1.3- diketonates substitution reactions

The reduction of pertechnetate with concentrated hydrochloric acid finally yields the tetravalent state, and no further reduction to the tervalent state takes place. Therefore, the tervalent technetium complex has usually been synthesized by the reduction of pertechnetate with an appropriate reductant in the presence of the desired ligand. Recently, the synthesis of tervalent technetium complexes with a new starting complex, hexakis(thiourea)technetium(III) chloride or chloropentakis(thiourea)technetium(III) chloride, has been developed. Thus, tris(P-diketonato)technetium(III) complexes (P-diketone acetylacetone, benzoyl-acetone, and 2-thenoyltrifluoroacetone) were synthesized by the ligand substitution reaction on refluxing [TcCl(tu)5]Cl2 with the desired P-diketone in methanol [28]. 

When a metal atom donates electron density to a bound ligand, usually by means of Ji-back bonding, electrophilic substitution reactions may be promoted. This is observed then usually with metals in low oxidation states and is therefore prevalent with organometallic complexes - and less with those of the Werner-type, where the metals are usually in higher oxidation states. Nevertheless there have been detailed studies of electrophilic substitution in metal complexes of P-diketones, 8-hydroxyquinolines and porphyrins. Usually the detailed course of the reaction is unaffected. It is often slower in the metal complexes than in the free ligand but more rapid than in the protonated form. 

Diketones substituted on the central carbon atom are less reactive under the above conditions. Satisfactory reaction occurs in pyridine or THF in the latter solvent cyclization accompanies the acylation despite the basic conditions (6OJOCIH0). 

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... 

REACTIONS OF METAL /i-DIKETONATES 15.4.4.1 Substitution Reactions at Carbon... 

Hydrolysis of metal / -diketone complexes is usually just the reverse of the preparative reaction but detailed study of such processes provides considerable insight into the mechanisms of inorganic substitution reactions. 

Hydrolysis (the reverse of the preparative reaction) and other exchange reactions of the metal /3-diketonates have been studied in some detail in order to probe the mechanisms of inorganic substitution reactions. As mentioned in Section 4.1, there has been intense study of the intramolecular rearrangements of M(dike)3 complexes. Decomposition of metal /1-diketonate complexes to give high-purity metals or metal oxides is of technological importance. 

Measurements of optical rotations have been used to follow the course of other substitution reactions (5, 5, 8, 70, 72). The loss or retention of optical activity or inversion during a substitution process gives useful information concerning the mode of attack and the symmetry of intermediates or activated complexes. Studies of racemization and isomerization have led to elucidating the mechanism of stereochemical rearrangements in the fine work of Fay and Piper 20) with metal complexes of unsymmetrical 1,3-diketones. 

Few ring substitution reactions have been reported and these are predominantly benzylic brominations such as that indicated in Eq. (71) for the generation of benzosilepin systems. Multibromination of benz-silacyclopentane has been effected, but attempts to hydrolyze the tetrabromo derivative to the diketone resulted in the expulsion of the silicon heteroatom, in contrast to the reactivity of the six-membered ring [Eq. (69)]. 

Other examples of electrophilic substitution reactions catalyzed by metal ions through mechanisms involving the promotion of proton loss and stabilization of enol forms of reactants include the bromination of /3-diketones ... 

Reactions of 1,3-Diketones. - ab initio Calculations have been carried out in a study of the single bond rotation within 3-hydroxyacrolein. Pulsed-laser photolyis of acetylacetone, in its enol form, shows that hydroxyl radicals are formed." The photochemical reactivity of some 1-acetonaphthones in the presence of piperidinopropenenitrile has shown both addition and substitution reactions. Photochemical ring expansion of ethyl 2,4-dioxo-3-(aryl)-aza-bicyclo[3.1.0]hexane-6-carboxylates afford ethyl N-(aryl)-2,6-dioxo-piperid-3-ene-4-carboxylates in 68-80% yields. ... 

Among the various derivatives of biomass, furan compounds obtained from furfural are important (200 000 t year ). A new family of furan diethers has been obtained by alkylation of 2,5-furandimethanol or furfuryl alcohol under the action of microwave irradiation with PTC solvent-free conditions (Scheme 10.79) [154]. Reaction times were improved by use of microwave irradiation, and the same conditions were extrapolated to the synthesis of a series of new furan diethers by alkylation of furfuryl alcohol by dihalides. 1,4-Diketones substituted by furans or thiophenes were synthesized by conjugate addition of aldehydes to a,j8-unsaturated ketones by irradiation without solvent in the presence of thiazolium halides and DBU adsorbed on alumina [155]. 

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