New synthesis of 5,5-dimethyl-3-oxo-4,5-dihydro-3<Emphasis Type="Italic">H</Emphasis>-pyrrole 1-oxide

A procedure has been developed for the synthesis of 5,5-dimethyl-3-oxo-4,5-dihydro-3H-pyrrole 1-oxide via oxidation of 2,2,6,6-tetramethyl-4-oxopiperidine 1-oxyl with chlorine in aqueous sodium perchlorate and subsequent ring contraction in the resulting 2,2,6,6-tetramethyl-1,4-dioxopiperidinium perchlorate in dilute perchloric acid. The relative rates of proton exchange between the CH₂ and CH groups in the title compound were determined by ¹H NMR spectroscopy in D₂O, and the acidity constant was estimated on the basis of the dependence of the UV spectrum upon pH (pK _a = 13.2).     

Photostabilization of commercial polypropylene by hindered piperidine compounds: An ESR and luminescence study

The photostabilization of commercial polypropylene by a hindered piperidine stabilizer, bis [2,2,6,6-tetramethyl-4-piperidinyl] sebacate (I) and by a model N-oxy radical compound, 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxy (II) is examined using ESR and luminescence spectroscopy. ESR spectroscopy shows that I operates through the formation of a stable nitroxyl radical in the polymer. On the other hand, II disappears rapidly during the early stages of photo-oxidation but continues to act as an effective stabilizer. A low steady-state equilibrium concentration of nitroxyl radicals is believed to be responsible for the high photostabilizing efficiency of I. Both compounds also inhibit the photolysis of the luminescent α,β-unsaturated impurity groups present in the polymer; possible mechanisms are discussed.     

The formation of nitroxyl radicals in reactions of dimethyldioxirane with 2,2,6,6-tetramethylpiperidine and 2,2,5,5-tetramethyl-3-imidazoline-3-oxide derivatives

The oxidation of secondary and tertiary amines, derivatives of 2,2,6,6-tetramethylpiperidine and 2,2,5,5-tetramethyl-3-imidazoline-3-oxide, by dimethyldioxirane results in the formation of nitroxyl radicals, which induce the decomposition of dimethyldioxirane. Chemiluminescence in the IR and visible regions during the reaction of dimethyldioxirane with 2,2,6,6-tetramethyl-4-hydroxypieeridine-1-oxyl was observed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2501–2503, December, 1998.     

Mechanism of disproportionation of 2,2,6,6-tetramethyl-1,4-dioxopiperidinium perchlorate in acidic aqueous medium

The cation of 2,2,6,6-tetramethyl-1,4-dioxopiperidinium perchlorate disproportionates in acidic aqueous medium to give piperidinoxyl and/or 1-hydroxypiperidine, nitrone, and acetone. A mechanism of this reaction was proposed and the rate constants for its particular steps were calculated.     

Reaction products of nitronyl nitroxyl radicals with acids

Study of the structures of compounds generated by the reactions of 4,4,5,5-tetramethyl-2-phenyl-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide with trifluoromethanesulfonic and picric acids demonstrated that these reactions produced salts of disproportionation products of nitronyl nitroxyl.     

Reactions of the paramagnetic quaternary Mannich base in an alkaline medium

The catalytic action of aqueous NaOH at 20 °C on 2,2,6,6-tetramethyl-3-(N-methyl-piperidiniomethyl)-4-oxopiperidine 1-oxyl iodide rapidly resulted in the formation (k = 57 L mol⁻¹ s⁻¹) of a paramagnetic ketone with an activated double bond: 2,2,6,6-tetramethyl-3-methylidene-4-oxopiperidine 1-oxyl. The latter underwent slow transformation into a nitroxyl biradical containing an activated double bond and a methylene bridge linking positions 3 and 3′. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 421–423, February, 2008.     

