Iron carbenes

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. 

The reductive behavior of iron-carbene porphyrins has been investigated. A 2g + FI" " reduction of thiocarbonyl complexes can lead to the corresponding thioformyl complexes in a reversible system [421]. 

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. 

Experimental Procedure 3.2.1. Cyclopropanation with an Iron Carbene Conqjlex 1,1-Diphenylcyclopropane [468]... 

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 3.2.3. 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 reactions of these iron carbene reagents with alkenes to give cyclopropanes are stereospecific. They also exhibit high syn stereoselectivity in many cases. Optically active derivatives have been reported that have chiral ligands on iron or chiral alkoxy groups on the prospective caibene center and which have been resolved with the iron itself as a chiral center. Resulting from this work have been some highly enantioselective cyclopropanations. 

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. 

Treatment of the iron carbene Fe(tetraphenylporphyrinXCCl2) with primary amines provided a novel route to the isocyanide derivatives Fe(tetra-phenylporphyrin)(NH2R)(CNR) (R = Pr1, Bu", Bu ) (96). 

Insertions of CNR (R = Bu, C6HM, Ph) into the iron-carbene bond in the ( /3-vinylcarbene)irontricarbonyl complex (100) formed the tf -vinylke-tenimine (101) which converted to 102 on alkylation with Me0S02F. The structural elucidation of 101 (R = C6Hn) was also reported (435). 

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... 

I(Cyclopentadienyl)dicarboayM phate, Cp(CO)2Fe+=CHSC H, phenylthio iron carbene is obttii C1CH2SC6H3 and then with (C H Cyclopentane annulation.2 by insertion into C-H bonds. Thi... 

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