John R. Moffatt

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.

Modelling of micellar effects upon substitution reactions with moderately concentrated hydroxide ion

Cationic micelles of cetyltrimethylammonium chloride and bromide (CTACl and CTABr) speed reactions of 2,4-dinitro-1-naphthyl chloride (DNNC) and p-nitrophenyl diphenyl phosphate (pNPDPP) with OH–. The pseudophase ion-exchange model fits the data for reaction with [OH–] < 0.05 mol dm–3, but the reaction is faster than predicted at higher ratios of [OH–]. The deviations from theory are largest with high ratios of [OH–] to [Cl–] or [Br–] and are larger in CTABr than in CTACl. A mass-action-like model with each anion binding independently to the micelle fits the data for 10–3–0.5 mol dm–3 OH–.

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)+.

Kinetic study of the effect of (OC)3Cr π-complexation upon the reactivities of tropone acetals and related tropylium cations

The (OC)3Cr π-complexes (4) and (5) of acyclic and cyclic acetals of tropone undergo acidcatalysed heterolysis in water to give cationic complexes (2) and (6) that are stable in solutions of pH < ca. 5. These cations are thermodynamically more stable than the corresponding uncomplexed alkoxytropylium cations (8) and (13) but stabilisation is less than that the resulting from (OC)3Cr complexation of the tropylium cation. The spiro-acetals (5a–c) have very similar reactivities towards heterolysis and are somewhat less reactive than the acyclic acetals (4a–d). These heterolysis reactions, and their reverse, are exo-stereospecific. Reactions of the acyclic acetals with aqueous solutions of high pH give the tropone complex (9), the acetal heterolysis step being rate-limiting. The complexed alkoxytropylium cations (2) and (7) undergo base-catalysed conversion in water into the tropone complex; the intermediate (hydroxy) alkoxy compounds do not build up during these reactions whose rates are relatively insensitive to the identity of the alkoxy group. The reactivity of water relative to that of hydroxide ion towards these cations is much lower than predicted by Swain–Scott and Ritchie nucleophilicity scales. In aqueous alkali, the complexed (hydroxyalkoxy) tropylium cations (6) give apparently exclusively the corresponding spiro-acetals (5). These acetals and cations co-exist in aqueous solutions of pH 6–7 and their interconversions in these solutions are much faster than formation of the tropone complex. Conversion of the cations (6) into the acetals (5) is catalysed by borate but not by Tris buffer. Salts speed heterolysis of the acetals (4) and (5) and inhibit reactions of the cations (2), (6), and (7). Comparisons of reactivities of the (OC)3Cr-complexed diethyl, ethylene, and trimethylene acetals of tropone with those reported earlier for uncomplexed analogues show that complexation does not change the overall pattern of reactivity but suggest that the transition states for heterolysis of the complexed acetals are ‘earlier’(more acetal-like).

Effects of alcohol-modified micelles on deacylation and nucleophilic aromatic substitution by azide ion

Alcohol-modified micelles and oil–water microemulsions have similar rate effects upon aromatic nucleophilic substitution by N3–. Relative values of k2m/kw for aromatic nucleophilic substitution and deacylation show that enhancements of the rate of deacylation can be ascribed wholly to increased reactant concentrations in the micelles, but that there is an additional catalytic effect upon nucleophilic aromatic substitution.Reactions of azide ion with 1-chloro-2,4-dinitronaphthalene and p-nitrophenyl benzoate in water containing dilute t-pentyl alcohol are accelerated by cationic micelles, but the rate enhancements are lower than in aqueous micelles. At constant [N3–], first-order rate constants for reaction of 1-chloro-2,4-dinitronaphthalene go through maxima with increasing concentration of cetyltrimethylammonium chloride or bromide (CTACl or CTABr). Similar behaviour was found with p-nitrophenyl benzoate in CTACl. Rate constants of reaction of the naphthalene substrate in cetyltrimethylammonium azide increase with increasing [surfactant]. Estimated rate constants of deacylation at the micellar surface are slightly lower than in the absence of surfactant, but are higher for reaction of 1-chloro-2,4-dinitronaphthalene.