography (DPP). All the reductions were irreversible and kinetically controlled. There is no indication that HClO₄ reacts with THF or OXP; with CF₃SO₃H the decrease of the signals due to the acid may indicate the formation of a product (sec. oxonium ion or ester) or it may be due merely to a modification of the mercury surface by the oxacyclic compound which produces a reduction in the kinetic current; no new signals appear.For the various DCAs with CF₃SO₃Et, 1,3-dioxepan (DXP) gave the clearest indication of reactions occurring and the signals have been assigned (tentatively) to EtDXP⁺ and to the hemiformal ester EtO(CH₂)₄OCH₂OSO₂CF₃. For 1,3-dioxolan (DXL) the picture was less clear, and there was no evidence that 1,3,6-trioxan (TXA) was alkylated.Our study of the DCA with acids gave no evidence that DXL interacts with HClO₄ or CF₃SO₃H, which is due to the exceptionally low basicity of DXL. The signals obtained with DXP and 1,3,6-trioxocan (TXC) are assigned to a molecular complex H₂A⁺-DCA, to the sec-oxonium ion HDCA⁺, and to the hemiformal ester HO(CH₂)₄OCH₂OA.It is concluded that the polarographic behaviour of all the species involved is so complicated that our original aim of distinguishing by this technique between sec. and tert. oxonium ions is not feasible.The sec. oxonium ions formed from cyclic ethers or DCA in conc. H₂SO₄, whose existence was indicated by the PMR spectra, could not be detected polarographically as their reduction potentials were outside the “potential window”.     

Electrospray ionization mass spectrometry of stable nitroxide free radicals and two isoindoline nitroxide dimers

Electrospray ionization (ESI) mass spectra were recorded for a range of substituted isoindoline nitroxides, two isoindoline nitroxide dimers and two piperidinyl nitroxides. In all cases the dominant molecular species arise from oxidation rather than protonation, an unusual process in ESI. Fragment ion spectroscopy was used to establish fragmentation mechanisms for the nitroxides under ESI conditions.     

Synthesis, reactivity, and ligand properties of a stable alkyl carbene

Stable tert-butyl diisopropylamino carbene is more nucleophilic but also more electrophilic than diamino carbenes; it undergoes cyclopropanation reactions and easily binds to metal fragments.     

Preparation and structural characterization of a new class of stable thioketones: ortho-hydroxythioacetophenones

ortho-Hydroxythioacetophenone and four structurally related ortho-hydroxyaryl methyl thioketones have been prepared from the corresponding ketones by reaction with gaseous H₂S and HCl in ethanol under strictly controlled reaction conditions. The remarkable stability of the new monomeric thioketones seems to be due to a strong intramolecular O-H?SC hydrogen bonding.     

Halogenation of 2-alkyl-2-azabicycloalkenes as a method for the synthesis of stable aziridinium salts of a new class

Addition of bromine and potassium dihaloiodates(i) to 2-alkyl-2-azabicyclo[2.2.1]hept-5-enes and 2-alkyl-2-azabicyclo[2.2.2]oct-5-enes affords quaternary ammonium salts containing the aziridine ring and the polyhalide anion. The possibility of using these salts for the synthesis of 6-substituted 2-alkyl-2-azabicyclo[2.2.1]heptanes has been shown.