Chromium complexes ferrocenyl

Ferrocenyl ligands, via zinc reagents, 9, 120 Ferrocenylmethyl phosphonium salts, with gold(I), 2, 274 Ferrocenylmonophosphine, in styrene asymmetric hydrosilylation, 10, 817 Ferrocenyl oxazolines, synthesis, 6, 202 Ferrocenylphosphines with chromium carbonyls, 5, 219 in 1,3-diene asymmetric hydrosilylation, 10, 824-826 preparation, 6, 197 various complexes, 6, 201 Ferrocenylselenolates, preparation, 6, 188 Ferrocenyl-substituted anthracenes, preparation, 6, 189 Ferrocenyl terpyridyl compounds, phenyl-spaced, preparation 6, 188 Ferrocifens... 

Thomas and co-workers59 have described the oxidation of methylthio-sub-stituted tricarbonyl-(ri6-arene)chromium(0) complex 26 with CHP in the presence of Ti(0-j-Pr)4/DET/H20 combination to give the corresponding sulfoxide in 90-95% ee, in a similar approach to that used by Kagan60 for the oxidation of aryl ferrocenyl sulfide 28 (Scheme 9). 

The octahedral complex Cr[CH3C(0)CHC(0)Fc]3 displays a single ferrocenyl oxidation at -1-0.58 V, vs. SCE (CH2CI2 solution), which is lower by about 60 mV than that of the free l-ferrocenyl-l,3-butanedionato ligand, thus suggesting that the chromium(III) diketonate donates electron density to the ferrocenyl appendices . As a consequence, the Cr(in) Cr(II) reduction, which is partially overlapped by the solvent discharge in Cr(acac)3, cannot be detected. The simultaneous oxidation of the three ferrocenyl units means that they are electronically isolated from each other. 

The absolute configurations of the product alcohols of acyclic ketone reductions can be reliably predicted for many oxidoreductases by using the Prelog rule (Scheme 6). More complex models are required for cyclic ketones, as will be seen later. For a given substrate, oxidoreductases are usually available that deliver hydride equivalents from opposite enantiotopic faces. This can be exploited to produce either alcohol enantiomer at will, as noted in Scheme 7 by the reduction of (13) to either (R)- or (S)-(14). Organometallic ketones can also be reduced with enantiotopic specificity. This is demonstrated by the conversions (Scheme 8) of the ferrocenyl and chromium carbonyl ketones (15) (16) and (17)... 

Scheme 7-1 shows a series of ferrocenyl-benzenetricarbonylchromium(i) complexes that have been studied by electrochemistry, in which the chromium atom assumes a pseudotetrahedral geometry. 

Table 7-2. Half-wave potentials (m. SCE) for the sequential oxidation of the chromium-ferrocenyl complexes (> -C5H5)Fe[( C5H4- R (> -C6H5)Cr(CO)3] shown in Scheme 7-1...

Like the chromium- and molybdenum-carbene complexes of the type illustrated in Scheme 7-3, the corresponding octahedral tungsten complexes exhibit only a one-electron oxidation, which is thought to be centered on both the ferrocenyl and the tungsten fragments. The redox potentials are summarized in Table 7-10. 

I. 10). Thus aryl ferrocenyl sulfoxides (36) (>99% ce) were prepared by CHP oxidation of (35), in presence of the titanium complex prepared under strictly defined conditions [72]. Similar oxidation was used by Gibson, nee Thomas, et al. for the preparation of sulfinyl substituted tricarbonyl (T -arene) chromium(O) complexes (38), with ee s of up to 86% [73]. It is remarkable that the conditions are mild enough to avoid overoxidation and destruction of the tricarbonylchromium moiety. 

For instance, organometaUic compounds such as hydrolytically labile chromium-tricarbonyl complexes, which are of interest as chiral auxiliary reagents for asymmetric synthesis [201], were easily resolved by PSL (Scheme 3.11) [202, 203]. A remarkable enhancement in selectivity was obtained when the acyl moiety of the vinyl ester used as acyl donor was varied. This concept was successfully employed for the resolution of 1-ferrocenylethanol, which cannot be well resolved via enzymatic hydrolysis due to the lability of 1-ferrocenyl acetate in aqueous systems [204, 205]. 

The same resolution can be efficiently carried out on structurally more complex compounds that are optically active for the helical structure of the molecule, i.e., bis(hydroxymethyl) thiaetherohelicene [160]. In this way, a nearly optically pure helical molecule can be obtained (Scheme 33). This method has been successfully applied also to resolution of organometallic compounds and a few interesting results are shown in Scheme 34. Tiicarbonyl [(ir (6)-cycloheptatriene)chromium(0)] and ferrocenyl alcohols have been examined as substrates of the lipase-catalyzed acylation, and the reaction can be accomplished in a highly enantioselective manner [161,162]. 

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