Flexible, Bowl-Shaped N-Heterocyclic Carbene Ligands: Substrate Specificity in Iridium-Catalyzed Ketone Hydrosilylation

Abstract

A series of benzimidazolium chlorides was synthesized as precursors to N-heterocyclic carbene ligands, with N-substituents varying in size from 3,5-xylyl (1a) to first-generation dendritic 3,5-bis(3,5-di-tertbutylphenyl)phenyl (1b), to the second-generation 3,5-bis[3,5-bis(3,5-di-tert-butylphenyl)phenyl]phenyl (1c). The dendritic side groups of 1b and 1c form a flexible, bowl-like cavity. Iridium complexes of 1a-c were synthesized and were shown to be catalytically active for the hydrosilylation of aryl methyl ketones. The dendritic ligands 1b and 1c effect a moderate level of substrate specificity in the competitive hydrosilylation of ketones of varying size. In the competitive hydrosilylation of acetophenone versus 3-(3,5-di-tert-butylphenyl)acetophenone, acetophenone is consumed approximately 3.7 times more quickly using the second-generation ligand 1c. Using the control ligand 1a, this ratio is 1.8.

Substrate‐Selective Catalysis

“…Chianese et al. reported the synthesis of bowl‐shaped N ‐heterocyclic carbene (NHC) ligands 154 – 156 (Figure 20) that induced substrate selectivity in the iridium‐catalyzed hydrosilylation of aryl methyl ketones 57…”

“…Chianese et al reported the synthesis of bowl-shaped N-heterocyclic carbene (NHC) ligands 154-156 ( Figure 20) that induced substrate selectivity in the iridium-catalyzed hydrosilylation of aryl methyl ketones. [57] The iridium complexes 157-159 of ligands 154-156 were synthesized (Scheme 25) and the activity of these complexes in the iridium-catalyzed hydrosilylation of acetophenone (160) was evaluated, in which the silyl ether 161 was the major product in all cases.…”

Asymmetric Zinc‐Catalyzed Hydrosilylation of Ketones and the Effect of Carboxylate on the Enantioselectivity

“…[2][3][4][5] AHS of prochiral ketones has been known since the early 1970s. Since then, several transition metals complexes-such as rhodium with chiral phosphine, oxazoline, or N-heterocyclic carbene complexes [6][7][8][9][10][11] ; iridium with C 2 -symmetric diamine, dithiourea, or N-heterocyclic carbene complexes 12,13 ; and titanium with chiral diamines 14,15 or binaphthol 16 complexes-have been proved to be effective catalysts. Chiral ammonium fluoride 17 as catalyst had also been reported.…”

“…Yellow solid 1. H NMR (400 MHz, CDCl 3 ) d 7.98 (s, 2H), 7.39 (s, 2H), 6.54 (d, J = 3.3 Hz, 2H), 6.42-6.23 (m, 2H), 3.40-3.23 (m, 2H), 1.88-1.68 (m, 6H), 1.50-1.32 (m, 2H) 13. C NMR (101 MHz, CDCl 3 ) d 151.26, 149.43, 144.40, 114.34, 111.36, 73.96, 33.01, 24.36.…”

Bulky, dendronized iridium complexes and their photoluminescence

“…A twofold Buchwald-Hartwig coupling between 1,2-diaminobenzene and 1-bromo-4-TIPSE benzene (5) produced functionalized 1,2-diaminobenzene 4 in high yield (89%). 36,37 Next, compound 4 was allowed to react with triethyl orthoformate and conc. HCl solution to afford TIPSE-functionalized 1,3-diphenyl benzimidazolium chloride 3 in high yield (86%).…”