17175-15-4Relevant articles and documents
Kinetic study on nucleophilic displacement reactions of phenyl Y-substituted phenyl carbonates with 1,8-diazabicyclo[5.4.0]undec-7-ene: Effects of amine nature on reaction mechanism
Park, Kyoung-Ho,Kim, Min-Young,Um, Ik-Hwan
, p. 77 - 81 (2016/01/20)
Second-order rate constants (kN) for nucleophilic displacement reactions of phenyl Y-substituted phenyl carbonates (7a-7l) with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 °C have been measured spectrophotometrically. The Br?nsted-type plot for the reactions of 7a-7l with DBU is linear with βlg = -0.48, indicating that the reactions proceed through a concerted mechanism, which is in contrast to the stepwise mechanism reported previously for the corresponding reactions with ethylamine (a primary amine) and piperidine (a secondary amine). The Hammett plots correlated with σ- and σo constants exhibit many scattered points. In contrast, the Yukawa-Tsuno plot results in an excellent linear correlation with ρY = 1.27 and r = 0.57, implying that a negative charge develops partially on the O atom of the leaving group in the transition state. The bulky DBU is less reactive than the primary and secondary amines toward substrates possessing a weakly basic leaving group. It has been concluded that steric hindrance exerted by DBU in the plausible intermediate (T±) forces the reactions to proceed through a concerted mechanism because expulsion of the leaving group from T± could reduce the steric hindrance.
Leaving-group substituent controls reactivity and reaction mechanism in aminolysis of phenyl y-substituted-phenyl carbonates
Kang, Ji-Sun,Song, Yoon-Ju,Um, Ik-Hwan
, p. 2023 - 2028 (2013/09/02)
A kinetic study is reported for the nucleophilic substitution reactions of phenyl Y-substituted-phenyl carbonates (5a-5k) with piperidine in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 °C. The plots of k obsd vs. [piperidine] for the reactions of substrates possessing a strong electron-withdrawing group (EWG) in the leaving group (i.e., 5a-5i) are linear and pass through the origin. In contrast, the plots for the reactions of substrates bearing a weak EWG or no substituent (i.e., 5j or 5k) curve upward, indicating that the electronic nature of the substituent Y in the leaving group governs the reaction mechanism. Thus, it has been suggested that the reactions of 5a-5i proceed through a stepwise mechanism with a zwitterionic tetrahedral intermediate (i.e., T±) while those of 5j and 5k proceed through a stepwise mechanism with two intermediates (i.e., T± and its deprotonated form T-). The slope of the Bronsted-type plot for the second-order rate constants (i.e., kN or Kk2) changes from -0.41 to -1.89 as the leaving-group basicity increases, indicating that a change in the rate-determining step (RDS) occurs. The reactions of 5a-5k with piperidine result in larger k1 values than the corresponding reactions with ethylamine. Copyright
Kinetic investigation of the phenolysis of phenyl 4-nitrophenyl and phenyl 2,4-dinitrophenyl carbonates
Castro, Enrique A.,Angel, Mauricio,Pavez, Paulina,Santos, Jose G.
, p. 2351 - 2354 (2007/10/03)
The reactions of phenyl 4-nitrophenyl carbonate (PDNPC) and phenyl 2,4-dinitrophenyl carbonate (PDNPC) with a homogeneous series of phenoxide anions are subjected to a kinetic investigation in water at 25.0 °C, ionic strength 0.2 mol dm-3 (KCl). Under phenoxide or total phenol excess over the substrate all these reactions obey pseudo first-order kinetics and are first order in phenoxide. The Bronsted-type plots for the nucleophilic rate constants are linear with slopes 0.61 and 0.49 for the phenolysis of PNPC and PDNPC, respectively. The magnitude of these slopes and the absence of curvature in the Brnsted plot at pKa = 7.1 for the PNPC reactions are consistent with concerted mechanisms (one step) for both reaction series. PDNPC is more reactive than PNPC toward phenoxide nucleophiles; this can be explained by the presence of a second nitro group in PDNPC, which (i) leaves its carbonyl carbon more positively charged than that of PNPC, making the former a better electrophile, and (ii) makes 2,4-dinitrophenoxide a better leaving group than 4-nitrophenoxide. The larger nucleophilic rate coefficients found in this work relative to those obtained in the concerted phenolysis of 4-nitrophenyl and 2,4-dinitrophenyl methyl carbonates is explained by a stronger electron withdrawal from PhO compared to MeO. Comparison of the concerted phenolysis of PNPC with the stepwise reactions of quinuclidines with the same substrate indicates that substitution of a quinuclidino group in a zwitterionic tetrahedral intermediate by a phenoxy group greatly destabilises the intermediate.
