Aluminum complexes 1,3-diketones

Figure 6. Reflection spectrum of an oxidized aluminum film exposed to 0.01 OM 2,4-pentanedione in ethanol. Also shown are the peak positions for transmission spectra of the pure diketone (AcAcH) and the aluminum complex (Al(AcAc)s).

Aluminum /3-diketonates have been much studied by nmr methods because of their stereochemical nonrigidity. Aluminum(III), Ga111, and Inm form stable complexes with di-, tri-, and hexadentate chelating ligands in which the metal is octahe-drally coordinated. These complexes are stable under physiological conditions. Complexes of the radioisotopes 67Ga (y, t1/2 = 3.25 d), Ga (/3+, = 68 min), and... 

A systematic study of the kinetics of vaporization of Al(acac)3 along with fluori-nated aluminum /3-diketonate complexes, Al(tfac)3 (5) and Al(hfac)3 (6), has been reported, and the saturation vapor pressures determined at 75-175°C [106]. 

Selective reduction of ketones.1 This reagent can be used to effect selective reduction of the more hindered of two ketones by DIBAH or dibromoalane. Thus treatment of a 1 1 mixture of two ketones with 1-2 equiv. of 1 results in preferential complexation of the less hindered ketone with 1 reduction of this mixture of free and complexed ketones results in preferential reduction of the free, originally more hindered, ketone. An electronic effect of substituents on a phenyl group can also play a role in the complexation. This method is not effective for discrimination between aldehydes and ketones, because MAD-complexes are easily reduced by hydrides. MAD can also serve as a protecting group for the more reactive carbonyl group of a diketone. The selectivity can be enhanced by use of a more bulky aluminum reagent such as methylaluminum bis(2-f-butyl-6-( 1,1-diethylpropyl)-4-methylphenoxide). 

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... 

Benzil (Bz-Bz) and other a-diketones react (i) with the aluminum hydride complex 208 to afford 209 (the diketone is reduced by the hydrido ligand) , (ii) with Green s GaF (predominately consisting of the salt, Ga(I)2[Ga(II)2l6]) to give 210 ° or (iii) with stannylenes 188 to afford 211 . ... 

The concept of in situ protection of the less hindered or more Lewis basic of two ketones to enable selective reduction of the usually less reactive groups has been successfully developed. The sterically hindered Lewis acid MAD (78) derived from BHT and trimethyl aluminum was used to coordinate preferentially to the less hindered ketone and DIBAL-H reduced the more hindered ketone that remained un-complexed. An approximate order of comparative reactivity for various classes of ketones has been established. The selectivity was improved by using the more hindered Lewis acid MAB (79) and/or di-bromoalane as the reducing agent. The discrimination between aromatic ketones is good but less successful between two dialkyl ketones. The chemoselectivity was demonstrated in the reduction of diketone (80) to keto alcohol (81) in 87% yield and excellent selectivity (equation 20). 

Tris[as-(diacetyltetracarbonylmanganese)] aluminum is preparedreadily by treating acetylpentacarbonylmanganese with 1 molar-equivalent of methyllithium at 0° followed by the addition of t/a molar-equivalent of anhydrous aluminum chloride. This complex is isostructural with tris(2,4-pentanedionato)aluminum (where 2,4-pentanedione = acetylacetone) except that the methine group is replaced formally by a Mn(C0)4 group, which suggests that the title compound is one example of a metallo-j3-diketonate type complex. 

This procedure has been shown recently to be a general preparative method for the synthesis of a large variety of metallo-/3-diketonate complexes of aluminum. In each case a diacylmetalate anion is prepared from an acyl complex and is then complexed to the aluminum atom. The preparation of the metallo-jS-diketonate complex presented here utilizes acetylpentacarbonylmanganese as the acyl complex. The preparation of this acetyl complex from acetyl chloride and sodium pentacarbonylmanganate(l-) is provided, also. 

One of the most versatile classes of ligands in coordination chemistry is that of the /3-diketonates, of which the most common is the acetylacetonate, (acac), Figure 9.1. The coordination chemistry of this ligand first appears in the literature in work by Combes in 1887-1894. Alfred Werner also published on the chemistry of the acac ligand in 1901. The acac ligand is remarkable in that it forms complexes with virtually any metal, including beryllium, lead, aluminum, chromium, platinum, and gadolinium. 

The complex (2) from (-butyl chloride and aluminum chloride transforms 1,5-diketones (1) into pyryllium salts (3).2... 

Complementary to the acylation of enolate anions is the acid-catalyzed acylation of the corresponding enols, where the regiochemistry of acylation can vary from that observed in base-catalyzed reactions. Although the reaction has been studied extensively in simple systems, it has not been widely used in the synthesis of complex molecules. The catalysts most frequently employed are boron trifluoride, aluminum chloride and some proton acids, and acid anhydrides are the most frequently used acylating agents. Reaction is thought to involve electrophilic attack on the enol of the ketone by a Lewis acid complex of the anhydride (Scheme 58). In the presence of a proton acid, the enol ester is probably the reactive nucleophile. In either case, the first formed 1,3-dicarbonyl compound is converted into its borofluoride complex, which may be decomposed to give the 3-diketone, sometimes isolated as its copper complex.164... 

In the case of the much more labile tris-diketonate complexes of aluminum and gallium, the techniques of study are more complex since it is impossible to isolate even partially resolved samples. The most probable mechanisms for these labile complexes appear to be certain twist processes along with bond rupture to give spy transition states. 

Strongly complexing P-diketones are currently employed to stabilize highly reactive metal alkoxides, such as W(OEt)6 [25]. Aluminum sec-butoxide, modified by ethylacetoacetate (etac), appears to be quite attractive as a precursor for the sol-gel synthesis of multicomponent ceramics, such as cordierite. Al(OsBu)2(etac) is more soluble and less reactive than the corresponding alkoxide [26]. 

In certain cases, the Lewis acidity of transition metal complexes, e.g., those of aluminum, is so low that they are relatively inactive toward epoxy resins, even at elevated temperatures. A series of patents issued to Toshiba Corp. describe the activation of organoaluminum compounds, such as jS-diketones (64), j8-ketoesters (65), and salicylate esters (66), by the addition of organosilane compounds with hydrolyzable groups as epoxy curatives. Excellent room temperature latency with rapid heat cures are claimed. 

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