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

Abstract

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.

Keywords

• benzylphosphonium salts
• Wittig olefination
• hydrogen bonding
• nitrostilbenes
• 2-arylindoles