Choice of Solvent (MeCN vs H₂O) Decides Rate-Limiting Step in S_NAr Aminolysis of 1-Fluoro-2,4-dinitrobenzene with Secondary Amines: Importance of Brønsted-Type Analysis in Acetonitrile

A kinetic study is reported for nucleophilic substitution reactions of 2,4-dinitro-1-fluorobenzene (DNFB) with a series of secondary amines in MeCN and H2O at 25.0 °C. The reaction in MeCN results in an upward curvature in the plot of kobsd vs [amine], indicating that the reaction proceeds through a rate-limiting proton transfer (RLPT) mechanism. On the contrary, the corresponding plot for the reaction in H2O is linear, implying that general base catalysis is absent. The ratios of the microscopic rate constants for the reactions in MeCN are consistent with the proposed mechanism, e.g., the facts that k2/k-1 < 1 and k3/k2 > 102 suggest that formation of a Meisenheimer complex occurs before the rate-limiting step and the deprotonation by a second amine molecule becomes dominant when [amine] > 0.01 M, respectively. The Brønsted-type plots for k1k2/k-1 and k1k3/k-1 are linear with βnuc values of 0.82 and 0.84, respectively, which supports the proposed mechanism. The Brønsted-type plot for the reactions in H2O is also linear with βnuc = 0.52 which has been interpreted to indicate that the reaction proceeds through rate-limiting formation of a Meisenheimer complex. DNFB is more reactive toward secondary amines in MeCN than in H2O. The enhanced basicity of amines as well as the increased stability of the intermediate whose charges are delocalized through resonance are responsible for the enhanced reactivity in the aprotic solvent

Kinetic Study on S_NAr 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

Energy Transfer in the Azobenzene−Naphthalene Light Harvesting System

We have investigated the model light harvesting systems (LHSs) A and B typifying energy transfer (ET) between a naphthalene, Np (donor, D), and an azobenzene, Az (acceptor, A), shown schematically in Scheme . These models were actualized as the naphthyl azo molecules 1 and 4 containing a methylene tether (Scheme ). The methoxy azo molecules 2 and 5, respectively, served as benchmarks for the assessment of ET. Photophysical data, including initial rate constants for photoisomerization (trans to cis, t-1 → c-1, and cis to trans, c-1 → t-1), the relevant c-1 → t-1 quantum yields, and fluorescence quenching with free naphthalene, 3, as D were measured. Therefore, (1) irradiation of 3 at (270 nm) to give 3* generates fluorescence at 340 nm that is 65% quenched by the trans isomer of 2 (t-2) and 15% quenched by c-2. Comparable naphthalenic fluorescence of c-1 (LH model A) is quenched beyond detectability. (2) Rates of photoisomerization were determined spectrophotometrically for c-1 → t-1 starting from the c-1 photostationary state as compared with the c-2 → t-2 benchmark. (3) Progressing toward more complex LH systems, the initial rate constants, ki, for c-4 → t-4 (LH model B), were measured as compared with the c-5 → t-5 benchmark. (4) A new criterion for ET (D → A) efficiency emerges that combines ki (c → t) ratios and light absorption on irradiation (at 270 nm) ratios. On the basis of this new criterion, both 1 and 4 exhibit virtually quantitative ET efficiency. (5) Quenching data of 1 (almost complete) and 4 (95%) and ET are discussed by comparison with the relevant model azoarenes, 2 and 5, respectively, and in terms of geometrical considerations. Implications for the extension of the results, notably the new criterion for ET efficiency, in these LH models A and B to the polymer and block copolymer D−(CRR′)n−A and D−(CRR′)n−A−(CR′′R′′′)m−D targets are considered

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

Pseudo-first-order rate constants (<i>k</i>_obsd) have been measured spectrophotometrically for nucleophilic substitution reactions of 1-X-2,4-dinitrobenzenes (<b>1a</b>–<b>d</b>, X = F, Cl, Br, I) with various primary and secondary amines in MeCN and H₂O at 25.0 ± 0.1 °C. The plots of <i>k</i>_obsd vs [amine] curve upward for reactions of <b>1a</b> (X = F) with secondary amines in MeCN. In contrast, the corresponding plots for the other reactions of <b>1b</b>–<b>d</b> with primary and secondary amines in MeCN and H₂O are linear. The Brønsted-type plots for reactions of <b>1a</b>–<b>d</b> with a series of secondary amines are linear with β_nuc = 1.00 for the reaction of <b>1a</b> and 0.52 ± 0.01 for those of <b>1b</b>–<b>d</b>. 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 (<i>E</i>) values for 1-X-2,4-dinitrobenzenes have been determined: <i>E</i> = −14.1 for X = F, <i>E</i> = −17.6 for X = Cl and Br, and <i>E</i> = −18.3 for X = I. These values are consistent with the range and order of <i>E</i> values for heteroaromatic superelectrophiles and normal 6-π aromatic electrophiles

