Abstract:Dehydrogenation of hydroaromatic compounds with quinones was reinvestigated in the light of recent criticism of the reaction mechanism. Kinetic and spectroscopic evidence shows that disappearance of substrate proceeds at the same rate as the product‐forming step. A mechanism consisting in fast formation followed by slow decomposition of an intermediate can be ruled out. The order of reactivities of 1,4‐cyclohexadiene (1), 1,4‐dihydronaphthalene (8) and 9,10‐dihydroanthracene (11) changes in going from benzoqui… Show more
“…Related orders of hydride abstraction rates from the dihydroarenes 3 , 4 , and 5 (Tables and ) have also been found in their reactions with other hydride acceptors (Table ). It should be noted, however, that for the reactions of dihydroarenes with quinones the possibility of concerted didehydrogenation with direct formation of arenes has been discussed. ,− …”
Section: Discussionmentioning
confidence: 99%
“…Previous kinetic investigations of hydride transfer reactions have often used tritylium ions as hydride abstractors. − Generally, these reactions follow second-order kinetics, ,,,− ,,,− and the rate constants depend only slightly on the nature of the negative counterion 17,18,23 or the solvent, , though a comparison of data from different sources sometimes proved to be problematic. Only a few studies were addressed to the evaluation of a quantitative relationship between the structure and the reactivity of hydride donors and hydride acceptors. ,,,,− …”
Benzhydryl cations were used as reference electrophiles to determine the hydride donor reactivities of unsaturated hydrocarbons. The kinetics of the reactions were followed by UV-vis spectroscopy and conductivity measurements, and it was found that the second-order rate constants for the hydride transfer processes were almost independent of the solvents or counterions employed. The rate constants correlate linearly with the previously published empirical electrophilicity parameters E of the benzhydrylium ions. Therefore, the linear free energy relationship log k(20 degrees C) = s(E + N) could be employed to characterize the hydride reactivities of the hydrocarbons by the nucleophilicity parameters N and s. The similarity of the slopes s for hydride donors and pi-nucleophiles allows a direct comparison of the reactivities of these different functional groups based on their nucleophilicity parameters N. Since nucleophilicity parameters of -5 < N < 0 have been found for a large variety of allylic and bisallylic hydride donors, a rule of thumb is derived that hydride transfer processes may compete with carbon-carbon bond-forming reactions when carbocations are combined with olefins of pi-nucleophilicity N < 0.
“…Related orders of hydride abstraction rates from the dihydroarenes 3 , 4 , and 5 (Tables and ) have also been found in their reactions with other hydride acceptors (Table ). It should be noted, however, that for the reactions of dihydroarenes with quinones the possibility of concerted didehydrogenation with direct formation of arenes has been discussed. ,− …”
Section: Discussionmentioning
confidence: 99%
“…Previous kinetic investigations of hydride transfer reactions have often used tritylium ions as hydride abstractors. − Generally, these reactions follow second-order kinetics, ,,,− ,,,− and the rate constants depend only slightly on the nature of the negative counterion 17,18,23 or the solvent, , though a comparison of data from different sources sometimes proved to be problematic. Only a few studies were addressed to the evaluation of a quantitative relationship between the structure and the reactivity of hydride donors and hydride acceptors. ,,,,− …”
Benzhydryl cations were used as reference electrophiles to determine the hydride donor reactivities of unsaturated hydrocarbons. The kinetics of the reactions were followed by UV-vis spectroscopy and conductivity measurements, and it was found that the second-order rate constants for the hydride transfer processes were almost independent of the solvents or counterions employed. The rate constants correlate linearly with the previously published empirical electrophilicity parameters E of the benzhydrylium ions. Therefore, the linear free energy relationship log k(20 degrees C) = s(E + N) could be employed to characterize the hydride reactivities of the hydrocarbons by the nucleophilicity parameters N and s. The similarity of the slopes s for hydride donors and pi-nucleophiles allows a direct comparison of the reactivities of these different functional groups based on their nucleophilicity parameters N. Since nucleophilicity parameters of -5 < N < 0 have been found for a large variety of allylic and bisallylic hydride donors, a rule of thumb is derived that hydride transfer processes may compete with carbon-carbon bond-forming reactions when carbocations are combined with olefins of pi-nucleophilicity N < 0.
