Green Catalytic Chemistry

We have developed a number of magnesium and zirconium catalyzed processes that begin to overcome the limitations of these oxophilic metal centers.

For example, a magnesium complex is an effective catalyst for the reduction of esters through hydroboration. The catalyst is proposed to involve a magnesium hydridoborate To^MMgH₂Bpin, which is crystallographically characterized and shown above. The catalytic system is selective for esters vs. olefins, nitro groups, nitriles, and heteroaromatics. In addition, this system can catalytically degrade polyesters, such as PLA. Kinetic studies show zero order dependence on HBpin and half-order dependence on ester, suggesting a reversible cleavage of the ester and a turnover-limiting unimolecular elimination of product.

A related catalyst derived from To^MMgMe mediates the deoxygenation of amides to amines. Amides are naturally abundant and easily synthesized, therefore an efficient route to amines involves this step. Although metal hydrides such as LiAlH₄ reductively deoxygenate amides, their selectivity, water sensitivity, and waste generation can limit their applications. In this catalytic reaction, HBpin and tertiary or secondary amides react to give amines and pinBOBpin.

To^MMgMe catalyzes the dehydrocoupling of amines and silane to give silazanes and hydrogen. Reaction of To^MMgMe and primary and secondary amines give magnesium amides such as To^MMgNHR and To^MMgNR₂. Magnesium amides and silane react to give silazane and “To^MMgH”. To^MMgH is not isolable, but its intermediacy is suggested by kinetic studies. Under catalytic conditions, even hydrazine and ammonia are selectively silylated in the presence of To^MMgMe as the precatalyst.

Because To^MMgH is not isolable, we trapped this species with B(C₆F₅)₃ to give To^MMgHB(C₆F₅)₃. This compound contains the HB(C₆F₅)₃ anion, which may contribute to chemistry similarly to H₂Bpin in the above hydroboration, but as a catalyst instead of a consumed reagent. In fact, To^MMgHB(C₆F₅)₃ is a catalyst for the hydrosilylation of α,β-unsaturated esters to give silyl ketene acetals.