Hypervalent Organotin(IV) Compounds as Efficient Catalysts for the Knoevenagel Condensation of Aldehydes

Hypervalent organotin(IV) catalyst: hypervalent organotin(IV) compounds show catalytic activity in the Knoevenagel condensation of aldehydes under mild reaction condition with a yield of product up to 97%. This study represents the first documented use of organotin(IV) compounds in facilitating the Knoevenagel condensation of aldehydes.

Abstract

We report the synthesis of six mononuclear organotin(IV) compounds, [R₂Sn(L¹)] [R = Ph (1), t-Bu (2), and Me (3)] and [R₂Sn(L²)] [R = Ph (4), t-Bu (5), and Me (6)] supported by two polydentate pro-ligands, (E)-2,4-dichloro-6-(((2-hydroxy-5-nitrophenyl)imino)methyl)phenol (H₂L¹ ) and (E)-2,4-dichloro-6-(((2-hydroxyphenyl)imino)methyl)phenol (H₂L² ). HR-MS, multinuclear (¹H, ¹³C, and ¹¹⁹Sn) NMR spectroscopy, and FT-IR spectroscopy were used to characterize all compounds (1–6). Structural investigation via single-crystal X-ray diffraction confirmed a mononuclear geometry for all compounds, each featuring a five-coordinate tin(IV) center. The Lewis acidity of the compounds (1–6) was examined through the Gutmann–Beckett method. The effectiveness of all Sn(IV) compounds (1–6) as catalysts for the Knoevenagel condensation of various aldehydes with malononitrile was assessed. In addition, a plausible mechanism is proposed based on the mechanistic studies and theoretical calculations.

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