Alkene-Iron Complexes

Cationic iron-alkene complexes also participate in an unusual cycloaddition process, wherein electron-deficient alkenes are attacked by nucleophilic o -allylic Fp complexes, generating stabilized carb-anions and cationic alkene-iron complexes. Attack of the carbanion on the alkene forming a five-membered ring completes this process (Scheme 13). Oxidative removal of the iron produces useful organic compounds.19-21... 

Neutral ri2-alkene-tetracarbonyliron complexes can be prepared from the corresponding alkene and nonacarbonyldiiron via a dissodative mechanism. The organic ligand in the alkene-iron complex is more easily attacked by nucleophiles than the corresponding free alkene due to the acceptor character of the tetracarbonyliron fragment. The reaction principle is demonstrated in Scheme 1.8 [30],... 

Neutral a-alkyliron complexes are obtained upon reaction of Na[Cp(CO)2pe] (5) with alkyl halides (9) (Scheme 6), and as with Collman s reagent this occurs in an Sn2 fashion with inversion of coirfiguration at the carbon atom. Epoxides also participate in this reaction, but tertiary alkyl halides are poor substrates. Alternatively, complexes (9) may be prepared by reaction of an appropriate metal alkyl with Cp(CO)2PeX (6). Typically complexes of this type are prepared in order to gain access to the synthetically nseful cationic rf--alkene iron complexes (Section 4.1.2). Also, nucleophilic addition of (5) to heteroatom-snbstituted alkyl halides (snch as methoxymethylchloride or chloromethyl methyl snllide) affords complexes of type (9) that can be converted to cationic... 

Alkenes. This reagent reacts with an alkyl bromide (or tosylate) to form an iron complex of type 2. Treatment with trityl tetrafluoroborate in CH2CI2 at 0 abstracts a hydride ion from a /3-carbon atom of 2 to form an alkene iron complex (3). The alkene 4 is liberated quantitatively and without isomerization by treatment with sodium iodide in acetone." Because of the size, (C6H5)3C" abstracts a hydride ion preferentially from a methyl rather than a methylene group. In addition, electronic factors may be involved. This elimination reaction is therefore useful for preparation of 1-alkenes. 

Scheme 4-20. Ti -Alkyliron complexes by reaction of (Ti -alkene)iron complexes with nucleophiles.

Scheme 4-21. Elimination of alcohol from P-alkoxyalkyl-Fp complexes and demetalation of the cationic (Ti -alkene)iron complexes.

Ethyl 3,3-dicyanoacrylate (150 mg, 1 mmol) dissolved in dichloromethane (3 mL) is added to a solution of the ri -allyliron complex (369 mg, 1 mmol) in the same solvent (7 mL). The reaction mixture is then stirred at room temperature for 20 h. After removal of the solvent under reduced pressure, the crude product is purified by chromatography with Florisil. The residue is applied to the column as a benzene solution and is eluted with petroleum ether (40-60 °C)/diethyl ether (1 1) to give the (Ti -alkene)iron complex mp 148.5-149 °C (decomp.) 341 mg (70%). ... 

Formation of neutral (ii -alkene)iron complexes is achieved by ligand exchange from carbonyliron complexes preferably nonacarbonyldiiron (Scheme 4-67). ... 

Scheme 4-72. Addition of nucleophiles to (Ti"-alkene)iron complexes.

In (alkene)iron complexes, either neutral or cationic, the electron density at the alkene is reduced. This allows for nucleophilic addition to such complexes. For example, malonates are added to give ethyl malonate derivatives after aqueous workup. Reaction of the intermediate anionic T -alkyl-Fp complexes with alkyl halides leads to migratory insertion of a carbonyl ligand and provides acylated products (Scheme 4-73). ... 

