Dynamic mixtures obtained by reversible covalent acylhydrazone formation of fragrance aldehydes and/or ketones and a hydrazide in water were found to be efficient delivery systems for the controlled release of highly volatile organic molecules.

Delivery systems generated by reversible hydrazone formation from hydrazine derivatives (see Fig. 1) and carbonyl compounds in H 2 O efficiently increase the long-lastingness of volatile aldehydes and ketones (R 1 R 2 C¼O) in various perfumery applications. The hydrazones are usually obtained in an (E) configuration at the imine double bond (NHN¼C) and, in the case of aliphatic acylhydrazones R'COÀNHÀN¼CR 1 R 2 (R' ¼ alkyl), as syn and anti conformers with respect to the amide bond (COÀNHN). An average free-energy barrier of ca. 78 kJ/mol was determined for the amide-bond rotation by variable-temperature 1 H-NMR measurements ( Fig. 2). In the presence of H 2 O, the hydrazone formation is entirely reversible, reaching an equilibrium composed of the hydrazine derivative, the carbonyl compound, and the corresponding hydrazone. Kinetic measurements carried out by UV/VIS spectroscopy showed that the same equilibrium was reached for the formation and hydrolysis of the hydrazone. Rate constants are strongly pH-dependent and increase with decreasing pH ( Table 1). The influence of the hydrazine structure on the rate constants is less pronounced than the pH effect, and the presence of surfactants reduces the rate of equilibration (Tables 1 and 3). The full reversibility of the hydrazone formation allows to prepare dynamic mixtures by simple addition of a hydrazine derivative to several carbonyl compounds. Dynamic headspace analysis on dry cotton showed that the presence of a hydrazine derivative significantly increased the headspace concentrations of the different carbonyl compounds as compared to the reference sample without hydrazine ( Table 4). The release of the volatiles was found to be efficient for fragrances with high vapor pressures and low H 2 O solubility. Furthermore, a special long-lasting effect was obtained for the release of ketones. The simplicity of generating dynamic mixtures combined with the high efficiency for the release of volatiles makes these systems particularly interesting for practical applications and will certainly influence the development of delivery systems in other areas such as the pharmaceutical or agrochemical industry.

Dynamic mixtures generated by reversible aminal formation of fragrance aldehydes with N,N-dibenzyl alkyldiamines in aqueous systems were found to be suitable delivery systems for the controlled release of bioactive volatiles.

The light-induced controlled release of fragrances from photolabile 2-oxo-(2-phenyl)acetates via Norrish Type II photofragmentation was evaluated by irradiation of the precursors in different solvents and on cotton in a typical fabric softener application. The desired photooxidation was found to work efficiently in water-based systems, and it tolerates the presence of oxygen. The formation of a certain amount of alcohol besides the desired aldehyde or ketone was attributed to further reaction of the photochemically released carbonyl compound, rather than to ester hydrolysis in an aqueous environment.

The use of alkyl phenyl ketones as delivery systems for the controlled release of fragrances was investigated by photoirradiation of undegassed solutions with a xenon lamp as well as natural sunlight. A large variety of precursor compounds was prepared efficiently in a few reaction steps from commercially available starting materials. The Norrish type II photofragmentation was found to be the predominant reaction pathway to yield the desired perfumery alkenes and acetophenones in polar and apolar solution. Systematic GC-MS analysis of the irradiated solutions allowed identification of a series of side products that are due to the presence of oxygen. A detailed analysis of the product distribution after irradiation was carried out for a series of 4-alkoxy-1-phenylbutanone derivatives. Besides the expected acetophenones, vinyl ethers and phenylcyclobutanols, the formation of alkyl formates, alcohols and 4-oxo-4-phenylbutanoates was observed. The product distribution as influenced by solvent polarity, precursor concentration and substituent effects was investigated. The utility of alkyl phenyl ketones as precursors for the light induced controlled release of fragrances under natural daylight conditions was also demonstrated.