Efficient organotin catalysts for urethanes: kinetic and mechanistic investigations

Abstract

Dibenzyltin bis(2-ethylhexanoate) 1 (4-Y—C₆H₄CH₂)₂Sn(OC(O)R¹)₂ [Y=H, 1a; MeO, 1b; Cl, 1c; Me, 1d; and R¹=MeCH₂CH₂CH₂CH(Et)—] were synthesized either from the reaction of corresponding dibenzyltin dichlorides with silver 2-ethylhexanoate or from the reaction of dibenzyltin oxides with 2-ethylhexanoic acid. Compound 1a was further utilized as a catalyst for the reaction of mono- and di-isocyanates [PhNCO, CH₃C₆H₃-2,4-(NCO)₂ and OCN(CH₂)₆NCO] with alcohols (primary, secondary, tertiary, cyclohexcyl, alkyl, allyl, benzyl and aryl) leading to the formation of the corresponding urethanes. The catalytic efficiencies of 1 vis-à-vis industrially known organotin catalysts have been determined through kinetic studies for the reaction of PhNCO and n-BuOH at various temperatures. Compounds 1a, 1c and 1d show higher efficiency than dibutyltin bis(2-ethylhexanoate). FTIR studies further provide mechanistic insights into the catalytic cycle, which comprises pre-coordination of isocyanate to tin(IV), formation of stannyl carbamate and generation of dibenzyl(alkoxy)carboxylate as the active catalyst.