Carbenes, Fischer

Organic Synthesis Using Transition Metals, Second Edition. Roderick Bates. 2012 John Wiley Sons, Ltd. Published 2012 by John Wiley Sons, Ltd. 

The a-protons of the alkyl substituent are acidic, just as the protons a- to an ester carbonyl group are acidic. Measurement of pK values for carbene complexes is not straightforward as such numbers are often determined in an aqueous environment in which the conjugate base of the carbene is unstable. A value of 12.5 for the methoxy substituted methyl chromium carbene in 1 1 H2O/ CH3CN appears reasonable. Values for 

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Chromium carbene complexes like 13, which are called Fischer carbene complexes, can conveniently be prepared from chromium hexacarbonyl 11 and an organolithium compound 12, followed by an O-alkylation step ... 

The 13C-NMR spectra of 4-7, 9-11 show a close similarity to the spectral data of analogous carbene complexes. The shift differences between the metal carbonyls of the silylene complexes and the related carbon compounds are only small. These results underline the close analogy between the silicon compounds 4-7, 9-11 and Fischer carbene complexes. This view is also supported by the IR spectral data. On the basis of an analysis of the force constants of the vco stretching vibration,... 

From Alkyl-Substituted Fischer Carbene Complexes. 23... 

Reaction of ayfi-Unsaturated Fischer Carbene Complexes with Alkenes,... 

Keywords Fischer carbenes Template synthesis Cocyclization Cycloaddition Cyclopentadienes Cyclopentenones Domino reactions... 

Synthesis of a,/MJnsatu rated Fischer Carbene Complexes... 

Scheme 1 Synthesis of a,/ -unsaturated Fischer carbene complexes 3 from (pentacarbonyl)-metallaacylates 2 [12-15]...

Due to the high a-C,H acidity in the alkoxyethylidene complexes 6 (e.g.,piCa=8 (R=Me)) [ 16], transformations via an enolate analog are possible and have been used to introduce additional functionality into the carbene side chain to access various Fischer carbene complexes [3]. The a,/J-unsaturated complex 8 could be obtained from 6 (R=Et) by an aldol-type condensation with benzaldehyde 7 in the presence of triethylamine and trimethylsilyl chloride (Scheme 2) [17]. This reaction proceeds completely diastereoselectively to yield only the trans-isomer. Analogously, binuclear complexes have been prepared from 6 and 1,3-and 1,4-phthaldialdehyde in good yields [17]. This type of condensation has... 

The possibility of being involved in olefin metathesis is one of the most important properties of Fischer carbene complexes. [2+2] Cycloaddition between the electron-rich alkene 11 and the carbene complex 12 leads to the intermediate metallacyclobutane 13, which undergoes [2+2] cycloreversion to give a new carbene complex 15 and a new alkene 14 [19]. The (methoxy)phenylcar-benetungsten complex is less reactive in this mode than the corresponding chromium and molybdenum analogs (Scheme 3). 

This reaction mode of alkynylcarbene complexes of type 23 undoubtedly provides the most convenient access to /J-amino-substituted a,/J-unsaturated Fischer carbene complexes 27 (X=NH2, NHR2, NR2). Fischer et al. reported the very first such addition of an amine to an alkynylcarbene complex of type 23 and observed a temperature-dependent competition between 1,4- and 1,2-addition [12]. In a later systematic study, de Meijere et al. found that in addition to the 1,4-addition products 30,1,2-addition-elimination (formal substitution)... 

Cocydizations of a,/MJnsaturatecl Fischer Carbene Complexes with Alkynes... 

Recently, Aumann et al. reported that rhodium catalysts enhance the reactivity of 3-dialkylamino-substituted Fischer carbene complexes 72 to undergo insertion with enynes 73 and subsequent formation of 4-alkenyl-substituted 5-dialkylamino-2-ethoxycyclopentadienes 75 via the transmetallated carbene intermediate 74 (Scheme 15, Table 2) [73]. It is not obvious whether this transformation is also applicable to complexes of type 72 with substituents other than phenyl in the 3-position. One alkyne 73, with a methoxymethyl group instead of the alkenyl or phenyl, i.e., propargyl methyl ether, was also successfully applied [73]. 

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

In 20 years of usage, a,/J-unsaturated Fischer carbene complexes demonstrated their multitalented versatility in organic synthesis, yet new reaction types are still being discovered every year. In view of their facile preparation and multifold reactivity, their versatile chemistry will undoubtedly be further developed and applied in years to come. The application of chirally modified Fischer carbene complexes in asymmetric synthesis has only begun, and it will probably be an important area of research in the near future. 

Keywords Fischer carbene complexes Cycloaddition reactions Carbocycles Heterocycles... 

The cydopropanation reaction of an unsaturated substrate is one of the most important strategies to access three-membered ring derivatives. The use of Fischer carbene complexes to perform this kind of cyclisation has become an important tool in organic synthesis [4]. In the next few sections the most significant features of this chemistry are briefly described. 

The ability of Fischer carbene complexes to transfer their carbene ligand to an electron-deficient olefin was discovered by Fischer and Dotz in 1970 [5]. Further studies have demonstrated the generality of this thermal process, which occurs between (alkyl)-, (aryl)-, and (alkenyl)(alkoxy)carbene complexes and different electron-withdrawing substituted alkenes [6] (Scheme 1). For certain substrates, a common side reaction in these processes is the insertion of the carbene ligand into an olefinic C-H bond [6, 7]. In addition, it has been ob-... 

Non-heteroatom-stabilised Fischer carbene complexes also react with alkenes to give mixtures of olefin metathesis products and cyclopropane derivatives which are frequently the minor reaction products [19]. Furthermore, non-heteroatom-stabilised vinylcarbene complexes, generated in situ by reaction of an alkoxy- or aminocarbene complex with an alkyne, are able to react with different types of alkenes in an intramolecular or intermolecular process to produce bicyclic compounds containing a cyclopropane ring [20]. 

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

The reactions of Fischer carbene complexes with 1,3-dienes (carbodienes or heterodienes) lead to the formation of cyclic products with different ring sizes depending upon both the nature of the reaction partners and the reaction conditions. Between these synthetically useful transformations are found [2c+2s], [3C+2S], [4S+1C], [3S+3C], [4S+2C], [4S+3C] and [2S+1C+1C0] cycloaddition reactions which will be summarised further on, in addition to the [2S+1C] cycloaddition processes here described. 

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