Chromium hexacarbonyl, reaction with iron complexes

Cyclopentadiene iron tricarbonyl has been prepared and decomposes thermally to the binuclear carbonyl [a -C5H5Fe(CO)2]2 [26o]. The binuclear iron complex may further react with cyclopentadiene or thermally decompose ( 200°) [27, 28] to give ferrocene. Monosubstituted ferrocenes may be prepared by the former reaction [27]. Chromium hexacarbonyl and cyclopentadiene at 280-350° react to give chromocene [29] the reaction is reversible since treatment of chromocene with carbon monoxide under pressure affords chromium hexacarbonyl, together with intermediate products such as [jr-CpCr(CO)3]2, [jr-Cp2Cr][3r-CpCr(CO)3] and, when hydrogen is also present, the cyclopentenyl complex jr-CsH5CrC5H7(CO)2, 4.1, is formed [30, 31, 32]. 

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. 

Generally the reaction of unsaturated aldehydes (aromatic, olefmic and acetylenic) with chiral boronates has provided homoallylic alcohols in low to moderate enantioselectivity [124]. However, the enantioselectivity of the allyl- and 2-bu-tenylborations of benzaldehyde and unsaturated aldehydes is significantly improved when a metal carbonyl complex is utilized as the substrate [131]. For example, the reaction of iron carbonyl-complexed diene 225, chromium carbonyl-complexed benzaldehyde 226 and dicobalt hexacarbonyl-complexed acetylene 227 all give significantly increa.sed allyl and 2-butenylboration selectivities compared to the parent aldehydes (Fig. 10-6). In the case of chiral substrates 225 and 226, these species can be obtained in enantioenriched form by kinetic resolution by use of the asymmetric allylboration reaction. 

The reaction, of metal carbonyls with 1,3-diketones generally results in a complete displacement of carbon monoxide accompanied by oxidation of the metal to yield 1,3-diketonato complexes. For example, iron pentacarbonyl, chromium hexa-carbonyl, and molybdenum hexacarbonyl afford FefCgHjOOs,1 Cr(CsHr02)8,2 and Mo(CgH702)s,2,s respectively, when allowed to react with 2,4-pentanedione. 

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