Advances in Organometallic Chemistry: 51

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Cationic dienyl-Fe CO 3 complexes are very reactive toward nucleophiles, and show considerable promise as synthetic intermediates [ ]; however, there are some problems associated with their use. Thermal and photochemical cyclization of diene-Fe CO 3 complexes with electron-deficient alkenes to yield spiro compounds has been suggested as an alternative [ ],. The stereocontrolled construction of quaternary [ ] or spiro carbon [ , ] centers remains a challenging problem in organic synthesis. Recent extension [ ] of the study [ ] describes a unique, iron-mediated, intra-molecular ene-type reaction, leading to asymmetric, diastereospecific construction of quaternary carbon centers, with formation of spirolactones and spirolactams in enantiomerically pure form.

Excellent stereocontrol during the ene-type coupling between diene- Fe CO 3 groups and alkene in thermally induced spirocyclization can be achieved by appropriate substitution at C-5 of the diene ring; here, an electron-withdrawing cyano group yields a product without racemization, and an electron-donating phenyl group gives rearranged products, also optically active.

Similarly, optically pure spirolactones can be prepared [ ]. This methodology permits application of asymmetric synthesis to a variety of spirocyclic, natural products and some new approaches to synthesis of natural products will be discussed later in the text. One of the most outstanding properties of transition metals is their ability to stabilize short-lived molecules as ligands in coordination compounds, and this is partially observed in metal-stabilized carbenes [ ].

Ligands bound through a disubstituted carbon atom are known collectively as carbenes e. Several theoretical studies have been carried out in attempts to evaluate the strength of, as well as the barrier to, rotation about the expected metal-carbon double bond in transition-metal carbene complexes [ ]. The structures of metal-carbene complexes may be understood in terms of various limiting forms that contribute to the stabilzation of the formally electron-deficient carbene carbon atom [ 74 , ].

For heteroatom-substituted carbenes, there is thus a significant positive charge on the heteroatom X or Y OR,NR , and substantial double-bond character between that heteroatom and the carbene atom. The complexes that contain metal-stabilized carbenes are known for almost all transition elements; the development of the metal carbene chemistry is credited to the brilliant research of Fischer and his students [ 32 , 33 , ]. The various transition-metal complexes can be divided into two groups on the basis of the chemical reactivity of the carbene atom [ — ].

Carbene ligands with heteroatom substituents e. Thus, in the Schrock-type alkylidene complexes the metal-coordinated sp 2 -carbon atom is nucleophilic in character and displays an ylide-type reactivity. However, the reactivity of any given carbene ligand varies considerably from complex to complex; for example, methylene ligands are usually nucleophilic, but can become electrophilic for complexes bearing a positive charge. Although carbene complexes have been studied intensively for more than 2 decades, it is only very recently that useful applications of these complexes to organic synthesis have been developed [ , , , — ].

Transition-metal carbene complexes are generally recognized as important, reactive intermediates in organometallic chemistry, in particular in certain catalytic reactions, e. Carbenes and carbene precursors are known to react with transition-metal complexes to produce alkyl [ ] and alkylidene [ ] complexes, as well as a host of compounds in which the intact precursor molecule is coordinated to the metal center [ , ].

As synthetically useful reagents, carbene complexes are not only suitable as carbene-transfer reagents but also undergo interesting cycloaddition with other ligands in the CO-ligand sphere. Their treatment requires techniques no more complicated than those used for Grignard reactions. In comparison to the wide application of chromium carbene complexes in organic synthesis [ 18 , 19 , 74 , , , — ], the Fischer-type carbene complexes of iron e.

Synthesis of neutral and cationic metal-carbene complexes has been discussed [ ]. Carbene complexes of type 21 have also been prepared by photochemical exchange of a CO ligand in Fe CO 5 with alkylidene [ ]. A few additional routes to 0-substituted, Fischer-type carbene complexes are described next.

For example, methoxycarbene complexes 24 are readily prepared by treatment of the bromide 22 with the appropriate lithium acetylide, to give 23 , followed by protonation of the corresponding vinylidene complexes, and subsequent addition of methanol [ — ]. Alternatively, 24 may be obtained from the corresponding acyl complexes 25 [ 61 , 64 ] following treatment with trimethyloxonium tetrafluoroborate [ ].

“Recent Developments in the Organometallic Chemistry of N-Heterocyclic Carbenes”

Cationic carbene complexes can also be prepared by the alkylation of neutral acyl complexes [ ]. For example, synthesis of important cyclopropanic reagent, e.

