All reagents and solvents were purchased from Aldrich Chemical Co. and used as received. Benzoxazine 1 was prepared according to the reported procedure, 5c and the purity is >95% as determined by 1 H NMR.Measurements. 1 H NMR spectra were taken on a Bruker AM-360 (360 MHz). Chemical shifts were reported in parts per million relative to TMS as an internal standard (δ TMS = 0) for 1 H NMR spectra. The solvent for NMR measurement was deuterated DMSO or deuterated chloroform. DSC studies were done on a DSC-Q20 thermal analyzer from TA Instruments with N 2 as a purge gas at a scanning rate of 10 °C/min. About 5 mg of samples was tested in high-pressure closed aluminum pans.General Procedures for Ring-Opening Polymerization of Benzoxazine 1. In a test tube, benzoxazine 1 (113 mg, 0.5 mmol) and a catalyst (1 or 5 mol %) were dissolved in acetone (∼0.2 mL). The mixture was dried at room temperature under high vacuum for ∼1 h and heated under the conditions showed above. Then the resulting mixture was subjected to routine analysis.

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Electrophilic and electrostatic catalysis have been identified as distinct contributions that affect the reactivity of radical anions in the reductive cleavage of alkyl aryl ethers. Two modes of mesolytic scission of these radical anions are possible: homolytic (dealkylation, a thermodynamically favored but kinetically forbidden process) and heterolytic (dealkoxylation). From our studies (alkali metal reductions, electrochemical studies, use of substrates with a preformed positive charge in certain positions of their structure) it can be concluded that the heterolytic scission is very much dependent on the electrophilic assistance by the counterion and it is only observed in contact ionic pairs with unsaturated cations (electrophilic catalysis). On the other hand, the homolytic scission is observed in solvent-separated ionic pairs, and it is especially efficient when the pair has a controlled topology with a tetralkylammonium cation (saturated cation) near the oxygen atom. The effect of the cation has, in this case, electrostatic origin (electrostatic catalysis), probably lowering the barrier of the intramolecular pi-sigma electron transfer process and thus reducing the kinetic control of the reaction in such a way that the thermodynamically more favorable process is produced.

Fragmentation reactions of radical anions (mesolytic cleavages) of cyanobenzyl alkyl ethers (intramolecular dissociative electron transfer, heterolytic cleavages) have been studied electrochemically. The intrinsic barriers for the processes have been established from the experimental thermodynamic and kinetic parameters. These values are more than 3 kcal/mol lower as an average than the related homolytic mesolytic fragmentations of radical anions of 4-cyanophenyl ethers. In the particular case of isomers 4-cyanobenzyl phenyl ether and 4-cyanophenyl benzyl ether, the difference in intrinsic barriers amounts to 5.5 kcal/mol, and this produces an energetic crossing where the thermodynamically more favorable process (homolytic) is the kinetically slower one. The fundamental reasons for this behavior have been established by means of theoretical calculations within the density functional theory framework, showing that, in this case, the factors that determine the kinetics are clearly different (mainly present in the transition state) from those that determine the thermodynamics and they are not related to the regioconservation of the spin density ("spin regioconservation principle"). Our theoretical results reproduce quite well the experimental energetic difference of barriers and demonstrate the main structural origin of the difference.

Couplings between two radical anions [1] or two radical cations [2] are common outcomes in electrochemical reactions and give rise to a doubly charged s-bonded dimeric species. In the case of delocalized p systems such as 9-cyanoanthracene, formation of a p-dimer intermediate before collapse of the two radical anions into a s-bonded species has been proposed, [2a-b] although the same event has been explained by one-step radical-anion dimerization [2c-d] (Scheme 1).Moreover, the formation of radical-anion dimers in the solid state, such as that of 7,7,8,, is well established. [3] Furthermore, it is known that the nucleophilic aromatic substitution (S N Ar) mechanism involves addition compounds (s complexes) as intermediates. [4] For this reaction, UV and NMR spectroscopic experiments suggested the existence of a p-complex intermediate prior to s-complex formation. [5] Definitive evidence was provided by the isolation of the p-complex intermediate in the S N Ar reaction of indole-3-carboxylate with 1,3,5-trinitrobenzene. [6] Thus, the formation of p-dimer intermediates in the s dimerization of radical anions remains controversial.In the reduction of 1,3,5-trinitrobenzene (1), Bock and Lechner-Knoblauch observed an irreversible wave, which was explained by formation of 1,3-dinitrobenzene and nitrite anion. [7] However, by bulk electrochemical reduction of 1 in acetone, Sosokin et al. isolated the s-bonded dimer 1,1'-dihydrobis(2,4,6-trinitrocyclohexadienyl) (3, Scheme 2) as its tetraethylammonium salt. [8] We report herein on a complete electrochemical (cyclic voltammetry and bulk electrolysis), spectroscopic, and synthetic investigation of the reduction of 1, providing conclusive evidence for the formation of a pdimer intermediate prior to formation of the s dimer and the reaction of this p dimer with N 2 to give an organic N 2 -fixation system.The electrochemical behavior of 1 is definitely different from those of nitrobenzene or dinitrobenzenes (see the Supporting Information). [9] Figure 1 a shows that, at low scan rates, 1 has one chemically irreversible reduction wave at À0.56 V versus SCE in acetonitrile (CH 3 CN, 0.1m nBu 4 NBF 4 , Ar atmosphere, 10 8C). The resulting follow-up product is oxidized at + 0.23 V. This oxidation wave only appears after a first reduction scan. The reduction wave becomes reversible at scan rates higher than 1800 V s À1 (E8 = À0.57 V, k s = 0.01 cm s À1 ). Peak-potential analysis of the reduction wave at low and high scan rates indicates a oneelectron process. The shape of the voltammograms (peak width) suggests fast electron transfer with kinetic control by chemical reaction. [10] The peak potential is concentrationdependent (22 mV per unit log c) and scan rate-dependent (23 mV per unit log v) in the concentration range 2-10 mm. These cyclic voltammetric data indicate dimerization of the radical anion of 1 through a second-order reaction pathway ([E + C2(A rr)] mechanism) to form 2, which is responsible for the oxidation wave at + 0.23 V (Scheme 2). [11] A dimerization rate c...