Cyclopentadiene complexes with iron

The [4+2] cycloaddition of chiral a,P-unsaturated ketones, derived from optically pure 2-hydroxy-3,3-dimethylbutyric acid, proceed with high selectivities and in excellent yield at low temperature in the presence of a Lewis acid, e.g. ZnCl2 or BF3-OEt2. Cycloaddition reactions of chiral at iron aciyloyl complexes with cyclopentadiene in the presence of ZnCl2 have been studied by Davies et al... 

Perhaps chemists will be able to mimic nature without duplicating the iron-sulfur-molybdenum structure. For example, a zirconium complex with tetramethyl cyclopentadiene can bind dinitrogen in a manner that breaks the NON bond, as shown below. Treatment with hydrogen gas results in formation of small amounts of ammonia. Although the yields are too low to make this a viable commercial process, researchers hope to make the process more efficient through chemical modifications and changes in conditions. 

The reactions of a,a -dibromoketones with iron carbonyls generate oxyallyliron complexes (75). These undergo cycloaddition with cyclopentadiene and furan, but with thiophene only products of electrophilic attack are obtained (78JA1765). Thus the oxyallyliron complex (75 R = Me R = H) reacts with thiophene to produce (76) in 37% yield. 

A titanium complex derived from chiral /V-arencsulfonyl-2-amino-1 -indanol [20], a cationic chiral iron complex [21], and a chiral oxo(salen)manganese(V) complex [22] have been developed for the asymmetric Diels-Alder reaction of oc,P-unsaturated aldehydes with high asymmetric induction (Eq. 8A.11). In addition, a stable, chiral diaquo titanocene complex is utilized for the enantioselective Diels-Alder reaction of cyclopentadiene and a series of a.P Unsaturated aldehydes at low temperature, where catalysis occurs at the metal center rather than through activation of the dienophile by protonation. The high endo/exo selectivity is observed for a-substituted aldehydes, but the asymmetric induction is only moderate [23] (Eq. 8A. 12). 

There is a certain analogy between the aromatic anions of cyclopentadienide (C5H5 ) and boratabenzene (C5H6B ). l-Methylbora-2,5-cyclohexadienehas a more acidic proton connected to the, sp3-hybridized ring carbon atom than cyclopentadi-ene, due to the same tendency of aromatic anion formation [252, 253]. The related 1-phenyl-1,4-dihydroborabenzene affords the lithium salt of 1-phenylborataben-zene on treatment with tert-butyllithium. Like metallic complexes such as ferrocene formed by cyclopentadiene, boratabenzene also forms such sandwich -complexes with iron and cobalt. The iron complex can be acetylated under Friedel-Crafts conditions. 

Enantioselective Diels-Alder reaction. Highly stereoselective Diels-Alder reactions can be achieved by use of the 4,4 -diphenylbis(oxazoline) 2b, prepared from (+)-phenylglycinol, as a chiral, bidentate ligand for iron salts. Thus reaction of Fel3 with 2b and I2 in CH3CN forms a complex presumed to be I-Fel3, which can catalyze reaction of 3-acryloyl-l,3-oxazolidin-2-one with cyclopentadiene at —50° to give the endo-adduct in 95% yield. The product is the 2R-enantiomer (82% ee). 

While the mercury compound shows localized N—Hg bonds, the TADB-system (which is isoelectronic with cyclopentadiene) forms a low-spin complex with iron. This was the first identified example of a sandwich-compounds of a boron-nitrogen ring. The complex is prepared by adding an etheric solution of TADBLi to a suspension of FeCl2 in tetra-hydrofuran. 

Oxidation of cyclobutadienotropone iron complex 282 generates the free ligand 534 (Scheme 140), which can be trapped as an adduct (535) with cyclopentadiene (78AJC1607). 

This latter reaction is exemplified by the reaction of [Fe(PF3)5] with cyclopentadiene, which, in contrast with the analogous iron carbonyl system, gives the stable hydrido complex [Fe(f/5-C5H5)(PF3)2H]. 

Diene complexes containing alkene or diene substituents undergo Diels Alder reactions in good yields. Hetero-Diels Alder reactions have also been reported. Chirahty transfer is observed upon reaction of chiral diene iron tricarbonyl complexes. Reaction of the chiral complex (101) with cyclopentadiene in the presence of a Lewis acid give (102) with a relatively high chirahty transfer from the metal complex (Scheme 162). 

The complexes are isolated, characterized and used as chiral Lewis acids. Dissociation of the labile ligand liberates a single coordination site at the metal center. These Lewis acids catalyze enantioselective Diels-Alder reactions. For instance, reaction of methacrolein with cyclopentadiene in the presence of the cationic iron complex (L = acrolein) occurs with exo selectivity and an enantiomeric excess of the same order of magnitude as those obtained with the successful boron and copper catalysts (eq 3). ... 

As with the pentadienes, the complex formed with cyclopentadiene is dependent upon the iron carbonyl employed. 

The chiral iron(III) Lewis acid 3, derived from an oxazoline ligand, catalyzes Diels-Alder reactions of A -acryloyl-l,3-oxazolidinone (1) and cyclopentadiene (2) with good enantiomeric excess30. Nickel complexes of chiral phosphanes also catalyze Diels-Alder reactions albeit with low enantiomeric excess, not exceeding 15% cc31. Much better results are achieved for cobalt complexes with chiral phosphanes in the presence of a Lewis acid31,32. 

However, under certain conditions C-H activation can also be facile and favorable with first-row transition metals. To illustrate this point, just two examples are given here. In 1952, Miller, Tebboth and Tremaine reported the formation of a very stable volatile complex, when cyclopentadiene was passed over reduced iron at 300 The product, dicyclopentadienyliron (later called ferrocene) obviously is a C H activation product of cyclopentadiene (Eq. 1). Much later, Timms was able to show that the same reaction could also be brought about at a temperature as low as —196 °C. There are many examples of similar reactions in the literature. They... 

Ironcarbonyl compounds are liable to form 7r-complexes with unsaturated compounds such as monoenes, butadiene, cyclobutadienes, and cyclopentadiene. In the case of diene, the two double bonds of 1,5-cyclooctadiene are able to coordinate to the metal, without a sterical strain as shown in Table 15.2. The two double bonds of 1,4-cyclohexadiene are difficult to bond to iron without strain, so 1,4-cyclohex-adiene isomerized to 1,3-cyclohexadiene. The iron atoms have such a high reactivity with diene compounds that the isomerization of the diene occurs [13,19,20,25]. 

Two distinct observations are made with respect to the hydrogenation of olefin 7i-complexes with catalysts such as raney nickel. When the olefin is firmly bound to a metal, complete catalytic hydrogenation is prohibited. Thus 7i-cyclopentadienylcobalt cyclooctatetraene (6-22), 7r-cyclopentadienyl tt-cyclopentadiene rhenium dicarbonyl (6-23), and Tr-cycloheptatriene iron tricarbonyl (6-24), each of which contains at least one uncomplexed double bond, readily undergo hydrogenation of the uncomplexed bonds only. The metal-olefin bond is preserved in each case. 

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