Solvation and reactivity of 2-adamantyl tosylate in fluorinated alcohol-water systems

Abstract

Fluorinated alcohols exhibit solvating properties contrary to those of aprotic dipolar solvents. Thus they slow a strong solvating ability towards anions (high ionizing power Y) and a weak solvating ability towards cations (low nucleophilicity N). A systematic study of commercial fluorinated alcohols has shown [1] that these properties are particularly pronounced in the case of hexafluoroisopropyl alcohol (HFP) which exhibits both the highest ionizing power and the lowest nucleophilicity. The binary mixtures HFPH 2O possess also peculiar solvating properties; moreover such media should promote various reactions of synthetic interest and especially cyclization-solvolysis [2]. The above considerations prompted us to examine the effect of water fraction on the ionizing power of HFPH 2O mixtures, i.e. on their ability to promote the heterolytic cleavage of C-leaving group bond during solvolysis. The results will be compared to those observed in EtOHH 2O and trifluoroethanol (TFE)H 2O systems (since, as a result of tis characteristics [1], TFE is representative of R fCH 2-OH and R fCHOHCH 3 fluorinated alcohols). For each solvent mixture the ionizing power has been determined from Y = log k/k o where k is the rate constant for solvolysis of 2-adamantyl tosylate in the solvent under examination and k o that in a reference medium: 80% v/v, ethanol/water. Hence, owing to its peculiar structure 2-adOTs is a suitable substrate for a meaningful determination of ionizing power: 1), the rate determining step is the cleavage of COTs bond for an extensive range of solvents including fluorinated alcohols 2), the internal ion pair return is negligible. In the light of experimental data it can be noted that HFPH 2O systems appear, at first sight to exhibit an unexpected behaviour. Although H 2O is considerably more ionizing than HFP, a decrease in the ionizing power of these mixtures is observed as water content increases. Consequently HFPH 2O systems show a lower ionizing power than each component taken alone. In sharp contrast, aqueous EtOH mixtures exhibit a ‘normal’ behaviour, since the ionizing power of the medium increases along with the proportion of water (which is more ionizing than EtOH). However Y always remains below the statistical value according to which Y EtOH-H 2O = Y H 2O X H 2O + Y EtOH (1 − X X 2O ). The binary TFE-H 2O mixtures show similar qualitative trend. However, increase in Y with respect to the proportion of water is much less pronounced than in the case of EtOH-H 2O mixtures (though the difference Y H 2O − Y TFE is large). Consequently the ionizing power of the mixture is much lower than the statistical value. The solvent effects upon the rate of ionization of a substrate such as 2-adOTs mainly depend on the ability of the solvent to interact with the leaving group (OTs) via hydrogen bonding [1]. The hydrogen bonding ability of binary alcohol-water systems arises not only from the interactions which appear between their hydrogen bond donor and acceptor sites. We will discuss how the peculiar structure of water (‘polymeric’ structure involving few donor and acceptor sites) and the structure of alcohols (increase of the donor ability and decrease of the acceptor ability in the following order EtOH, TFE, HFP) account for the whole range of Y variations observed.