Controlled Release of Perfumery Alcohols by Neighboring‐Group Participation. Comparison of the Rate Constants for the Alkaline Hydrolysis of 2‐Acyl‐, 2‐(Hydroxymethyl)‐, and 2‐Carbamoylbenzoates

Abstract

AbstractA series of 2‐acylbenzoates 1 and 2, 2‐(hydroxymethyl)benzoates 3, 2‐carbamoylbenzoates 4–6, as well as the carbamoyl esters 7 or 8 of maleate or succinate, respectively (see Fig. 2), were prepared in a few reaction steps, and the potential use of these compounds as chemical delivery systems for the controlled release of primary, secondary, and tertiary fragrance alcohols was investigated. The rate constants for the neighboring‐group‐assisted alkaline ester hydrolysis were determined by anal. HPLC in buffered H2O/MeCN solution at different pH (Table 1). The rates of hydrolysis were found to depend on the structure of the alcohol, together with the precursor skeleton and the structure of the neighboring nucleophile that attacks the ester function. Primary alcohols were released more rapidly than secondary and tertiary alcohols, and benzoates of allylic primary alcohols (e.g., geraniol) were hydrolyzed 2–4 times faster than their homologous saturated alcohols (e.g., citronellol). For the same leaving alcohol, 2‐[(ethylamino)carbonyl]benzoates cyclized faster than the corresponding 2‐(hydroxymethyl)benzoates, and much faster than their 2‐formyl and 2‐acetyl analogues (see, e.g., Fig. 4). Within the carbamoyl ester series, 2‐[(ethylamino)carbonyl]benzoates were found to have the highest rate constants for the alkaline ester hydrolysis, followed by unsubstituted 2‐(aminocarbonyl)benzoates, or the corresponding isopropyl derivatives. To rationalize the influence of the different structural changes on the hydrolysis kinetics, the experimental data obtained for the 2‐[(alkylamino)carbonyl]benzoates were compared with the results of density‐functional computer simulations (Table 2 and Scheme 4). Based on a preliminary semi‐empirical conformation analysis, density‐functional calculations at the B3LYP/6‐31G** level were carried out for the starting precursor molecules, several reaction intermediates, and the cyclized phthalimides. For the same precursor skeleton, these simple calculations were found to model the experimental data correctly. With an understanding of the influence of structural parameters on the rate constants obtained in this work, it is now possible to influence the rates of hydrolysis over several orders of magnitude, to design tailor‐made precursors for a large variety of fragrance alcohols, and to predict their efficiency as controlled‐release systems in practical applications.