Aluminium‐Catalyzed Selective Hydroboration of Esters and Epoxides to Alcohols: C−O Bond Activation

Abstract

In this work, the molecular aluminium dihydride complex bearing an N, N’‐chelated conjugated bis‐guanidinate (CBG) ligand is used as a catalyst for reducing a wide range of aryl and alkyl esters with good tolerance of alkene (C=C), alkyne (C≡C), halides (Cl, Br, I and F), nitrile (C≡N), and nitro (NO2) functionalities. Further, we investigated the catalytic application of aluminium dihydride in the C−O bond cleavage of alkyl and aryl epoxides into corresponding branched Markovnikov ring‐opening products. In addition, the chemoselective intermolecular reduction of esters over other reducible functional groups, such as amides and alkenes, has been established. Intermediates are isolated and characterized by NMR and HRMS studies, which confirm the probable catalytic cycles for the hydroboration of esters and epoxides.

Vanadium-catalysed regioselective hydroboration of epoxides for synthesis of secondary alcohols

“…10 However, the substrate scope and functional group tolerance for the current vanadium catalyst are not as good as those known catalysts. [8][9][10][11][12][13] Based on previous reports 16,17 and the current results, a plausible mechanism that may operate for the regioselective hydroboration of epoxides is proposed (Scheme 6). As evidenced previously, vanadium complex 1 would first react readily with HBpin to generate a vanadium hydride species (A) by releasing TMSCH 2 Bpin from the reaction.…”

“…12 In a recent work by Nembenna and coworkers, an aluminum dihydride catalyst supported by a N,N′-chelated conjugated bis-guanidinate ligand enabled the hydroboration of epoxides with high regioselectivity for primary alcohols at 70 °C. 13 In addition, rather early pioneering work by Brown and Yoon has revealed the existence of catalytic activity of NaBH 4 for anti-Markovnikov BH 3 addition to epoxides. 14 Among the catalysts described above, little progress has been made using transition metals as catalysts for hydroboration of epoxides except for the initial contributions based on Ni and Fe catalysts.…”

“…12 In a recent work by Nembenna and coworkers, an aluminum dihydride catalyst supported by a N , N ′-chelated conjugated bis-guanidinate ligand enabled the hydroboration of epoxides with high regioselectivity for primary alcohols at 70 °C. 13 In addition, rather early pioneering work by Brown and Yoon has revealed the existence of catalytic activity of NaBH 4 for anti-Markovnikov BH 3 addition to epoxides. 14…”

“…Comparing to other catalysts enabling regioselective hydroboration of epoxides reported thus far, 5–13 the current vanadium catalyst which features the first early transition-metal catalyst for Markovnikov-selective epoxide hydroboration shows higher reactivity ( i.e. lower catalyst loading and milder conditions) than those containing main-group metals, except the dimeric NaHBEt 3 /tpy complex.…”

“…10 However, the substrate scope and functional group tolerance for the current vanadium catalyst are not as good as those known catalysts. 8–13…”