Grignard reagents-Iron acetylacetonate

In the course of a study on creation of a library of a great number of hetaryl ketones and related derivatives, Szewczyk et al. <2001AGE216> elaborated a ruthenium-catalyzed transformation of heterocycles with activated C-H bond by reaction with olefins and carbon monoxide. Thus, 253 gave 254, albeit in very poor yield. Synthetically, the more straightforward iron-catalyzed transformation was described by Fiirstner et al. <2002JA13856>. These authors reacted 255 with a Grignard reagent in the presence of Fe(acac)3 to afford the 7-alkyl-substituted derivative 256 in reasonable yield (acac = acetylacetonate). 

Twenty-five years later, a dramatic improvement was reported by Fiandanese, Marchese and coworkers ° . They discovered that excellent yields of ketone were obtained when diethyl ether is replaced by THF. Moreover, iron acetylacetonate is used as a catalyst instead of iron(III) chloride because it is not hygroscopic and easier to handle. The scope of the procedure is very large and the reaction occurs highly chemoselectively under mild conditions (0 °C). It should be noted that excellent yields are obtained from stoichiometric amounts of Grignard reagents (Table 3). 

In 2002, Figad re and coworkers reported the mono-reduction of 2-aryl (or heteroaryl)-1,1-dibromo-l-alkenes (Scheme 23). The reaction is achieved with one equivalent of isopropylmagnesium chloride in the presence of iron(III) acetylacetonate. Pure ( )-alkenyl bromides are obtained. With two equivalents of alkyl Grignard reagent, the mono-substituted product is obtained in moderate yield. 

Yamamoto and coworkers studied the substitution of ally lie phosphates by Grignard reagents in the presence of copper or iron salts. Only the Sn2 product is formed under copper catalysis whereas, in the presence of iron(III) acetylacetonate, the Sn2 product is generally obtained with an excellent selectivity (Scheme 49). It should be noted that aryl-, alkenyl-, aUcynyl- and aUcyhnagnesium halides can be used successfully. 

Ruthenium, the homologue of iron in this group, was also shown to form complexes quite early. Ruthenocene, Ru( 5H5)2, is obtained by treatment of the acetylacetonate of tervalent ruthenium with five times the theoretical quantity of the Grignard reagent (206), or, better, by the action of cyclopentadienyl sodium on ruthenium trichloride in tetrahydrofuran (47). It forms pale yellow scales which sublime at 120° and melt at 200°. Its properties are closely similar to those of ferrocene it is soluble in organic solvents, and in the absence of air is not attacked by bases or by sulfuric or hydrochloric acid. Oxidation converts it into the pale yellow [Ru( 5H6)2] + ion. 

Alkyl di-Grignard reagents can be reacted selectively with aliphatic and aromatic acid chlorides in the presence of tris(acetylacetonate)iron(III) [83]. The best conditions utilized alkyl chlorides in THF. Formation of alcohols is avoided in this one-step catalyzed... 

Base-catalyzed elimination of -acetoxy sulfones is highly stereoselective, leading to ( )-alkenyl sulfones which undergo transition metal-catalyzed coupling with Grignard reagents with retention of configuration to provide a stereoselective synthesis of trisubstituted alkenes. Either nickel(II) acetylacetonate, tris(acetylacetonato)iron(III), or iron(III) chloride can be used as the catalyst (eq 17). ... 

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