Organotin(IV) Compounds as Catalysts for the Solvent-Free Synthesis of Naphthofurans

Four distinct organotin(IV) compounds, [R₂Sn(L¹)] [R = Ph (1) and n-Bu (2)] and [R₂Sn(L²)] [R = Ph (3) and n-Bu (4)], have been synthesized from two polydentate proligands such as 2-((2-hydroxy-3-methoxybenzylidene)amino)-4-methyl phenol (H₂L¹) and 4-(tert-butyl)-2-(((2-hydroxy-5-methylphenyl)imino)methyl)phenol (H₂L²). The organotin(IV) compounds were synthesized by the reaction of organotin(IV) halides such as Ph₂SnCl₂ and (n-Bu)₂SnCl₂ with both proligands using Et₃N as a base. All compounds were fully characterized using FT-IR spectroscopy; ¹H, ¹³C{¹H}, and ¹¹⁹Sn NMR spectroscopy; HRMS spectrometry; and single-crystal X-ray diffraction analysis. The Lewis acidity of all compounds was determined by the Gutmann–Beckett method. The catalytic activities of all of the compounds were investigated for the synthesis of naphthofurans from trans-β-nitrostyrene derivatives and β-naphthol or α-naphthol under solvent-free conditions. The maximum yield of naphthofurans is up to 95%.