Tricarbonyl iron protonation

The first ri -allyliron complex was obtained by Emerson and Pettit treating ri -butadiene(tricarbonyl)iron with Bronsted acid such as tetrafluoroboric acid to obtain the coordinatively unsaturated cationic T -allyl(tricarbonyl)iron complex. In the presence of carbon monoxide, tricarbonyl(T -diene)iron complexes can be protonated to give tetracarbonyl(Ti -aIlyl)iron complexes (Scheme 4-77). ... 

Hunt s group (50, 51) have pioneered the application of the Cl source to organometallics such as the iron tricarbonyl complex of heptafulvene, whose electron impact spectrum shows (M—CO)+ as the heaviest ion, in contrast to the methane Cl spectrum with the ion as base peak. Boron hydrides (52) and borazine (53) have also been studied. The methane Cl spectrum of arenechromium and -molybdenum (54) show protonation at the metal giving a protonated parent or molecular ion. Risby et al. have studied the isobutane Cl mass spectra of lanthanide 2,2,6,6-tetramethylheptane-3,5-dionates[Ln(thd)3] (55) and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-oetanedione [H(fod)] lanthanide complexes (56). These latter complexes have been suggested as a means of analysis for the lanthanide elements. 

Recently, in a study of a large number of organo-iron compounds 120), it has been found possible to treat the isomer shift as being due to a sum of partial isomer shifts 8,. from individual ligands which for 7r-cyclopentadienyl derivatives could be correlated with proton NMR chemical shifts in the same compounds. The Mossbauer spectra of two cases of theoretical interest are worthy of mention. First, the spectra of both the mono- and binuclear iron tricarbonyl derivatives of cyclooctatetraene 115) show similar 8 and A values. The small 8 values are consistent with zero oxidation state for the iron atom and essentially complete covalent bonding between the iron and the tt electrons of the ring, so that, at least for the mononuclear... 

A bimetallic iron tricarbonyl cation C8H9Fe2(CO)6 + (IV) is derived from cyclooctatriene (157). The Mossbauer resonance of spectrum of (IV) and the related cycloheptatrienyl-Fe2(CO)6 cation (V) (73) suggest that these cations may be considered as containing a bis(7r-allyl) and an allyl-diene system, respectively. In the second case (V), rapid valence tautomerism is invoked to account for the unique proton NMR signal in solution. 

The tropylium iron tricarbonyl was eventually synthesized in 1964 by protonation of methyl tropyl ether-iron tricarbonyl. 

Similarly protonation of diphenylfulveneiron tricarbonyl generates a substituted 7r-cyclopentadienyl cation (241, 242). Hydride abstraction from cyclopentadieneiron tricarbonyl releases the 77-cyclopentadienyl cation complex (172). The Mossbauer spectra of the [CpFe(CO)3]1 cation and related iron carbonyl cations have been determined (121). 

Tricarbonyl(cyclooctatetraene)iron forms red-brown air-stable crystals which melt at 94°. It is insoluble in water but dissolves readily in organic solvents. It is easily sublimed. X-ray studies indicate that the metal is bound to two double bonds only, and the unsaturation is also shown by the protonation in concentrated sulfuric or other strong acid solutions to give the tricarbonylbicyclo[5.1.0]-octadieniumiron ion, C8H9Fe(CO)3 , and by the formation of a Diels-Alder adduct with tetracyanoethylene. ... 

The binuclear iron complex (C8H8)Fe2(CO)6 had been expected from the reaction, but the chair conformation (XXXV), which was subsequently found for this substance, was entirely unexpected (65j 66j 67). In this complex, each end of the cyclooctatetraene ligand behaves as a butadiene-type (n = 4) ligand, and bond distance measurements indicate very little tt-tt interaction between the two halves of the ring. The proton NMR spectrum of the complex in solution exhibits two resonances of equal intensity, while the infrared spectrum is very similar to the spectrum of butadiene-iron tricarbonyl and similar diene complexes (105). 