“…Deprotonation of the substrate by DDQH − can afford the corresponding dehydrogenated product, [23,24] as featured in the formation of (hetero)aromatic compounds from unsaturated precursors, [25] aldehydes and ketones from activated alcohols, [26] and oxocarbenium and iminium species from ethers and amines, respectively. [27,28] Alternatively, a carbocationic intermediate can undergo intramolecular rearrangement (for example, a Wagner-Meerwein rearrangement [29] ) or be subject to intra- or intermolecular addition reactions, leading to oxidative C–H functionalization products.…”
Section: Ddq-catalyzed Oxidations Of Organic Substratesmentioning
Lead In
Quinones are common stoichiometric reagents in organic chemistry. High potential para-quinones, such as DDQ and chloranil, are widely used and typically promote hydride abstraction. In recent years, many catalytic applications of these methods have been achieved by using transition metals, electrochemistry or O2 to regenerate the oxidized quinone in situ. Complementary studies have led to the development of a different class of quinones that resemble the ortho-quinone cofactors in Copper Amine Oxidases and mediate efficient and selective aerobic and/or electrochemical dehydrogenation of amines. The latter reactions typically proceed via electrophilic transamination and/or addition-elimination reaction mechanisms, rather than hydride abstraction pathways. The collective observations show that the quinone structure has a significant influence on the reaction mechanism and have important implications for the development of new quinone reagents and quinone-catalyzed transformations.
“…All the desired conditions are satisfied by the surface-mediated hydride-transfer reaction of 1,4-cyclohexadiene (CHD), as the reagent, with (C 6 H 5 ) 3 C + that can be generated by chemisorption of (C 6 H 5 ) 3 CCl on silicas, aluminas, titanium dioxides, or aluminosilicates . The hydride-transfer reaction of CHD with (C 6 H 5 ) 3 C + BF 4 - in dichloromethane solution as well as that of CHD with chemisorbed (C 6 H 5 ) 3 C + on solid acids yield quantitatively (C 6 H 5 ) 3 CH and benzene (Chart ). − 1 Hydride-Transfer Reaction of 1,4-Cyclohexadiene with Surface-Coordinated Triphenylmethylium…”
The specific rate constant k′ of the surface-mediated hydride-transfer reaction of 1.4-cyclohexadiene with triphenylmethylium is strongly dependent on the nature of the solid acid catalyst used. The catalysis of this hydride-transfer reaction by 30-moderately strong solid acid catalysts, e.g., silicas, aluminas, aluminosilicates, and titanium dioxide particles, has been studied. Generation of the triphenylmethylium when chlorotriphenylmethane is chemisorbed to the solid acid catalyst was used for the kinetic measurement. The individual, pseudo-first-order rate constant, k, of the hydride-transfer reaction increases linearly with the amount of solid acid catalyst used. The specific rate constant, k′, based on the surface area of the solid acid, was used to quantifiy the catalytic activity of the solid acids. k′ increases in the order silicas < aluminosilicates < titanium dioxides = aluminas. Ln k′ can be be correlated with the corresponding surface polarity parameters of the solid acid, e.g., Reichardt's E T (30), Gutmann's acceptor number AN, and Kamlet-Taft's parameters R (hydrogen-bond-donating acidity) and π* (dipolarity/polarizability). It is shown that the catalytic activity of a moderately strong solid acid catalyst can be characterized by Gutmann's acceptor number AN or the Kamlet-Taft R value. The different classes of solid acids, e.g., silicas, aluminas, and aluminosilicates, respectively, give different dependencies of k′ as a function of their solid acid acidity parameters.
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