Iron, carbonyl compounds protonation

The following acid-catalyzed cyclizations leading to steroid hormone precursors exemplify some important facts an acetylenic bond is less nucleophilic than an olelinic bond acetylenic bonds tend to form cyclopentane rather than cyclohexane derivatives, if there is a choice in proton-catalyzed olefin cyclizations the thermodynamically most stable Irons connection of cyclohexane rings is obtained selectively electroneutral nucleophilic agents such as ethylene carbonate can be used to terminate the cationic cyclization process forming stable enol derivatives which can be hydrolyzed to carbonyl compounds without this nucleophile and with trifluoroacetic acid the corresponding enol ester may be obtained (M.B. Gravestock, 1978, A,B P.E. Peterson, 1969). 

In a remarkable synthetic feat, Tard et al. reported in 2005 the constmction of the complete six-iron metallosulfur framework of the [FeFe]-hydrogenase H-cluster shown as complex 2 in Figure 6.5 [104]. However, this compound differs from the enzyme active site in that it does not feature a rotated structure with an open apical coordination site and abridging carbonyl ligand. Such a rotated structure is now thought to be one of the necessary features for efficient catalysis by the iron carbonyl complexes because it promotes protonation at a terminal coordination site instead of a bridging position. As expected, based on these grounds, the model in this experiment is an unremarkable catalyst. 

Due to the low oxidation state of the metal in carbonyliron complexes and ferrates, these species can be applied for the reduction of various carbonyl compounds. Initially, these reagents have been applied in stoichiometric amounts. First examples describe the hydrogenation of a,p-unsaturated carbonyl compounds by carbonyl(hydrido)ferrate complexes to give saturated carbonyl compounds or saturated alcohols. Low valent iron species for the reduction of carbonyl compounds and imines can also be generated in situ from iron(II) chloride and lithium powder in the presence of 4,4 -di-rert-butylbiphenyl. Catalytic versions have been developed subsequently. Thus, pentacarbonyliron functions as a precatalyst for the hydrogenation of aldehydes and ketones in the presence of a tertiary amine as solvent (Scheme 4-322). The catalytically active system probably consists of (tetracarbonyl)(hydrido)ferrate and the protonated amine. ... 

The same ethylidene ruthenium complex, as well as its iron congener, is alternatively obtained through direct protonation of the dimetallacycles 64a (M = Fe) and 64b (M = Ru) (64). In this case, the carbonyl alkyne carbon-carbon bond is broken irreversibly to give the cationic /x, 17s-vinyl complexes 65a and 65b, which undergo nucleophilic attack by hydride (NaBFLi) to produce complexes of methylcarbene (63a,b) (Scheme 21a). Deuterium-labeling experiments prove that the final compounds arise from initial hydride addition to the /3-vinylic carbon of 65. However, isolation of small amounts of the 7j2-ethylene complex 66 indicates that hydride attack can also occur at the a-vinylic carbon (64). 

By reaction of cationic carbonyl complexes with lithium carbanions, neutral acyl complexes are prepared. Whereas treatment of [> -CpFe(CO)3]BF4 with (a) PhLi gives the expected > -CpFe(CO)2 [C(0)Ph] in 80% yield, with (b) MeLi only traces of > -CpFe(CO)2 [C(0)Me] can be detected . This complex and other phosphane-substituted acyl compounds of the type f -CpM(CO)L[C(0)Me] [M = Fe, Ru L = CO, PPh3, P(hex)j], as well as >/ -CpMo(CO)2P(hex)3[C(0)Me] (prepared by different routes), are protonated with and alkylated with [R3 0]BF4 reversibly, yielding cationic hydroxy- and alkoxy(methyl)carbene complexes, respectively . The formation of the ( + )- and ( —)-acetyl complex / -CpFe(C0)(PPh3)[C(0)Me] from the ( + )-and ( —)-conformers of optically active > -CpFe(C0XPPh3)[C(0)0-menthyl] and MeLi occurs with inversion of configuration at the asymmetric iron atom . 

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

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