Another important development in cyclooctatetraene-metal chemistry has concerned the protonation of C8H8-metal derivatives 59, 221). The complex (C8H8)Fe(CO)3 is readily protonated in strong acids to yield salts of the type [(C8H9)Fe(CO)3] X , where X = Cl, CIO4, or BF4. The proton NMR spectrum indicates the complex contains the bicyclo-[5.1.0]-octadienyl-iron tricarbonyl cation (XXXIX), and this formulation is supported by the observation that when the tetrafluoroborate salt of... 

The mechanism of these substitution reactions can be readily rationalized in a manner which completely parallels the accepted electrophilic mechanism of benzene and other aromatic systems. The electrophile, R", adds to the cyclobutadiene ligand to produce the 7r-allyl-Fe(CO)3 cationic intermediate (XVI) loss of a proton from this intermediate generates the substituted cyclobutadiene -Fe(CO)3 complex. We have previously isolated salts of the 7r-allyl-iron tricarbonyl cation (XVII), as well... 

Synthesis of the parent homotropone 34 was achieved starting from tricarbonyl(cycloocta-tetraene)iron complex 32 via protonation and formation of the bicyclo[5.1.0]octadienylium cation complex 33. Nucleophilic addition of hydroxide and oxidation was followed by oxidative decomplexation with cerium(IV). ... 

Cycloheptatriene does not react with Fe(CO)s to give the expected dicarbonyl complex 7i -C7H8Fe(CO)2, but forms instead a mixture of tricarbonyl complexes derived from cycloheptatriene and cyclohepta-1,3-diene (20, 35). This work, together with protonation and related reactions of the free double bond of 7r-C7HsFe(CO)3, have been summarized by Pettit and Emerson (118). More recently, Pettit and co-workers (101) have shown that (7-methoxycycloheptatriene)Fe(CO)3 is protonated by fluoro-boric acid with loss of methanol to give the ir-tropylium-iron tricarbonyl cation. 

Protonation of cyclobutadieneiron tricarbonyl by fluorosulphuric acid in sulphur dioxide at — 80°C occurred on iron to give a o-n-bonded structure (308), similar in type to that formed by protonation of butadieneiron tricarbonyl. The H and n.m.r. spectra were both in accord with structure (308). In particular, the iron bonded... 

The splitting of the absorption maximum at 327 mp of vitamin A acetate into two peaks on ir-complex formation can be said to be due to a "ir-complex effect". The NMR spectrum of the compound was found to be very complex. However, it was possible to assign all strong peaks to the proposed structure by a comparison with the NMR spectrum of vitamin A acetate. The peaks due to the protons around the six-membered ring of the vitamin A structure kept their position on ir-complex formation. Therefore the six-membered ring may not participate in ir-complex formation with the iron tricarbonyl group. However, the exact location of ir -bonding in the vitamin A structure could not be determined unequivocally. 

This has an interesting consequence with the -q -cyclohexadienyl complex 10.1 (Scheme 10.12). As acylation is initially on iron, the iron and the acetyl group are cis in the -intermediate 10J9. The proton that is lost must be cis to iron (perhaps via transfer to iron in a reverse of the acylation sequence). The more acidic proton, Hb, a- to the newly installed acetyl group is trans to iron and, therefore, not available. The proton lost, therefore, is Ha on the other side resulting in net movement of the diene system to give diene complex 10.40. Better yields are obtained with the more electron rich monotriphenylphosphine complex 10.41 (L = PPhs), than the tricarbonyl complex (L = CO). ... 

Another example of electrophilic chemistry is provided by the myrcene complex 10.48 (Scheme 10.15). The iron tricarbonyl complex 10.48 undergoes cyclization via a carbocation 10.49 on acid treatment. The 16e rr-allyl complex 10.50 produced may then lose a proton (in a way similar to benzene in electrophilic cyclization) to give a V-diene complex 10.51 or gain an additional ligand, CO if supplied, to give a stable ISe-ir-allyl complex 10.52. 

