scholarly journals Reactive intermediates. Some chemistry of quinone methides

2000 ◽  
Vol 72 (12) ◽  
pp. 2299-2308 ◽  
Author(s):  
Y. Chiang ◽  
A. J. Kresge ◽  
Y. Zhu
Keyword(s):  
Acid Catalysis ◽  
Isotope Effects ◽  
Reactive Intermediates ◽  
Quinone Methide ◽  
Carbonyl Carbon Atom ◽  
Carbonyl Carbon ◽  
Quinone Methides ◽  
Rate Determining Step ◽  
Energy Relationships ◽  
Acid Catalyzed

Quinone methides were produced in aqueous solution by photochemical dehydration of o-hydroxybenzyl alcohols (o-HOC6H4CHROH; R = H, C6H5, 4-CH3OC6H4), and flash photolytic techniques were used to examine their rehydration back to starting substrate as well as their interaction with bromide and thiocyanate ions. These reactions are acid-catalyzed and show inverse isotope effects (kH+/kD+ < 1), indicating that they occur through preequilibrium protonation of the quinone methide on its carbonyl carbon atom followed by rate-determining capture of the benzyl carbocations so formed by H2O, Br-, or SCN-. With some quinone methides (R = C6H5 and 4-CH3OC6H4) this acid catalysis could be saturated, and analysis of the data obtained in the region of saturation for the example with R = 4-CH3OC6H4 produced both the equilibrium constant for the substrate protonation step and the rate constant for the rate-determining step. Energy relationships comparing the quinone methides with their benzyl alcohol precursors are derived.

[4+n] Annulation Reactions Using ortho-Halomethyl Anilines as Aza-ortho-Quinone Methide Precursors

Synthesis ◽  
2021 ◽  
Author(s):  
Xiao-Yu He ◽  
Yu-Hong Ma ◽  
Qing-Qing Yang ◽  
Wen-Jing Xiao
Keyword(s):  
Convenient Method ◽  
Reactive Intermediates ◽  
Quinone Methide ◽  
Synthetic Chemistry ◽  
Important Class ◽  
Quinone Methides ◽  
Aniline Derivatives

Aza-ortho-quinone methides are an important class of reactive intermediates, which have found broad applications in synthetic chemistry. Recently, 1,4-elimination of ortho-halomethyl aniline derivatives has emerged as a new powerful and convenient method for aza-ortho-quinone methide generation. This review will highlight their recent applications as aza-ortho-quinone methide precursors in annulation reactions to access various biologically important nitrogen-containing heterocycles. The general mechanisms are briefly discussed as well.

Vinyl ether hydrolysis. XIII. The failure of I-strain to produce a change in rate-determining step

1980 ◽  
Vol 58 (2) ◽  
pp. 124-129 ◽  
Author(s):  
Y. Chiang ◽  
W. K. Chwang ◽  
A. J. Kresge ◽  
S. Szilagyi
Keyword(s):  
Vinyl Ether ◽  
Acid Catalysis ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Mineral Acid ◽  
Buffer Solutions ◽  
Rate Determining Step ◽  
Kinetic Isotope ◽  
Acetic Acid Buffer ◽  
Hydrolysis Of

Rates of hydrolysis of 1-ethoxy-3,3,5,5-tetramethylcyclopentene and 1-methoxy-2,3,3,5,5-pentamethylcyclopentene measured in mineral acid and formic and acetic acid buffer solutions show general acid catalysis and give large kinetic isotope effects in the normal direction (kH/kD > 1). This indicates that these reactions proceed by the conventional mechanism for vinyl ether hydrolysis in which proton transfer from the catalyzing acid to the substrate is rate-determining, and that the I-strain in these substrates is insufficiently great to shift the reaction mechanism to rapidly reversible substrate protonation followed by rate-determining hydration of the ensuing cationic intermediate.

Correlation of rates of uncatalyzed and hydroxide-ion catalyzed ketene hydration. A mechanistic application and solvent isotope effects on the uncatalyzed reaction

2000 ◽  
Vol 78 (4) ◽  
pp. 508-515
Author(s):  
John Andraos ◽  
A Jerry Kresge
Keyword(s):  
Carbon Atom ◽  
Rate Constants ◽  
Isotope Effects ◽  
Nucleophilic Attack ◽  
Carbonyl Carbon Atom ◽  
Basic Solution ◽  
Carbonyl Carbon ◽  
Hydroxide Ion ◽  
Solvent Isotope ◽  
Solvent Isotope Effects

Rates of hydration of a number of ketenes were measured in neutral and basic solution using flash photolytic techniques, and rate constants for their uncatalyzed, kuc, and hydroxide-ion catalyzed, kHO, reactions were determined. These results, plus additional data from the literature, were found to provide the remarkably good correlation log kuc = -3.21 + 1.14 log kHO, which spans 10 orders of magnitude in reactivity and includes 31 ketenes. This good correlation implies that uncatalyzed and hydroxide-ion catalyzed ketene hydraton occur by similar reaction mechanisms, which for the hydroxide-ion catalyzed process is known to involve nucleophilic attack on the carbonyl carbon atom of the ketene. Rate constants for phenylhydroxyketene, on the other hand, do not fit this correlation, which suggests that the mechanistic assignment upon which these rate constants are based may not be correct. Solvent isotope effects on these uncatalyzed ketene hydrations are weak; most are less than kH/kD = 2. It is argued that these isotope effects are largely, if not entirely, secondary in nature and that they are consistent with both a reaction mechanism in which nucleophlic attack of a single water molecule on the ketene carbonyl carbon atom produces a zwitterionic intermediate and also a mechanism that avoids this intermediate by passing through a cyclic transition state involving several water molecules.Key words: ketene hydration, rate correlation, nucleophilic attack, solvent isotope effects, phenylhydroxyketene.

