Tetrabutylammonium Borohydride

Protonolysis of BH4– Leads to Intermediacy of BH3-σ(H2) That Evolves H2 and Furnishes Borane as the Key Reducing Agent

“…Although the reduction of carbonyl fragments with borane is well-known, [70][71][72][73][74]101 the explicit formation and involvement of borane in mediating the reduction of ketones with borohydrides have not been reported, albeit sporadically alluded to in the literature. 12,49 For example, boranes generated in situ by reaction of BH 4 − with dichloromethane 12 or in reactions where boranes act as catalysts 49…”

“…Although it has been noted that reductions performed in aprotic solvents are sluggish, 11−13 such differences in reactivity in protic vs aprotic solvents are often alluded to solvent effects and/or solubility. 12,14,15 The choice of alcohol as a solvent is so widely accepted that it constitutes the standard protocol taught in the undergraduate organic laboratory. 16−20 This choice is however uneconomical because the B−H fragments are unstable toward protonolysis (leading to immediate H 2 -evolution 21−24 ), thus requiring excess BH 4…”

Enantio- and Diastereoselective Mannich Reactions of ß-Dicarbonyls by Second Stage Diastereoconvergent Crystallization

“…Substrate-controlled diastereoselective reduction of ß-keto ester product 3a occurred efficiently using NBu 4 BH 4 to provide a good yield of a single diastereomer of amino-alcohol 6 after the minor diastereomer was purged via flash column chromatography (Scheme 3c, left). 21 Although Lewis acid-directed reduction of ß-keto ester products resulted in poor diastereoselectivity (≤2:1 in all cases; see Supporting Information for further details), 22 ß-keto amide product 5d was reduced using a MnBr 2 /NaBH 4 combination to provide a single diastereomer of the complementary amino-alcohol 7 (Scheme 3c, right). 23 The relative stereochemistry of amino-alcohols 6 and 7 was Table 2.…”

Crystallization-Enabled Stereoconvergent Michael Additions of β-Keto Esters to Nitroolefins

“…Having developed a highly diastereo-and enantioselective reaction platform, we sought to exploit the polyfunctional nature of the products in a variety of secondary transformations (Scheme 3). The ketone was reduced with a noncoordinating borohydride to yield β-hydroxy ester 4 in good yield and excellent dr. 40 An X-ray diffraction study of alcohol 4 was conducted to assign the absolute stereochemistry as (1R,2R,3S). The nitro group of alcohol 4 was further reduced to the amine, 41 which spontaneously cyclized to form β-hydroxy lactam 5.…”