Organometallic complexes of tris(oxazolinyl)borate and mixed N-heterocyclic carbene-oxazolinylborate: Synthesis and catalysis

In order to enhance the reactivity of the organometallic complexes our group recently synthesized that bear the ligand of tris(oxazolinyl)phenylborate, such as ToM and ToP (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; ToP = tris(4S-isopropyl-2-oxazolinyl)phenylborate), the steric and electronic properties of the ligands are modified. The modification made to the ligand of ToM is replacing one of the three oxazoline rings in the ligand with an imidazole ring, which upon deprotonation forms a carbene moiety, hence increasing the electron-donating ability of the ligand. Therefore, a new set of mixed N-heterocyclic carbene-oxazoline scorpionate ligands, bis(4,4-dimethyl-2-oxazolinyl)(1-tert-butylimidazolyl)phenylborate (PhB(OxMe2)2ImtBuH, OxMe2 = 4,4-dimethyl-2-oxazoline; ImtBuH = 1-tert -butylimidazole) and bis(4,4-dimethyl-2-oxazolinyl)(1-mesitylimidazolyl)phenylborate (PhB(OxMe2)2ImMesH, ImMes H = 1-mesitylimidazole) are synthesized, followed by the isolation of a variety of the zinc, rhodium, and iridium complexes bearing these ligands. The zinc complexes, {PhB(OxMe2)2ImMes}ZnMe, and {PhB(OxMe2)2ImMes}ZnEt, are spectroscopically and crystallographically characterized, in which the zinc ethyl complex reacts with oxygen to provide an isolable mononuclear zinc ethylperoxide complex {PhB(OxMe2)2ImMes}ZnOOEt. From the potassium salts of the ligands, a number of rhodium and iridium complexes are also synthesized, in which the rhodium dicarbonyl complex, {PhB(Ox Me2)2ImMes}Rh(CO)2, and the iridium isocyanide complex, {PhB(OxMe2)2ImMes}Ir(CO)CN tBu, undergo oxidative addition of phenylsilane under thermal conditions in darkness to give the corresponding metal silyl complexes, {PhB(Ox Me2)2ImMes}RhH(SiH2Ph)CO and {PhB(OxMe2)2ImMes}IrH(SiH2Ph)CN tBu, respectively, which demonstrates the enhanced activity of the ligand. Later, the rhodium silyl complex forms an electrophilic, coordinatively unsaturated rhodium silylene complex with Lewis acid, B(C6F 5)3, which catalyzes the deoxygenation reactions of carbonyl-containing compounds, especially esters, in the presence of phenylsilane.;The other modification is made to the ligand of ToP. In order to achieve high enatioselectivity in catalysis, the original oxazoline rings in ToP are replaced with the more sterically enhanced oxazolines, 4S-isopropyl-5,5-dimethyl-2-oxazoline, which has shown high enantioselectivity when incorporated in the intramolecular alkene hydroamination catalyst, {PhB(C5H4)(Ox iPr,Me2)2}Zr(NMe2) 2. The resulting ligand, tris(4S-isopropyl-5,5-dimethyl-2-oxazolinyl)phenylborate (ToP*), is then synthesized from the natural amino acid, L-Valine. The rhenium and manganese carbonyl complexes, To P*Re(CO)3 and ToP*Mn(CO)3, are spectroscopically and crystallographically characterized to assess the steric and electronic features of the ligand. The zinc hydride complex, To P*ZnH, is then used as catalyst for cross-dehydrocoupling of silanes and alcohols and moderate enantioselectivity is achieved.