Efficient catalysis of Suzuki–Miyaura C[sbnd]C coupling reactions with palladium(II) complexes of partially hydrolyzed bisimine ligands: A process important in environment context

Abstract

Potentially hexadentante [O−,N,E:E,N,O−] chalcogenated bisimine ligands L1–L3 have been synthesized by reaction of 1,1′-(4,6-dihydroxy-1,3-phenylene)bisethanone with H2N(CH2)2SPh, H2N(CH2)2SePh and H2N(CH2)2TeC6H4-4-OMe respectively. The L1–L3 react with Na2PdCl4 resulting in their partial hydrolysis, which appears to be metal-promoted. Of the two [(CH3)CN(CH2)2EAr] fragments of L1–L3, one is converted to (CH3)CO and H2N(CH2)2EAr eliminated. The hydrolysis products 1-[C(CH3)N(CH2)2SPh]-3-[C(CH3)O]-4,6-[OH]2C6H2 (L1′), 1-[C(CH3)N(CH2)2SePh]-3-[C(CH3)O]-4,6-[OH]2C6H2 (L2′) and 1-[C(CH3)N(CH2)2TeC6H4-4-OMe]-3-[C(CH3)O]-4,6-[OH]2C6H2 (L3′) have formed complexes [PdCl(L′–H)] (1, 3 and 5). The other product of hydrolysis H2N(CH2)2EAr (L″) reacted with Na2PdCl4 yielding the complexes [PdL”Cl2] (2, 4 and 6). All the complexes (1–6) were found thermally and air stable. Complexes 1, 3 and 5 have been investigated as catalysts for Suzuki–Miyaura CC coupling reactions. The catalytic activities of 1 and 3 which are palladium complexes of S- and Se-containing Schiff base derivatives respectively, were found good for the Suzuki–Miyaura cross-coupling of aryl bromides with phenylboronic acid under mild reaction conditions. The Pd(II) complex (3) of selenated ligand was found active to catalyze the coupling of 2-chlorobenzaldehyde and 3-chlorotoluene. The activity of Te analog was found to be the lowest one as it failed in catalyzing the coupling of electronically deactivated aryl bromides.