title: The effect of pressure on hydrogen transfer reactions with quinones.
abstract: The effect of pressure on the oxidation of hydroarenes 3-9 with 2,3-dichloro-5,6-dicyano-1,4-quinone (DDQ; 1 a) or o-chloranil (10), leading to the corresponding arenes, has been investigated. The activation volumes were determined from the pressure dependence of the rate constants of these reactions monitored by on-line UV/Vis spectroscopic measurements in an optical high-pressure cell (up to 3500 bar). The finding that they are highly negative and only moderately dependent on the solvent polarity (DeltaV( not equal ) = -13 to -25 in MTBE and -15 to -29 cm(3) mol(-1) in MeCN/AcOEt, 1:1) rules out the formation of ionic species in the rate-determining step and is good evidence for a hydrogen atom transfer mechanism leading to a pair of radicals in the rate-determining step, as was also suggested by kinetic measurements, studies of kinetic isotope effects, and spin-trapping experiments. The strong pressure dependence of the kinetic deuterium isotope effect for the reaction of 9,10-dihydroanthracene 5/5-9,9,10,10-D(4) with DDQ (1 a) can be attributed to a tunneling component in the hydrogen transfer. In the case of formal 1,3-dienes and enes possessing two vicinal C--H bonds, which have to be cleaved during the dehydrogenation, a pericyclic hydrogen transfer has to considered as one mechanistic alternative. The comparison of the kinetic deuterium isotope effects determined for the oxidation of tetralin 9/9-1,1,4,4-D(4)/9-2,2,3,3-D(4)/9-D(12) either with DDQ (1 a) or with thymoquinone 1 c indicates that the reaction with DDQ (1 a) proceeds in a stepwise manner through hydrogen atom transfer, analogously to the oxidations of 1,4-dihydroarenes, whereas the reaction with thymoquinone 1 c is concerted, following the course of a pericyclic hydrogen transfer. The difference in the mechanistic courses of these two reactions may be explained by the effect of the CN and Cl substituents in 1 a, which stabilize a radical intermediate better than the alkyl groups in 1 c. The mechanistic conclusions are substantiated by DFT calculations.
authors.affiliation: Institut für Organische Chemie der Universität Duisburg-Essen, Universitätsstrasse 5, 45117 Essen, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Wurche
authors.forename: Wilhelm
authors.initials: W
authors.lastname: Sicking
authors.forename: Reiner
authors.initials: R
authors.lastname: Sustmann
authors.forename: Frank-Gerrit
authors.initials: FG
authors.lastname: Klärner
authors.forename: Christoph
authors.initials: C
authors.lastname: Rüchardt
mesh_terms: D016227:Benzoquinones / Q000138:chemical synthesis / Q000737:chemistry; D002703:Chloranil / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D006859:Hydrogen / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011312:Pressure; D011809:Quinones / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D016227:Benzoquinones; D011809:Quinones; D002703:Chloranil; C000928:dichlorodicyanobenzoquinone; D006859:Hydrogen; C039499:2-chloranil