Another option to gain access to T -allyliron complexes is the removal of a leaving group from substituted neutral (T] -alkene)iron complexes. This procedure starts from allylic alcohols, allylic ethers, or carboxylates, which are complexed with nonacarbonyldiiron and subsequently treated with tetrafluoroboric acid to aflbrd the cationic Ti -allyl(tetracarbonyI)iron complexes (Scheme 4-78). The reaction is stereoselective with respect to the geometry of the starting allyl alcohols. ( )-AlIyl alcohols give anti products, whereas syn complexes are formed from (Z)-allyl alcohols. The alkene complexes can also be generated in situ to afford the q -allyliron complexes after treatment with acid. ... 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

Reduction of unsaturated organic substrates such as alkenes, alkynes, ketones, and aldehydes by molecular dihydrogen or other H-sources is an important process in chemistry. In hydrogenation processes some iron complexes have been demonstrated to possess catalytic activity. Although catalytic intermediates have rarely been defined, the Fe-H bond has been thought to be involved in key intermediates. 

Scheme 56 Isomerization reaction of 1-alkene catalyzed by an iron complex...

Iron complexes with the pentadentate ligand 3 derived from pyridyl and prolinol building blocks containing a stereogenic center were reported from the group of Klein Gebbink (Scheme 4) [34]. In alkene oxidations with hydrogen peroxide,... 

The hydration of propylene with sulfuric acid catalyst in high-temperature water was investigated using a flow reaction system.31 The major product is isopropanol. A biopolymer-metal complex, wool-supported palladium-iron complex (wool-Pd-Fe), has been found to be a highly active catalyst for the hydration of some alkenes to the corresponding alcohols. The yield is greatly affected by the Pd/Fe molar ratio in the wool-Pd-Fe complex catalyst and the catalyst can be reused several times without remarkable change in the catalytic activity.32... 

The key feature of efficient metathesis catalysts seems to be their ability to form, before the [2 + 2] cycloaddition step, a n complex with the alkene (Figure 1.7). Comparison of catalyst 1 with the iron complex 3 shows that the latter, although cationic, will not be able to bind to an olefin, because this would give rise to a complex with 20 valence electrons. A similar argument can be used... 

Alkene synthesis. A regio- and stereoselective alkene synthesis is formulated in equations (I)—(III). The first step involves formation of the alkyliron complex 1. Treatment of 1 with trityl tetrafluoroborate abstracts a -hydrogen to give a cationic iron complex 2. Liberation of the free alkene is effected by Nal in acetone. This sequence is useful because 1- and 2-alkyliron complexes (1) are converted into 1-alkcncs exclusively 3-alkyliron complexes are converted exclusively into the less stable (Z)-2-alkenes. The paper includes some 20 examples of alkenes prepared in this way. 

Removal of the 0-substituted Fp group can be achieved by conversion into the cationic alkene-Fp complex using Ph3CPF6 and subsequent treatment with iodide, bromide or acetonitrile. Oxidative cleavage with ceric ammonium nitrate in methanol provides the methyl esters via carbon monoxide insertion followed by demetallation. The [3 + 2]-cydoaddition has been successfully applied to the synthesis of hydroazulenes (Scheme 1.11) [34]. This remarkable reaction takes advantage of the specific nucleophilic and electrophilic properties of V-allyl-, cationic t 5-dienyl-, cationic ri2-alkene- and ti4-diene-iron complexes, respectively. 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

The suggestion for the catalytic cycle is presented in Scheme 4.18. Initially, an unsaturated iron complex is formed by expulsion of both dinitrogen molecules. Next, coordination of the alkene takes place, which is preferred since activation of hydrogen is also feasible. After alkene coordination, oxidative addition of hydrogen yields a formally 18-electron complex. Insertion of the alkene gave an alkyl complex, which recreated the starting complex via reductive elimination. Notably, the alkene complex also supports an isomerization of the double bond, hence an extension of possible intermediates is conceivable. 

Most examples in the literature on hydrosilylation with iron complexes as catalyst concern Fe(CO)5 or related iron carbonyl compounds [41]. The first use of iron pentacarbonyl was reported for the reaction of silicon hydrides with alkenes at 100-140 °C to form saturated and unsaturated silanes according to Scheme 4.20 [42, 43]. 

The intermolecular [2 + 2]-cycloaddition of alkenes and alkynes utilizing an iron complex as a catalyst was reported by Rosenblum and Scheck [48]. The application of the [CpFe(CO)2]BF4 complex (Scheme 9.21) gave the desired cyclobutene derivatives 29 in up to 53% yield. 

---