Cyclopentadienones, iron complexes

The reaction of two alkynes in the presence of pentacarbonyliron affords via a [2 + 2 + 1]-cycloaddition tricarbonyl(ri4-cyclopentadienone)iron complexes (Scheme 1.6) [5, 21-23]. An initial ligand exchange of two carbon monoxide ligands by two alkynes generating a tricarbonyl[bis(ri2-alkyne)]iron complex followed by an oxidative cyclization generates an intermediate ferracyclopentadiene. Insertion of carbon monoxide and subsequent reductive elimination lead to the tricarbonyl(T 4-cyclopentadienone)iron complex. These cyclopentadienone-iron complexes are fairly stable but can be demetallated to their corresponding free ligands (see Section 1.2.2). The [2 + 2 + l]-cycloaddition requires stoichiometric amounts of iron as the final 18-electron cyclopentadienone complex is stable under the reaction conditions. 

A solution of the tricarbonyl(r -cyclopentadienone)iron complex (96.6 mg, 0.239 mmol) in acetonitrile (120 mL) is irradiated using a 150 W medium-pressure mercury lamp (Heraeus TQ 150, Pyrex filter) at -30 °C for 45 min with concomitant injection of argon into the solution. Then, air is injected into the cold purple solution for 5 min. Filtration through a short path of Celite, evaporation of the solvent, and flash chromatography of the residue on silica gel provides the free ligand as yellow needles mp 26.5-27 57.5 mg (91%). ... 

Using this cycloaddition strategy, a series of differently substituted tricarbonyl (T] -cyclopentadienone)iron complexes has been prepared and tested as catalysts for the oxidation of alcohols to ketones. [2+2+1] Cycloaddition of 1,8-diyne derivatives promoted by pentacarbonyliron in an carbon monoxide atmosphere affords functionalized hydroazulenes as exemplified in Scheme 4-9. These compounds constitute a structural... 

In order to get unsymmetrically substituted monocyclic (cyclopentadienone)iron complexes, one may temporarily connect two different alkynes via a silyl ether tether to... 

Cyclopentadienone iron alcohol complexes like 37 were generated from the reactions of [2,5-(SiMe3)2-3,4-(CH2)4(ri -C4COH)]Fe(CO)2H (36) and aromatic aldehydes [47]. This process can be used for the iron-catalyzed hydrogenation of aldehydes (Fig. 18 and Fe-H Complexes in Catalysis ). 

Casey CP, Guan H (2009) Cyclopentadienone iron alcohol complexes synthesis, reactivity, and implications for the mechanism of iron-catalyzed hydrogenation of aldehydes. J Am Chem Soc 131 2499-2507... 

The reaction between acetylenes and ruthenium carbonyls produces a series of n complexes with cyclic ligands which, as in the iron system, have either the metal or a CO group incorporated into the ring. Accordingly, 3-hexyne 536) and hexafluoro-2-butyne 90) react with Ru3(CO)i2 to give the (substituted cyclopentadienone)tricarbonylruthenium complexes with structures presumably comparable to those of the iron complexes (93-95). Although diphenylacetylene will not react directly with Ru3(CO)i2 to produce this type of complex 536), it can be prepared 90) by treating Ru3(CO)i2 with tetracyclone in benzene under reflux. 

In 2014, Feringa, Barta and co-workers reported an IV-alkylation reaction of amines with aliphatic alcohols catalyzed by an iron cyclopentadienone complex 16 [130]. A catalytic cycle was proposed based on in situ NMR studies (Scheme 25). Complex 16 was first transformed to the active complex 17 by addition of oxidant MejNO. Complex 17 was then reduced to 18 by the alcohol. 16 and 18 acted as the catalysts to dehydrogenate the alcohol and hydrogenate the imine intermediates. In 2015, Zhao and co-workers realized an AgF-assisted amination of secondary alcohols using complex 16 as the catalyst [131]. In the same year. Wills and coworkers demonstrated a similar iron complex for A-alkylation of amines with alcohols [132]. 

The iron carbonyl promoted cyclocarbonylation of 2-hydroxy-a, -diynes to give (i) -cyclopentadienone)iron tricarbonyl complexes such as (30) has been described i. ... 

Cyclopentadienones are formed in a [2+2+1] cycloaddition reaction of diynes with homoleptic carbonyliron complexes. This method is described in more detail in Section 2.1.2. The majority of (diene)iron complexes are obtained by addition of... 

Substituted cyclopentadienones react with iron carbonyls to form stable, diamagnetic 7r-co triplexes of the type [Fe(CO)3(cyclopentadienone)] (215). The proposed structure is shown in (XX). These complexes undergo reactions typical of metal carbonyls, e.g., displacement of carbon monoxide by tertiary phosphines, but the carbonyl group of the ligand does not show reactions characteristic of a keto-group. These complexes are also formed by interaction of acetylenes with iron carbonyls (see Section VI,C). Interaction of tetracyclone and Fe3(CO)i2 gives unstable complexes which contain the sandwich anion [Fe(tetracyclone)2]2 analogous to the anion (XXV) (215). 

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. 

In the case of tt complexes of substituted cyclopentadienones, such as the iron tricarbonyl derivatives prepared by Weiss and H libel (30), qualitative molecular-orbital theory (20) predicted a considerable reduction of the ketonic carbonyl bond order. It was observed that the ketonic carbonyl frequency dropped by as much as 65 cm-1, in agreement with theory. A similar explanation can also be provided in terms of valence bond theory (Fig. 14). It has been suggested that n complexing of arenes such as benzene results in loss of aromaticity of the ring in contrast to the dicyclopentadienyl... 

A problem is that the Pauson-Khand reaction uses two equivalents of cobalt. More efficient versions, many of them catalytic, using other metals have been developed. These include carbonyl complexes of titanium, molybdenum, tungsten (Scheme 7.15), rhodium and ruthenium (Scheme 7.16). Rhodium, iridium and iron (Scheme 7.17) have also been used with two alkynes to give cyclopentadienones, often as complexes 7.59. A version of the Pauson-Khand reaction employing a nickel catalyst and an isonitrile in place of CO has been developed. The product is an imine, which can be hydrolysed to a cyclopentenone. 

Iron cyclopentadienone complexes have recently emerged as alternatives to Ru(II) catalysts, although their main focus is on ketone reduction rather than imine reduction [117]. 

Scheme 25 Proposed mechanism for iron cyclopentadienone complexes-catalyzed Al-alkylation...

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