Tricarbonyl iron complexes 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). ... 

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). 

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). ... 

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of variously substituted tricarbonyl(cyclohexadiene)iron complexes (A. J. Birch, B. J. Chauncy, and D. J. Thompson, unpublished results, 1975) and with the large D/H (deuterium/hydrogen) isotope effect, i.e., ratedetermining protonation at Fe (Whitesides and Nielan, 1975).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... 

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. " ... 

This method can be employed to the formation of acyclic and cyclic Ti -dienyl-iumiron complexes. T) -Cycloheptadienyliumiron and n -qrclooctadienyliumiron complex salts are readily available using this procedure starting from T -cycloheptatriene- or (T -cyclooctatriene)iron complexes, respectively. Protonation of tricarbonyl(cyclooctatetraene)iron by noncoordinating acids is known to afford cyclopropane annulated Ti -cycloheptadienyliumiron complexes. This procedure has been exploited for the synthesis of c -2-(2 -carboxycyclopropyl)glycines. ... 

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. 

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). 

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... 

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. 

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. 

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). ... 

Linear Alkenes and Alkynes.—Protonation of the tricarbonyliron complex of 2,4-dimethylpenta-l,3-diene (3) might be expected to occur at C-1 to give eventually the cationic complex (4) since there is no bulky substituent at C-2 or bulky a n-substituent to interfere with carbonyl ligands on the iron. However, these steric requirements do not seem to be dominant since protonation of (3) occurs at C-4 to give (5) rather than (4). " Further, if the o-metry of the tricarbonyl cation is closely related to that of the parent diene... 

On protonation, the complex CgHgFe(CO)3 undergoes rearrangement to the bicyclo-[5,l,0]-octadienyl iron tricarbonyl cation, 6.11, in which the hydrocarbon is acting as a 5-electron ligand [30, 31, 32]. The postulated mechanism for the reaction is ... 

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