Lithium Bromide/HBpin: A Mild and Effective Catalytic System for the Selective Hydroboration of Aldehydes and Ketones

Abstract

The catalytic hydroboration of aldehydes and ketones with HBpin was examined using simple and commercially available metal salts (Li, Na, and K). Among the tested salts, LiBr (0.5-1.0 mol%) was found to be an efficient catalyst for the hydroboration of various aldehydes and ketones at room temperature. Further, the chemoselective hydroboration of aldehydes over ketones was also demonstrated.

Molecular Main Group Metal Hydrides

“…Analogous studies in tetrahydrofuran were proposed to result in a complex equilibrium between several different boron-containing species in which the trialkoxy­borohydride, [( t BuO)­B­(H)­pin] − , was the major species participating in hydride delivery (Scheme ). On the basis of this precedent, a number of closely related studies have been described during the last decade, derived variously from the use of NaOH, n BuLi, − (also imines, alkynes, nitriles, and carbodiimides), , NaH, KH, Li­[O t Bu], K­[O t Bu], K 2 CO 3 , KCH 2 Ph, M­[N­(SiMe 3 ) 2 ] (M = Li, Na, K), LiBr, , MF (M = K, Cs), 2,6-di- tert -butylphenolate lithium, 1,1-dilithioferrocene, ( Dipp Nacnac)­Li, and l-lithio-2-alkyl-1,2-dihydropyridine . In a very recent advance, Newman and Melen have shown that the simple bases, lithium di- iso -propylamide and tert- butoxide catalyze the hydroboration of ketones, but with some enantioselectivity (er up to 70:30) when used in conjunction with enantiopure BINOL-type ligands .…”

s-Block Metal Catalysts for the Hydroboration of Unsaturated Bonds

“…Out of all tested substrates, only hydroboration of aldehydes and ketones provided the desired products. With as little as 0.5–1 mol % of LiBr, various substrates bearing aliphatic and aromatic substituents were reduced under mild reaction conditions and in short reaction times …”

“…Thus, excellent yields were obtained for a wide range of N-aryl ketimines. Furthermore, after successfully applying lithium bromide for hydroboration of aldehydes and ketones in the presence of cheap and readily available lithium bromide 53, 97 An et al utilized this catalytic system for hydroboration of imines. Various aryl-protected aldimines and ketimines were hydroborated to afford the corresponding amines under mild reaction conditions and in remarkably short reaction times.…”

“…With as little as 0.5−1 mol % of LiBr, various substrates bearing aliphatic and aromatic substituents were reduced under mild reaction conditions and in short reaction times. 97 Very recently, Maron, Venugopal, and co-workers reported (Me 6 TREN)−magnesium alkoxide complex 54 and magnesium dialkoxide 55 as active catalysts for the hydroboration of ketones. 98 The sterically hindered magnesium complex 54 was employed as a model catalyst to explore the role of magnesium alkoxides for ketone hydroboration (Scheme 25).…”

“…Thus, excellent yields were obtained for a wide range of N -aryl ketimines. Furthermore, after successfully applying lithium bromide for hydroboration of aldehydes and ketones in the presence of cheap and readily available lithium bromide 53 , An et al. utilized this catalytic system for hydroboration of imines.…”