Quinone stability substituent effects

TV-Ethyl substitution had very little effect on the measured rate constant, whereas a 4-methyl substituent increased the rate constant by a factor of ca 100. In this case the initial product (identified by Bamberger) is the iminocyclohexadienol 39, which slowly hydrolyses to the quinone 40. These substituent effects suggest that in the transition state the developing positive change is located mostly at the 4-position (stabilized by the 4-Me substituent) and very little on the nitrogen atom (no stabilization by a TV-Et substituent), so that the intermediate is more properly described by the iminium ion. This is supported by an earlier observation46 that whilst full incorporation of 180 from the solvent H2180 occurs in the product, there is no detectable 180 incorporation into the reactant phenylhydroxylamine. 

Anthracene (404) and its derivatives are reported to yield 9,10-sndoperoxides (405). No mechanistic studies were made with respect to the influence of substituents and their positions on the reactivity of the anthracenes toward oxygen, except those discussed earlier. However, substituent effects have been observed with regard to the thermal stability of the anthracene endoperoxide and its thermal transformation reactions, which either lead to quinone formation or to evolution of oxygen and reformation of the hydrocarbon (Table XIII). 

Effects of oxygen substitutents in an aromatic ring upon an exocyclic rather than endocyclic carbocation charge center have also been measured. The possibility of comparing HO, MeO, and O substituent effects for the benzylic cations is provided by recent studies of quinone methides, including the unsubstituted / -quinone methide 23, which may be considered as a resonance-stabilized benzylic cation with a /xoxyanion substituent. 

The features of an effective quinone co-catalyst should be i) a reduction potential value (E°) ranging between 0,4 and 0,5 V, to make possible both the reduction and oxidation processes ii) the absence of substituents in positions 2 and 3 on the ring to allow an efficient Pd(0) binding and stabilization Hi) the presence of hydrophobic substituents to ensure a selective partition of the quinone in the organic layer of the biphasic system iv) its high stability toward oxidation. 

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