Carbene iron complexes

Interestingly, [Ee(F20-TPP)C(Ph)CO2Et] and [Fe(p2o-TPP)CPh2] can react with cyclohexene, THF, and cumene, leading to C-H insertion products (Table 3) [22]. The carbenoid insertion reactions were found to occur at allylic C-H bond of cyclohexene, benzylic C-H bond of cumene, and ot C-H bond of THF. This is the first example of isolated iron carbene complex to undergo intermolecular carbenoid insertion to saturated C-H bonds. 

Experimental Procedure 2.1.3. Preparation of an Iron Carbene Complex by a-Abstraction of Hydride Dicarbonyl(T 5-cyclopentadienyl)(phenylthiocarbene) iron hexafluorophosphate [179]... 

Few cationic iron carbene complexes have been characterized spectroscopically [55,104,464], because most such compounds quickly degrade at room temperature [466]. Besides elimination of the alkylidene ligand, one major decomposition pathway can be disproportionation [459] (Figure 3.14). 

Dimethylsulhde can be eliminated from a-(dimethylsulfonium)alkyl complexes simply by heating (Figure 3.15). The resulting, very reactive, electrophilic iron carbene complexes cannot usually be isolated but are generated directly in the presence of a suitable reactant, e.g. an olefin. Cationic nickel [475] and tungsten [476] carbene complexes have been prepared by similar routes. 

Table 3.7. Intramolecular C-H insertion reaction of cationic iron carbene complexes generated in situ by S-alkylation of 1-(phenylthio)alkyl complexes (see Experimental Procedure 3.2.3).

Hence, cationic iron carbene complexes such as Cp(CO)2Fe =CHCHZR, in which Z is an electron-withdrawing group, might also be suitable for intermolecular cyclopropanation or C-H insertion reactions. The use of such carbene complexes in organic synthesis has not yet been thoroughly investigated, but could fruitfully supplement the chemistry of acceptor-substituted carbenes. 

Experimental Procedure 3.2.1. Cyclopropanation with an Iron Carbene Complex 1,1 -Diphenylcyclopropane... 

Experimental Procedure 3.2.3. Intramolecular C-H Insertion of an Iron Carbene Complex 8-Phenylbicyclo[4.3.0]nonan-2-one... 

The cyclic, cationic iron carbene complex 6 can be prepared and isolated if the counterion is a nonhalide species, such as hexafluorophosphate or tetrafluoroborate95 Treatment of com-... 

It has been demonstrated35 that enolate trapping by the electrophilic iron-carbene complex 1 provides 2. Alkylation at sulfur followed by a-elimination again generates a carbene, which then inserts stoichiometrically 1,5 into the C —H bond to give the octahydroindenone 3. Several other applications of this cyclization have been published 3fi. 

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). 

In a related strategy, Helquist and coworkers used thioether-substituted Fp complexes and applied the intermediate iron-carbene complexes to cyclopropana-tion (Scheme 1.17), C—H insertion and Si—H insertion reactions [48]. 

A cyclopentane annelation by intramolecular C—H insertion of intermediate cationic iron-carbene complexes has been applied to the synthesis of the fungal metabolite ( )-sterpurene [49]. 

Nevertheless, Hossain and coworkers also investigated the carbene transfer reaction using a chiral bimetallic iron-carbene complex, which also exhibits excellent... 

The generation of four-membered ring systems can be accomplished by a cycloaddition process under photochemical conditions or with special substrates under thermal conditions. Iron-vinylidene complexes belong to such a class of special substrates where a thermal [2 + 2]-cycloaddition is possible. If imines are used, a hetero-[2 + 2]-cycloaddition with an iron-vinylidene complex leads to an iron-carbene complex attached to an azetidine ring system, as reported by Barrett and coworkers (Scheme 9.20) [46, 47]. The oxidation of these iron-carbene complexes leads to [3-lactams 27. Interestingly, the application of 2-thiazolines generates penam... 

Most efforts to explore the reactivity of ruthenium carbene complexes have employed the alkoxycarbene species so readily synthesized from the inter- or intramolecular reaction of vinylidene complexes with alcohols. These electrophilic alkoxycarbene complexes exhibit only limited reactivity at Ca, primarily with hydride reagents. For example, treatment of the 2-oxacyclopentylidene complex 97 with NaAlH2(OCH2CH2OMe)2 affords the neutral 2-tetrahydrofuranyl complex (98) [Eq. (89)] (55), as was anticipated from similar reductions of iron carbene complexes (87). 

In comparison to the neutral analogs, there are many cationic iron carbene complexes (48) that have been isolated or prepared in situ. In general, only those complexes in which one of the carbene substituents is a good electron-donor group (i.e. X or Y = OR, SR, or NHR) are stable enough to be isolated. There are a number of approaches these carbene complexes, and many of the earliest methods - still in current use - are summarized in a 1987 review by Brookhart. (Readers should also consult the 1991 review by Fatiadi). ... 

Another tactic that has been used to prepare iron carbene complexes is the protonation of vinyl iron complexes. For example, vinyl iron (55) is readily converted to the carbene (equation 8). The acidity of the iron carbene has been demonstrated as well, as treatment of (56 L = PPhs) with pyridine affords (55). ... 

In contrast to the neutral iron carbene complexes (47), cationic methylidene, ethyhdene, dimethylallylidene, andben-zylidene complexes (68) and (69) (Y = H, Me, CH=CMc2, Ph) react with alkenes to afford cyclopropanes in good yields (Scheme 23). h74,s4,93 ethylidene and benzyli-... 

These compounds are nucleophihc at the noncoordinated sulfur, and undergo alkylation there with iodomethane or benzyl bromide to give initially (128) (which may be isolated as the PFe salt) and ultimately (129) (equation 28). They also react with electron-deficient alkynes, possibly by a dipolar mechanism, to afford cyclized iron carbene complex (130). Finally, compounds (129) are subject to attack by some nucleophiles hydride attack occurs initially at the metal, but ultimately gives ) -dithioester complexes, such as (131). ... 

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