Nucleophilic substitution—continued kinetics

The research on the reaction mechanism of the oxidative polycondensation of 2,6-dimethylphenol (DMP) was also continued in the years around 1990. In several publications a Dutch research group reported on kinetic studies dedicated to the 02-promo ted polycondensation of DMP catalyzed by copper tetramethyl 1,2-diamino-ethane complexes [233-236] or by copper complexes of imidazole [237,238]. The speculative mechanistic scheme was based on dimeric copper complexes such as (147a) which were assumed to incorporate a DMP anion (147b) which was oxidized to yield a phenoxy cation and to coordinate another DMP molecule (148a). The growing step was then assumed to consist of a nucleophilic substitution at the phenoxy cation (149) with liberation of a reduced dimeric copper complex (148b). This complex was believed to be... 

Discussion of the borderline region between the pair mechanism is considered to be not proven. A kinetic study of the solvolyses of a range of secondary tosylates in a range of solvents has been interpreted as showing a range of nucleophilic solvent assistance with a spectrum of 5n1/ S n2 character. 

Sulphur.—Discussion continues as to whether nucleophilic substitution at sulphur(n) is a one-stage associative process or involves an intermediate adduct of significant lifetime. There is kinetic evidence for an intermediate in reactions of p-nitrobenzenesulphenyl chloride with aniline and with substituted anilines, but no evidence for an intermediate was obtained in... 

Hence Spj2 means that the reaction involves Substitution by a Nucleophile and that it follows second-order kinetics, i.e. two species are involved in the rate-determining step. The S 2 mechanism involves only one continuous step. 

As in the previous volume, this chapter reviews kinetic and mechanistic studies of the stoichiometric reactions of coordinated hydrocarbons with nucleophiles and electrophiles, together with some related processes such as cycloadditions. Nucleophilic addition and substitution, in particular, continue to attract considerable interest, especially with dienyl- and arene-metal substrates. Even here, however, quantitative studies still lag far behind the extensive synthetic literature. However, fundamental reactivity patterns and parameters are beginning to emerge from the mechanistic studies which should assist in the design of rational syntheses. 

The first thianthrene dication (32) has been isolated. Work on the thianthrene cation radical (33) continues. Its reaction with aliphatic amines yielded the sulphimides (34 X = NR) phenols and aromatic amines were attacked in the pflra-position to afford (34 X = / -HOCeH4 or p-R2NC H4), and enolizable ketones yielded the j8-keto-sulphonium salts (34 X = CHR COR ). The latter compounds reacted with nucleophiles Y to give the a-substituted ketones YCHR COR, and might prove to be useful intermediates for their synthesis. Reinvestigation of the kinetics of the reaction of (33) with phenol and with anisole has provided evidence which appears to disprove the disproportionation mechanism postulated earlier for these reactions. 

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