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.

Di-μ-chlorobis[hydrogen bis(diphenylphosphinito)(1–)-P,P']dipalladium, [Pd2Cl2{(C6H5)2PO.H.OP(C6H5)2}2]

“…The preparation of (I) from diphenylphosphine oxide and sodium chloropalladate:has . beenreported previously (Dixon & Ra'ttray, 1971 Table 3 and are slightly shorter than those observed in the enols of/3-diketones, for which a distance greater than 2.45/~ is usually found (Emsley, 1984).…”

3,3-Dibenzylpentane-2,4-dione, C19H20O2

“…The stereochemistry of the molecule is determined by the two benzyl substituents on C3. In the solid state the molecule possesses a distorted S conformation (Emsley, 1984). The deviation from the ideal S conformation in which C1-C5, O1 and 02 are in a single plane, is determined by the dihedral angle of 62.1 (2) ° between the plane defined by O1, C1, C2 and C3, and that through 02, C3, C4 and C5.…”

The nature of resonance-assisted hydrogen bonds: a quantum chemical topology perspective

“…[25][26][27][28] The proton donor and proton acceptor groups in RAHBs are connected through a chain of p-conjugated double bonds, and the standard explanation of the noticeable strength of these interactions is based on a chemically intuitive reasoning in which the conjugated bonds display an equalization of lengths through a pseudo-ring. This description of RAHBs is consistent with the deeply rooted idea that the existence of equivalent resonance structures is a stabilizing feature of a molecular system.…”