Synthesis and properties of nicotinic acid derivatives containing a 2,2,6,6- tetramethylpiperidine 1-oxyl residue

N¹-(Nicotinoyl)-N²-4-(2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl)hydraxine was obtained by condensation of nicotinoyl hydrazide with 2,2,6,6-tetramethyl-4-oxopiperidine 1-oxyl. Acylation of 2,2,6,6-tetramethyl-4-hydroxypiperidine 1-oxyl with nicotinoyl chloride gives nicotinic acid 2,2,6,6-tetramethyl-1-oxyl 4-piperidyl ester. A spin-labeled analog of nicotinamide was obtained by condensation of nicotinoyl azide with 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl. The synthesis of 1-N-(-D-ribofuranoside)-3'-N[4-(2,2,6,6-tetramethylpiperidine-1-oxyl)pyridinecarboxamide from 2,2,6,6-tetramethyl-4-nicotinoylaminopiperidine 1-oxyl and 2,3,5-tri-O-benzoyl--D-ribofuranosyl bromide proceeds without damage to the iminoxyl radical. The preparation of the corresponding spin-labeled nucleotide is hindered by destruction of the iminoxyl radical during ion-exchange chromatography.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 352–355, March, 1976.     

Interaction of chlorine dioxide with nitroxyl radicals

The formation of charge transfer complexes between chlorine dioxide and nitroxyl radicals (2,2,6,6-tetramethylpiperidin-1-oxyl, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, 4-oxo-2,2,6,6-tetramethylpiperidin-1-oxyl, 4-methoxy-2,2,6,6-tetramethylpiperidin-1-oxyl, 4-acetylamido-2,2,6,6-tetramethylpiperidin-1-oxyl, 2,2,5,5-tetramethyl-4-phenyl-3-imidazolin-1-oxyl, and bis(4-methoxyphenyl) nitroxide) in acetone, acetonitrile, n-heptane, diethyl ether, carbon tetrachloride, toluene, and dichloromethane was found by spectrophotometry at –60—+20 °C. The thermodynamic parameters of complex formation were determined. The radical structure affects its complex formation ability. The charge transfer complex is transformed into the corresponding oxoammonium salt.     

Reactions of stabilizer compounds. IV. ESR studies of the formation of stable nitroxyl radicals in the photooxidation of polypropylene containing hindered amine light stabilizers

ESR studies of the formation of stable nitroxyl radicals in the photooxidation of polypropylene containing 2,2,6,6-tetramethyl-4-hydroxypiperidine (II) and the commercial light stabilizer bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate (III) are described. Spectra of the stable nitroxyl radicals in solid polymer and in solution after extraction from the polymer were obtained. The extract from photooxidized polypropylene containing the bifunctional stabilizer (III) showed only the mononitroxyl radical. The possible formation of polymer-bonded nitroxyl radicals from (III) is discussed.     

Photo-stabilisation of commercial polypropylene by piperidine compounds: The rôle of stable free radicals

The photo-stabilisation of commercial polypropylene by a hindered piperidine stabiliser, bis [2,2,6,6-tetramethyl-4-piperidinyl] sebacate, a model piperidine compound, 2,2,6,6-tetramethyl-4-piperidinol, and its corresponding N-oxy derivative is examined using luminescence and ESR spectroscopy. All three compounds inhibit the photolysis of the luminescent α,β-unsaturated carbonyl impurity groups present in the polymer. ESR spectroscopy shows that the hindered piperidine stabiliser operates by the same mechanism as that of the stable hindered piperidino-N-oxy radical compound. Possible mechanisms whereby the piperidine compounds inhibit the photolysis of the α,β-unsaturated carbonyl groups are also discussed.     

Synthesis of AB2 miktoarm star-shaped copolymers by combining stable free radical polymerization and atom transfer radical polymerization

A stable nitroxyl radical functionalized with two initiating groups for atom transfer radical polymerization (ATRP), 4-(2,2-bis-(methyl 2-bromo isobutyrate)-propionyloxy)-2,2,6,6-tetramethyl-1-piperidinyloxy (Br₂-TEMPO), was synthesized by reacting 4-hydroxyl-2,2,6,6-tetramethyl-1-piperidinyloxy with 2,2-bis-(methyl 2-bromo isobutyrate) propanoic acid. Stable free radical polymerization of styrene was then carried out using a conventional thermal initiator, dibenzoyl peroxide, along with Br₂-TEMPO. The obtained polystyrene had two active bromine atoms for ATRP at the ω-end of the chain and was further used as the macroinitiator for ATRP of methyl acrylate and ethyl acrylate to prepare AB₂-type miktoarm star-shaped copolymers. The molecular weights of the resulting miktoarm star-shaped copolymers at different monomer conversions shifted to higher molecular weights without any trace of the macroinitiator, and increased with monomer conversion.     