Evidence for a Catalytic Six-Membered Cyclic Transition State in Aminolysis of 4‑Nitrophenyl 3,5-Dinitrobenzoate in Acetonitrile: Comparative Brønsted-Type Plot, Entropy of Activation, and Deuterium Kinetic Isotope Effects

A kinetic study for reactions of 4-nitrophenyl 3,5-dinitrobenzoate (<b>1a</b>) with a series of cyclic secondary amines in acetonitrile is reported. Plots of the pseudo-first-order rate constant (<i>k</i>_obsd) vs [amine] curve upward, while those of <i>k</i>_obsd /[amine] vs [amine] exhibit excellent linear correlations with positive intercepts, indicating that the reaction proceeds through both uncatalyzed and catalyzed routes. Brønsted-type plots for uncatalyzed and catalyzed reactions are linear with β_nuc = 1.03 and 0.69, respectively. The Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧ values measured for the catalytic reaction with morpholine are −0.80 kcal/mol and −61.7 cal/(mol K), respectively. The negative Δ<i>H</i>^⧧ with a large negative Δ<i>S</i>^⧧ suggests that the reaction proceeds through a highly ordered transition state (i.e., a six-membered cyclic transition state, which includes a second amine molecule that accepts a proton from the aminium moiety of the zwitterionic tetrahedral intermediate and simultaneously donates a proton to the aryloxyl oxygen of the nucleofuge with concomitant C–OAr bond scission). This proposal is consistent with the smaller β_nuc value for the catalyzed reaction as compared to the uncatalyzed reaction. An inverse deuterium kinetic isotope effect (DKIE) value of 0.93 and a contrasting normal primary DKIE value of 3.23 for the uncatalyzed and catalyzed routes, respectively, also support the proposed cyclic transition state

Spirooxazine to Merooxazine Interconversion in the Presence and Absence of Zinc: Approach to a Bistable Photochemical Switch

A spironaphthoxazine (SO) photoswitch was synthesized, and its photochromic behaviors were investigated. SO underwent reversible ring-opening/closure isomerization between a spirocyclic isomer (closed form) and a merocyanine (MO isomer, open form) upon ultraviolet light irradiation. For the model SO in this work, the thermal equilibrium is substantially shifted toward the spirocyclic isomer even at −30.0 °C. However, addition of zinc, as Zn(ClO4)2, exerted an important effect on the thermal reversion process from the open (MO) to the closed form (SO). Kinetic analysis showed that thermal reversion with zinc is retarded more than 13-fold, significantly improving bistability. Moreover, introduction of zinc to the spirooxazine−merooxazine (SO−MO) system resulted in a new absorption band readily distinguishable from the bands arising from spirooxazine and merooxazine. For the first time, to the best of our knowledge, the microscopic rate constants for: MO photogeneration from SO (k1), thermal reversion of MO to SO (k2), complexation of MO with zinc (k3) and for dissociation of the complex, MO−Zn (k4), as well as for the ionization equilibria of Zn(ClO4)2 have been evaluated. The preferred transoid structures of MO and those of MO−Zn derived from the preferred MO structures are considered. Although the kinetic study does not permit elucidation of the nature of zinc binding to MO to give MO−Zn, nor the precursor isomers of MO, a DFT calculational study in progress should shed light on the structure and relative stability of these essential intermediates

Spirooxazine to Merooxazine Interconversion in the Presence and Absence of Zinc: Approach to a Bistable Photochemical Switch

A spironaphthoxazine (SO) photoswitch was synthesized, and its photochromic behaviors were investigated. SO underwent reversible ring-opening/closure isomerization between a spirocyclic isomer (closed form) and a merocyanine (MO isomer, open form) upon ultraviolet light irradiation. For the model SO in this work, the thermal equilibrium is substantially shifted toward the spirocyclic isomer even at −30.0 °C. However, addition of zinc, as Zn(ClO4)2, exerted an important effect on the thermal reversion process from the open (MO) to the closed form (SO). Kinetic analysis showed that thermal reversion with zinc is retarded more than 13-fold, significantly improving bistability. Moreover, introduction of zinc to the spirooxazine−merooxazine (SO−MO) system resulted in a new absorption band readily distinguishable from the bands arising from spirooxazine and merooxazine. For the first time, to the best of our knowledge, the microscopic rate constants for: MO photogeneration from SO (k1), thermal reversion of MO to SO (k2), complexation of MO with zinc (k3) and for dissociation of the complex, MO−Zn (k4), as well as for the ionization equilibria of Zn(ClO4)2 have been evaluated. The preferred transoid structures of MO and those of MO−Zn derived from the preferred MO structures are considered. Although the kinetic study does not permit elucidation of the nature of zinc binding to MO to give MO−Zn, nor the precursor isomers of MO, a DFT calculational study in progress should shed light on the structure and relative stability of these essential intermediates

