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 ToMMgH2Bpin, 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 ToMMgMe 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.
ToMMgMe catalyzes the dehydrocoupling of amines and silane to give silazanes and hydrogen. Reaction of ToMMgMe and primary and secondary amines give magnesium amides such as ToMMgNHR and ToMMgNR2. Magnesium amides and silane react to give silazane and “ToMMgH”. ToMMgH 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 ToMMgMe as the precatalyst.
Because ToMMgH is not isolable, we trapped this species with B(C6F5)3 to give ToMMgHB(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, ToMMgHB(C6F5)3 is a catalyst for the hydrosilylation of α,β-unsaturated esters to give silyl ketene acetals.