Acetylacetone rings

The acetylacetonate ring is attacked by a variety of sulfur electrophiles (25, 26). For example, reaction of these chelates with thiocyanogen proceeds smoothly at —10° to give high yields of the tristhiocyanochelates (XIII). The chemical reactivity of the thiocyano groups on these rings has not yet been studied. 

Aryl sulfenyl chlorides attack the acetylacetonate ring without catalysis. The resulting aryl sulfide chelates (XV) are unusual in that they form remarkably stable clathrate complexes with benzene (26). 

Friedel-Crafts acylations of the metal acetylacetonate rings are much slower than the electrophilic substitutions described above, probably because of the considerable steric bulk at the reaction site. Furthermore, the strongly acidic conditions during the reaction and subsequent hydrolysis step give rise to considerable degradation, particularly in the case of the more sensitive chromium and cobalt chelates. This consideration places severe limitations on the reaction conditions that can be employed. 

Rhodium acetylacetonate differed considerably from the other metal chelates in the acetylation reaction (26). Under the same conditions that had given extensive acetylation of the cobalt and chromium acetylacetonates, the rhodium chelate reacted very slowly and formed only a small amount of the monoacetylated compound (XX). Fortunately, the hydrolytic stability of rhodium acetylacetonate is such that the Friedel-Crafts reaction can be carried out under vigorous conditions that would rapidly degrade the chromium and cobalt chelates. Thus treatment of rhodium acetylacetonate with acetyl chloride and aluminum chloride in dichloroethane afforded the mono- and diacetylated chelates (XX and XXI). No triacetylated chelate was isolated from this reaction. In a similar manner butyryl-and benzoyl-substituted rhodium chelates (XXIII and XXIV) have been prepared. These and other experiments indicate that the rhodium acetylacetonate ring is less reactive than the cobalt or chromium rings. 

Attempted alkylations of these acetylacetonate rings using benzyl and allyl chloride resulted only in the recovery of the starting material. 

The substitution of the remaining acetylacetonate rings provided additional evidence of the assigned aldehyde chelate structures and opened the way for the preparation of a number of unsymmetrically substituted chelate rings. 

In the search for a reactive functional group which could be substituted on the acetylacetonate ring, chloromethylation of these chelates was attempted. The initially formed products were too reactive to be characterized directly. Treatment of rhodium acetylacetonate with chloromethyl methyl ether in the presence of boron trifluoride etherate afforded a solution of a very reactive species, apparently the chloromethyl chelate (XXX) (26). Hydrolytic workup of this intermediate yielded a polymeric mixture of rhodium chelates, but these did not contain chlorine On the basis of evidence discussed later on electrophilic cleavage of carbon from metal chelate rings and on the basis of their NMR spectra, these polymers may be of the type shown below. Reaction of the intermediate with dry ethanol afforded an impure chelate which is apparently the trisethyl ether (XXXI). Treatment of the reactive intermediate with other nucleophiles gave intractable mixtures. 

Several of the unusual chemical properties of functional groups on metal acetylacetonate rings may be explained in terms of the considerable steric hindrance afforded the central carbon of the chelate ring by the flanking methyl groups. To examine this hypothesis the preparation of chelates of formyl acetone (XXXVIII) and malonaldehyde (XXXIX) was undertaken. 

Figure 11-4 The trimeric structure of nickel(II) acetylacetonate. The unlabeled circles represent O atoms and the curved lines connecting them in pairs represent the remaining portions of the acetylacetonate rings.

The first example of a reaction in which a metal acetylacetonate ring was substituted without metal-ligand bond rupture was reported in 1925 (171). The treatment of tris(2,4-pentanediono)chromium(III) with bromine in chloroform gave the tribromo compound ... 

Although the aldehyde group substituted in an aromatic nucleus is quite reactive, it was found to be surprisingly unreactive when substituted in a metal acetylacetonate ring. Positive Fehling and Tollens tests were given by these formylated compounds, but all attempts to oxidize these aldehyde groups on a preparative scale were unsuccessful (54). 

Fig. 25-G-2. Sketch indicating the trimeric structure of nickel acetylacetonate. The unlabelled circles represent oxygen atoms, and the curved lines connecting them in pairs represent the remaining portions of the acetylacetonate rings. [Reproduced by permission from J. C. Bullen, R. Mason and P. Pauling, Inorg. Chem., 1965, 4, 456.]...

Addition of hexafluorobut-2-yne to the co-ordinated cyclo-octadiene in [Rh(cod)Cl]2 produces a new tricyclic system. Two moles of hexafluorobut-2-yne react with [Ir(acac)(cod)], one molecule inserting itself between the metal atom and an olefinic carbon to produce an iridiacyclopentene ring, the second molecule adding 1,4 to the iridium acetylacetonate ring. ... 

Tetrafluoroethylene reacts with Fe(CO)3(butadiene) by a two-step oxidative addition process to give (43). The reaction of the rhodium(i) complex Rh(acac)(cod) with hexafluorobut-2-yne results both in trimer-ization of this alkyne, and either its insertion into a y-bonded acetyl-acetonate intermediate or a concerted addition to the acetylacetonate ring, to produce (44), RhCl3,3H20 reacts with 2-methylallyl alcohol, in the presence of 4-methylpyridine (pic), probably via two insertion reactions, to... 

---