Base metal mediated acid-free one pot synthesis of aldehydes and dialdehydes directly from alkyl/aryl/heteroaryl-methyl halides in aqueous medium

Abstract

Herein, we demonstrated the first examples of three new base metal catalyst systems that efficiently convert the aryl and alkyl methyl halides directly into aldehydes. The developed catalyst systems are very economical as they employ the cheap and commercially available base metal complexes namely, nickel acetate, ferrous acetate and ferric acetate as catalysts and hexamethylenetetramine as the reagent. Unlike the conventional methods the developed synthetic strategies i) avoid the use of stoichiometric amount of acids and employs very cheap base metal catalysts, ii) yield the aldehyde products in a single step by avoiding isolation of substituted hexamethylenetetramine, iii) work in pure aqueous medium iv) found to be active not only for the synthesis of aryl aldehydes but also for the challenging aliphatic aldehydes, heteroaromatic aldehydes as well as dialdehydes, and v) yield the aldehyde products even from starting from cheaper methyl chloro compounds thus reducing the cost of the process further. The recyclability of all the catalyst systems was tested and found that the systems were considerably active till the seventh cycle. DFT studies revealed that a considerable reduction in the HOMO-LUMO gap was observed upon coordination of HMTA to the metal centre. Among all the complexes considered for DFT studies, Ni(OAc)₂.HMTA had the lowest HOMO-LUMO gap and the same was reflected in its catalytic activity, which gave the aldehyde product in highest yield in comparison to other catalysts.