Biphasic autoxidation of tetralin catalyzed by surface-active transition metal complexes

Biphasic autoxidation of tetralin has been carried out using surface-active tetramethylethylenediamine complexes of manganese, chromium, and nickel as catalysts, tetralin as the substrate and organic phase, and dodecyl sodium sulfate as emulsifier. Advantages of the biphasic reaction over the homogeneous and heterogeneous counterparts include avoidance of the use of a troublesome solvent, ease of catalyst recovery and substrate recycle, and attainment of high reactivity, selectivity, and reproducibility under mild reaction conditions (T similar to 60 degrees C, P similar to 1 atm). The main reaction products are alpha-tetralone and alpha-tetralol. The selectivity for the former decreases from 95% with the chromium complex to 90% with the nickel complex and 60% with the manganese complex, and the activity varies in a reverse order. The biphasic reaction stops at a bulk tetralin conversion of 35% due to the buildup of inhibitive, higher oxidation products. Similar product inhibition has been reported in one-liquid-phase systems. The biphasic scheme, however, permits a more convenient recovery and recycle of the catalyst and unreacted substrate. The reaction order with respect to oxygen decreases from 1.0 to 0 above an oxygen pressure of 0.15 atm. The reaction order with respect to catalyst decreases from 2.0 or 1.4 to 1.0 and then 0 with increasing metal concentration. Manganese switches role from catalyst to inhibitor above a threshold metal concentration, as indicated by a lengthening of the induction period of the reaction. A generalized reaction mechanism is proposed which yields model results in good agreement with the experimental findings.