Intramolecular CH‐Hydrogen Bonding During the Dissociation of the Oxaphosphetane Intermediate Facilitates Z/E‐Selectivity in Wittig Olefination

The Z-selective synthesis of novel nitrostilbenes has been reported. An intrinsic role of intramolecular hydrogen bonding during the dissociation of OPA-intermediate is critical for the stereoselective Wittig olefination. DFT calculations and X-ray measured intramolecular CH hydrogen bonding distances strongly support the observed phenomenon. Further, the current methodology has been utilized to synthesize medicinally potential salicylate-methyl-ester-based 2-arylindole derivatives.

Abstract

Herein, DFT studies corroborating experimental results revealed that the shortest intramolecular hydrogen bonding distance of cis/trans-oxaphosphetane (OPA) oxygen with the CH-hydrogen of a triphenylphosphine phenyl ring provides good evidence for the attained olefin Z/E-selectivity in Wittig olefination of the studied examples. 2-Nitrobenzaldehyde, 3-nitrobenzaldehyde, 2-nitro-3-bromobenzaldehyde, 2-nitro-5-bromobenzaldehyde and 2-nitro-5-arylbenzaldehydes provided Z-nitrostilbenes with (2-chloro-4-hydroxy-3-methoxy-5-(methoxycarbonyl)benzyl) triphenylphosphonium chloride as the major products. However, 4-nitrobenzaldehyde and 2-nitro-6-bromobenzaldehydes furnished E-nitrostilbenes as the major products in high yields. Furthermore, the DFT computed intramolecular CH1/CH2-hydrogen bond distances with Cl/NO₂ of selected stilbene derivatives were in good agreement with intramolecular hydrogen bond distances measured from single-crystal X-ray diffraction measurements.