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-CHdouble bond; length as m-dashN–(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-CHdouble bond; length as m-dashN–(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-CHdouble bond; length as m-dashN–(C6H4–2SMe)] (1a) and [(2,4,6-Me3-C6H2)–CHdouble bond; length as m-dashN–(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.