Instability of 2,2-di(pyridin-2-yl)acetic acid. Tautomerization versus decarboxylation

The DFT calculations at the B3LYP level with 6-311G** basis set were carried out in order to reveal whether tautomerization or decarboxylation is responsible for the instability of 2,2-di(pyridin-2-yl)acetic (DPA) and 1,8-diazafluorene-9-carboxylic (DAF) acids. The carboxyl protons in both compounds are involved in the intramolecular hydrogen bonds (the pyridine nitrogen atoms are the hydrogen bond acceptors). Although formation of two intramolecular OH···N hydrogen bonds in the enols of both carboxylic acids enables effective electron delocalization within the quasi rings (···HO − C = C − C = N), only ene-1,1-diol of DAF has somewhat lower energy than DAF itself (ΔE is ca. 7 kcal mol-1). DPA and its enediol have comparable energies. Migration of the methine proton toward the carbonyl oxygen atom (to form enediols) requires overstepping the energy barriers of 55-57 kcal mol-1 for both DPA and DAF. The enaminone tautomers of the acids, formed by migration of this proton toward the pyridine nitrogen atom, are thermodynamically somewhat more stable than the respective enediols. The energy barriers of these processes are equal to ca. 44 and 62 kcal mol-1 for DPA and DAF, respectively. Thus, such tautomerization of the acids is not likely to proceed. On the other hand, the distinct energetic effects (ca. 15 kcal mol-1) favor decarboxylation. This process involves formation of (E)-2-(pyridin-2(1H)-ylidenemethyl)pyridine and its cyclic analogue followed by their tautomerization to (dipyridin-2-yl)methane and 1,8-diazafluorene, respectively. Although the later compound was found to be somewhat thermodynamically more stable, kinetic control of tautomerization of the former is more distinct

‘Oxygen-Consuming Complexes’–Catalytic Effects of Iron–Salen Complexes with Dioxygen

[(salen)FeIII]+MeCN complex is a useful catalyst for cyclohexene oxidation with dioxygen. As the main products, ketone and alcohol are formed. In acetonitrile, [(salen)FeII]MeCN is rapidly oxidized by dioxygen, forming iron(III) species. Voltammetric electroreduction of the [(salen)FeIII]+MeCN complex in the presence of dioxygen causes the increase in current observed, which indicates the existence of a catalytic effect. Further transformations of the oxygen-activated iron(III) salen complex generate an effective catalyst. Based on the catalytic and electrochemical results, as well as DFT calculations, possible forms of active species in c-C6H10 oxidation have been proposed

Halide-Promoted Dioxygenolysis of a Carbon–Carbon Bond by a Copper(II) Diketonate Complex

A mononuclear Cu­(II) chlorodiketonate complex was prepared, characterized, and found to undergo oxidative aliphatic carbon–carbon bond cleavage within the diketonate unit upon exposure to O₂ at ambient temperature. Mechanistic studies provide evidence for a dioxygenase-type C–C bond cleavage reaction pathway involving trione and hypochlorite intermediates. Significantly, the presence of a catalytic amount of chloride ion accelerates the oxygen activation step via the formation of a Cu–Cl species, which facilitates monodentate diketonate formation and lowers the barrier for O₂ activation. The observed reactivity and chloride catalysis is relevant to Cu­(II) halide-catalyzed reactions in which diketonates are oxidatively cleaved using O₂ as the terminal oxidant. The results of this study suggest that anion coordination can play a significant role in influencing copper-mediated oxygen activation in such systems

Anion Effects in Oxidative Aliphatic Carbon–Carbon Bond Cleavage Reactions of Cu(II) Chlorodiketonate Complexes

Aliphatic oxidative carbon–carbon bond cleavage reactions involving Cu­(II) catalysts and O₂ as the terminal oxidant are of significant current interest. However, little is currently known regarding how the nature of the Cu­(II) catalyst, including the anions present, influence the reaction with O₂. In previous work, we found that exposure of the Cu­(II) chlorodiketonate complex [(6-Ph₂TPA)­Cu­(PhC­(O)­CClC­(O)­Ph)]­ClO₄ (<b>1</b>) to O₂ results in oxidative aliphatic carbon–carbon bond cleavage within the diketonate unit, leading to the formation of benzoic acid, benzoic anhydride, benzil, and 1,3-diphenylpropanedione as organic products. Kinetic studies of this reaction revealed a slow induction phase followed by a rapid decay of the absorption features of <b>1</b>. Notably, the induction phase is not present when the reaction is performed in the presence of a catalytic amount of chloride anion. In the studies presented herein, a combination of spectroscopic (UV–vis, EPR) and density functional theory (DFT) methods have been used to examine the chloride and benzoate ion binding properties of <b>1</b> under anaerobic conditions. These studies provide evidence that each anion coordinates in an axial position of the Cu­(II) center. DFT studies reveal that the presence of the anion in the Cu­(II) coordination sphere decreases the barrier for O₂ activation and the formation of a Cu­(II)–peroxo species. Notably, the chloride anion more effectively lowers the barrier associated with O–O bond cleavage. Thus, the nature of the anion plays an important role in determining the rate of reaction of the diketonate complex with O₂. The same type of anion effects were observed in the O₂ reactivity of the simple Cu­(II)–bipyridine complex [(bpy)­Cu­(PhC­(O)­C­(Cl)­C­(O)­Ph)­ClO₄] (<b>3</b>)

Anion Effects in Oxidative Aliphatic Carbon–Carbon Bond Cleavage Reactions of Cu(II) Chlorodiketonate Complexes

Aliphatic oxidative carbon–carbon bond cleavage reactions involving Cu­(II) catalysts and O₂ as the terminal oxidant are of significant current interest. However, little is currently known regarding how the nature of the Cu­(II) catalyst, including the anions present, influence the reaction with O₂. In previous work, we found that exposure of the Cu­(II) chlorodiketonate complex [(6-Ph₂TPA)­Cu­(PhC­(O)­CClC­(O)­Ph)]­ClO₄ (<b>1</b>) to O₂ results in oxidative aliphatic carbon–carbon bond cleavage within the diketonate unit, leading to the formation of benzoic acid, benzoic anhydride, benzil, and 1,3-diphenylpropanedione as organic products. Kinetic studies of this reaction revealed a slow induction phase followed by a rapid decay of the absorption features of <b>1</b>. Notably, the induction phase is not present when the reaction is performed in the presence of a catalytic amount of chloride anion. In the studies presented herein, a combination of spectroscopic (UV–vis, EPR) and density functional theory (DFT) methods have been used to examine the chloride and benzoate ion binding properties of <b>1</b> under anaerobic conditions. These studies provide evidence that each anion coordinates in an axial position of the Cu­(II) center. DFT studies reveal that the presence of the anion in the Cu­(II) coordination sphere decreases the barrier for O₂ activation and the formation of a Cu­(II)–peroxo species. Notably, the chloride anion more effectively lowers the barrier associated with O–O bond cleavage. Thus, the nature of the anion plays an important role in determining the rate of reaction of the diketonate complex with O₂. The same type of anion effects were observed in the O₂ reactivity of the simple Cu­(II)–bipyridine complex [(bpy)­Cu­(PhC­(O)­C­(Cl)­C­(O)­Ph)­ClO₄] (<b>3</b>)