Abstract
This work involves a facile synthesis of three (S) -proline-based organocatalysts with C2 symmetry and their effects in enantioselective aldol reaction of acetone with substituted aromatic aldehydes. Moderate enantioselectivities (up to 61% ee) were obtained depending on the nature of the substituents on the aryl ring. Computational calculations at HF/6-31 + G(d) level were employed to underline the enantioselectivity imposed by all the organocatalysts. Higher calculations at B3LYP/6-311 ++ G(d,p) scrf=(solvent=dichloromethane)//B3LYP/6-31 + G(d) levels of theory were also performed for the aldol reaction of acetone with benzaldehyde and 4-nitrobenzaldehyde catalyzed by 1. The computational outcomes were consistent with those produced by experimental results and they were valuable to elucidate the mechanism for the observed stereoselectivity.
Highlights
The valuable role of nature is recognized in the selectivity and activity in biochemical processes occurring in cells
Enantioselective organocatalysis developed as an alternative to those involving metal ions and has become the focus of current research [5,6,7,8] after a long history of proline-catalyzed intramolecular aldol reaction of an achiral triketone with high enantioselectivity, a pioneering reaction developed in the 1970s, called the Hajos–Parrish–Eder–Sauer–Wiechert reaction [9,10]
Precursor 1a was synthesized in 60% yield by reaction of an excess of ( S)-Boc-proline with 2,6-diaminopyridine
Summary
The valuable role of nature is recognized in the selectivity and activity in biochemical processes occurring in cells. This phenomenon has inspired scientists to develop small synthetic models that could rival those presented by nature. The C-C bond-forming reactions are very important in synthetic organic chemistry since they are involved in constructing the frame of very complex molecules. The design of efficient organocatalysts for asymmetric aldol reactions constitutes a significant and active area in the field of synthetic organic chemistry in recent years [12,13,14,15,16,17,18,19,20,21,22]. The main requirements in designing these types of catalysts are the proline frame
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