C. David Johnson

Hydroxy Schiff base-oxazolidine tautomerism: apparent breakdown of Baldwin's rules

Abstract The tautomerism between hydroxy Schiff bases, X-C6H4CR=NCMe2CH2OH (R=H, Me, C6H5; X is a range of substituents) and X-C6H4,CR=NC6H4 (o-CH2OH), and the corresponding ring closed systems, oxazolidines and dihydrobenzoxazines, has been investigated. In all cases where equilibria can be determined the process is shown to be extremely facile, despite the fact that Baldwins rules are contravened. Information about the structure of some of these molecules in the crystalline state is provided by solid state NMR. Previous work on the equilibration of 1-methyl-1,3-oxazolidine and its iminium tautomer in trifluoroacetic acid is shown to be complicated by trifluoroacetate formation, but the overall conclusion that this 5- endo - trig process is also rapid, appears to be correct. Some mechanistic consequences of these deductions are discussed.

Diels–Alder reaction of fumaronitrile and cyclopentadiene in water: the influence of cosolutes

Rates of reaction are reported for the title compounds in aqueous solution. The influence of added salts on the rate of reaction is recorded: alkali-metal salts advance the rate, while tetraethylammonium chloride retards it. Sodium dodecyl sulphate and cetyltrimethylammonium bromide also retard the reaction, while the effect of cyclodextrins depends on their cavity size. The explanation of these phenomena in terms of the hydrophobic effect is considered and extended by measurement of ΔH‡ and ΔS‡ parameters for the reaction.

Stereoelectronic effects in ring closure reactions

Abstract The mechanistic criterion of reversed substituent effects in reactions, formally classified as 5-endo-trig and 6-endo-trig , is examined.

Transition state variation in the menshutkin reaction

Data for reaction of CH3I and CD3I with substituted pyridines were previously argued to indicate a variant transition state as rate of reaction was varied, but the fact that both sets of results follow LFERs may be taken as evidence against such a conclusion.

On the compatibility of the reactivity–selectivity principle and the Ritchie N+ relationship

Consideration of cross correlations existing within the framework of Ritchies N+ scale reveals that the variations in slope of the LFERs resulting from the treatment are sufficient to account for the range of selectivity ratios observed in this area of reactivity, and do not require any intrinsic link between reactivity (stability) and selectivity.

Ring closure to ynone systems: 5- and 6-endo- and -exo-dig modes

The rates of cyclisation of 2-hydroxy-2-methyl-5-arylpent-4-yn-3-ones in trifluoroacetic acid are reported. An approximate ρ value of –4.0 (using σ+) and a kinetic isotope effect, KTFA/KTFA – d, of 4.2 are observed, which suggest that the reaction proceeds via rate-limiting triple-bond protonation and does not entail carbonyl protonation along the reaction co-ordinate. The base-catalysed ring closure occurs via the vinylic carbanion. A similar mechanistic picture appears to be involved in the cyclisation of 1-(2-hydroxyphenyl)-3-arylprop-2-yn-1-ones which yield a mixture of flavone and aurone in base, whilst in acid only the flavone is observed. In contrast with recent theoretical studies, the base-catalysed results appear to conform to Baldwins original proposal of acute-angle approach of a nucleophile to a triple bond.

Selectivity and the Ritchie equation

The rates of addition of six primary amines to various electrophiles in aqueous solution have been analysed using a log k–log k type LFER treatment. Observed behaviour is in accord with the reactivity–selectivity principle and in quantitative agreement with previous work, questioning the validity of the Ritchie equation over large ranges of Lewis acidity. It is suggested that it is the range of electrophile stability considered, and not the intrinsic reactivity of the electrophiles themselves, which determines the degree of conformity to Ritchie type constant selectivity relationships.

Kinetic acidity function Hc. Part 3. Applicability to hydration of styrenes

Variation of rate of hydration of substituted styrenes with acidity has been measured in aqueous sulphuric acid solutions. This reaction proceeds through a rate-limiting carbon protonation step. Linear Pfluger plots of unit slope result from the data. These appear to validate the applicability of an acidity function to the results; possible sources of error in this and related conclusions are carefully discussed.

Aromaticity and tautomerism. Part IV. Free energy: enthalpy correlations for protonation of pyridine bases and azine N-oxides and temperature variation of the H0 and HA acidity functions

Thermodynamic parameters for the protonation of nine substituted pyridines and nine azine N-oxides are obtained from basicity measurements of the bases at 25, 40, 60, 80, and 90° using the spectrophotometric method. The protonations of both series of bases follow linear ΔH0–pKa correlations. The spectrophotometric measurements are also used to assess the temperature variation of the H0 and HA acidity functions and the relationship HA=mH0+C is found to be approximately temperature independent.

The kinetics and mechanism of the electrophilic substitution of heteroaromatic compounds. Part XXXVIII. Hydrogen exchange of isoxazoles and isothiazoles

Acid-catalysed deuteriodeprotonation rates are recorded for isoxazole, isothiazole, and some methyl derivatives. The rates are extrapolated to give rate constants at 100° and pH 0 which are compared with those for other heteroaromatic ring systems to provide quantitative estimates of relative reactivity. Quantitative effects of methyl groups provide evidence for the relative degree of bond-fixation, and indicate that the aromaticity increases in the series isoxazole, pyrazole, isothiazole.