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Electrophilic or Fischer-type iron-carbenes are usually subject to a two-fold nucleophilic attack, resulting in either exchange of the heteroatom substituents on the carbene carbon, or ligand substitution at the metal. The general patterns of reactivity of electrophilic carbene complexes are summarized in Scheme 5.

Generally, interaction of carbene complexes with alkynes, metathesis, and cyclopropanation reactions are important reactions of metal carbenes. However, it is the carbene-transfer reactions, and particularly, cyclopropanation reactions, which probably proceed through metallic intermediates, that are of the most importance in organic synthesis.

New coupling reactions of conjugated 1,3-dienes with iron-carbene complexes to give 1,3-diene Fe CO 3 derivatives have recently been reported by the Semmelhack group [ ]. Another synthetically interesting new reaction of iron-carbene complexes with alkynes has been reported by the same group [ 17 , ]. It has been demonstrated [ 17 ] that iron-carbene complexes, e. This is a remarkable reaction, with a strong potential for future synthesis; however, the mechanism by which it occurs is not yet known, although a somewhat complex proposal can explain the products formed [ 17 , ].

The chemistry of metal-carbene complexes including those of iron and chromium has been extensively investigated, and many such complexes have found impressive synthetic utility [ , — , — ]. In contrast, the chemistry of nitrene ligands to iron or chromium, e. This is a new synthesis adventure, and no doubt more studies in this direction will be forthcoming.

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The limits of stability of strained hydrocarbons, including cyclopropanes, have been substantially clarified over the course of the past 30 years [ , ]. The chemistry of the cyclopropyl group has recently been discussed at length [ ]. The torsional strain in the three-membered ring imparts a high degree of reactivity, which can lead to the fragmentation of cyclopropanes; this process is apparently regio- and stereo-chemically controlled [ ].

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Because of the various cleavage reactions and rearrangements in which they participate, cyclopropanes frequently serve as valuable synthetic intermediates leading to other ring systems, or to acyclic products. The cyclopropane ring system is also seen as an important structural feature among many classes of naturally occurring compounds [ 66 ]. Fused, and bridged, polycyclic systems containing a cyclopropane ring have been shown to be useful intermediates in organic synthesis [ , ].

New synthetic approaches to carbocycles via intramolecular cyclopropanation reaction have recently been discussed [ ], and a recent summary on cyclopropanes as building blocks for organic synthesis has appeared [ ]. However, the reaction of Fischer carbene complexes with enyne substrates yields bi- and tricyclic cyclopropane-containing carbon skeletons [ ]. The paper also discusses various modes of metal carbene-alkene and metal carbene-alkyne couplings, including intra- and intermolecular cyclopropanation reactions.

A cyclopropanation reaction involving transition-metal carbene complexes has recently been reviewed [ 66 , ].

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  • Among the large number of methods developed for the synthesis of cyclopropanes, the majority of them may be placed in two broad categories: 1 addition of carbenes, carbenoids, or related species to alkenes to yield intermediates for carbene-transfer reactions and 2 intramolecular coupling, or alkylation reaction [ , ].

    Electrophilic, cationic complexes of iron are efficient cyclopropanating agents, particularly for unfunctionalized alkenes. These complexes are unstable, and are usually generated and used in situ. As observed earlier by Pettit et al. Several methods have been developed for the preparation of more-stable metal carbenes suitable for cyclopropanation [ ]; some of these require replacement of the simple methylidene group by an ethylidene [ ], isopropylidene [ , ], allyhdene [ , ], or benzylidene [ ] group.

    Brookhart has used methoxyalkyliron complexes Scheme 11 to generate relatively stable ethylidene [ , ], benzylidene [ 66a , , ], and cyclopropylidene [ ] transfer reagents. Note the generation of the required cationic iron complex 49, following treatment of the methoxyalkyliron complex 48 with trimethylsilyl triflate at low temperature, and then with alkenes, to give the corresponding cyclopropanes 50, 51 and Recently, more-stable metal carbenes have been synthesized, and applied as cyclopropanation reagents in synthetic organic chemistry.

