Pressure independence of stereomutation and fragmentation in the bis-spirocyclobutanes, anti- and syn-2,9-dicyanodispiro[5.0.5.2]tetradeca-1,8-dienes.

Pressure dependent rate constants and volumes of activation for stereomutational interconversions of the cyclobutanes, anti- and syn-2,9-dicyanodispiro[5.0.5.

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 (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.

Stereochemical memory in the regioselective and diastereoselective rearrangement of tricyclo[3.3.0.0 2,4]octanes (housanes) by electron transfer (1,3-cyclopentanediyl radical cations) and acid (cyclopentyl carbenium ions) and silver-ion catalysis.

The electron-transfer-catalyzed rearrangement of the housanes 5 affords regioselectively only the two cyclopentenes 6 (CH(3) migration) and 7 (R migration) by 1,2-migration of the two groups at the methano bridge to the methyl terminus. The 1,2-shift of the CH(3) group prevails, and the rearrangement ratio is essentially insensitive to the migratory aptitude of the R substituent. This stereochemical memory effect derives from the conformational impositions on the stereoelectronic requirements during the 1,2-migration in the 1,3-radical-cation intermediates.