Acetylacetone complexes, electrophilic

The electrophilic substitutions of acetylacetonate complexes have been taken as suggesting aromatic character in the chelate ring. Results with seventeen different 1,3-diketonatochromium(III) complexes were recently held to support this suggestion (176-178 equation 43).784 The bromination of tris(l,l,l-trifluoro-2,4-pentanedionato)chromium(III), previously claimed to be unreactive,785 has been reported.786... 

Although the methyl groups in acetylacetonate complexes retard some reactions by steric hindrance, they provide some electronic enhancement in reactions with electrophiles and furthermore protect the donor oxygen atoms from electrophilic attack. These properties have been discerned by a comparison of the numerous reactions of acetylacetone complexes with the relatively few successful reactions of complexes of formylacetone and malondialdehyde. 

Electrophilic attack at the co-ordinated ligand has been demonstrated for acetylacetone complexes of platinum and of palladium. Several reactions... 

The bromination of tris(acetylacetonato)chromium(III) was first reported by Reihlen.781 There have been many studies of electrophilic substitution at complexes of both acetylaceton-ate and its derivatives this work has been extensively reviewed.782,783 Some typical reactions are outlined below (equation 42). In this section, we shall briefly mention some more recent work the interested reader is recommended to study the extensive, although somewhat dated, review by Collman,782 and Mehrotra s book.783... 

In the above examples, the nucleophilic role of the metal complex only comes after the formation of a suitable complex as a consequence of the electron-withdrawing effect of the metal. Perhaps the most impressive series of examples of nucleophilic behaviour of complexes is demonstrated by the p-diketone metal complexes. Such complexes undergo many reactions typical of the electrophilic substitution reactions of aromatic compounds. As a result of the lability of these complexes towards acids, care is required when selecting reaction conditions. Despite this restriction, a wide variety of reactions has been shown to occur with numerous p-diketone complexes, especially of chromium(III), cobalt(III) and rhodium(III), but also in certain cases with complexes of beryllium(II), copper(II), iron(III), aluminum(III) and europium(III). Most work has been carried out by Collman and his coworkers and the results have been reviewed.4-29 A brief summary of results is relevant here and the essential reaction is shown in equation (13). It has been clearly demonstrated that reaction does not involve any dissociation, by bromination of the chromium(III) complex in the presence of radioactive acetylacetone. Furthermore, reactions of optically active... 

The electrophilic substitution of P-diketonate complexes appears to occur as for arenes, and a process involving initial coordination of the electrophile, followed by an intramolecular group transfer, has not been observed, although it has been postulated for the reaction of copper(II) acetylacetonate with thioacetals (equation 14).31... 

Neutral Complexes. Interaction of acetylacetone and hydrous Rh2G3 gives the trisacetylacetonate, which has been resolved into enantiomeric forms. It undergoes a variety of electrophilic substitution reactions of the coordinated ligand, such as chlorination. The stereochemistry and racem-ization of the cis- and trans-isomers of the unsymmetrical trifluoroacetyl-acetonate have been studied by nmr spectroscopy the compound is extremely stable to isomerization. 

In 2004, we reported the Cobalt-catalyzed hydrohydrazination of olefins with di-ferf-butyl azodicarboxylate (5) and phenylsilane (Scheme 4.1). Our approach was based on a stepwise introduction of a hydride and an electrophilic nitrogen source, instead of the more classical approach based on electrophilic activation of the olefin followed by addition of a hydrazine nucleophile. This solution to override the inherently low reactivity of aUcenes was first introduced by Mukaiyama for the related Cobalt-catalyzed hydroperoxidation reaction. The introduction of new Cobalt-catalyst 4 was the key for an efficient hydrohydrazination reaction, as the Cobalt-complexes with acetylacetonate-derived ligands used by Mukaiyama promoted direct reduction of the azodicarboxylate. 

Figure 7 illustrates the electrochemial redox chemistry in acetronitrile for several coordination complexes of iron [Fe (MeCN)4, Fe CL, and Fe (acac)s (acac = acetylacetonate)] in relation to that for two iron organometallics [Fe (Cp)2 and Fe (CO)s (iron-pentacarbonyl) both stable 18-electron systems]. In MeCN, Fe (MeCN)4" is the only charged species of the group. It is reversibly oxidized (II/III couple E1/2, -I-1.6 V vs SCE). The uncharged Fe Cb molecule is reversibly reduced (Ill/n couple Ei/2, -1-0.2 V vs SCE) to giveFe Cl, which is reduced by an irreversible two-electron process to iron metal (Ep,c -L5 V vs SCE). The more basic Fe (acac)3 molecule is reversibly reduced (ni/n couple Ei/2, -0.7 V vs SCE), but does not exhibit a second reduction peak. The III/II reduction potentials for these three coordination complexes are a measure of their relative electrophilicity (Lewis acidity). 

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