CpFe 2 -> system

Stabilized carbon nucleophiles (e g. from /3-diketones, jS-keto esters, malonate esters, etc.) can be arylated by substitution for chloride on the arene in arene-Fe-Cp cation complexes (equation 112). A base is necessary, and two heterogeneous systems are favored potassium carbonate in dimethyl formamide (DMF) or potassium fluoride prepared on Celite-545. As usual in the CpFe+ system, detachment of the substituted arene requires somewhat extreme conditions, usually pyrolytic sublimation at 200 °C. ... 

Some transition metal systems M(CO)R react with a wide range of L, including phosphites, phosphines, arsines, stibines, organic amines, iodide, and CO, to mention a few, yielding the corresponding acyls. Other systems, e.g., CpFe(CO)2R (2S), display a marked selectivity toward various L. Certain unsaturated molecules L [SOj (239), CF2=Cp2 (238), inter alia] insert themselves into the M—R bond instead of effecting the reaction shown in Eq. (8). 

Rather limited information is available on how the nature of R affects the rate of CO insertion, all other factors being constant. A generalization that ethylmetal complexes react faster than the corresponding methyl carbonyls derives from investigations on four systems RIr(CO)2(AsPh3)Cl2 (92), RMn(CO)5 (51), CpFe(CO)2R (98), and CpMo(CO)jR (80). When R = Et the reactions with CO or P and As donor ligands proceed at least 6 times faster than when R = Me. 

The ultraviolet light-induced decarbonylation has been successfully effected in several types of transition metal acyl. It is particularly important when applied to those systems which do not eliminate CO under thermal conditions, e.g., CpFe(CO)2COR (141). Most of the work has been of a synthetic rather than mechanistic variety, however. 

In the system [Fe(CO)2Cp]2/55 the main products are FeCp2, the borabenzene derivative 56, and two triple-decked complexes CpFe[/x-(2-EtC4H3BPh)]FeCp and CpFe[/u,-(2-ViC4H3BPh)]FeCp with a central borole ligand (52). 

Table k. Comparative performances of the Fe CCO), CpFe(C0)2l2 and Co2(C0)g-NaY systems (C- selectivities expressed as mole percent of CO converted into the desired product). 

There are no routes yet to homoleptic metal isocyanide anions. If one considers the interesting products obtained from methyl iodide additions to molybdenum (43) and manganese (44) carbonyl isonitrile anions, negatively charged isocyanide complexes should have some interesting chemistry. Also, now that a simple route to [CpFe(CNR)2]2 complexes has been devised (45), the synthesis of the anion [CpFe(CNR)2] could provide a route to a range of products including heterometal-metal bonded systems. 

The intramolecular iron-catalyzed Alder-ene reaction of enynes in the carbocy-clization reaction was recently reported by Furstner et al. (Scheme 9.8) [20], A low-valent cyclopentadienyliron catalyst, specifically the [CpFe(C2H4)2][Li(tmeda)] complex, is a reactive catalyst for enyne cydoisomerization reactions. Bicyclic products, also incorporating large ring systems, are thereby accessible, and the Thorpe-Ingold effect seems to be helpful for these types of reactions. 

The formation of epoxides is a well-investigated synthetic problem and two approaches, either from a double bond system by transfer of oxygen starting from an alkene, or carbene transfer to a carbonyl group, have attracted much interest. The use of the CpFe(CO)2+ fragment was also investigated by Hossain and coworkers with a view to its use for the synthesis of epoxides (Scheme 9.15) [30, 31]. However, the CpFe(CO)2+ fragment not only catalyzes the carbene transfer, but also acts as... 

Apart from relatively simple linear trinuclear structures such as that of the above [CpFe(CO)2]2Zn, Zn[Co(CO)4]2, [Fe(SiR3)(CO)3(PR3)CdBr]2, and related compounds with FeCd(/x-X)2CdFe and Fe—Cd—Fe groups,108 there are cyclic systems such as 15-XXII, 15-XXIII, and 15-XXIV, and a compound containing a Zn404 cubane core, [MeOZnFe(CO)2Cp]4. 

Kinetic studies on the sulfur dioxide insertion have been few and most have utilized liquid SO2 as the reaction medium. The most exhaustive investigation in neat SO2 was conducted on the systems CpFe(CO)2R and various (R Cp)Fe(CO)2R (77, 74, 75) studied cursorily were CpMo(CO)3R, CpRu(CO)2R, RMn(CO)5, and RRe(CO)5 71, 76). Kinetic studies in organic solvents have been confined to a few CpFe(CO)2R complexes 71, 77). Of the many SO2 reactions of the main group metal-carbon systems, only those involving some MesSnR aryl and benzyl compounds were examined kinetically 51, 52, 99). 

