Chiral Tetra-coordinate Aluminum Cation in Catalysis

Abstract

Although the application of cationic aluminum complexes in catalysis has been extensively investigated, the incorporation of an aluminum cation into a chiral environment is less explored. In this study, a C 2 -symmetric chiral tetra-coordinate aluminum cation [3] + bearing two chiral amino alcohol-derived ligands was prepared from commercially available starting materials in two steps. The high Lewis acidity of [3] + exceeding that of B(C 6 F 5 ) 3 was verified by a competition binding experiment. Preliminary catalytic activity studies of [3] + revealed that the tetracoordinate aluminum cation is effective in accomplishing high conversion in all cases. However, the presence of a labile N−Al bond diminishes the enantioselectivity of the activation process.

Chiral Bis(oxazoline) Ligand‐Supported Alkyl Aluminum Cations

“…As shown in Table 2, the putative base‐free BOX ligand‐coordinated alkyl aluminum cation of H with R 1 =R 3 =Me and R 2 =H displays significantly higher FIA than those well‐documented strong Lewis acids as well as aluminum cation G . With the experimental and computational findings, we can conclude that [ 3 a – b ][B(C 6 F 5 ) 4 ] and [ 4 c – d ][B(C 6 F 5 ) 4 ] should be more acidic than cation G , an aluminum cation that has been proven to be more Lewis acidic than B(C 6 F 5 ) 3 [18] …”

“…Besides the Gutmann–Beckett method, we also calculated the solvation corrected fluoride ion affinity (FIA) following the published procedures [18,25] . As shown in Table 2, the putative base‐free BOX ligand‐coordinated alkyl aluminum cation of H with R 1 =R 3 =Me and R 2 =H displays significantly higher FIA than those well‐documented strong Lewis acids as well as aluminum cation G .…”

“…] should be more acidic than cation G, an aluminum cation that has been proven to be more Lewis acidic than B(C 6 F 5 ) 3 . [18] Knowing that [a] Acceptor number (AN) [b] [ [a] The value was calculated from the linear equation. + and are relatively ineffective in controlling the enantioinduction process, the existence of a weakly solvated aluminum cation is essential for hydrosilylation of un-activated alkene, a reaction that is quite challenging for main-group catalysts.…”

“…They performed well in catalyzing asymmetric conjugate addition, aldol reaction, and carbonylation of epoxide, respectively. Recently, our group has developed a chiral tetra‐coordinate aluminum cation ( G ), which is an efficient catalyst in hydroboration of ketones, Diels‐Alder reaction, and Michael addition, despite moderate enantioselectivity presenting [18] . Regarding the lack of chiral aluminum cation catalyst, specifically for the alkyl aluminum complexes, we sought to explore new possibilities of this species.…”