Cyclopropane iron carbene

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

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. 

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. 

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 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-... 

Finally, a racemic cyclopropanation process has also been developed that utilizes an iron Lewis acid catalyst (72) that presumably proceeds through an iron carbene intermediate (73) (Scheme 28). The catalyst is activated by reaction with diazo compounds to produce an intermediate (74) that loses dinitrogen see Dinitrogen Dinitrogen Complexes) to afford the cyclopropane. This chemistry has been extended to the production of epoxides and aziridines and has recently been reviewed. ... 

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. 

Electrophilic iron-carbene complexes have been used to synthesize cyclopropanes. These undergo selective addition to the more electron-rich double bond as, for example, in 2-phenyl-sulfonyl-1,3-dienes. An example is the formation of bicyclo[4.1.0]heptene 19. 

The cyclopropanation of non-functionalized alkenes requires even stronger electrophilic metal carbenes as provided by non-heteroatom-stabilized group 6 carbene complexes 17 or cationic iron carbene complexes the reaction is highly syn-selective (Scheme 18). Iron carbenes bearing optically active phosphine ligands allowed for an efficient enantioselective cyclopropanation. [34]... 

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. 

The iron Lewis acid complex, [CpFe(CO)2(THF)][BF4], has been shown to catalyze the reaction of ethyl diazoacetate and phenyldiazomethane with olefins to form cyclopropanes. The selectivity of the reaction suggests the involvement of an intermediate iron carbene [Gp(C0)2Fe =GH(G02Et) ]. ... 

Aryldiazomethane can also be used for iron porphyrin-catalyzed alkene cyclopropanation [55]. For example, the treatment of p-tolyldiazomethane with styrene in the presence of [Fe(TTP)] afforded the corresponding arylcyclopropapane in 79% yield with a high transicis ratio of 14 1 (eq. 1 in Scheme 11). Interestingly, when bulkier mesityldiazomethane was used as carbene source, ds-selectivity was observed (cisitrans = 2.0 1). Additionally, mesityldiazomethane was found to react with frans-p-styrene, the latter was found not to react with EDA or trimethyl-silyldiazomethane under the similar reaction conditions, to give l-mesityl-2-methyl-3-phenylcyclopropane in 35% yield. Trimethylsilyldiazomethane is also an active carbene source for [Fe(TTP)]-catalyzed cyclopropanation of styrene, affording l-phenyl-2-trimethylsilylcyclopropane in 89% yield with transicis ratio of 10 1 (eq. 2 in Scheme 11). 

Interestingly, the cyclopropanation of styrenes with EDA catalyzed by the half sandwich iron complex [CpFe(CO)2(THF)] BF4 afforded cyclopropanes in good yields and with ds-selectivity cisitrans = 80 20) [62]. With phenyldiazomethane as a carbene source, excellent cA-selectivity (92-100%) was achieved (Scheme 15) [63]. 

The molybdenum complex 1, a typical high-valent Schrock-type carbene, efficiently catalyzes the self-metathesis of styrene. On the other hand, the cationic iron complex 3 does not induce metathesis but stoichiometrically cyclopropanates styrene. The tungsten complex 2, again a Fischer-type carbene complex, mediates... 

Acid-catalyzed dealkoxylation is particularly suitable for the preparation of highly reactive, cationic iron(IV) carbene complexes, which can be used for the cyclopropanation of alkenes [438] (Figure 3.11). Several reagents can be used to catalyze alkoxide abstraction these include tetrafluoroboric acid [457-459], trifluoroacetic acid [443,460], gaseous hydrogen chloride [452,461], trityl salts [434], or trimethylsilyl triflate [24,104,434,441,442,460], In the case of oxidizing acids (e.g. trityl salts) hydride abstraction can compete efficiently with alkoxide abstraction and lead to the formation of alkoxycarbene complexes [178,462] (see Section 2.1.7). 

Because electrophilic carbene complexes can cyclopropanate alkenes under mild reaction conditions (Table 3.1) [438,618-620], these complexes can serve as stoichiometric reagents for the cyclopropanation of organic compounds. Thoroughly investigated carbene complexes for this purpose are neutral complexes of the type (C0)5M=CR2 (M Cr, Mo, W) and cationic iron(IV) carbene complexes. The mechanism of cyclopropanation by electrophilic carbene complexes has been discussed in Section 1.3. 

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