Inhibition of the anthraquinone photosensitised oxidation of polypropylene by a hindered piperidine stabiliser: A spectroscopic study

A hindered piperidine stabiliser, bis{2,2,6,6-tetramethyl-4-piperidinyl}-sebacate, has been found to inhibit the photo-sensitising effect of anthraquinone in polypropylene. On flash photolysis in n-hexane solution transient formation due to the semi-quinone radical (AH·) at 370 nm and subsequent formation of the hydroquinone (9,10-dihydroxyanthracene (AH₂) were found to be effectively inhibited by the corresponding nitroxyl radical but not by the amine. The absence of the hydroquinone photoproduct was confirmed using fluorescence spectroscopy. With the aid of phosphorescence spectroscopy it is concluded that the nitroxyl radical is the effective stabilising species and operates by a radical scavenging mechanism to form the hydroxylamine, and not by excited state quenching.     

Synthesis and structure of <Emphasis Type="Italic">trans</Emphasis>-2,2,4,6,6,8,8-heptamethyl-2,3,5a,6,7,8,9,9a-octahydro-1<Emphasis Type="Italic">H</Emphasis>-pyrido[4,3-<Emphasis Type="Italic">b</Emphasis>][1,4]diazepin-7-oxyl

trans-3,4-Diamino-2,2,6,6-tetramethylpiperidin-1-oxyl at the disstillation in a vacuum at >100°C suffered a partial thermal decomposition with the formation of two new nitroxyl radicals, bicyclic trans-2,2,4,6,6,8,8-heptamethyl-2,3,5a,6,7,8,9,9a-octahydro-1H-pyrido[4,3-b][1,4]diazepin-7-oxyl prevailing. This radical was also obtained by the reaction of the initial diaminopiperidinoxyl with mesityl oxide. According to XRD data the radical has trans-located substituents at the bridging bond of the bicycle. Taking into consideration the asymmetric atoms C^5a and C^9a the synthesized radical is a mixture of two enantiomers. Each of them in the crystalline state forms a pair of diastereomers distinguished by the position of the hydrogen atom at the atom N¹.     

Synthesis of Polystyrene‐block‐poly(butyl acrylate) Copolymers Using Nitroxide‐Mediated Living Radical Polymerization in Miniemulsion

Living free‐radical butyl acrylate polymerization in miniemulsion was initiated by polystyrene bearing a nitroxyl end group to yield polystyrene‐block‐poly(butyl acrylate) block copolymers. Polystyrene macroinitiator was obtained using different initiating systems (potassium persulfate or benzoyl peroxide) in the presence of 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) or the more water‐soluble 4‐hydroxy‐2,2,6,6‐tetramethylpiperidin‐N‐oxyl (OH‐TEMPO). The nitroxide water‐solubility has an important influence in determining molecular weight distribution and controlling the growth of the second block.     

Controlled radical polymerization of styrene in the presence of nitronyl nitroxides

Bulk radical polymerization of styrene in the presence of nitronyl nitroxides (2-(4-substituted phenyl)-4,4,5,5-tetramethyl-4,5-dihydroimidazolyl-1-oxyl 3-oxide) was studied. All nitronyl nitroxides, like other nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine 1-oxyl radical (TEMPO), act as reversible radical scavengers. The efficiency of controlling the polymerization is affected by the substituent at the 4′-position. The efficiency increases with electron donating strength of 4′-substituents, at least at the beginning of the reaction. However, the thermal stability of nitronyl nitroxides decreases in the same order. Thus, TEMPO is more suitable than nitronyl nitroxides for controlled/“living” radical polymerization of styrene.     

