The composition, structure and hydrogen bonding of the β-diketones

Abstract

Proton transfer and hydrogen bonding are two aspects of the chemistry of hydrogen that respectively govern the behaviour and structure of many molecules, both simple and complex, from water to DNA. The fl-dicarbonyls exhibit both of these features, and in ways which have singled them out for detailed study for many years. They provide the best known examples of keto-,~ enol tautomerism, with the advantage of slow proton transfer and high concentrations of the enol tautomers in most cases. These enols are stabilized by intramolecular OHO hydrogen bonds which at various times have been thought of as being centred, linear hydrogen bonds that are somehow incorporated into the delocalised zt system to give "aromatic" systems. Research involving structural, spectroscopic and computational techniques has deepened our understanding of these compounds and changed our picture of them. The hydrogen bonding is surprisingly strong, surprising since it is neither centred, nor linear, nor involved in the ring's delocalized bonding, although it is certainly coupled to it. This review deals with controversies that have surrounded the fl-dicarbonyls and discusses the current view that the enol tautomers are a double-minimum potential well with a low energy barrier. The review ends with a brief took at the fl-thioxoketones, which provide an analogous system based on a heteronuclear SHO hydrogen bond in the enol or enethiol tautomer.