Stability and reactivity of ferrocenyl(2,4,6-trimethoxyphenyl)carbenium salts

Abstract

Ferrocenyl(phenyl)methanols, Fc–CHΦOH [Fc=Fe(η 5-C 5H 5)(η 5-C 5H 4), Φ a=2,4,6-(MeO) 3C 6H 2 ( 1a); Φ b=2,6-(MeO) 2C 6H 3 ( 1b)], were prepared by the reactions of FcCHO with ΦLi. 1a reacted with only a slight excess of perchloric acid or tetrafluoroboric acid even in methanol and in 1 M HCl acid to give dark violet crystals of the carbenium salts, [FcCHΦ a]X [X=ClO 4 ( 2a), BF 4 ( 2a′)]. 1b reacted with these acids in diethylether to give the carbenium salts, [FcCHΦ b]X [X=ClO 4 ( 2b), BF 4 ( 2b′) (dark red–brown crystals)] in good yields. The p K R + values could be measured in HCl acid for 1a (3.0) and for 1b (1.6). 2a and 2b were labile in hot 2-propanol to give the reduced compounds, FcCH 2Φ. 1a and 1b reacted with 1,3,5-trimethoxybenzene, Φ aH, in 2-methyl-2-propanol containing only a slight excess of HCl acid to give diphenyl(ferrocenyl)methanes, FcCHΦ a 2 and FcCHΦ aΦ b, respectively. While 1a decomposed in neat 1,3-dimethoxybenzene, Φ bH, in the presence of acid, 1b reacted to give 2,4-dimethoxyphenyl(2,6-dimethoxyphenyl)ferrocenylmethane, FcCHΦ bΦ d [Φ d=2,4-(MeO) 2C 6H 3]. The 1H-NMR spectrum of 2a was temperature-dependent and indicated that the free rotations of both Fc and Φ a groups in 2a existed at r.t. but were frozen at −60°C.