Marutirao M. Mhala

A comparison of the stability and reactivity in solution of the tropylium cation and its π-Cr(CO)3 complex

Abstract π-Complexation of the tropylium cation with a Cr(CO) 3 group greatly reduces its reactivity towards addition of methanol and enhances to a smaller degree its rate of formation by acid heterolysis of tricarbonyl(η-7- exo -methoxycycloheptatriene)chromium ( 2 ); the metal-complexed carbocation 1 is stable in aqueous solutions of pH ⩽ 7, but at higher pH neutral products form irreversibly. Conversion of 2 into 1 in MeCN/H 2 O ( 1 1 w/w) is general acid-catalysed.

The effects of single- and twin-tailed ionic surfactants upon aromatic nucleophilic substitution

Reactions of OH– with 2,4-dinitro-1-chloro-benzene and -naphthalene have been examined in solutions of didodecyldimethylammonium chloride and hydroxide. Rate effects were analysed quantitatively in terms of distribution of reactants between water and the colloidal particles. Second-order rate constants at the surface of the particles are very similar to those in normal aqueous micelles of cetyltrimethylammonium hydroxide, chloride, and bromide and p-octyloxybenzyltrimethylammonium bromide and are slightly higher than in water. Similar observations were made on the reaction of OH– with 2,4-dinitro-1-fluorobenzene.

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

π-Complexation of the tropylium cation (Tr)+ with an (OC)3Cr group increases thermodynamic stability (ΔpKR +ca. 4.3 in methanol) and reduces reactivity towards abstraction of methoxide ion from Malachite Green methyl ether (MG)OMe (krel.ca. 110) in MeNO2–MeCOEt (40 : 60 v/v) and nucleophilic exo-addition of methanol (krel.ca. 2 100) in methanol. The organometallic cation (1a) is stable in aqueous solutions of pH 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)+.