Palladium-catalyzed reaction of 2-iodoferrocenyl alcohols with internal alkynes: Synthesis of functionally 1,2-disubstituted ferrocenes and ferroceno-pyrans

The reaction of racemic (2-iodoferrocenyl)methanol with internal alkynes in the presence of (dppf)PdCl2 and i-Pr2NH produces alkenyl-substituted ferrocene carboxaldehydes in moderate yields. All reactions are carried out at 100 or 120 °C for different reaction times (between 6 and 26 h) in a screw-cap Pyrex bottle. The scope and limitations of this reaction are studied by employing variously substituted 11 internal alkynes. The reactions are regioselective with alkynes having a sterically crowded substituent such as t-butyl and trimethylsilyl groups. Moreover, racemic 1-(2-iodoferrocenyl)ethanol derivatives are synthesized as two diastereomers. Both diastereomers are reacted with internal alkynes in the presence of (dppf)PdCl2 and i-Pr2NH at 120 °C to afford alkenyl-substituted acetylferrocenes and ferroceno-pyrans in moderate to good yields. According to the alkyne employed, different reaction times (between 6 and 55 h) are necessary to drive the reactions to completion. Mechanisms are also suggested for the formation of observed products.