Paul A. Newman

Substitution meta - to an aromatic hydroxyl group

Abstract It is suggested that the bromination of 4-methylphenol in superacid solvents involves a 1,2-shift of bromine within a protonated bromodienone.

Electrophilic substitution with rearrangement. Part 9. Dienones derived from brominations of o-, m-, and p-cresol

Regiospecific protodebromination of ring-substituted bromophenols derived from 2-, 3-, or 4-methylphenol can be effected by heating them with aqueous hydrogen iodide; the synthetic scope of this reaction has been explored. These di- and poly-bromophenols can generally be converted by further bromination in aqueous acetic acid into dienones, which have now been shown to have the 4-bromo-2,5-dienone rather than the 2-bromo-3,5-dienone structure. The rearrangements of these dienones to ring-substituted polybromophenols by treatment with sulphuric acid have been investigated; where more than one product is formed, the regioselectivity differs from that prevailing in the corresponding direct bromination of the phenol with liquid bromine. The alternative rearrangements of these dienones in aprotic solvents with and without illumination have been compared with results obtained by reaction of the methylphenols with bromine under the same conditions. Characteristic differences between the behaviours of 2-, 3-, and 4-methyl-substituted compounds reflect the specific reactions available to the particular dienones.

Solvolytic elimination and hydrolysis promoted by an aromatic hydroxy-group. Part 2. The hydrolysis of 2-bromo-4-dibromomethylphenol in 95% 1,4-dioxane

The reaction of 2-bromo-4-dibromomethylphenol with water in slightly aqueous (95%) 1,4-dioxane has been examined kinetically by using u.v. spectroscopy, which provides evidence for the transient formation of the quinone methide, 2-bromo-4-bromomethylenecyclohexa-2,5-dienone, during the hydrolysis. The final product is 3-bromo-4-hydroxybenzaldehyde. The rate of disappearance of starting material is independent of acidity, but is reduced by added or developing bromide ion. The rate of the loss of bromide ion from the phenol is very much greater than that of the corresponding reaction of 2-bromo-4-dibromomethylanisole under the same conditions. An estimate of the value of σp-OH+(–1.36), made by using these relative rates, is larger than the value (–0.92) based on relative rates of aromatic electrophilic substitution. Solvent kinetic isotope effects on these reactions are reported; the theoretical implications of variation in the substituent parameter for the hydroxy-group is discussed in terms of solvent effects on H–O hyperconjugation.

Enhancement of the conjugative electron-releasing power of the hydroxy-group shown by the kinetics of solvolysis of 2-bromo-4-dibromomethylphenol and of 2-bromo-4-dibromomethyl-anisole

Abstract The relative rates and kinetic forms for the solvolyses of 2-bromo-4-dibromomethyl-phenol and its methyl ether lead to a new estimate (σ + = −1.6) of the electron-releasing power of the hydroxy-group.

Electrophilic substitution with rearrangement. Part 10. Some products of bromination of 2.4-dimethylphenol and of 4-t-butyl-2- methylphenol

The reaction of 2,4-dimethylphenol with bromine gives first 6-bromo-2,4-dimethylphenol, which according to the conditions of further bromination can give 4,6-dibromo-2,4-dimethylcyclohexa-2,5-dienone; a mixture of 5,6with some 3,6-dibromo-2,4-dimethylphenol; 6-bromo-4-bromomethyl-2-methylphenol with none of the 2-bromomethyl-4-methyl isomer; or a mixture from which 6-bromo-2,4-bis(bromomethyl)phenol can be isolated. The corresponding dienone from 6-bromo-4-t-butyl-2-methylphenol reacts by more complex pathways, and the products include those of de-t-butylation. The probable mechanisms involved in these reactions are discussed.

Solvolytic elimination and hydrolysis promoted by an aromatic hydroxy group. Part 1. The reaction of 2,6-dibromo-4-dibromomethylphenol and of 2,6-dibromo-4-bromomethylene-cyclohexa-2,5-dienone with water in 95% 1,4-dioxane

The reaction of 2,6-dibromo-4-dibromomethylphenol with water to give the quinone methide, 2,6-dibromo-4-bromomethylenecyclohexa-2,5-dienone, together with 2,6-dibromo-4-hydroxybenzaldehyde has been studied in 95% 1,4-dioxane. U.v. spectroscopy has been used to study the kinetic form of the reaction. At very low acidities, rapid conversion into the quinone methide occurs through the aryl oxide ion, and is followed by addition and replacement to give the aldehyde. At acidities greater than 10–3M, reaction of the un-ionised phenol is dominant and follows a conventional SN1 pathway through a mesomeric carbocationic intermediate. Subsequent loss of a proton from this intermediate to give the quinone methide is competitive with its capture by the solvent (to give in several steps the related aldehyde) and by bromide ion (to give starting material).The mass-law constant for capture of the carbocation by bromide ion is very large (ca. 104 in this solvent), so that relatively small concentrations of hydrogen bromide convert the quinone methide back into the dibromomethylphenol by 1,6-addition. Solvent kinetic isotope effects on the individual reaction stages are reported. The rate of solvolysis of the phenol is much greater than would be predicted by the use of conventional linear free-energy parameters.

Naphthalene tetrachlorides and related compounds. Part 11. trans-1,1,3,4-Tetrachlorotetralin-2-one, its hydrate, and related compounds

X-Ray crystallographic analysis of 1,1,3,4-tetrachlorotetralin-2,2-diol has shown that this compound is the trans-isomer, which can be reversibly dehydrated to give trans-1,1,3,4-tetrachlorotetralin-2-one of previously unknown configuration, produced nearly quantitatively by chlorination of 2-naphthol in acetic acid. The 1H and 13C spectra of these compounds are reported; most of the angular-dependent coupling constants accord with expectations, the value of 3J(13C-1, 1H-3) being low, probably because the coupling path includes a carbonyl carbon atom. The rather high value of 3J(H-3, H-4) is discussed. The properties of some related ketodichlorides and ketotetrachlorides are reported.