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

Abstract

The synthesis and characterization of two new ferrocenyl Schiff bases: [Fc-CH N–(CH 2) n –(C 4H 3S)] ( 2) {Fc represents (η 5-C 5H 5)Fe(η 5-C 5H 4)– and n = 1( 2a) or 2( 2b)} containing the thienyl (C 4H 3S) 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(η 3-1-Ph–C 3H 4)(μ-Cl)] 2 in the presence of a slight excess K[PF 6] produced [Pd(η 3-1-Ph–C 3H 4){Fc-CH N–(CH 2) n -(C 4H 3S)}][PF 6] { 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-CH N–(C 6H 4–2SMe)] ( 1a) and [(2,4,6-Me 3-C 6H 2)–CH N–(C 6H 4–2SMe)] ( 1b) has allowed to establish the importance of the nature of the substituents on the imine group on the regioselectivity of the process.