Beyond alkyl transfer: synthesis of main group metal (Mg, Ca, Zn) silyl and tris(oxazolinyl)borato complexes and their stoichiometric and catalytic reactions with borane Lewis acids and carbonyls
Recently, the fundamental knowledge of main group metal chemistry has grown. This progress is crucial for the further development of main group metal compounds in silicon chemistry and catalysis and for advancing their applications as green alternatives to many rare earth and transition metal compounds. This thesis focuses on reactivity beyond the well-documented alkyl-transfer applications for main group metals, and it highlights examples of reactions with Lewis acids and the reduction of carbonyls.
A series of magnesium, calcium, and zinc organometallic compounds are synthesized, and their characterization, stoichiometric reactions, and catalytic activity are described. A novel silyl ligand, -Si(SiHMe2)3, and the ancillary ligand ToM (tris(4,4-dimethyl-2-oxazolinyl)phenylborate) are employed in the synthesis of these homoleptic and heteroleptic species. The silyl anion, KSi(SiHMe2)3, is prepared from the reaction of KOtBu and Si(SiHMe2)4. A single crystal X-ray diffraction study shows that the structure is made up of a chain of alternating potassium cations and Si(SiHMe2)3 anions with K coordinated to the central Si atoms and the three Si-H moieties oriented toward the next K atom.
This unique silyl ligand incorporating beta-SiH functionality undergoes salt metathesis reactions with main group metal halides (MgBr2, CaCl2, ZnCl2) to afford disilyl compounds. While the disilyl magnesium and calcium compounds are THF adducts, the disilylzinc compound is free of coordinating solvent. All three compounds undergo coordination by N-donor ligands, and zinc is also stabilized by the N-heterocyclic carbene 1,3-di-t-butylimidazol-2-ylidene (ImtBu). Interestingly, spectroscopic characterization and X-ray analysis reveal that all of the main group metal silyl compounds contain classic 2-center-2-electron bonding, and the beta-SiH groups do not show evidence of nonclassical interactions. Yet they undergo beta-hydrogen abstraction with the Lewis acids PhB(C6F5)2 or B(C6F5)3.
The monomeric magnesium and zinc silyl complexes containing an ancillary ligand are also synthesized via salt metathesis with ToMMgBr or ToMZnCl, and similarly give classical interactions between the metal center and the silyl ligand. However, ToMMg-Si(SiHMe2)3 and ToMZn-Si(SiHMe2)3 react with the Lewis acids B(C6F5)3 and CO2 via contrasting pathways and yield different products. The reactivity of ToMZn-Si(SiHMe2)3 was also compared to the analogous compounds ToMZn-CH(SiHMe2)2 and ToMZn-N(SiHMe2)2 in order to explore some of the similarities and differences in reactivity among Zn-element bonds.
The cationic complex ToMMgHB(C6F5)3, which is accessible from the reaction noted above between ToMMg-Si(SiHMe2)3 and B(C6F5)3 or from the reaction of ToMMgMe, PhSiH3, and B(C6F5)3, is an effective precatalyst for the 1,4-hydrosilylation of unsaturated esters. Silyl ketene acetals are isolated in high yield from a range of alpha,beta-unsaturated esters and hydrosilanes. However, an alpha-alkyl group in the ester is essential. In the presence of an alpha-proton, rapid polymerization is observed as a competing pathway with 1,4-hydrosilylation.
The precursor to ToMMgHB(C6F5)3, ToMMgMe, is also an efficient precatalyst for the reduction of carbonyls. Under catalytic conditions, tertiary and secondary amides are reduced to amines using pinacolborane (HBpin) as a reductant. The optimized reaction conditions reveal that excess HBpin gives reduction at room temperature in excellent yield. Interestingly, this system is the first example of a hydroboration of amides to amines and the first magnesium-catalyzed amide reduction.