Although other metal alkyl systems have not been studied nearly as exhaustively as CpFe(CO)2R, they seem to manifest a parallel behavior. Thus, for CpMo(CO)3R, the reactivity toward neat SO2 at — 48°C to... 

A comparison of the isoelectronic CpFe(CO)2R an CpCr(NO)2R is also noteworthy. When R = CHaPh or Ph, the latter system inserts SO2 much more rapidly than the former (61, 71). This may be a result of the lower formal oxidation state of chromium(O) than of iron(II) and is consistent with an electrophilic nature of the insertion. 

Presented here are mechanistic conclusions derived from the data discussed in the preceding parts of this section. They apply to CpFe-(CO)aR and related coordinatively saturated systems. 

Mechanistic aspects of SO2 insertion reactions have been studied by several groups, espedally for CpFe(CO)2R systems. While the intimate details of the mechanism are still not explained totally, some aspects are well understood. On the basis of kinetic studies, Jacobson and Wojdcki proposed a bimolecular electrophilic process in dilute organic solvents, first order in SO2 and CpFe(CO)2R, to rationalize observed kinetics. The reaction is characterized by large negative zlS values (typically -50 to -60 e.u.), which have been attributed to developing charges in the transition state and to extensive solvation by S02 The proposed mechanism, shown below, involves electrophilic... 

For example, in [CpFe(CO)2(propene)]+, the equilibrium constant is 19 in favor of exo however, in the [CpMo(NO)(CO)(propene)]+ system, the constant is only 1.7. The orientational preference induced by the NO relieves... 

The work of Darensbourg et al. has superseded these early model systems. A series of model compounds with the core unit Fe(CO)2(CN) or Fe(CO)(CN)2 were found to reproduce the unique IR absorption spectra of [FeNi]-hydrogenases very well. The IR spectrum of the [FeNi]-hydrogenase enzyme from D. gigas in the A state exhibits bands at 1947, 2093, and 2083 cm . The IR spectrum of the iron(II) model complex K [CpFe(CO)(CN)2] in acetonitrile exhibits absorption bands at 1949, 2094, and 2088 cm which are assigned to the vco, the symmetric vcn and the asymmetric voj, respectively. The energies and peak-widths at half-maximum of this absorption are sensitive to the oxidation state of the iron center, to the medium and to the counter-ion. Polar media produce broad bands with peak width at half-maximum of the vco band of 17 cm in water. The use of non-polar solvents is required to achieve the narrow (4 cm ) peak width at half-maximum observed for the enzyme. 

Metallocarboxylate anions, in particular, are very effective oxide transfer agents.17,19 Reaction of Li2W(C0)5(C02) with additional C02 results in formation of W(CO)6 and Li2C03, a reaction described as reductive disproportionation. This dianion would transfer oxide to CpFe(CO)3+BF4 labeling studies showed that oxygen, but not carbon, of the C02 ligand was incorporated into the iron product [CpFe(CO)2]2. Intermediate metalloanhydrides have been proposed to rationalize intramolecular oxide transfer from coordinated C02 to coordinated CO in several systems. 

Addition of a variety of nucleophiles to rf-alkene and alkyne ligands has been investigated, particularly when these ligands are complexed with iron and palladium. Reactions shown in equation 8.39 demonstrate well the typical stereochemistry resulting from the trans mode of attack by external nucleophiles on t 2—7t systems.56 Careful analysis of the reaction of amines with ( )- and (Z)-2-butenyl iron complexes (the CpFe(CO)2 group is abbreviated Fp, which is pronounced fip ) showed the stereochemistry to be cleanly anti.51... 

Reaction of Ru3(CO)i2 with excess AIH3 in THF followed by acidification with 1 M H2SO4 yields hydrocarbons (CH4, C2H6, etc.), corresponding to reduction of —30% of the CO ligands originally present d. Similar results are obtained with M(CO)g systems (M = Cr, Mo, W) . Acid decomposition of Mg[CpFe(CO)2]-4THF also yields small amounts of hydrocarbons . In both of these studies, Lewis acid coordination of CO as in 1 is believed to play an important role. 

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