Fischer carbene complexes reactivity

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). 

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. 

S+3C] Heterocyclisations have been successfully effected starting from 4-amino-l-azadiene derivatives. The cycloaddition of reactive 4-amino-1-aza-1,3-butadienes towards alkenylcarbene complexes goes to completion in THF at a temperature as low as -40 °C to produce substituted 4,5-dihydro-3H-azepines in 52-91% yield [115] (Scheme 66). Monitoring the reaction by NMR allowed various intermediates to be determined and the reaction course outlined in Scheme 66 to be established. This mechanism features the following points in the chemistry of Fischer carbene complexes (i) the reaction is initiated at -78 °C by nucleophilic 1,2-addition and (ii) the key step cyclisation is triggered by a [l,2]-W(CO)5 shift. 

A pathway may be considered which involves a double regioselective alkyne insertion followed by a stereoselective cyclisation to undergo a novel [3+2+2]-cyclisation. These examples illustrate the scope in which the reactivity of Fischer carbene complexes can be tuned in a qualitative manner by transmetalation. 

One of the most unique applications of CM to reagent synthesis is described in a report by Zhang and Herndon, where catalysts 2 and 5 were used to effect the homologation of homoallylic Fischer carbene complexes (Equation (4)). The E/Z ratios observed in these reactions using catalyst 5 were typically 9 1. Highly reactive, electron-neutral olefins were... 

Other reactions for which a discussion of their structure-reactivity behavior in terms of the PNS has provided valuable insights include nucleophilic addition and substitution reactions on electrophilic alkenes, vinylic compounds, and Fischer carbene complexes reactions involving carbocations and some radical reactions. 

A concise synthesis of highly substituted furans, pyrroles, butenolides, and 2-butene-4-lactam esters starts from alkynyl adducts of a Fischer carbene complex 21 (Scheme 27) < 1998JOC3164>. Incorporation of an aldehyde yields a reactive vinyl tungstencarbonyl complex 22 that can be oxidatively transformed to an ester group, furnishing the furan carboxylic ester 23. 

The Fischer carbene complexes can be described by resonance forms such as (II) and (III), and are characterized by quite long M—C and by rather short C—X bonds, which show some double bond character. The ionic forms give the complexes increased reactivity towards polar reagents, for example to attack by nucleophiles at the carbene carbon. 

There are essentially three different types of transition metal carbene complexes featuring three different types of carbene ligands. They have all been named after their first discoverers Fischer carbenes [27-29], Schrock carbenes [30,31] and WanzUck-Arduengo carbenes (see Figure 1.1). The latter, also known as N-heterocycUc carbenes (NHC), should actually be named after three people Ofele [2] and Wanzlick [3], who independently synthesised their first transition metal complexes in 1968, and Arduengo [1] who reported the first free and stable NHC in 1991. Fischer carbene complexes have an electrophilic carbene carbon atom [32] that can be attacked by a Lewis base. The Schrock carbene complex has a reversed reactivity. The Schrock carbene complex is usually employed in olefin metathesis (Grubbs catalyst) or as an alternative to phosphorus ylides in the Wittig reaction [33]. 

Barluenga, J., Fananas, F. J. Metalloxy Fischer Carbene Complexes An Efficient Strategy to Modulate Their Reactivity. Tetrahedron 2000, 56,4597-4628. 

The synthesis and reactivity of titanoxo units as fragments of transition metal Fischer carbene complexes have been reviewed.1349 Other reviews have appeareed covering structurally characterized organometallic hydroxo complexes of transition metals including mono- and bis-Gp titanium derivatives.809... 

Despite the caveat expressed above, the most common reaction that Fischer carbene complexes undergo is attack by a nucleophile at Ccarbene. It is interesting that such reactivity should occur, because partial charge calculations typically indicate a higher positive charge at the carbon in CO ligands than at Ccarbene.46 The key to understanding the electrophilicity of these complexes, however, is the... 

Electrophilic transition-metal-carbene complexes (Fischer carbene complexes) serve as formal carbene transfer reagents in reactions with alkenes to give functionalized cyclopropanes. This reaction behavior is well documented for alkoxycarbene complexes of elements of group In contrast, aminocarbene complexes exhibit a different reactivity over a wide range of conditions and [2 + 1] cycloadditions to alkenes represent exception. 

As pointed out earlier, the reactivity of pentacarbonyl-type Fischer carbene complexes towards nucleophiles is much higher than that of carboxylic esters. For example, Ki for MeO addition to 11 (equation 56) is approximately 2 X 10 -10 higher than for MeO addition to methylbenzoate. A comparison of K for n-PrS additiontoll(1.06 X lO" M ) with Guthrie s estimate of 7.9 X 10 " ... 

Relative to their proton basicities, the thiolate ions are much more reactive towards Fischer carbene complexes than alkoxide ions. For example, for the reaction... 

As discussed in the section Alcohols and alkoxide ion nucleophiles, the most useful measure of reactivity of a reaction system is its intrinsic barrier or intrinsic rate constant because, in comparing different systems, these quantities correct for potential differences in the thermodynamics of the reactions. A comparison of intrinsic rate constants for the deprotonation of Fischer carbene complexes by buffer bases with those for the deprotonation of other carbon acids is revealing. A representative hst of such values is given in Table 26. They show the well-known... 

Scheme 44. Rhodium-catalyzed reactivity enhancement of a Fischer carbene complex.

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