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EPR spectroscopy and product isolation have been used to investigate the initiation of spontaneous copolymerizations of donor and acceptor monomers. The results establish the presence of tetramethy ...

The reaction between an acceptor olefin (symbolized as CC and substituted by at least one conjugatively electron-withdrawing group) and N-tert-butylhydroxylamine in the presence of t-BuNO as an oxidant gives an aminoxyl which formally is a spin adduct of HC–C˙ and t-BuNO and in the appropriate cases identical to the aminoxyl formed in the thermal or photochemical reaction between CC and α-phenyl-N-tert-butylnitrone (PBN). The acceptor olefins studied were N-phenylmaleimide, maleimide, N-methylmaleimide, maleic anhydride, diethyl fumarate, diethyl 2,3-(2H2)fumarate, diethyl maleate, trimethyl ethylenetricarboxylate, 1,4-benzoquinone, fumaronitrile, methyl acrylate, methyl methacrylate and acrylonitrile.

Spin adducts, formally derived from tricyanomethyl radical attachment to α-phenyl-N-tert-butylnitrone (PBN; IUPAC name: N-benzylidene-tert-butylamine N-oxide), have been generated by various methods, such as oxidation of a mixture of tricyanomethanide ion and PBN by tris(4-bromophenyl)aminium ion or bromine, photo-oxidation of a mixture of tricyanomethane and PBN with 2,4,6-tris(4-methoxyphenyl)pyrylium ion as a sensitizer, or photolysis of chlorotricyanomethane and PBN at –30 °C, the low temperature being necessary to avoid fast concurrent cycloaddition with PBN. Both the C- and N-connected spin adducts, (NC)3C-PBN˙ and (NC)2CCN–PBN˙, have been characterized, as has an aminoxyl formed by elimination of hydrogen cyanide from the former species, (NC)2CC(Ph)N(O˙)But. For comparison, similar experiments have been performed using carbamoyldicyanomethanide ion and carbamoylchlorodicyanomethane and the spin adduct H2NCO(CN)2C-PBN˙ has been characterized.The redox properties of tricyanomethanide ion, carbamoyldicyanomethanide ion, chlorotricyanomethane and carbamoylchlorodicyanomethane have been studied by cyclic voltammetry. For chlorotricyanomethane, the redox reactivity has also been evaluated by its propensity to generate radical cations from aromatic substrates ArH in 1,1,1,3,3,3-hexafluoropropan-2-ol. Tricyanomethanide ion has E[(CN)3C˙/(CN)3C–]rev at 1.35 V (vs. Ag/AgCl) in acetonitrile while chlorotricyanomethane with Epc at about 0.6 V and a capability to oxidize compounds with redox potentials up to ca. 1.5 V to radical cations, emerges as belonging to the strongest neutral organic electron acceptors.

The reaction between aromatic compounds ArH and halogenating agents, viz. iodine chloride, chlorine, bromine, iodine, N-bromosuccinimide and N-chlorosuccinimide, in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) has been investigated. EPR spectroscopy established that these reagents produced persistent radical cations ArH˙+ from ArH with Erev(ArH˙+/ArH) up to 1.6, 1.3, 1.4, 1.1, 1.5 and 1.2 V vs. Ag/AgCl, respectively. Cyclic voltammetry of the halogenating species shows that no effect of complexation with halide ion is observed in HFP, as expected from its capacity to drastically attenuate nucleophilic reactivity, and that the cathodic peak potentials Epc (referenced to the internal ferricinium/ferrocene redox couple) are significantly or remarkably higher in HFP than in acetonitrile. For N-bromosuccinimide, the difference amounts to almost 1 V.The persistency of the radical cations in HFP is such that the kinetics of reactions between a halogenating agent, such as iodine chloride or bromine, and ArH, such as 1,4-dimethoxybenzene [Erev(ArH˙+/ArH) = 1.50 V vs. Ag/AgCl] or 1,4-dimethoxy-2,3-dimethylbenzene [Erev(ArH˙+/ArH) = 1.16 V], have been studied at room temperature over periods of hours. The initial concentration of the radical cation corresponds to yields in the range of 40–100%, depending on the reaction conditions. It is thus possible to establish that the radical cation decays via two pathways, one being the well known oxidative substitution reaction with halide ion. The second mechanism involves halogen atom transfer from the halogenating agent (Cl atom from ICl, Br atom from bromine). In the case of the radical cation of 1,4-dimethoxy-2,3-dimethylbenzene reacting with bromide ion or bromine, the latter reaction is >102 times faster.