A mechanism for the stereospecific monodeuteration of tricarbonyl(l-car-bomethoxycyclohexa- ,3-diene)iron [Eq. (129)] is suggested in Scheme 11. This mechanism is consistent with all the available data on stereospecific deuterations of variously substituted tricarbonyl(cyclohexadiene)iron complexes (A. J. Birch, B. J. Chauncy, and D. J. Thompson, unpublished results, 1975) and with the large D/H (deuterium/hydrogen) isotope effect, i.e., ratedetermining protonation at Fe (Whitesides and Nielan, 1975). It should be noted that the principle of microscopic reversibility may not strictly apply in... 

The isomerization to the 1,3-form of cyclohepta-1,4-diene co-ordinated to iron was the subject of earlier kinetic investigations and the preliminary communication has been followed up by a full report. Fluxional behaviour in diazepineiron tricarbonyls (37) has been shown to require proton transfer via an intermolecular... 

An -ray structure determination of a cationic intermediate (8) isolated during the acylation of [Fe(CO)8(rraAw, ra j-hexa-2,4-diene)] establishes that Friedel-Crafts acylation involves stereospecific endo attack. The stereochemistry of reaction parallels protonation of tricarbonyl(diene)iron, cyclopentadienyl(cyclohexa-l,3-diene)rhodium complexes, and ( j -cyclopentadienyl)rhodium complexes of limonene (9), a-phellandrene (10), and carvone (11). ... 

T)-tritylcyclo-octatetraene) iron has been prepared in two steps from tricarbonyl octatetraene)iron. Treatment with cerium(iv) salts released free tritylcyclo-octatriene and protonation gave the tricarbonyl(l-5-ri-6-tritylcyclo-octa-2,4,6-trienylium)iron cation (220). Thermolysis at 160 °C gave tricarbonyl[2,3 4,5-Ti-7-tritylbicyclo[4,2,0]-octa-2,4,7-triene]iron. ... 

In 1973, the homo- and copolymerization of it-(2,A-hexa- eay acrylate)tri-carbonyliron with styrene, methyl acrylate, acrylonitrile, and vinyl acetate was reported by Pittman and coworkers. It was reported that these polymers (47) could be protonated to produce the Jt-allyliron derivatives. Nakamura and coworkers have also reported the synthesis and elecirochemical properties of polymers containing dienes coordinated to iron tricarbonyl moieties in the polymer sidechains. ... 

Which protons are most strongly shielded in butadiene iron tricarbonyl, C4H6 Fe(CO)3 ... 

At present it is understood that cyclo-octatetraene iron tricarbonyl may have a different structure in solution than it does in the crystalline state. Indeed, different protons are detected at different temperatures, which has led to such descriptions as ring whizzing, dynamic equilibrium, and stereochemical nonrigidity in an attempt to describe the bonding. 

Gycloheptatrienes in protic solvents are reported to react with Fe(GO)s and a catalytic amount of NaBH4 to produce ( 7" -l,3-diene)iron tricarbonyl complexes. Pearson and Ghidu have demonstrated that stereospecific cyclization of iron tricarbonyl diene complexes with pendant alkenes and arenes proceeds via protonation of a double bond vicinal to the iron tricarbonyl diene moiety. This methodology has been used to diastereoselectively produce polycycles from iron tricarbonyl-stabilized pentadienyl carbocations. " ... 

As mentioned earlier, a precise interpretation of the NMR spectrum of butadiene-iron tricarbonyl is rendered difficult because of the unknown effects of the iron tricarbonyl group upon the shielding of the protons on the ligand. Nevertheless, the spectra of a fairly large number of compounds have now been studied and certain patterns can be seen which can be of considerable utility in assigning a structure to an unknown compound. 

Reported first by Green and co-workers 12) the protons in butadiene-iron tricarbonyl fall into two widely separated regions, the two central protons appearing at low field with respect to the terminal ones. In cyclic dienes (XII), the protons on the central carbons of the diene system (Ca and C3) lie... 

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