Cyclopalladation of ferrocene tertiary amines: Experimental, theoretical, and reactive studies toward Suzuki-Miyaura cross-coupling reaction

Abstract

The ferrocene tertiary amines of general formula [(η5C5H5)Fe(η5C5H4)CH2N(CH3)CH2C(R)CHC6H5, RH (3a) and RCH3 (3b)] were conveniently prepared from the corresponding ferrocenylimines [(η5C5H5)Fe(η5C5H4)CH2NCHC(R)CHC6H5, RH (1a) and RCH3 (1b)] in two steps. Metalation of 3a, 3b with Li2PdCl4 and CH3COONa, followed by treatment with PPh3, gave the ortho palladium (II) complexes Pd{[(η5C5H5)Fe(η5C5H3)CH2N(CH3)CH2CHCHC6H5]}ClPPh3 (5a) and Pd{[(η5C5H5)Fe(η5C5H3)CH2N(CH3)CH2C(CH3)CHC6H5]}ClPPh3 (5b), respectively. However, Palladation of ferrocenylimines 1a and 1b afforded the heteroannular palladium (II) complexes. This protocol proved that the isomerism around the CN moiety might indicate that the palladium entered into the lower ferrocene ring, giving the heteroannular palladium (II) complexes. Additionally, theoretical studies indicated that the CH bond in the substituted ferrocene ring for the ligands 3a and 3b was prior to metalation. As for the catalyst, the palladacycles showed high efficiency in Suzuki-Miyaura cross-coupling reaction using low catalyst loading 0.1 mol%.