Nucleophilic catalysis with π-bound nitrogen heterocycles:: Synthesis of the first ruthenium catalysts and comparison of the reactivity and the enantioselectivity of ruthenium and iron complexes

Three ruthenium complexes that bear pi-bound nitrogen heterocycles have been synthesized. It is established that these complexes serve as effective nucleophilic catalysts for a range of processes, including the acylation of alcohols with diketene, the ring opening of azlactones, and the addition of alcohols to ketenes; their activity is comparable to or somewhat greater than the corresponding iron catalysts. The relative efficiency of the ruthenium complexes as asymmetric catalysts is also evaluated: in the kinetic resolution of secondary alcohols, ruthenium is markedly less effective than iron, but in the deracemization/ring opening of azlactones, ruthenium is slightly more enantioselective. This study documents for the first time the impact of the metal on the reactivity and on the enantioselectivity of nucleophilic catalysts based on pi-bound nitrogen heterocycles.