A kinetic study of the self-degradation of o-carboranylalanine to nido-carboranylalan in solution

The spontaneous degradation of racemic o-carboranylalanine [3-(1,2-dicarbac loso-dodecaboran(12)-1-yl)-2-aminopropanoic acid], a carborane analogue of the amino acid phenylalanine, to the corresponding diastereomeric, racemic pairs of nido-carboranylalanine [containing the dodecahydro-7,8-dicarba-nid o-un-decaborate (-1) cage] has been studied in buffered water-methanol solu tion at three temperatures. The parent compound and the two diastereomeric reaction products were separated off-line by liquid chromatography. The reaction was found to be first order with respect to o-carboranylalanin e. The rate is pH dependent with a broad maximum around pH 5, where the zwi tterion form predominates. Rate constants and activation parameters for the formation of the two nido-compounds were determined. The activation entrop ies close to zero (0 and 3 J K-1 mol(-1), respectively) show that the loss of one boron atom from o-carboranylalanine is a unimolecular process.