Boric acid as a precatalyst for BH₃-catalyzed hydroboration

Abstract

Cheap and air-stable boric acid is shown to be a good precatalyst for BH3 hydroboration of esters and carbonates.

Transborylation-Enabled Boron Catalysis

“…Fontaine used B –O/ B –H transborylation for the borane-catalysed reduction of esters, lactones, and carbonates to alcohols (Scheme 11a ). 66 B –O/ B –H transborylation between HBpin and B(OH) 3 generated the catalyst, BH 3 (Scheme 11b ). The reaction was proposed to proceed through hydroboration of the ester by BH 3 , 67 giving a bis(alkoxy)borane 29 , and B –O/ B –H transborylation of the bis(alkoxy)borane 29 with HBpin formed a mono(alkoxy)borane 30 and the borate pinacol ester product 31 .…”

Diastereoselective, Catalytic Access to Cross‐Aldol Products Directly from Esters and Lactones

“…Dialkylboranes (HBR 2 ) have been used for the stoichiometric reduction of enones to boron enolates [10a-d] and the stoichiometric reduction of esters and lactones to alcohols. [12,13] Although never isolated or inter- cepted at this stage, the reduction of an ester to an alcohol by a borane proceeds through an aldehyde intermediate. Thus, the potential for the catalytic generation of both the nucleophilic (boron enolate) and electrophilic (aldehyde) coupling partners of an aldol-type reaction using a single borane catalyst was postulated, and for the first time the reaction of high oxidation-state carbonyl reagents could be used to give aldol-type products (Figure 1c).…”

“…In all cases the formation of a β‐hydroxy carbonyl product requires an aldehyde or ketone coupling partner. Dialkylboranes (HBR 2 ) have been used for the stoichiometric reduction of enones to boron enolates [10a–d] and the stoichiometric reduction of esters and lactones to alcohols [12, 13] . Although never isolated or intercepted at this stage, the reduction of an ester to an alcohol by a borane proceeds through an aldehyde intermediate.…”

Boron-Catalyzed, Diastereo- and Enantioselective Allylation of Ketones with Allenes

“…The mechanism of catalytic turnover was investigated as both allylic borane 4 and borinic ester 5 could plausibly undergo transborylation, B–C and B–O transborylation respectively, with only transborylation of the borinic ester 5 enabling turnover and catalyst regeneration. − Reaction of allylic borane 4 with HBpin under catalytic reaction conditions gave no B–C transborylation with only the recovery of starting material (Scheme b). Reaction of borinic ester 5 with HBpin under catalytic reaction conditions gave boronate ester 6 and regeneration of the catalyst H- B -9-BBN, observed by 11 B NMR spectroscopy (see Supporting Information).…”