Julian M. Dust

Mechanistic Assessment of SNAr Displacement of Halides from 1-Halo-2,4-dinitrobenzenes by Selected Primary and Secondary Amines: Brønsted and Mayr Analyses

Pseudo-first-order rate constants (k(obsd)) have been measured spectrophotometrically for nucleophilic substitution reactions of 1-X-2,4-dinitrobenzenes (1a-d, X = F, Cl, Br, I) with various primary and secondary amines in MeCN and H(2)O at 25.0 ± 0.1 °C. The plots of k(obsd) vs [amine] curve upward for reactions of 1a (X = F) with secondary amines in MeCN. In contrast, the corresponding plots for the other reactions of 1b-d with primary and secondary amines in MeCN and H(2)O are linear. The Brønsted-type plots for reactions of 1a-d with a series of secondary amines are linear with β(nuc) = 1.00 for the reaction of 1a and 0.52 ± 0.01 for those of 1b-d. Factors governing reaction mechanisms (e.g., solvent, halogen atoms, H-bonding interactions, amine types) have been discussed. Kinetic data were also analyzed in terms of the Mayr nucleophilicity parameter for the amines with each aromatic substrate. Provisional Mayr electrophilicity parameter (E) values for 1-X-2,4-dinitrobenzenes have been determined: E = -14.1 for X = F, E = -17.6 for X = Cl and Br, and E = -18.3 for X = I. These values are consistent with the range and order of E values for heteroaromatic superelectrophiles and normal 6-π aromatic electrophiles.

Water and hydroxide ion pathways in the σ-complexation of superelectrophilic 2-aryl-4,6-dinitrobenzotriazole 1-oxides in aqueous solution. A kinetic and thermodynamic study

As part of our continuing studies of the highly electron-deficient nature of nitrobenzofuroxans, nitrobenzofurazans and related heterocycles, we report here a kinetic and thermodynamic study of σ-complexation for a series of 2-aryl-4,6 -dinitrobenzotriazole 1-oxides (3a–e) over a large pH range in aqueous solution. The reaction series represents a modulation in electrophilic properties of the benzotriazole moiety in formation of the corresponding hydroxy σ-adducts (4a–e). Analysis of the data has allowed dissection of observed rates into forward (kH2O1, kOH−2) and reverse (kH+−1, k−2) rate constants as well as the obtention of pKa values for H2O addition to the benzotriazole moiety. Our results reveal that 3a–e are superelectrophilic compounds with respect to 1,3,5-trinitrobenzene (TNB) as a standard electron-deficient aromatic, but less superelectrophilic compared to 4,6-dinitrobenzofuroxan (DNBF). Some data pertaining to buffer catalysis of the formation and decomposition of the adducts together with solvent deuterium isotope effects for these pathways are also reported. From these results, it is concluded that adduct formation occurs via general base catalyzed water attack: the general bases include notably H2O, HCO3−, CO32− as well as OH−. This contrasts with the situation for the σ-complexation of DNBF where HCO3− and CO32− were found to act as nucleophilic catalysts whereas OH− functioned as a general base catalyst. This contrasting behaviour provides further evidence that the dinitro-activated carbocyclic ring of the benzotriazoles 3a–e ranks somewhat lower in electrophilic/superelectrophilic properties compared to that in DNBF. Altogether, the results provide a basis for understanding the relationship between the superelectrophilic reactivities, as evidenced by the contrasting kinetic and thermodynamic properties of the systems at hand, and the varied abilities of these substrates to react in pericyclic Diels–Alder reactions.

Kinetic Study on SNAr Reaction of 1-(Y-Substituted-phenoxy)-2,4-dinitrobenzenes with Cyclic Secondary Amines in Acetonitrile: Evidence for Cyclic Transition-State Structure

A kinetic study is reported for SNAr reactions of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes (1a-1h) with amines in MeCN. The plots of pseudo-first-order rate constant versus amine concentration curve upward, indicating that the reactions are catalyzed by a second amine molecule. The Brønsted-type plots for the reaction of 1-(4-nitrophenyl)-2,4-dinitrobenzene (1a) with secondary amines are linear with βnuc = 1.10 and 0.85 for the uncatalyzed and catalyzed reactions, respectively, while the Yukawa-Tsuno plots for the reactions of 1a-1h with piperidine result in excellent linear correlations with ρY = 1.85 and r = 0.27 for the uncatalyzed reaction and ρY = 0.73 and r = 0.23 for the catalyzed reaction. The catalytic effect decreases with increasing amine basicity or electron-withdrawing ability of the substituent Y in the leaving group. Activation parameters calculated from the rate constants measured at five different temperatures for the catalyzed reaction of 1a with piperidine are ΔH(‡) = 0.38 kcal/mol and ΔS(‡) = -55.4 cal/(mol K). The catalyzed reaction from a Meisenheimer complex (MC(±)) is proposed to proceed through a concerted mechanism with a cyclic transition-state rather than via a stepwise pathway with an anionic intermediate, MC(-). Deuterium kinetic isotope effects provide further insight into the nature of the concerted transition state.

Deceleration of thermal ring closure in a glass-forming mexylaminotriazine-substituted merocyanine (MC) linked to intramolecular hydrogen bonding

The design and synthesis of a glass-forming nitrobenzospiropyran (SP) functionalized with a mexylaminotriazine group is reported herein. The title compound was synthesized in 63% overall yield by the carbamylation of a 2-hydroxyethyl SP precursor and an amino-functionalized glass-forming triazine precursor. As a photoswitch, the colourless SP undergoes ring opening upon irradiation at 365 nm to the coloured merocyanine (MC) form. The negative solvatochromism (i.e., hypsochromic shift increasing with solvent polarity) and kinetics of thermal ring closure of the MC form were studied spectrometrically in four solvents: toluene, tetrahydrofuran, N,N-dimethylformamide and 1-propanol. Results were compared to those for previously studied related merocyanines of 6-nitroBIPs. The observed thermal reversion rate from the MC to the SP form in non-polar solvents was surprisingly slow, and is believed to be a result of intramolecular hydrogen bonding. Kinetics in solvents that can accept hydrogen bonds and analysis of activation parameters (Ea, A, ΔH‡, ΔS‡, ΔG‡) highlights the role of intramolecular hydrogen bonding for this MC derivative. Thus, the large negative ΔS‡ (−32.1 J mol−1 K−1) found for the MC reversion in toluene suggests a tighter transition state that reflects the strength of hydrogen bonding in the MC that inhibits the trans–cis isomerization that is the rate-determining step in ring closure.