Diene-iron bonding

The butadiene moiety in (IV) is presumed to be planar, or nearly so, and the iron atom lies below this plane and approximately equidistant from the four carbon atoms of the diene system. The nature of the diene-iron bonding was presumed to involve interaction of the Fe atomic orbitals with -n molecular orbitals of the diene system as a whole and was therefore more analogous to the 77 bonding in ferrocenes, rather than the a type implied in Reihlen s structure (II). The conjugated nature of the diene system was considered to be an essential feature necessary for the formation of iron derivatives of this type. 

The double bond transposition could also be achieved by the conversion of an intermediate for PGA2 synthesis into a 1,3-diene iron tricarbonyl complex from which PGC2 was synthesized in four steps. The Fe(CO)3 diene complex which survived the Wittig reaction was cleanly removed by Collins reagent in the subsequent step (Ref. 10). 

Isolated double bonds can be oxidatively cleaved in systems containing a conjugated diene moiety if it is protected as a tricarbonyl(diene)iron complex44. Dienal 39 was acquired in 49% yield by a two-step osmylation-periodate cleavage sequence (equation 27). In contrast, ozonolysis of the polyene complexes is reported to lead to destruction of the complex. 

Iron carbonyls have been used in stoichiometric and catalytic amounts for a variety of transformations in organic synthesis. For example, the isomerization of 1,4-dienes to 1,3-dienes by formation of tricarbonyl(ri4-l,3-diene)iron complexes and subsequent oxidative demetallation has been applied to the synthesis of 12-prostaglandin PGC2 [10], The photochemically induced double bond isomerization of allyl alcohols to aldehydes [11] and allylamines to enamines [12,13] can be carried out with catalytic amounts of iron carbonyls (see Section 1.4.3). 

Diene-iron tricarbonyl complexes undergo C—H bond insertion reac-... 

Acyclic tricarbonyl(diene)iron complexes may be prepared by reaction of a 1,3-diene with (1), (2), or (3) either thermally, photochemically, or under the influence of ultrasonic stirring (Scheme 62). Ligand transfer species, such as (benzylideneacetone)Fe(CO)3, (l-aza-1,3-diene)Fe(CO)3, or bis( -cyclooctene)Fe(CO)3 are also useful for the complexation of dienes nnder mUd reaction conditions. The thermal reaction of nnconjugated dienes with (1) generally results in double bond migration... 

The chemistry of diene iron tricarbonyl complexes described above has been in a number of total syntheses. An iterative stereospecific 1,3-migration of the iron tricarbonyl moiety was used to prepare compounds with multiple chiral centers. An example of one iteration can be seen in Scheme 168. Ester hydrolysis of (105) and protection of the resulting alcohol gives (106). Reduction of the nitrile with DIBAL-H followed by olefination furnished (107). Treatment of (107) with a base resulted in the migration of iron toward the nitrile to give (108). The uncomplexed double bond can... 

Cyclic 1,3-diene iron tricarbonyl complexes eliminate hydrogen on electron impact to give predominant odd electron ions with iron bonded to an aromatic system 57) These same molecules eliminate hydrogen and iron on photolysis to give aromatic hydrocarbon products. 

Previous syntheses of tricarbonyl( /-diene)iron complexes have relied mainly on the reaction of Fe(CO)s, Fc3(CO)i2, or Fc2(CO)9 with the free diene. The use of the first two carbonyls suffers from the prolonged reflux times and/or ultraviolet irradiation necessary to obtain reaction and the consequent low yields and mixtures of complexes obtained with heat- and ultraviolet-sensitive dienes. The latter reagent, although utilized at lower temperatures, may react with polyenes (n > 3) to give mixtures containing, in addition to the expected product, binuclear derivatives containing a metal—metal bond.2... 

While a great number of tricarbonyl( -diene)iron complexes have been reported and their reactivity investigated, much less is known of the corresponding heterodiene complexes. In recent years, synthesis of several tricar-bonyl(heterodiene)iron systems involving r] coordination of the heterodiene unit has been achieved. Among the tetracarbonyl(/ -olefin)iron complexes prepared by Weiss was tetracarbonyl(cinnamaldehyde)iron, which converts on heating to the //-bonded tricarbonyl(cinnamaldehyde)iron. The preparation and synthetic utility of (benzylideneacetone)tricarbonyl iron, an analogous complex of an ar,/9-unsaturated ketone, are reported here. 

In contrast to the very large number of tricarbonyl( -diene)iron complexes described in the literature,thg corresponding ruthenium compounds have received very little attention. This may reflect the well-documented tendency of ruthenium to form metal—metal bonds as opposed to iron. particular, while the metal—metal bonds in Fe3(CO)i2 are easily broken, Ru3(CO)i2 undergoes a variety of reactions in which the Rus cluster is retained. 