A KINETIC ISOTOPE EFFECT STUDY OF THE TSCHUGAEFF REACTION

1961 ◽  
Vol 39 (2) ◽  
pp. 348-358 ◽  
Author(s):  
Richard F. W. Bader ◽  
A. N. Bourns
Keyword(s):  
Isotope Effects ◽  
Strong Support ◽  
Effect Study ◽  
Sulphur Atom ◽  
Carbonyl Carbon ◽  
Cyclic Transition ◽  
Rate Determining Step ◽  
Kinetic Isotope ◽  
Cyclic Transition State ◽  
Step Mechanism

Three different kinetic isotope effects have been measured in the thermal decomposition of S-methyl-trans-2-methyl-1-indanyl xanthate: the S32/S34 effect for the thio-ether sulphur atom, the S32/S34 effect for the thion sulphur, and the C12/C13 effect for the carbonyl carbon. The results of these measurements provide strong support for a two-step mechanism, in which the rate-determining step is the removal through a cyclic transition state of the cis beta hydrogen atom by the thion sulphur atom.

Brønsted Acid Catalyzed Domino 1,6-Addition/Intramolecular Cyclization­ Reactions: Diastereoselective Synthesis of Dihydro­coumarin Frameworks

Synthesis ◽  
2017 ◽  
Vol 50 (06) ◽  
pp. 1307-1314 ◽  
Author(s):  
Guang-Jian Mei ◽  
Zheng Cao ◽  
Gui-Xiang Zhou ◽  
Chun Ma ◽  
Kang Jiang
Keyword(s):  
Hydrogen Bonding ◽  
Crucial Role ◽  
Acid Catalysis ◽  
Bronsted Acid ◽  
Brønsted Acid ◽  
Diastereoselective Synthesis ◽  
Quinone Methides ◽  
Hydrogen Bonding Interactions ◽  
Brönsted Acid ◽  
Acid Catalyzed

An efficient domino 1,6-addition/intramolecular cyclization reaction between para-quinone methides (p-QMs) and azlactones under Brønsted acid catalysis was established. A series of highly functionalized dihydrocoumarins were constructed in good to excellent yields (up to 96%) with excellent diastereoselectivities (all >20:1 d.r.). In this process, the Brønsted acid plays a crucial role not only in activating the two substrates, but also in controlling the diastereoselectivity of the reaction via hydrogen-bonding interactions. In addition, this protocol demonstrates the great practicability of utilizing p-QMs in domino reactions.

Multiple changes in rate-determining step in the acid and base catalyzed cyclizations of ethyl N-(p-nitrophenyl)hydantoates caused by methyl substitution

1999 ◽  
Vol 77 (5-6) ◽  
pp. 849-859
Author(s):  
Iva B Blagoeva ◽  
Anthony J Kirby ◽  
Asen H Koedjikov ◽  
Ivan G Pojarlieff
Keyword(s):  
Steric Hindrance ◽  
Acid Catalysis ◽  
Isotope Effects ◽  
Base Catalysis ◽  
Ethoxy Group ◽  
First Order ◽  
Tetrahedral Intermediate ◽  
General Base ◽  
Rate Determining Step ◽  
Acid And Base

The slopes of the pH-rate profiles for the cyclization of 2-methyl- and 2,3-dimethyl hydantoates 1-NPU and 2-NPU between pH 1 and 7 change from 1 to 0 and then back to 1. A reaction first order in H+ was observed with the latter compound. The 2,2,3-trimethyl derivative 3-NPU showed only one reaction first order in OH-, but complex acid catalysis is described by slopes 0, -1, 0, and finally -1 again. The cyclizations were general base catalyzed, with Brønsted β values of 0.5-0.6. The OH- catalysis at higher pH for 1-NPU and 2-NPU showed inverse solvent kinetic isotope effects and deviated from the Brønsted relationships, while that for 3-NPU showed a normal effect and complied with the Brønsted relationship. The accelerations due to the gem-dimethyl effect were lost with the OH- and general base-catalyzed reactions of 3-NPU. This behaviour is due to a change from the rate-determining formation of the tetrahedral intermediate with 1-NPU and 2-NPU to the rate-determining breakdown with 3-NPU, due to steric hindrance to protonation of the leaving ethoxy group. The OH- reaction at higher pH involves attack of the ureide anion with 1-NPU and 2- NPU, becoming concerted with deprotonation when catalyzed by general bases and changing to acid inhibition of the anion of the tetrahedral intermediate at low pH. With 3-NPU at higher pH, T- is in equilibrium and the conjugate acids of the general bases accelerate its breakdown by protonating the ethoxy group. Acid catalysis of the cyclization of 3-NPU at higher pH is also protonation of the leaving group from T0 changing to the rate-determining formation of T at lower pH. The latter mechanism is preferred for the cyclization of 2-NPU.Key words: gem-dimethyl effect, mechanism, general base catalysis, proton transfer, steric hindrance.

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