New π-Acceptor Ligands in Homogeneous Catalysis
A challenge in homogeneous catalysis is the search for metal catalysts with extreme activity. For this reason we try to increase metal complex reactivity employing electronic activation through novel types of π-acceptor ligands. Together with the catalysis research at BASF we have developed new classes of electronically activating ligands based on the phosphabenzene nucleus.[43] The corresponding rhodium complexes display high activity on hydroformylation of poorly reactive internal alkenes (Scheme 13).
Scheme 13: Isomerizing hydroformylation of internal alkenes with phosphabenzene/rhodium-catalysts and non-isomerizing hydroformylation of internal alkenes with phosphabarrelene/rhodium catalysts.
In many cases high catalyst activity in hydroformylation goes along with a high tendency towards alkene isomerization. This is desirable if an isomerizing hydroformylation is the synthetic goal. However, if a position selective hydroformylation of an internal alkene is needed, alkene isomerization is a severe problem. To overcome this problem we developed new phosphabarrelene/rhodium catalysts (Scheme 13).[53,71,74] These systems allow for the first time the hydroformylation of internal alkenes with exceptional activity and simultaneous suppression of alkene isomerization which opens new perspectives for the use of the hydroformylation as an attractive carbon-carbon bond forming reaction in industry and laboratory.