Iron heteroatom-substituted

Heteroatom-substituted (Fischer-type) carbene complexes are mostly used as stoichiometric reagents. For this reason only carbene complexes of reasonably cheap metals, such as chromium, molybdenum, tungsten, or iron have found broad application in organic synthesis. 

In 2002, an iron-activated nucleophilic aromatic substitution on a solid phase (resin-bound) was published by Ruhland et al. [89]. They demonstrated its first application by the synthesis of a library of o-, m- and p-heteroatom-substituted... 

Neutral cr-alkyliron complexes are obtained upon reaction of Na[Cp(C0)2pe] (5) with alkyl halides (9) (Scheme 6), and as with Collman s reagent this occurs in an Sn2 fashion with inversion of configuration at the carbon atom. Epoxides also participate in this reaction, but tertiary alkyl halides are poor substrates. Alternatively, complexes (9) may be prepared by reaction of an appropriate metal alkyl with Cp(C0)2peX (6). Typically complexes of this type are prepared in order to gain access to the synthetically useful cationic rf--alkene iron complexes (Section 4.1.2). Also, nucleophilic addition of (5) to heteroatom-substituted alkyl halides (such as methoxymethylchloride or chloromethyl methyl sulfide) affords complexes of type (9) that can be converted to cationic... 

Iron (III) chloride is a common catalyst used in electrophilic aromatic substitutions. In addition to those applications outlined above for the construction of aromatic C-C bonds, such salts have also been used for the introduction of heteroatom-based functional groups at the aromatic ring [47]. 

Many complexes of conjugated ketones are also known, such as the iron tricarbonyl complexes of substituted cyclopentadienones 30), although reaction with chromium hexacarbonyl occurs only if phenyl substituents are available for tt complexing 31). A common difficulty of preparing complexes of heterocyclics is the ability of the heteroatom to form o bonds with the metal. 

The reactivity of five-membered rings with one heteroatom to electrophilic reagents has been quantitatively compared. Table 1 shows that the rates of substitution for (a) formylation by phosgene and V,iV-dimethylformamide, (b) acetylation by acetic anhydride and tin(IV) chloride, and (c) trifluoroacetylation with trifluoroacetic anhydride (71AHC(13)235) are all in the sequence furan > tellurophene > selenophene > thiophene. Pyrrole is still more reactive as shown by the rate for trifluoroacetylation, by the relative rates of bromination of the 2-methoxycarbonyl derivatives (pyrrole > furan > selenophene > thiophene), and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3]+ (Scheme 5) (2-methylindole ss V-methylindole > indole > pyrrole > furan > thiophene (73CC540)). 

Several porphyrin analogues with the pyrrolic nitrogens substituted by heteroatoms have been synthesized, but only the oxa analogues (23 X = O, Y = NH) are reported to form stable metal complexes (Figure 8). 60 Formation of a Zn complex of the thia analogue (23 X = S, Y = NH) requires the presence of a large excess of Zn" ion. An iron complex of dithiaporphyrin (23 X = Y = S) is also known. 

Substitution of complexed dienols (244) or dienol acetates with carbon or heteroatom nucleophiles, in the presence of a Lewis acid, occurs with retention of configuration (Scheme 69). (Alkyl aluminum reagents act as both nucleophile and Lewis acid in this process). This reaction is believed to proceed via stereospecific ionization, with anchimeric assistance from the iron, to generate the transoid pentadienyl cation (247) followed by attack of the weak nucleophile on the face opposite to iron. The cross-conjugated pentadienyl cation can also be generated the substitution of (2-acetoxymethyl-l,3-butadiene)Fe(CO)3 (193) has previously been discussed (Section 6.1.1). 

Substitution of either A1 or Si with various heteroatoms changes acid strength from the extremely weak acidity of borosilicates to the superacid-like strength of certain aluminosilicates. The acid sites of Ga- and Fe-silicates are weaker than those of their Al-analogs [35]. Several shape selective commercial processes use hetwoatom substituted molecular sieve catalysts. Iron-substituted pentasils (Encilite) are used for xylene isomerization and for producing ethylbenzene fi om benzene and ethanol [36,37]. 

Acid mediated elimination of cyclic (dienyl ether)- and (dienol)Fe(CO)2L complexes leads to the formation of (cyclodienyl)Fe(CO)2L cations (equation 26 and 27) . Protonation of (pentadienol)- or (pentadienyl ether)Fe(CO)3 complexes generates the corresponding (pentadienyl)Fe(CO)3+ cations 167 (Scheme 41). Lillya and coworkers have demonstrated that ionization of the hydroxyl substituent occurs with anchimeric assistance from iron, and that isomerization of the initially generated transoid pentadienyl cation 168 to the more stable cisoid cation occurs with retention of configuration about the Cl—C2 bond . The in situ generated transoid pentadienyl cations may also undergo reaction with heteroatom, hydride or carbon nucleophiles to afford substituted ( , -diene)Fe(CO)3 products (169) . Acyclic (pcntadicnyl)MCp cations (M = Rh, Ir) may be prepared by acidic dehydration of (dienol)MCp complexes. ... 

Over the past few years, transition metal-catalyzed C-H functionalization methods for the formation of the C-C bonds have been well explored, many of which have included a handful of impressive strategies and synthetically valuable heterocycles when the starting materials contain heteroatoms. Among them, iron salts showed particular fascination for such purposes. In 2010, Liang and coworkers reported a new iron-catalyzed dual C-H activation between aryl C(sp )-H and vinyl C(sp )-H bond activation to build substituted indoles in the presence of Cu(OAc)2-CuCl2 (Scheme 9.13) [16]. This reaction showed highly functional group tolerance and exhibited different properties from the palladium-catalyzed reactions of this type. 

The addition of various carbon and heteroatom nucleophiles to (bicyclo[5.1.0]-octadienyl)iron cations has been reported. Mostly, the nucleophile attacks at the terminus of the dienyl system to form substituted (bicyclo[5.1.0]octa-2,4-diene)iron complexes. The nucleophile stereoselectively attacks from the rear side of the dienyl system, opposite to the iron atom. The method can be used for the synthesis of cis-2-(2 -carboxycyclopropyl)glycine (CCG-III) (Scheme 4-180). ... 

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