Gabriello Illuminati

The behaviour of ferrocene and ruthenocene in weakly to strongly protic media. Implications on the mechanism of substitutions involving proton as the electrophile

Abstract The interaction of ferrocene and ruthenocene with proton donors has been investigated by NMR spectroscopy in a broad range of protic media, including such weakly protic agents as chloroform, moderately acidic systems such as TFA, and more strongly acidic systems such as conc. H2SO4. The results indicate that these metallocenes undergo two kinds of interactions with acidic media, viz., π-hydrogen bonding in which the Cp ring is the electron donor and, in the most strongly protic solvents, metal protonation. The (H0) 1 2 of ruthenocene and a group of ferrocene derivatives have been determined by a combined extractive-spectrophotometric technique. These values show that the metal atom acts as a very weak base. The H/D exchange rate of ruthenocene in conc. D2SO4 decreases on increasing acid concentration, which rules out the metal-protonated species as an intermediate in the reaction.

Reaction of ethynylferrocene with arenesulphenyl chlorides

The reaction of ethynylferrocene with arenesulphenyl chlorides [ArSCl; Ar = Ph, p-ClC6H4, p-MeC6H4, p-MeOC6H4, p-NO2C6H4, or 2,4-(NO2)2C6H3] follows an unusual pathway; the main outcome is substitution of the acetylenic proton, rather than electrophilic addition to the triple bond. The following compounds were isolated and identified: FcCCSAr, FcCClCHSAr, and FcCClCH2 or FcCOCH3(Fc = ferrocenyl), depending on the reaction conditions. A mechanistic scheme is proposed.

The methoxymercuration of some styrene derivatives

Abstract Product analyses have been carried out and reaction rates and activation parameters determined for the methoxymercuration of some styrene derivatives and related compounds in methanol. The behaviour of α- and β-alkyl substituted styrenes generally parallels that of the similarly substituted vinylferrocenes. The anomalous reactivity of β,β-dimethylstyrenes is suggested to arise from the structural features of this compound. The reaction of the stilbenes was found to be stereospecific. The results are consistent with an unsymmetrically bridged structure for the cationic intermediate possibly involving varying degrees of bridging depending on substrate.

The competing methoxymercuration and ring-mercuration reactions of some alkylvinyl-substituted ferrocenes

Abstract Product analysis and kinetic data for the reaction of some alkylvinylferrocenes with mercuric acetate in methanol are reported. Depending on the location and nature of the alkyl group(s), exclusive addition or ring-substitution reactions are observed as well as both reactions together. This behaviour is interpreted in terms of the relative polar and steric effects. Styrene has also been investigated under similar conditions to provide comparisons of mechanistic significance. The methoxymercuration reaction seems to proceed via a slowly formed, possibly bridged intermediate, with weak carbonium ion character; with the ferrocene derivatives as substrates it appears to be a complex process as suggested by an anomalous temperature-dependence behavior.

The kinetics and mechanism of the protodeboronation of ferroceneboronic acid in moderately concentrated sulfuric acid

Ferroceneboronic acid has been characterized by its UV and NMR spectrum and its dissociation constant in water/ethanol solvents has been determined. The kinetics of protodeboronation have been studied in the same solvents containing moderate concentrations of sulfuric acid; the H0 acidity function has been evaluated throughout the range of the reaction media used. The log of the rate of reaction of ferroceneboronic acid depends linearly with unit slope on H0. The solvent hydrogen isotope effect, the temperature dependence and the nature of acid catalysis have also been investigated. The reaction probably proceeds by an A-SE2-type mechanism.

Ring-opening reactions. Part 4. The role of strain and stereochemical effects on the elimination and substitution reactions of small rings; the reactivity of 1,1-dimethylaziridinium systems

The kinetic behaviour of 1,l -dimethyl- and 1,1,2,3-tetramethyl-aziridinium ions has been compared in the light of the strong tendency of 1,1,2,ω-tetramethyl cyclic ammonium ions to undergo ring opening via the exocyclic elimination reaction. Surprisingly, with the aziridinium member of the latter series, ring-opening substitution takes place more readily than exocyclic elimination by a large factor. The operation of combined strain and stereochemical rate-enhancing effects in substitution, as suggested by previous work on the 1,1-dimethyl cyclic ammonium ions, is now confirmed.

The addition of 2, 4-dinitrobenzenesulfenyl chloride to some alkyl-substituted vinylferrocenes

Abstract The addition of 2, 4-dinitrobenzenesulfenyl chloride to alkyl-substituted vinylferrocenes has been investigated in CH2Cl2, 1, 1, 2, 2-tetrachloroethane and acetic acid solutions. Product analyses were carried out by VPC and PMR techniques and reaction rates measured by a spectrophotometric method. The orientation appears to be determined by the structural features of the intermediate episulfonium ion rather than by the site of the primary attack of the electrophile on the starting substrate. A comparison with the results previously obtained with the methoxymercuration reaction has a bearing on the mechanism of the latter reaction.

An inverted relationship between stability and rate of formation of Meisenheimer-type adducts from dinitrothiophen and trinitrobenzene derivatives

A σ-adduct (II) is formed by the action of the methoxide ion on 2,4-dinitrothiophen; the equilibrium and rate constants for this process have been determined at 25° in methanol and the striking differences between the dinitrothiophen and the trinitrobenzene systems are discussed.

The kinetics of ring-opening of 1,1-dimethylaziridinium ion with sodium methoxide in methanol. The anomalous SN2 reactivity of small rings

The current belief that ring strain relief is the major driving force in the reactivity of small rings disproved by quantitative data obtained for the SN2 ring-opening reaction of cyclic ammonium ions.

Ring-closure reactions. Part 15. Solvent effects on cyclic aralkyl ether formation by intramolecular Williamson synthesis

Product analysis, rate data, and effective molarity (e.m.) values have been obtained for the formation of catechol polymethylene ethers by the intramolecular alkylation of o-ω-bromoalkoxyphenoxides in Me2SO–water (99 : 1, v/v). Twelve ring sizes were investigated in the range 6–32. Comparison with similar data for the reaction in EtOH–water (75 : 25, v/v)(except for n 32) showed that the e.m. values are largely independent of solvent despite the large solvent effect observed in both the cyclization reaction and the related intermolecular model reaction. Small effects only were observed in the medium ring region. The main possible factors playing a role in solvent effects on intramolecular cyclization have been critically analysed in the light of the data.