The relative electrophilic reactivities of tropylium cation and its (OC)3M π-complexes: kinetic studies of alkoxide transfer and reversible nucleophilic addition

Abstract

π-Complexation of the tropylium cation (Tr)⁺ with an (OC)₃Cr group increases thermodynamic stability (ΔpK_{R +}ca. 4.3 in methanol) and reduces reactivity towards abstraction of methoxide ion from Malachite Green methyl ether (MG)OMe (k^rel.ca. 110) in MeNO₂–MeCOEt (40 : 60 v/v) and nucleophilic exo-addition of methanol (k^rel.ca. 2 100) in methanol. The organometallic cation (1a) is stable in aqueous solutions of pH < ca. 6, but at higher pH is converted irreversibly into (OC)₃Cr complexes (5) and (6) of ditropyl. The same complexes are formed on reaction of (1a) with 1,8-bis(dimethylamino)naphthalene. The relative rates of transfer of methoxide ion from (MG)OMe to {(η-Tr)Cr(CO)₃}⁺, {(η-Tr)Mo(CO)₃}⁺, and {(η-Tr)W(CO)₃}⁺ in MeNO₂–MeCOEt (40 : 60 v/v) are 1 : 10 : 6, respectively. The rates of transfer to (Tr)⁺ of alkoxide ion from (η-7-exo-alkoxycycloheptatriene)Cr(CO)₃ complexes (2a–d) in MeCN decrease through the series: alkoxy = methoxy > ethoxy > isopropoxy > t-butoxy, but the overall rate change is only about five-fold. In methanol, the 7-exo-methoxycycloheptatriene complex (2a) is about ten times more reactive towards acid heterolysis than is methyl tropyl ether. This conversion is general acid-catalysed. in aqueous solutions of pH > ca. 6, the rate of spontaneous heterolysis of the ether (2a) is substantially faster than that of consumption of the resulting cation (1a) which increases with increasing pH. The 7-endo-methoxy stereoisomer (3) is inert to acid heterolysis in aqueous solutions to give the cation (1a), but undergoes decomplexation to give (Tr)⁺.