The free energy of activation for basal-apical CO exchange in tricarbonyl(l-methoxycyclohexa-l,3-diene)iron is 7.3 0.2 kcal mol which is almost identical to that in tricarbonyl(hexa-l,3-diene)iron. Higher barriers are found in n -bonded heterodiene complexes e.g. for compound (31) AG = 14.0 kcal mol whereas the a-n and a-a azadiene derivatives (32 R=Bu ) and (33 R =PrO have barriers of less than 9 kcal moh. ... 

C/5-2,6 octadiene would be the most appropriate small molecular model for the repeat unit in m-poly(butadiene). However, in light of the fact that FeCCO) catalyzes double bond migration and isomerization any isomeric octadiene monomer should serve as an appropriate model for the polymer reaction. We reacted 1,7-octadiene with FeCCO) and obtained a simple infrared spectrum which was nearly identical to that for the reaction product of c/5-poly (butadiene) with Fe(C0)5, i.e., both materials displayed two discrete peaks in the carbonyl region, a sharp peak at 2050 cm and a broad absorption ( "30 cm HWHH) centered at 198O cm" (See Fig. 2). The broad unresolved character of the 1980 cm" band, as compared to that in rj -butadiene iron tricarbonyl which shows discrete sharp peaks at 1980 and 1990 cm in addition to the sharp peak at 2056 cm", can be understood in terms of the variety of isomeric diene iron tricarbonyls which can be formed in the reaction of FeCCO) with either c/5-poly(butadiene) or 1,7-octadiene. 

Shortly afterwards, in 1960, Fischer discovered that addition of triphenyl-carbenium tetrafluoroborate to T -cyclohexa- 1,3-diene iron tricarbonyl 4 produced the novel in -cyclohexadienyl iron tricarbonyl cation 5 as a stable salt, and the reactivity of such compounds towards C-C bond formation was soon being explored. The application of organometallic complexes to organic synthesis had begun. 

Reports on the synthesis of diene complexes using Fe2(CO)9 are more common. Reaction of 2-phenylsulfonyl-1,3-cyclohexadiene with 2equiv. of Fe2(GO)9 in refluxing ether formed the [l-(phenylsulfonyl)-l,3-cyclohexa-diene]iron(0)tricarbonyl isomer 26. The reaction is catalyzed by 1-aza-1,3-butadiene. In a separate study, 1-aza-1,3-butadienes were shown to effect a quantitative catalytic complexation of cyclohexadienes with Fe2(GO)9. Activities are greatly enhanced in the presence of aryl rings bonded to nitrogen. 

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. " ... 

High-frequency shoulders on Vs(CO) and Vas(CO) bands in the FTIR spectra of an organometallic probe derived from tricarbonyl(l-4- n -5-pyridinocyclohexa-l,3-diene)iron hexafluorophosphate have been observed io on incubation with a-chymotrypsin and were assigned to an iron-complex covalently bonded to enzyme-NH2 groups. Curve-fitting analysis of the spectra enabled the bound complex to be detected even in the presence of the unbound probe and its FTIR spectrum to be calculated and compared with spectral data arising from similar interactions between the tricarbonyliron moiety and twelve aminoacids and polylysine. 

The main point here is that the CgHg ring adopts a chair conformation in the complex with each iron atom again clearly associated with a diene unit. In this conformation little 7r-type interaction is to be expected between the two diene units bonded to iron. It is also of interest that the disposition of the three carbonyl groups about each iron atom is the same as observed in the earlier studies. 

Comparison of Observed and Calculated Bond Distances between Terminal Carbons of Diene-Iron Tricarbonyls and Adjacent Carbons of Substituent Groups... 

Addition of an T -allyl-Fp complex to this compound affords an T -aIlyl-Fp-substituted cycloheptatriene system. Two double bonds are involved in an (T -diene)iron complex. The remaining free double bond of the silyl enol ether attacks as a nucleophile at the cationic r -alkene-Fp moiety to form an (Tj -diene)iron complexed cyclopentane annulated cycloheptadienone. Treatment with CAN in methanol under carbon monoxide atmosphere releases the methoxycarbonyl-substituted free ligand (Scheme 4-25). Reaction of the Ti -dienyliumiron intermediate of Scheme 4-25 with an ( , Z)-isomeric mixture of ri -crotyl-Fp proceeds with high diastereoselectivity. Four new stereogenic centers are formed in the course of this formal [3+2] cycloaddition. A hetero [3+2] cycloaddition is also feasible between T -ailyl-Fp complexes and aromatic aldehydes in the presence of zinc chloride or titanium(IV) chloride to provide tetrahydrofuran derivatives (Scheme 4-26). A 1,2-shift of the iron complex fragment occurs in the course of this reaction. Employment of imines affords the corresponding pyrrolidines. ... 

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