Schiff bases containing ferrocenyl and thienyl units and their utility in the palladium catalyzed allylic alkylation of cinnamyl acetate

The synthesis and characterization of two new ferrocenyl Schiff bases: [Fc-CHN–(CH2)n–(C4H3S)] (2) {Fc represents (η5-C5H5)Fe(η5-C5H4)– and n = 1(2a) or 2(2b)} containing the thienyl (C4H3S) group are reported. NMR studies indicate that 2 have the anti-(E) conformation in solution and the X-ray crystal structure of 2a confirms that it also adopts the anti-(E) form in the solid state. Ligands 2 have been tested in the palladium catalyzed allylic alkylation of (E)-3-phenyl-2-propen-1-yl (cinnamyl) acetate using sodium diethyl 2-methylmalonate as nucleophile. The reaction of 2 with [Pd(η3-1-Ph–C3H4)(μ-Cl)]2 in the presence of a slight excess K[PF6] produced [Pd(η3-1-Ph–C3H4){Fc-CHN–(CH2)n-(C4H3S)}][PF6] {n = 1(5a) or 2(5b)}, which are the intermediates of this catalytic process. NMR studies of 5 reveal the coexistence of several isomers in solution. The stoichiometric reactions of 5 with the nucleophile are also reported. The comparison of the results obtained for 2, [Fc-CHN–(C6H4–2SMe)] (1a) and [(2,4,6-Me3-C6H2)–CHN–(C6H4–2SMe)] (1b) has allowed to establish the importance of the nature of the substituents on the imine group on the regioselectivity of the process.

Graphical abstractThe utility of the novel ferrocenyl Schiff bases [Fc-CHN–(CH2)n–(C4H3S)] (2) {Fc represents (η5-C5H5)Fe(η5-C5H4)– and n = 1(2a) or 2(2b)} in the palladium catalyzed allylic alkylation of (E)-3-phenyl-2-propen-1-yl (cinnamyl) acetate using sodium diethyl 2-methylmalonate as nucleophile is described. The study of the solution behaviour of the key intermediates of the process [Pd(η3-1-Ph–C3H4){Fc-CHN–(CH2)n–(C4H3S)}][PF6] {n = 1(5a) or 2(5b)} and their reactivity with the nucleophile are also reported.Figure optionsDownload full-size imageDownload as PowerPoint slide