Solvolysis of Benzyl Azoxytosylate and the Effect of Added Bases and Nucleophiles in Aqueous Trifluoroethanol and Aqueous Acetonitrile

Article abstract of DOI:10.1021/ja00241a025

The rates and products of reaction of benzyl azoxytosylate (1b) in 1:1 (v/v) aqueous trifluoroethanol containing sodium perchlorate, sodium thiocyanate, sodium iodide, sodium bromide, sodium chloride, sodium hydroxide, sodium acetate, perchloric acid, and imidazole (buffered and unbuffered) have been measured at 42 deg C as part of an investigation into its mechanism of solvolysis.Nonbasic solutes give only small rate effects (some rate enhancing, others rate retarding), but, if nucleophilic, they lead to substitution products-the classic evidence of an S_N1 reaction mechanism.After the initial rate-determining fragmentation, about half of the total solvolytic reaction proceeds through an electrophilic benzylic intermediate which is sufficiently long-lived to be trapped by nucleophilic solutes such as thiocyanate and the halide anions to give benzyl thiocyanate and benzyl halides.The other half gives the solvent-derived products benzyl alcohol and benzyl trifluoroethyl ether by a route which is not affected by dilute nonbasic solutes.Sodium acetate, which leads to negligible formation of benzyl acetate, and imidazole lead to the formation of trifluoroethyl tosylate; imidazole also produces N-tosylimidazole.These two base-induced bimolecular reactions involve nucleophilic attack at the sulfur of the tosyl group and involve electronic polarization of 1b in the opposite sense from that in the unimolecular fragmentation.One of the minor products in the presence of bases from the trappable intermediate of the solvolysis reaction is benzaldehyde, which suggests that the intermediate is C6H5CH2ON2⁺, a new type of reactive electrophile.It is not yet certain whether the half of the unimolecular fragmentation reaction which does not proceed through the trappable intermediate involves another electrophilic intermediate which is simply too short-lived to be intercepted by dilute nucleophiles or whether about half of the initial fragmentation is followed by a concerted uncoupled capture of the nascent benzyl cation by solvent.Replacing a small proportion of the trifluoroethanol in the reaction medium by the more nucleophilic ethanol does not have a drastic effect upon the overall course of the reaction, and a very similar mechanism also appears to be operative in aqueous acetonitrile.