Synthesis and Characterization of a Tripodal Tris(nitroxide) Aluminum Complex and Its Catalytic Activity toward Carbonyl Hydroboration

Abstract

An aluminum complex of a tripodal tris-(nitroxide)-based ligand has been prepared and characterized. The complex has the ability to participate in metal−ligand cooperative catalysis, which has been exploited for the hydroboration of both aldehydes and ketones. This represents new, transition-metal-like reactivity for an earth-abundant metal.

Recent Advances in Aluminum Compounds for Catalysis

“…C. R. Graves et al [29] . prepared an aluminum compound of a tripodal tris‐(nitroxide)‐based ligand ( 18 ).…”

Asymmetric Magnesium‐Catalyzed Hydroboration by Metal‐Ligand Cooperative Catalysis

“…In this regard, recently Graves and co-workers reported asimilar activation mode of HBpin by tripodal trisnitroxide aluminum catalyst. [12] Following the experimental results,weperformed DFT calculations to gain insight into the reaction pathway and to define the origin of the enantioselectivity.Based on the calculated results,wepropose the following reaction pathway. Thereaction starts with the formation of the active catalyst by coordination of the ligand to MgBu 2 and synchronous elimination of butane,astep exergonic by 66.4 kcal mol À1 (see Scheme S1 in the Supporting Information).…”

“…From the different chiral (R)-(+ +)-BINOL scaffolds (1a-h)tested, magnesium complexes of ligand 1a provided the best enantioselectivity (entries 2vs. [7][8][9][10][11][12][13]. Finally,w ei nvestigated the effect of alkali salt additives as they can have an influence on the performance.However, only as light increase in enantioselectivity was observed (entry 14 vs.2 ).…”

Asymmetric Magnesium‐Catalyzed Hydroboration by Metal‐Ligand Cooperative Catalysis

“…Based on the literature 11 B NMR values and the Lewis acidity/basicity affinity, the shift in the 11 B NMR activation of the HBpin is most probably due to a dual Mg−O and O−B coordination. In this regard, recently Graves and co‐workers reported a similar activation mode of HBpin by tripodal trisnitroxide aluminum catalyst . Following the experimental results, we performed DFT calculations to gain insight into the reaction pathway and to define the origin of the enantioselectivity.…”