Catalytic Oligomerization of Isoprene in Solution. Coordination Features of Phosphorus Ligands in the Ni (0) Complexes

The oligomerization of isoprene in HMPA catalyzed by NiCbL2(L: phosphorus ligands)NaBH4 has been kinetically investigated by varying the kind and amounts of phosphorus ligands and the amounts of NaBH4. Triphenylphosphine (PPh3), which formed the most active catalyst, is removed from the coordination sphere of the complex under the high isoprene. concentration, giving a negative reaction order in isoprene. In contrast, alkyl phosphines, the better coordinating ligands, give zero order kinetics, although the intrinsic activities of their complexes are rather limited. A small excess of triphenylphosphine in the reaction medium changes the order to zero, allowing the highest activity even in a high isoprene/Ni ratio beyond 1000. The reaction mechanism, the selectivity, and the activity order of phosphorus ligands are discussed, assuming that the elimination of the oligomeric product is rate-determining. Catalytic dimerization reactions of conjugated dienes have been intensively studied since the pioneer work of Wilke group 1,2) using various systems that contain nickel ion of various coordination structures, however their practical application is rather limited because of their insufficient catalytic activity, selectivity, and life-time, especially for isoprene. An exception is the excellent activity of Fe(acac)a-AIEta-Schiff-base catalyst. 3) The present authors have reported the enhanced activity of the NiCb(PPh3h-NaBH4 catalyst system by the addition of amine in the presence of suitable amounts of water using benzene as the solvent.H,b) However, the total catalytic turnover number was still very limited, so that the complete conversion of isoprene could never be achieved when the mole ratio of substrate charged to the catalyst was over only 100. Total catalytic turnover number as well as activity were found recently to be improved to a significant extent in some aprotic polar solvents such as hexamethylphosphorotriamide (HMPA), where the number could be beyond 1000. ) In the present study, the catalysis in HMPA was further investigated from the kinetic