Date of Award
Doctor of Philosophy
Aaron D Sadow
Multidentate oxazoline based ligands have played a critical role in transition metal chemistry. They have several key advantages over many other ligands: (i) strong chelating ability, (ii) chiral versions readily synthesized from commercially available amino acids, (iii) the chirality is in close proximity to the metal center. With this in mind, we set out to design new bis(oxazoline) ligands that would cyclometalate with late-metal centers and tris(oxazoline) ligands for d0 metal centers.
We synthesized new achiral [bis(4,4-dimethyl-2-oxazolinyl)propane], ProboxMe2, and chiral [bis(4R-phenyl-2-oxazolinyl)propane], ProboxPh, ligands for use as pincer complex proligands. While the isopropyl derivative, ProboxiPr, was known, no transition metal complexes containing these ligands have been reported. We attempted to synthesize palladium pincer complexes with ProboxMe2 and ProboxiPr but isolated thermally robust dipalladium(II) macrocycles of the form [(Probox)PdCl2]2. Attempts with rhodium provided a similar rhodium(I) macrocycle with ProboxPh. Finally, a rhodium(III) pincer complex is obtained with ProboxMe2. The spectroscopic and structural characteristics along with their reactivity are described.
Since cyclometalation of Probox based ligands proved to be much more difficult than anticipated, we changed gears to tris(oxazolinyl)phenylborate ligands. Our group had reported the synthesis of achiral tris(4,4-dimethyl-2-oxazolinyl)phenylborate, ToM, and chiral tris(4S-isopropyl-2-oxazolinyl)phenylborate, ToP, along with the synthesis of several metal complexes containing these ligands. Unfortunately, we found that the isopropyl group in ToP, when coordinated to a metal, rotates away from the metal center, presumably to avoid steric interactions with the metal center, thus lessening the stereochemical control during asymmetric transformations. We decided to design a new chiral tris(oxazolinyl)phenylborate ligand with tert-butyl groups at the 4-position on the oxazoline ring with the anticipation that the tert-butyl group will remain in close proximity to the metal center. Magnesium and calcium complexes bearing this ligand have been prepared and used as catalysts for the asymmetric hydroamination/cyclization of aminoalkenes with moderate success. We observed high conversion to the corresponding pyrrolidines with % ee's that were significantly higher than any other group 2-catalyst system to date.
ToMMgMe was found to be an efficient precatalyst for the cross-dehydrocoupling of amines and silanes. With evidence gathered through kinetic investigations, including a Hammett plot, a new mechanism was proposed that involves a nucleophilic attack of the magnesium amide on the silane followed by hydrogen transfer to magnesium and displacement of the newly formed silazanes. Rapid protonolysis of the magnesium hydride with amine completed the catalytic cycle. Additionally, kinetic studies on the action of ToMMgMe with PhSiH3 ¬were also conducted. These included Eyring analysis, isotope effect, and a Hammett plot; these data provided evidence that this process is very similar to the Si-N bond forming reactions catalyzed by ToMMgMe. Additionally, preliminary studies on catalytic hydrosilylation using ToMMgMe and ToMMgMe with B(C6F5)3 as precatalysts are reported.
Steven Ryan Neal
Neal, Steven Ryan, "From pincers to scorpionates: The synthesis and reactivity of oxazoline based transition metal and alkaline earth metal complexes" (2012). Graduate Theses and Dissertations. 12417.