Synthesis of non-water-soluble and water-soluble Pd(II) complexes as catalysts for methoxycarbonylation reaction of olefins.

Abstract

The treatments of (E)-2-(((2-methoxyethyl)imino)methyl)phenol (L1H), (E)-2-(((2-hydroxyethyl)imino)methyl)phenol (L2H) (E)-3,5-di-tert-butyl-2-(((2-hydroxyethyl)imino)methyl)phenol (L3H) with Pd(OAc)2 afforded the corresponding palladium complexes [Pd(L1)2] (Pd1), [Pd(L2)2] (Pd2) and [Pd(L3)2) (Pd3) respectively in excellent yields. Successful formation of these structures was confirmed by the use of 1H-NMR, 13C-NMR, FT-IR, ESI-MS and X-ray crystallography. In addition, reactions of water-soluble sodium 4-hydroxy-3-(((2-methoxyethyl)imino)methyl)benzenesulfonate (L4H), sodium 4-hydroxy-3-(((2-hydroxyethyl)imino)methyl)benzenesulfonate (L5H) and sodium (E)-3-(((2-(dimethylamino)ethyl)imino)methyl)-4-hydroxybenzenesulfonate (L6H) with Pd(OAc)2 gave the respective water-soluble palladium complexes [Pd(L4)2] (Pd4), [Pd(L5)2] (Pd5) and [Pd(L6)2] (Pd6) in moderate yields. Molecular structures of complexes Pd1 and Pd3 confirmed the formation of N^O bis(chelated) square planar complexes. All the palladium complexes (Pd1-Pd6) were found to be active in the methoxycarbonylation of 1-hexene, achieving percentage yields of up to 90% and regio-selectivity of up to 78% in favor of linear esters. The performance of the catalysts was found to be dependent on their architecture, due to electronic and steric parameters of the complexes. For instance, Pd1 with methoxy substituent gave higher activity compared to Pd2 with electron withdrawing pendant arm. On the other hand, Pd3 exhibited higher compositions of linear ester as in comparison to other complexes. The observed results due to the steric encumbrance noted in Pd3. Similarly, the type of acid promoter, and amount of phosphine used influenced the reactivity and selectivity of the catalysts both at higher and lower ratios. For instance, high phosphine ratios were observed to favour linear esters due to steric hindrance factors. Branched esters were notably favoured as the chain length was increased. Whereas diminished activities were observed at lower amount of acid due to the formation of palladium black. Moreover, different olefin substrates were investigated using the most active Pd4 complex, the results revealed varied catalytic performance, which is a clear indication of the dependance of activity on the identity of the alkene. The water-soluble catalysts (Pd4-Pd6) were observed to have lower catalytic activity compared to the non-water soluble analogues. However, enhanced catalytic performance was noted in biphasic media. The complexes could be recycled up to four runs with minimal loss of activity and change in regio-selectivity.