Unveiling the Long-Awaited Aldehyde Intermediate in Oxidative Dephosphorylation: A Unique Approach to Access Useful Carboxaldehydes.

Oxidative dephosphorylation reactions usually generate symmetrically substituted alkenes from organophosphonates. Even the ubiquitous presence of oxygen can produce such alkenes inadvertently as a side product in small amounts from Wittig/Horner-Wadsworth-Emmons (HWE) reactions. So far, aldehydes have been expected to be the on-pathway intermediate since their discovery, but there was no substantial experimental evidence to support this. Herein, aldehyde intermediates are judiciously trapped before forming alkene on dephosphorylation reactions of diverse phosphonates. Consequently, a unique synthetic strategy has been established to access structurally diverse aldehydes by oxidizing phosphonate carbanions using molecular oxygen. This is the first report to trap and isolate aldehydes since the historical discovery of Wittig/HWE reactions. Only ketone/amide was isolated as a support, but no aldehyde. The optimization studies offer 48-84 % reaction yields of such aldehydes. The mechanistic studies are validated with reaction mixture H and P-NMR studies. Along with other various aldehydes, a unique class of structurally diverse anthracenyl aldehydes exhibiting intense fluorescence has been identified. The extended process to bis-phosphonates led to bis-carboxaldehyde forming in 52-68 % yield within 30 min. The scope is further extended to achieve 2,1,3-benzothiadiazole-linked phosphonate, functionalized with carboxaldehyde.