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

Ferrocenyl(phenyl)methanols, Fc-CH Phi OH [Fc = Fe(eta(5)-C5H5)(eta(5)-C5H4), Phi(a) = 2,4,6-(MeO)(3)C6H2 (1a); Phi(b) = 2,6-(MeO)(2)C6H3 (1b)], were prepared by the reactions of Fc-CHO with Phi Li. 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-CH Phi(a)]X [X = ClO4 (2a), BF4 (2a')]. 1b reacted with these acids in diethylether to give the carbenium salts, [Fc-CH Phi(b)]X [X = ClO4 (2b), BF4 (2b') (dark red-brown crystals)] in good yields. The pK(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(2)Phi. 1a and 1b reacted with 1,3,5-trimethoxybenzene, Phi(a)H, in 2-methyl-2-propanol containing only a slight excess of HCl acid to give diphenyl(ferrocenyl)methanes, Fc-CH Phi(2)(a) and Fc-CH Phi(a)Phi(b), respectively. While 1a decomposed in neat 1,3-dimethoxybenzene, Phi(b)H, in the presence of acid, 1b reacted to give 2,4-dimethoxyphenyl(2,6-dimethoxyphenyl)ferrocenylmethane, Fc-CH Phi(b)Phi(d) [Phi(d) = 2,4-(MeO)(2)C6H3]. The H-1-NMR spectrum of 2a was temperature-dependent and indicated that the free rotations of both Fc and Phi(a) groups in 2a existed at r.t. but were frozen at -60 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.