Iron-Catalyzed Transformations

SCHEME 5.13 Enantioselective Michael addition/hydrogenation using chiral ruthenium complexes. 

SCHEME 5.14 Fe(in)-catalyzed cascade aza-Cope rearrangement/Maimich cyclization of 2-hydroxyhomoallyl tosylamine. 

SCHEME 5.16 Fe(III)-catalyzed three-component synthesis of qninohnes. 

SCHEME 5.17 Possible mechanism of FeClj-catalyzed three-component coupling/ hydroarylation/dehydrogenation of aldehydes, aUcynes, and anilines. 

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The chemistry of iron has been reviewed in COMC (1982) and COMC (1995)312-314 as well as in Comprehensive Coordination Chemistry 7/.315 More recent reviews cover iron-catalyzed transformations with samarium(ll) iodide,18d the chemistry of tricarbonyliron-diene complexes,316 and iron-catalyzed reactions in organic synthesis in general.317... 

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). 

The oxidative imination of sulfides and sulfoxides via nitrene transfer processes leads to N-substituted sulfilimines and sulfoximines. This reaction is interesting as chiral sulfoximines are efficient chiral auxiliaries in asymmetric synthesis, a promising class of chiral ligands for asymmetric catalysis and key intermediates in the synthesis of pseudopeptides [169]. However, very few examples of such iron-catalyzed transformations have been described. 

Scheme 4-243. Iron-catalyzed transformation of acyl chlorides and di-...

Saturated hydrocarbons are the main constituents of petroleum and natural gas. Mainly used as fuels for energy production they also provide a favorable, inexpensive feedstock for chemical industry [74]. Unfortunately, the inertness of alkanes renders their chemical conversion challenging with respect to selectivity. Clearly, the development of new and improved methods for the selective transformation of alkanes belongs to the central goals of catalysis. Iron-catalyzed processes might be a smart tool for such transformations (for reviews see [75-77]). 

Iron-catalyzed cross-coupling reactions of various acyl chlorides or thioesters with Grignard reagents have been pioneered by Marchese et al. and other research groups.322 These transformations provide general and convenient access to a wide range of ketones and have been further extended to the use of a supported iron(lll) complex.323... 

These indicate two different ways of carbon monoxide conversion both processes are highly exothermic. Iron catalyzes the transformation according to Eq. (3.8),... 

Scheme 8.2 Iron-catalyzed Ferrier transformation of glucals to 2,3-unsaturated glycosides.

So far, only a single report, by Gorman and Tomlinson of an iron-catalyzed DA reaction with inverse electron demand, has appeared [86]. The transformation of a 4-oxobutenoate (43) as a rather electron-poor hetero-1,3-diene and an enol ether as the electron-rich dienophile can be seen as an extreme example of a diastereoselective hetero-DA reaction controlled by an iron catalyst (Scheme 9.32). 

The yields for this transformation are good and the diastereoselectivities are generally excellent however, the scope of the iron-catalyzed DA reaction with inverse electron demand seems to be limited. 

Bedford and coworkers disclosed iron-catalyzed Suzuki-Miyaura-type coupling reactions of benzyl bromides with sodium tetraphenylborate in the presence of 5 mol% 15 as the catalyst and 10 mol% of dianisylzinc as a promoter. No reaction occurred in its absence. The coupling furnished 38-88% yield of 3 (entry 26) [66]. The transformation proceeds probably by initial B-Zn transmetalation. The resulting arylzinc transfers the aryl group to the iron catalyst as in the Negishi couplings above. An aryliron(I) complex was proposed to be formed initially. 

Finally, mention should be made of the use of iron porphyrins as catalysts in the Polonovski reaction. An added feature of this approach is that the yV-oxide can act as an oxygen source for other synthetically useful, iron porphyrin catalyzed transformations. 

Several metal-catalyzed transformations of 2 f-azirines to indoles (e.g., with palladium, rhodium, and iron) are discussed later in their respective chapters. Some excellent reviews of 2 f-azirine chemistry are available [5-7]. 

It should be noted that addition of Lewis acidic salts, such as MgBt2, is critical in order to achieve an effective catalytic transformation when using arylzinc compounds. This observation indicates that the difficult step of the catalytic cycle is the transmetaUation of the aryl group from the zinc reagent to the catalytically active iron complex [42]. While the involvement of an intermediate radical species or a single electron-transfer process is suspected, mechanistic details of these iron-catalyzed cross-coupling reactions remain unclear. 

Group 8 Iron and Ruthenium. Iron is the second most abundant metal in the earth crust (4.7%) and iron compoimds are relatively nontoxic and very cheap and efficient Lewis acid catalysts that recently has been attracted much attention in a great deal of useful organic transformations. Since 1979 (39), iron-catalyzed aldol reactions have been studied extensively (40). Iron porphyrin (41), (bda)Fe(CO)3 and (COT)Fe(CO)3 (42), and FeCla (43,44) were fovmd to be... 

Table 1 Iron salts-catalyzed transformations of persilylated deriva...

Iron salts are easily accessible, inexpensive and abundant and the metal itself is non-toxic. Their use should therefore become attractive from an economic and environmental point of view in a wide variety of carbohydrate transformations, in either stoichiometric applications or as a catalyst. As stated in the introduction, this review concentrates on a few transformations promoted by ferric salts used as Lewis acids in our laboratories and does not present exhaustive work done in carbohydrate chemistry with these salts. Many more other applications have been reported. However, their uses could be far more developed for fast and selective transformations of carbohydrates to useful new molecular constructs. Besides the acidic properties of iron(iii) presented here, iron chemistry is rich and could be particularly fruitful with carbohydrates in generating new types of complexes for regioselective transformations or in carbon-carbon forming reactions based on iron-catalyzed cross-coupling reactions. The glycochemistry community should certainly expect many more useful accomplishments in the near future. 

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