Iron complexes, carbene reactions with alkenes

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

In 1982, Casey and co-workers reported the first reactions that could be considered hydrocarbations because they involved the direct C-H bond addition across the C-C double bond of alkenes. They showed that the cationic bridging iron methylidyne complex undergoes this type of reaction with alkenes with anti-Markovnikov regioselectivity. No other hydrocarbation reactions had been reported until recently, when Kubiak and co-workers reported hydrocarbation reactions of a nickel carbene complex with alkenes. Thus, the dicationic aminocarbene complex 31 reacts with ethylene, resulting in a complete conversion to the ethylcarbene complex (Scheme 1). 

The cyclopropanation reactions of the cationic iron carbene complexes occur most efficiently with alkenes of normal electronic characteristics. Veiy electron deficient alkenes such as a,(3-unsaturated carbonyl compounds are veiy poor substrates. Veiy electron rich alkenes such as enol ethers react rapidly, but the expected cyclopropanes generally cannot be isolated if they are indeed formed, they apparently undergo further reactions, peihaps promoted by the metallic species present in the reaction mixtures. 

Dicarbonyl(ri5-cyclopentadienyl)iron-alky] complexes represent useful precursors for iron-carbene complexes [47]. For example, iron-carbene complexes are intermediates in the acid-promoted reaction of Fp-alkyl ether derivatives with alkenes to provide cyclopropanes via a [2 + l]-cycloaddition (Scheme 1.16). 

Stereoselectivity) is observed however, for ethylidene complexes of Fe(CO)(PR3)Cp (69) the products reflect trans selectivity. This difference in stereoselectivity has been suggested to be dependent upon which conformer is more reactive. The reaction of a chiral-at-iron cationic carbene complex (70) with styrene or vinyl acetate affords optically active cyclopropane products with high enantioselectivity (Scheme 24). h >3 intramolecular cyclopropanation, as in the case of (71), proceeds moderately well for the formation of norcarane-type ring systems however, intramolecular C-H insertion is a competing pathway when the alkene is highly... 

Cyclopropanation reactions of nonheteroatom-stabilized carbenes have also been developed. The most versatile are the cationic iron carbenes that cyclopropanate alkenes with high stereospecificity under very mild reaction conditions. The cyclopropanation reagents are available from a number of iron complexes, for example, (9-alkylation of cyclopentadienyl dicarbonyliron alkyl or acyl complexes using Meerwein salts affords cationic Fischer carbenes. Cationic iron carbene intermediates can also be prepared by reaction of CpFe(CO)2 with aldehydes followed by treatment with TMS-chloride. Chiral intermolecular cyclopropanation using a chiral iron carbene having a complexed chromium tricarbonyl unit is observed (Scheme 61). 

The reaction of alkenes with Fischer carbene complexes most typically leads to cyclopropane products however, the formation of a three-membered ring product from a reaction with an alkyne has been observed on only one occasion. The reaction of the cationic iron-carbene complex (199) with 2-butyne presumably leads to the formation of the cyclopropene (200), which was unstable with respect to hydride abstraction by the starting carbene complex and the ultimate product isolated from this reaction was the cyclopropenium salt (201) and the benzyl-iron complex (202). Cyclopropene products have never been observed from Group 6 carbene complexes despite the extensive investigations of these complexes with alkynes that have been carried out since the mid 1970s. 

Photolysis of hydroxy Fischer carbene complexes (96) (Scheme 20) in the presence of alcohols under several atmospheres of carbon monoxide gives low to moderate yields of a-hydoxy esters (97). It is proposed that the reactions proceed via ketenes formed from the liberated or complexed carbenes and CO. In some cases, acetals formed via thermal decomposition of the carbenes are the major products. Photolysis of iron porphyrin carbene complexes results in cleavage of the iron-carbon double bond, producing a four coordinate iron(II) porphyrin and the free carbene. The carbenes can be trapped in high yield with a variety of alkenes. 

The reactivity of carbene-metal complexes, amongst others the reactivity with respect to alkenes and alkynes, has been reviewed by Dotz Just like free carbenes the coordinated carbenes add to triple bonds to give cyclopropene derivatives. Other reaction products, however, are also possible. For instance, the carbene ligand of chromium complex 23 reacts with diphenylacetylene to a mixture of products, including naphthalene derivative 24 and furan derivative 25 (equation 18). A carbonyl ligand has participated. Molecular orbital calculations by Hofmann and Hammerle " on this system reveal that the reaction would pass through an y-vinylcarbene type of complex (26) instead of through a planar chromacyclobutene 27. The subsequent steps to yield either phenol or furan could involve vinylketene 28, but this still is a matter of debate. Similar, but more selective, furan syntheses have been observed for carbene complexes based on iron and cobalt. ... 

Enantiomerically pure iron carbene complexes have been used for carbene transfer reactions to alkenes, e.g. vinyl acetate and styrene, at low temperature to furnish cyclopropanes with moderate cis/trans selectivity in high optical yield (75-95% ee). A two-step reaction mechanism has been proposed to explain the origin of enantioselectivity. ... 

Alkene Cyclopropanation. Once this reagent has been prepared (or purchased commercially) it can be used directly in reactions with a wide range of alkenes without the need for using any further activation agents in order to obtain cyclopropanes (eq 2). An iron carbene complex is apparently generated as a reactive intermediate upon dissociation of dimethyl sulfide which ultimately recombines with the iron moiety to give the principal byproduct of the reaction. 

In a first step, a cationic (carbene)iron complex is formed that provides the carbene carbon atom for a cycloaddition reaction with the alkene. The yields are moderate. A good selectivity for the c/5-product is observed for the transformation of styrene. In case of methylstyrene, the c/s-selectivity is low. The same iron complex also catalyzes the cyclopropanation of several other alkenes, including the trisubstituted 2-methyl-but-2-ene with phenyldiazomethane at room temperature to give the corresponding cyclopropanes with high c/s-selectivity (Scheme 4—297). The cisitrans ratio of this reaction is altered in favor of the tram cyclopropane product when the reaction is performed with a silica gel-bound iron complex. ... 

N-substituted iron porphyrins form upon treatment of heme enzymes with many xenobiotics. The formation of these modified hemes is directly related to the mechanism of their enzymatic reactivity. N-alkyl porphyrins may be formed from organometallic iron porphyrin complexes, PFe-R (a-alkyl, o-aryl) or PFe = CR2 (carbene). They are also formed via a branching in the reaction path used in the epoxidation of alkenes. Biomimetic N-alkyl porphyrins are competent catalysts for the epoxidation of olefins, and it has been shown that iron N-alkylporphyrins can form highly oxidized species such as an iron(IV) ferryl, (N-R P)Fe v=0, and porphyrin ir-radicals at the iron(III) or iron(IV) level of metal oxidation. The N-alkylation reaction has been used as a low resolution probe of heme protein active site structure. Modified porphyrins may be used as synthetic catalysts and as models for nonheme and noniron metalloenzymes. 

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