Rationale on the Abnormal Effect of Temperature on the Enantioselectivity in the Asymmetric Borane Reduction of Ketones Catalyzed by L-Prolinol

✍ Scribed by Jiaxi Xu; Tiezheng Wei; Shrong-Shi Lin; Qihan Zhang

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: German
• Weight: 81 KB
• Volume: 88
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.200490292

✦ Synopsis

Abstract

The effect of temperature on the enantioselectivity of the oxazaborolidine‐catalyzed asymmetric borane reduction of ketones was investigated in the presence of (5__S__)‐3‐oxa‐1‐aza‐2‐borabicyclo[3.3.0]octane (=(3a__S__)‐tetrahydro‐1__H__,3__H__‐pyrolo[1,2‐c][1,3,2]oxazaborole; 1a) and its 2‐methoxy derivative (1b) as catalysts, which were synthesized from L‐prolinol with borane and trimethyl borate, respectively. The results indicate that the two catalysts induce a different temperature‐dependent enantioselectivity. The enantioselectivity of the B‐unsubstituted (5__S__)‐3‐oxa‐1‐aza‐borabicyclo[3.3.0]octane (1a) increases with increasing temperature, while its B‐methoxy‐substituted derivative 1b shows the highest enantioselectivity at ca. 50°. (5__S__)‐3‐Oxa‐1‐aza‐2‐borabicyclo[3.3.0]octane (1a) is more likely to dimerize than its 2‐methoxy derivative 1b. The conversion rates of L‐proline to L‐prolinol in the presence of different amounts of borane were also determined in this study.