The Effect of Pressure on Hydrogen Transfer Reactions with Quinones

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 (ΔV≠ = −13 to −25 in MTBE and −15 to −29 cm3 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‐D4 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‐D4/9‐2,2,3,3‐D4/9‐D12 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. The oxidation of hydroarenes to the corresponding arenes with quinones (see picture) has been investigated under high‐pressure conditions up to 3500 bar. The results provide good evidence for a hydrogen atom transfer mechanism at high pressure.