    Thus, Helquist et al. The reaction is stereospecific, giving cis -cyclopropanes with cis -alkenes, and trans -cyclopropanes with trans -alkenes. The ethylidene group is similarly transferred when the related methylphenylsulfonium ethyliron complex is used; in this case also, the stereochemistry of the alkene is maintained [ ]. The precise mechanism of the cyclopropanation reaction is yet to be determined. It has been clearly established that, in the transfer reaction, the electrophilic carbene complex attacks the nucleophilic alkene with substantial charge development in the transition state.

    A transition state involving unsymmetrical attack, with partial charge build-up on only C-2 and not a more-symmetrical model where equal charge builds up to C-1 and C-2 of the alkene, has been advanced [ 66 ]. The future challenging chemistry of, for example, metallo-bis methylene phosphoranes, e.

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    The mechanism of this reaction and its relevance to the cyclopropanation reactions of cationic metal carbenes and alkenes has been discussed [ ]. The unrecrystallized 53 35g, mmol as a yellow powder was placed in a mL, round-bottomed flask equipped with a magnetic stirring bar. The combined filtrates were concentrated by rotary evaporation, and the crude product was purified by flash chromatography silica gel, hexane. The colorless oil was distilled through a short-path apparatus to give 8.

    Transfer of the carbene ligand from optically active transition-metal-carbene complexes to alkenes represents a potentially useful and general method for the enantioselective synthesis of cyclopropanes [ ], and this task was successfully accomplished by the Brookhart group [ ]. The starting complexes were obtained as a pair of iron acyl diastereoisomers, e.

    These were converted into the desired cationic iron-carbene complexes 62 and 63 via intermediates 60 and 61, as shown in Scheme The efficient transfer of the ethylidene group from these two diastereoisomeric ethylidene complexes, e. Since two diastereoisomers, e. A study [ ] discussed at length the role of the metal e. With iron-ethylidene complexes having a chiral phosphine in place of one CO ligand, high asymmetry induction was expected and observed [ ].

    These complexes transfer the carbene moiety to alkenes to give cyclopropanes often with high enantioselectivity [ ]. Additional syntheses of optically active cyclopropanes by the use of chiral butadiene-tricarbonyliron complexes have been reported [ — ]; particularly, preparation in this way of formylcyclopropanes of high enantiomeric purity provided the key intermediate for the synthesis of low toxicity insecticide pyrethroids [ , ].

    A brief overview of reactions of cyclopropane derivatives in the presence of iron pentacarbonyl, Fe CO 5 , or its dimer Fe 2 CO 9 , is included for continuity.

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    This exciting chemistry has been studied by Sarel, and many metal-induced rearrangements of, and insertions into, cyclopropyl alkenes have been reviewed by that author [ 44 ]. Studies [ 44 ] led to discovery of at least five distinctly different modes of metal-mediated reactions of vinylcyclopropanes, depending on the substrate and the reaction parameters. These modes are as follows. Photoinduced acyl-metal insertions into the vinylcyclopropane system.

    More examples of acyl-metal insertions into vinylcyclopropane systems have been reported [ ]. For example, photoreaction of cyclopropylacetylene 80 with Fe CO 5 gave rise to benzoquinones 81 and 82 originating from the insertion of carbonyls between the molecules of acetylene while leaving the cyclopropane ring itself intact [ ]. The special features characterizing the interaction between zerovalent transition metals e. Recently, Gompper and Bartmann [ 43 ] extended this series, and reported the preparation of complexes, e. Asymmetric synthesis is a widely pursued goal in modern synthetic chemistry [ — ].

    Also, the importance of the spatial arrangement of molecular groups for asymmetric recognition has recently been emphasized [ ]. Chiral synthesis is currently among the most exciting areas of organic chemistry [ — ]. Organotransition-metal complexes form a class of very important compounds, not only because of their inherently interesting physical and chemical properties but also because of the significant impact they are having on synthetic organic chemistry [ 79 , — ].

    Many novel synthetic applications of organotransition-metal complexes have been reported in the recent literature, which provides methods for synthetic transformations difficult or impossible to achieve by more conventional routes. Consequently reactions of iron-carbene complexes with nucleophiles can form two diastereomers.

    The pyramidal nature of the phosphorous atom in organophosphine complexes comprising the PPh 3 ligand has been reported [ ]. Recently Gladysz et al. Introduction of the Fe CO 3 group onto a substituted cyclohexadiene makes the molecule asymmetric. Should these complexes be produced in optically active form, any synthesis utilizing them would be asymmetric [ ].

    However, the main recent interest in iron acetyl complexes and related metal acyl complexes lies in their novel synthetic applications.