29650-96-2Relevant academic research and scientific papers
The nonenzymatic decomposition of guanidines and amidines
Lewis, Charles A.,Wolfenden, Richard
supporting information, p. 130 - 136 (2014/01/23)
To establish the rates and mechanisms of decomposition of guanidine and amidine derivatives in aqueous solution and the rate enhancements produced by the corresponding enzymes, we examined their rates of reaction at elevated temperatures and used the Arrhenius equation to extrapolate the results to room temperature. The similar reactivities of methylguanidine and 1,1,3,3-tetramethylguanidine and their negative entropies of activation imply that their decomposition proceeds by hydrolysis rather than elimination. The influence of changing pH on the rate of decomposition is consistent with attack by hydroxide ion on the methylguanidinium ion (k2 = 5 × 10 -6 M-1 s-1 at 25 C) or with the kinetically equivalent attack by water on uncharged methylguanidine. At 25 C and pH 7, N-methylguanidine is several orders of magnitude more stable than acetamidine, urea, or acetamide. Under the same conditions, the enzymes arginase and agmatinase accelerate substrate hydrolysis 4 × 1014-fold and 6 × 1012-fold, respectively, by mechanisms that appear to involve metal-mediated water attack. Arginine deiminase accelerates substrate hydrolysis 6 × 1012-fold by a mechanism that (in contrast to the mechanisms employed by arginase and agmatinase) is believed to involve attack by an active-site cysteine residue.
Solvolytic Behavior of Aliphatic Carboxylates
Matic, Mirela,Denegri, Bernard,Kronja, Olga
supporting information, p. 1477 - 1486 (2015/10/05)
The leaving group abilities (nucleofugalities) of a series of aliphatic carboxylates have been obtained by determining the nucleofuge-specific parameters (Nf and sf) from solvolysis rate constants of X,Y-substituted benzhydryl carboxylates in a series of aqueous ethanol mixtures by applyication of the linear free energy relationship (LFER) equation: log k = sf (Ef + Nf). These values can be employed to compare reactivities of carboxylates with those of other leaving groups previously included in the nucleofugality scale, and also to estimate the solvolysis rates of various carboxylates. It is confirmed that the inductive effect is the most important variable governing the reactivities of halogenated carboxylates in solution. Moreover, both the Hammett correlation and the solvolytic activation parameters have revealed a strong influence of the inductive effect on the nucleofugality of alkyl-substituted carboxylates. The reaction constants (sf) indicate that carboxylate substrates with weaker leaving groups solvolyze via later, more carbocation-like, transition states, which is in accord with the Hammond postulate. In addition, due to the weaker demand for solvation of transition states that produce more strongly stabilized benzhydrylium ions, in which more efficient charge delocalization occurs, the reaction constants (sf) obtained with most of the leaving groups investigated here increase as the polarity of the solvent decreases.
Elimination Reactions of (E)- and (Z)-Benzaldehyde O-Pivaloyloximes. Transition-State Differences for the Syn and Anti Eliminations Forming Nitriles
Cho, Bong Rae,Cho, Nam Soon,Lee, Sang Kook
, p. 2230 - 2233 (2007/10/03)
Elimination reactions of (E)- and (Z)-benzaldehyde O-pivaloyloximes 1 and 2 with DBU in MeCN have been investigated kinetically. The reactions are second order and exhibit substantial values of Hammett ρ and kH/kD values, and an E2 mechanism is evident. The rate of elimination from 2 is approximately 20 000-fold faster than that from 1. For reactions of 1 with DBU in MeCN, a Hammett ρ value of 2.4 ± 0.1, kH/kD = 2.7 ± 0.3, ΔH? = 12.5 ± 0.2 kcal/mol, and ΔS? = -31.0 ± 0.6 eu have been determined. The corresponding values for 2 are ρ = 1.4 ± 0.1, kH/kD = 7.8 ± 0.3, ΔH? = 8.8 ± 0.1 kcal/mol, and ΔS? = -23.6 ± 0.4 eu, respectively. The results indicate that the nitrile-forming anti eliminations from 2 proceed via a more symmetrical transition state with a smaller degree of proton transfer, less negative charge development at the β-carbon, and greater extent of triple-bond formation than that for the syn elimination.
Gas-Phase Nucleophilic Displacement Reactions
Olmstead, William N.,Brauman, John I.
, p. 1653 - 1662 (2007/10/03)
Displacement reactions of each of a variety of anionic nucleophiles reacting with each of a variety of neutrals have been studied by pulsed ion cyclotron resonance (ICR) spectroscopy.Rate constants for these reactions are interpreted in terms of a three-step reaction sequence.RRKM calculations are used to obtain information about the energy of transition states.The origin of the barrier to reaction in solution is discussed.