Spirooxazine to Merooxazine Interconversion in the Presence and Absence of Zinc: Approach to a Bistable Photochemical Switch

A spironaphthoxazine (SO) photoswitch was synthesized, and its photochromic behaviors were investigated. SO underwent reversible ring-opening/closure isomerization between a spirocyclic isomer (closed form) and a merocyanine (MO isomer, open form) upon ultraviolet light irradiation. For the model SO in this work, the thermal equilibrium is substantially shifted toward the spirocyclic isomer even at −30.0 °C. However, addition of zinc, as Zn(ClO4)2, exerted an important effect on the thermal reversion process from the open (MO) to the closed form (SO). Kinetic analysis showed that thermal reversion with zinc is retarded more than 13-fold, significantly improving bistability. Moreover, introduction of zinc to the spirooxazine−merooxazine (SO−MO) system resulted in a new absorption band readily distinguishable from the bands arising from spirooxazine and merooxazine. For the first time, to the best of our knowledge, the microscopic rate constants for: MO photogeneration from SO (k1), thermal reversion of MO to SO (k2), complexation of MO with zinc (k3) and for dissociation of the complex, MO−Zn (k4), as well as for the ionization equilibria of Zn(ClO4)2 have been evaluated. The preferred transoid structures of MO and those of MO−Zn derived from the preferred MO structures are considered. Although the kinetic study does not permit elucidation of the nature of zinc binding to MO to give MO−Zn, nor the precursor isomers of MO, a DFT calculational study in progress should shed light on the structure and relative stability of these essential intermediates

Spirooxazine to Merooxazine Interconversion in the Presence and Absence of Zinc: Approach to a Bistable Photochemical Switch

A spironaphthoxazine (SO) photoswitch was synthesized, and its photochromic behaviors were investigated. SO underwent reversible ring-opening/closure isomerization between a spirocyclic isomer (closed form) and a merocyanine (MO isomer, open form) upon ultraviolet light irradiation. For the model SO in this work, the thermal equilibrium is substantially shifted toward the spirocyclic isomer even at −30.0 °C. However, addition of zinc, as Zn(ClO4)2, exerted an important effect on the thermal reversion process from the open (MO) to the closed form (SO). Kinetic analysis showed that thermal reversion with zinc is retarded more than 13-fold, significantly improving bistability. Moreover, introduction of zinc to the spirooxazine−merooxazine (SO−MO) system resulted in a new absorption band readily distinguishable from the bands arising from spirooxazine and merooxazine. For the first time, to the best of our knowledge, the microscopic rate constants for: MO photogeneration from SO (k1), thermal reversion of MO to SO (k2), complexation of MO with zinc (k3) and for dissociation of the complex, MO−Zn (k4), as well as for the ionization equilibria of Zn(ClO4)2 have been evaluated. The preferred transoid structures of MO and those of MO−Zn derived from the preferred MO structures are considered. Although the kinetic study does not permit elucidation of the nature of zinc binding to MO to give MO−Zn, nor the precursor isomers of MO, a DFT calculational study in progress should shed light on the structure and relative stability of these essential intermediates

Spirooxazine to Merooxazine Interconversion in the Presence and Absence of Zinc: Approach to a Bistable Photochemical Switch

A spironaphthoxazine (SO) photoswitch was synthesized, and its photochromic behaviors were investigated. SO underwent reversible ring-opening/closure isomerization between a spirocyclic isomer (closed form) and a merocyanine (MO isomer, open form) upon ultraviolet light irradiation. For the model SO in this work, the thermal equilibrium is substantially shifted toward the spirocyclic isomer even at −30.0 °C. However, addition of zinc, as Zn(ClO4)2, exerted an important effect on the thermal reversion process from the open (MO) to the closed form (SO). Kinetic analysis showed that thermal reversion with zinc is retarded more than 13-fold, significantly improving bistability. Moreover, introduction of zinc to the spirooxazine−merooxazine (SO−MO) system resulted in a new absorption band readily distinguishable from the bands arising from spirooxazine and merooxazine. For the first time, to the best of our knowledge, the microscopic rate constants for: MO photogeneration from SO (k1), thermal reversion of MO to SO (k2), complexation of MO with zinc (k3) and for dissociation of the complex, MO−Zn (k4), as well as for the ionization equilibria of Zn(ClO4)2 have been evaluated. The preferred transoid structures of MO and those of MO−Zn derived from the preferred MO structures are considered. Although the kinetic study does not permit elucidation of the nature of zinc binding to MO to give MO−Zn, nor the precursor isomers of MO, a DFT calculational study in progress should shed light on the structure and relative stability of these essential intermediates