The kinetics of cycloaddition between α-phenyl-N-tert-butylnitrone (PBN; IUPAC name: N-benzylidene-tert-butylamine N-oxide) and six activated nitriles R–CN, viz. dibromo- and dichloro-malononitrile (1a and 1b), chlorotricyanomethane (1c), tetracyanomethane (1d), trichloroacetonitrile (1e) and carbamoylchlorodicyanomethane (5), to give 2-tert-butyl-3-phenyl-5-R-2,3-dihydro-1,2,4-oxadiazoles 2a–e have been determined, as well as the similar reaction between 4-NO2PBN and 1c. The second-order rate constants for reaction between PBN and 1 in acetonitrile fall (relative rate constant in parenthesis) in the order of 1d (5.4 × 103) > 1c (3.1 × 102) > 1b (8) > 1a (1) > 5 (not given) > 1e (9 × 10–3). The rate constants are slightly larger in non-polar solvents. The rate constants of the PBN–1c and PBN–1d cycloadditions (0.15 and 2.6 dm3 mol–1 s–1, respectively) are of the same magnitude as those recently used to classify thioketones as ‘superdipolarophiles’.X-Ray crystal structures are reported for 2-tert-butyl-3-phenyl-5-(dichlorocyanomethyl)-2,3-dihydro-1,2,4-oxadiazole (2b) and 2-tert-butyl-3-phenyl-5-(chlorodicyanomethyl)-2,3-dihydro-1,2,4-oxadiazole (2c).

The temperature-dependent EPR line shapes from the methyl rotor of the X irradiation-induced CH3Ċ(COOH)2 radical in powder MMA (methyl malonic acid) and the X irradiation-induced radical CD3Ċ(COOH)2 in methyl-specifically deuterated powder MMA are studied experimentally for the temperature range 4.8 K (5 K) to 77 K (65 K). The hydrogenated system is simulated using a quantum inertial dynamical model with a hindering potential and three-site exchange rotation. The deuterated system is simulated using a classical three-site exchange model. The results show that due to the increase in moment of inertia, the tunneling frequency is negligible for the deuterated rotor, resulting in a stopped rotor low-temperature spectrum, while being sufficiently large for the hydrogenated system for this to exhibit tunneling. From the low-temperature deuterated analogue spectrum, the potential twist angle is estimated to δ = ±50° ± 2° (+n·60°, n ∈ Z). The site-exchange activation energy of the deuterium rotor is observed to b...

Abstract. Improved procedures for the synthesis of chlorotricyanomethane by reaction of potassium tricyanomethanide with sulfuryl chloride in sulfolane and of carbamoylchlorodicyanomethane by reaction of potassium carbamoyldicyanomethanide with sulfuryl chloride in dichloromethane are described.

The reaction between N-tert-butyl-α-phenylnitrone (PBN) and carboxylic acids has been studied. Two mechanisms are discernible: the generation of PBN˙+ by oxidation of PBN with a photochemically produced excited state [from either 2,4,6-tris(4-methoxyphenyl)pyrylium ion 2+ or tetrachlorobenzoquinone 4], followed by reaction with RCOOH, or the addition of RCOOH to PBN to give a hydroxylamine derivative, followed by thermal oxidation by a weak oxidant. The latter sequence is the Forrester–Hepburn mechanism. In this mechanism, neither 2+ nor 4 is effective as an oxidant, whereas bromine could be used. Thus only oxidants with redox potentials 0.1 V (SCE) are reactive enough to oxidize the intermediate hydroxylamine. This behaviour is in agreement with the redox reactivity of hydroxylamines.For the cyclic nitrone, 4,5-dihydro-5,5-dimethylpyrrole 1-oxide (DMPO), acyloxyl spin adducts have been prepared by the photochemical route.The reaction between dibenzoyl peroxide and PBN to give PhCOO–PBN˙ is not catalysed by added PhCOOH. It could be shown that the rate of formation of PhCOO–PBN˙ is compatible with the rate of thermal decomposition of dibenzoyl peroxide. Thus dibenzoyl peroxide does not support the Forrester–Hepburn mechanism, in agreement with its redox potential of ca. -0.2 V.

The reactions of N-tert-butyl-α-phenylnitrone (PBN) and 5,5-dimethyl-4,5-dihydropyrroline N-oxide (DMPO) with a series of N-heteroaromatic bases have been studied in different solvents (benzene, dichloromethane, acetonitrile) in the presence of such oxidants as PbO2, chloranil and tetrabutylammonium dodecatungstocobalt(III)ate. In all cases spin adducts were formed and their corresponding EPR signals were recorded. The formation of the spin adducts, which mostly involves nucleophilic addition of a base to the nitrone and subsequent oxidation (Forrester–Hepburn mechanism) is also discussed and compared to conventional spin trapping and to ‘inverted spin trapping'. In order to obtain further evidence for the mechanism involved some reactions were carried out using other oxidants, such as galvinoxyl and 2,2-diphenyl-1-picrylhydrazyl (DPPH).The reaction mechanisms are discussed on the basis of the redox potentials of the studied nucleophiles. Particular attention is given to the benzotriazolyl–DMPO˙ spin adduct, whose generation and decay rates were determined in the presence of some benzoquinones having different redox potentials.