“Living” free radical copolymerization of styrene and N-vinylcarbazole

Poly[styrene-co-(N-vinylcarbazole)] copolymers with controlled molecular weights and narrow polydispersities were synthesized by nitroxide-mediated “living” free radical copolymerization using an initiator/capping agent system consisting of benzoyl peroxide (BPO) and the stable nitroxyl radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO). The copolymerization behaves in a “living” fashion and allows the synthesis of poly[styrene-co-(N-vinylcarbazole)]/polystyrene block copolymers via a controlled chain-extension reaction of the prepared copolymers with styrene.     

S<Stack><Subscript>N</Subscript><Superscript>H</Superscript></Stack> Reaction of lithiated nitronyl nitroxide with quinoline <Emphasis Type="Italic">N</Emphasis>-oxide

The S_N^H reaction of lithiated 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide with quinoline N-oxide affords 4,4,5,5-tetramethyl-2-(1-oxidoquinolin-2-yl)-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide.     

Catalytic thermal oxidation of phenolic antioxidants by hindered piperidine compounds

The interactions of two hindered piperidine compounds, bis(2,2,6,6,-tetramethyl-4-piperidinyl) sebacate and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxy, with various phenolic antioxidants, during the thermal processing of polypropylene have been examined using ultraviolet-visible absorption, infra-red and ESR techniques. Evidence is presented to show that the stable N-oxy radical generated during thermal processing catalytically oxidises phenolic antioxidants to their corresponding quinone forms. The implications of this interaction, particularly with regard to its effect on light stability, are discussed.     

The photolysis in polymer matrices of dyes containing a benzothioxanthene chromophore linked with a hindered amine

Novel dyes based on a benzothioxantheneimide chromophore covalently linked with a sterically hindered amine (HAS) were prepared and their light stability was tested in polymer matrices. The following dyes: 2-(2,2,6,6-tetramethyl-4-piperidyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXINH) and N-alkoxy derivative 2-(1-(1′-phenylethoxy)-2,2,6,6-tetramethyl-4-piperidyl)-thioxantheno[2,1,9-dej] isoquinoline-1,3-dione (BTXINOR) were prepared. For comparison the parent dye without HAS structural unit benzothioxanthene-3,4-dicarboxylic anhydride and the N-alkyl derivative 2-(1-dodecyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXID) and the stable nitroxyl radical 2-(1-oxo-2,2,6,6-tetramethyl-4-piperidyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXINO) were also tested. Their spectral properties, absorption and fluorescence have been examined in polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC) and in isotactic polypropylene (PP). The light stability of these dyes and model compounds were examined in thin polymer films. The photolysis rate was monitored by UV spectroscopy and for all additives under study it was in the range 10⁻⁴–10⁻³ h⁻¹. The rate of decomposition was the lowest for the parent amine BTXINH in PMMA, PS and PVC. The rate constants of photolysis are about 10 times higher for all adducts and the lowest rate of decomposition in PP matrix was observed for BTXINOR. A distinct stabilization effect of HAS structural unit on the dye decomposition was not observed. The light stability of the dyes was more influenced by the selection of the polymer. The photolysis proceeds rather fast in PP, and moderately in PS and PVC compared to PMMA.     

Reaction of Nitroxides with Sulfur Containing Compounds II [1]. Preparation of Nitroxides Bearing an Isothiocyanate Substituent in View of the Nitroxyl Group Reduction with Thiophosgene

 The reaction of thiophosgene with 2,2,6,6-tetramethylpiperidine-1-oxyl (used as a model nitroxyl radical) was examined. 2,2,6,6-Tetramethylpiperidine and 2,2,6,6-tetramethyl-1-hydroxypiperidine were identified as products. The reaction is not competitive with the reaction of thiophosgene with an amino group. Thus, three nitroxides with an isothiocyanate group were synthesized from thiophosgene and the nitroxides containing the amino substituent.