Convenient procedures for the asymmetric reductions utilizing α,α-diphenyl- pyrrolidinemethanol and borane complexes generated using the I2/NaBH4 system

Ultrasound Mediated Functional Group Reduction and Chemoselective Studies by DMAB

“…The reduction of amino acids 30 is considered important transformation in organic synthesis, the amino alcohol obtained in this transformation plays vital role in asymmetric synthesis 28 and peptide synthesis. In the present study the reduction of three amino acids were focused namely valine, proline and leucine (entries 11-13, Table 1).…”

“…The addition of acetic acid 24 , mineral acid 25 and Lewis acid 26 is required to activate the stable amine borane complexes like triethylamine borane and pyridine borane. Kanth et al, prepared set of amine borane complexes from N,N-dialkyl anilines and N,N-dialkylamines, which is significantly more reactive than most other amine boranes [27][28][29] , this can be explained based on steric effects and electronic property of the groups attached to nitrogen atom which influence their reducing property 27,30,31 . Preparation of these amine borane complexes were cumbersome, which involves alkylation of aniline or mono alkyl aniline, which is delicate and time consuming 31,33 .…”

Recent development and improvement for boron hydride-based catalytic asymmetric reduction of unsymmetrical ketones

“…Based on the reaction of NaBH 4 with BF 3 ÁOEt 2 , I 2 , TMSCl, MeI or Me 2 SO 4 in THF to easily produce BH 3 ÁTHF, simple and convenient OAB reductions through in situ 'one-pot' generation of both OABs and borane species have been developed and from a practical view point, this is highly desirable. Although the first 'one-pot' reduction of acetophenone carried out with NaBH 4 and I 2 (method A) in the presence of (S)-2 in THF gave 65% ee, 11 the reduction using TMSCl or BF 3 ÁOEt 2 (method B), 12,24 and SnCl 4 or SnCl 2 (method C) 13 in place of I 2 provided 85-98% ee for aryl methyl ketones and 2-chloroacetophenone. Similar reduction using n-Bu 4 NBH 4 with MeI (method D) afforded high enantioselectivity.…”

Catalytic Asymmetric Reduction of Prochiral Ketones with Chiral β-Amino Alcohol N-Boranes and the Corresponding Tris(oxazaborolidine)borazines

“…In recent decades, a significant number of reports have been published on the synthesis of oxazaborolidines and their catalytic efficiencies in asymmetric ketone reductions. [4][5][6] In these studies, the oxazaborolidine catalyst was usually formed in situ through the complexation of a borane with an amino alcohol.…”

“…To determine the optimal conditions for the reaction, we initially tested a series of reducing agents for the asymmetric reduction of acetophenone (Scheme 2). The use of lithium aluminum hydride, sodium borohydride, or sodium borohydride in combination with diiodine 5 gave racemic products in moderate to good isolated yields (Table 1, entries 1-3, 8, and 9). The enantioselectivity was significantly improved when boranetetrahydrofuran was used as the source of borane (entries 4-6 and 10-13), and there was also an increase in the yield of the isolated product.…”