The photolysis of a solution of the spin trap, α-phenyl-N-tert-butylnitrone (PBN, 1) with 2,3-dichloro-4,5-dicyano-1,4-benzoquinone (DDQ) in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) containing 6% trifluoroacetic acid produces a persistent radical species, the six-line EPR spectrum of which has aN = 2.00 and aH = 2.78 mT. The half-life of this radical was ca. 20 min at 22 °C. A similar spectrum was obtained from PBNs substituted by an electron-withdrawing group (NO2, F) in the 4-position, whereas PBNs with an electron-donating group (4-MeO, 4-Me2N, 3,4-OCH2O) upon oxidation gave multi-line spectra which could be assigned to the corresponding radical cations.

The valence bond configuration mixing (VBCM) model has been applied to predict the regioselectivity of radical cations of non-alternant systems, viz. dibenzofuran, dibenzothiophene, azulene, acenaphthylene and fluoranthene, toward reaction with nucleophiles. The reactivity factors of the VBCM model, the coefficients of the transfer orbital (TO) and spin densities of the lowest triplet state of each compound, have been calculated by means of high-level quantum chemical theory (CASPT2). The predictions have been compared with available experimental data. Since the expected regiochemical outcome of reactions of radical cations of this type with nucleophiles is distinctly different from that of the electrophilic reactions of the parent compounds, the approach offers a new way of testing the VBCM model. From a practical point of view, direct access to otherwise difficult to synthesize derivatives of these compounds via radical cation reactions should be an attractive possibility.

The persistent benzofuran radical cations B1˙+ –B4˙+ have been generated by electrochemical or chemical oxidation. The lifetimes of the radical cations have been determined by UV spectroscopy and cyclic voltammetry investigations, and their structures established by EPR and ENDOR spectroscopy.

The reactions of two fluorinating reagents, XeF2 and N-fluorodibenzenesulfonamide [(PhSO2)2N–F], with several spin traps have been investigated. In dichloromethane, the strong oxidant XeF2 cleanly gives fluoro spin adducts with N-tert-butyl-α-phenylnitrone (PBN) or 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) according to a mechanism mediated by the radical cation of the spin trap. In both cases, further fluorination takes place with replacement of the α hydrogen by fluorine.The much weaker oxidant (PhSO2)2N–F reacts with PBN or DMPO in dichloromethane giving both the fluoro adduct and an adduct formally derived from an N-centred radical, assigned the structure of PhSO2N(F)–PBN˙ or (PhSO2)2N–DMPO˙, respectively. This type of reaction proceeds by a version of the Forrester–Hepburn mechanism, in which an acid HA, in this case HF, initially adds to the nitrone function to give a hydroxylamine derivative which is oxidized by (PhSO2)N–F giving the fluoro spin adduct, a proton and the highly labile radical anion (PhSO2)2N–F˙- . By decomposition of the latter to PhSO2(F)N- and PhSO2˙, conditions are set up for propagation of the reaction by a new molecule of HA [now PhSO2(F)NH] and thus formation of the PhSO2(F)N spin adduct.

The generation and reactions of aromatic radical cations by photolysis of ArH-tetranitromethane in 1,1,1,3,3,3-hexfluoropropan-2-ol (HFP) at room temperature has been investigated. The primary radical cations (ArH⊙†) were detectable by EPR spectroscopy in most cases where Eo(Ar⊙†/ArH) 1.5 V (vs. Ag/AgCl). Secondary radical cations were obtained from pentamethylbenzene and durene (1,2,3,4,5,6,7,8-octamethyl- and 1,2,4,5,6,8-hexamethyl-anthraceneradical cation), from certain naphthalenes (dehydrodimer radical cations, Ar−Ar⊙†) and from 2,3-dimethylanisole (the radical of a nitroso derivative). The persistence of the radical cations in the presence of trinitromethanide ion is seen as a consequence of the stabilization of the nucleophile by HFP. This assumption was supported by preparative results on 1-methoxynaphthalene and 1,4-dimethylnaphthalene, where the trinitromethanide ion initiated pathway was completely eliminated in HFP, contrary to results earlier obtained in dichloromethane.