Iron complexes alkylation--protonation

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

The base used may be hydroxide, alkoxide, carbonate, alkyl lithium, or alumina (13, 20, 24, 41, 49). The reaction is the reverse of the vinylidene synthesis by protonation of the n-acetylide, and the two complexes form a simple acid-base system. For the iron complexes in Eq. (2), the pK has been measured at 7.78 (in 2 1 thf-H20) (24). 

We considered the possibility of the positive effect of small amounts of water on the rate of the transformation of iron complexes with R4NBr and probably on the parameters SPEH and C in the ethylbenzene oxidation, catalyzed (Fe(III)(acac)3 + I NBr. Outer sphere coordination of H20 molecules may promote the stabilization of intermediate zwitterions L2(L ML1 02 ) and as a consequence the increase in the probability of the regioselective addition of 02 to nucleophilic y-C atom of (acac)- ligand can be expected [14], It is well-known that the stability of zwitterions increases in the presence of the polar solvents [14], The H - bond formation between H20 molecule and zwitterion may also promote the proton transfer inside of the zwitterion followed by the zwitterion conversion into the products via Scheme 1 [15], It is known the cases of the increase in the ratio of alkylation s products on y-C atom of the R4N(acac) in the presence of insignificant additives of water ( 10 3 MOJib/n) as compared to the alkylation s reaction in the non proton solvents [16],... 

A second general approach to (carbene)iron complexes is the removal of a-positioned leaving groups in alkyl congeners. Neutral T -ct-alkoxyalkyliron complexes can be protonated to cause alcohol elimination providing the corresponding cationic (carbene)iron complexes (Scheme 4-53). " ... 

Using an acetoxy function as leaving group a to the (t -diene)iron complex, this reaction has been employed for the synthesis of the C7-C24 segment of macrolactin 1-Ethoxy-substituted T) -cyclohexadienyliumiron complexes can also be obtained by alkylation of (Ti -cyclohexadienone)iron complexes with triethyloxonium cations (Scheme 4-162). Another general approach to ri -dienyliumiron complexes proceeds via protonation of a noncomplexed double bond in [(l-4-Ti)-l,3,5-triene]iron complexes (Scheme 4-163). - ... 

H NMR spectroscopy studies of iron(IIl) a-alkyl and o-aryl porphyrins have been very important in elucidating spin states. Alkyl and most aryl complexes with simple porphyrin ligands (OEP, TPP, or TTP) are low spin, S — I /2 species. NMR spectra for the tetraarylporphyrin derivatives show upheld resonances for the porphyrin pyrrole protons (ca. — 18 to —35 ppm), and alternating upfield and downfield hyperfine shifts for the axial alkyl or aryl resonances. For -alkyl complexes, the a-protons show dramatic downfield shifts (to ca. 600 ppm), upfield shifts for the /3-protons (—25 to — 160 ppm) and downfield shifts for the y-protons (12 ppm). The cr-protons of alkyliron porphyrins are not usually detected as a result of their large downfield shift and broad resonance. These protons were first detected by deuterium NMR in the dcuterated complexes Fe(TPP)CD3 (532 ppm) and Fe(TPP)CD2CDi (562, -117 ppm). ... 

Diazoalkanes are u.seful is precursors to ruthenium and osmium alkylidene porphyrin complexes, and have also been investigated in iron porphyrin chemistry. In an attempt to prepare iron porphyrin carbene complexes containing an oxygen atom on the /(-carbon atom of the carbene, the reaction of the diazoketone PhC(0)C(Ni)CH3 with Fe(TpCIPP) was undertaken. A low spin, diamagnetic carbene complex formulated as Fe(TpCIPP)(=C(CH3)C(0)Ph) was identified by U V-visible and fI NMR spectroscopy and elemental analysis. Addition of CF3CO2H to this rapidly produced the protonated N-alkyl porphyrin, and Bit oxidation in the presence of sodium dithionitc gave the iron(II) N-alkyl porphyrin, both reactions evidence for Fe-to-N migration processes. ... 

The active enzyme abstracts a hydrogen atom stereospecifically from the intervening methylene group of a PUFA in a rate-limiting step, with the iron being reduced to Fe(II). The enzyme-alkyl radical complex is then oxidized by molecular oxygen to an enzyme-peroxy radical complex under aerobic conditions, before the electron is transferred from the ferrous atom to the peroxy group. Protonation and dissociation from... 

Treatment of the potentially electrophilic Z-xfi-unsaturated iron-acyl complexes, such as 1, with alkyllithium species or lithium amides generates extended enolate species such as 2 products arising from 1,2- or 1,4-addition to the enone functionality are rarely observed. Subsequent reaction of 2 with electrophiles results in regiocontrolled stereoselective alkylation at the a-position to provide j8,y-unsaturated products 3. The origin of this selective y-deproto-nation is suggested to be precoordination of the base to the acyl carbonyl oxygen (see structures A), followed by proton abstraction while the enone moiety exists in the s-cis conformation23536. 

Rhenium-acyl complexes, such as 1, are isoelectronic with the iron-acyl complexes discussed above and many reactivity patterns are common to the two groups of compounds. Treatment of complex 1 with strong bases, such as butyllithium or lithium diisopropylamide, results in abstraction of a cyclopentadienyl proton which is followed by rapid migration of the acyl ligand to the cyclopentadienyl ring to produce the metal-centered anion 384. Alkylation of 3generates a metal-alkyl species, such as 4. 

Rapid development of this area followed the discovery of routes to these complexes, either by ready conversion of terminal alkynes to vinylidene complexes in reactions with manganese, rhenium, and the iron-group metal complexes (11-14) or by protonation or alkylation of some metal Recent work has demonstrated the importance of vinylidene complexes in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes (15). Interconversions of alkyne and vinylidene ligands occur readily on multimetal centers. Several reactions involving organometallic reagents may proceed via intermediate vinylidene complexes. 

The decrease of electron density at the iron centers caused by protonating or alkylating thiolate donors strongly shifts the redox potentials of corresponding complexes, which were obtained from cyclic voltammograms. Table III summarizes the results (and assignments) for [Fe(CO)(NnS4)] and its alkylated derivatives (89). 

The a-protons of iron acyl complexes are acidic and these can be deprotonated with Lithium diisopropylamide (LDA) or with n-butyllithimn. Thus the corresponding enolates are readily functionalized and undergo reaction with alkyl halides, aldehydes, disulfides, trimethylsilyl chloride, and epoxides to afford the corresponding a-derivatized products. " Early work on racemic complexes revealed that these transformations occur in a highly diastereoselective fashion,... 

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