Metal-carbenes

Fischer Carbenes Fischer recognized the first carbene complexes in 1964. They were formed by the attack of an alkyllithium on a metal carbonyl followed by methylation (Eq. 11.1). Going back to the bonding picture mentioned above, we saw that the methoxy substituent will also help stabilize the empty p orbital on the carbene carbon by ir donation from one of the lone pairs on oxygen. Resonance form 11.3, which is probably the dominant one in the heteroatom stabilized Fischer carbenes, shows the multiple character of this bond. This effect is responsible for the restricted rotation often observed for the heteroatom-carbene carbon bond in NMR studies. For example cis and trans isomers 11.8 and 11.9 of methoxymethyl carbenes are rapidly interconverting at room temperature (Eq. 11.2), but can be frozen out in the proton NMR at -40 C.  

Isonitriles are very sensitive to nucleophilic attack, and a wide range of bis-heteroatom-stabilized carbenes can be obtained  

Chugaev obtained carbene complexes very similar to these as early as 1915, but was not able to assign the right structure, given the methods available at that time. Acetylides L M—C CR are unexpectedly good bases via their canonical form L M =C=C R. They can react with acid in alcohol solution to give the carbenes shown in Eq. 11.7. An intermediate vinylidene cation probably undergoes nucleophilic attack by the alcohol. In this case the usual order of attack shown in Eq. 11.3 (Nu , then E ) is inverted. 

Electrophilic abstraction from an alkyl complex (Eq. 11.4a) is illustrated in the reactions of Eqs. 11.8 and 11.9 Eq. 11.9 is driven by Ae high Si—F bond strength. 

Alkylidenes can sometimes be made from organic carbene precursors such as diazomethane or R2CCl2.  

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Many other organometaUic compounds also react with carbonyl groups. Lithium alkyls and aryls add to the ester carbonyl group to give either an alcohol or an olefin. Lithium dimethyl cuprate has been used to prepare ketones from esters (41). Tebbe s reagent, Cp2TiCH2AlCl(CH2)2, where Cp = clyclopentadienyl, and other metal carbene complexes can convert the C=0 of esters to C=CR2 (42,43). 

These transition metal carbenes, prepared in 66-97% yield from amino acid esters, are cleaved by acid hyrolysis (CF3CO2H, 20°, 80% yield 80% AcOH M = W, BBr3, -25°). ... 

Heterocycles as ligands in asymmetric catalytic metal carbene transformations 98CRV911. 

New catalysts and methods for enantioselective metal carbene reactions in syntheses of O- and N-heterocycles 98PAC1123. 

Enantiocontrol in syntheses of y-lactones catalyzed by metal carbenes 99IZV16. 

MII Transition Metal Carbene Complexes (F. R. Kreissel, ed.), VCH,... 

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. 

However, formation of the metal carbene complex was not observed in pure, halide-free [BMIM][Bp4], indicating that the formation of carbene depends on the... 

Another means of in situ metal-carbene complex formation in an ionic liquid is the direct oxidative addition of the imidazolium cation to a metal center in a low oxidation state (see Scheme 5.2-2, route b)). Cavell and co-workers have observed oxidative addition on heating 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPli3)4 in refluxing THF [32]. The Pt-carbene complex formed can decompose by reductive elimination. Winterton et al. have also described the formation of a Pt-car-bene complex by oxidative addition of the [EMIM] cation to PtCl2 in a basic [EMIM]C1/A1C13 system (free CP ions present) under ethylene pressure [33]. The formation of a Pt-carbene complex by oxidative addition of the imidazolium cation is displayed in Scheme 5.2-4. 

In the light of these results, it becomes important to question whether a particular catalytic result obtained in a transition metal-catalyzed reaction in an imidazolium ionic liquid is caused by a metal carbene complex formed in situ. The following simple experiments can help to verify this in more detail a) variation of ligands in the catalytic system, b) application of independently prepared, defined metal carbene complexes, and c) investigation of the reaction in pyridinium-based ionic liquids. If the reaction shows significant sensitivity to the use of different ligands, if the application of the independently prepared, defined metal-carbene complex... 

Another approach is a stepwise mechanism that involves the initial formation of a metal carbene followed by the formation of a four-membered metallocycle species ... 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

A decade after Fischer s synthesis of [(CO)5W=C(CH3)(OCH3)] the first example of another class of transition metal carbene complexes was introduced by Schrock, which subsequently have been named after him. His synthesis of [((CH3)3CCH2)3Ta=CHC(CH3)3] [11] was described above and unlike the Fischer-type carbenes it did not have a stabilizing substituent at the carbene ligand, which leads to a completely different behaviour of these complexes compared to the Fischer-type complexes. While the reactions of Fischer-type carbenes can be described as electrophilic, Schrock-type carbene complexes (or transition metal alkylidenes) show nucleophilicity. Also the oxidation state of the metal is generally different, as Schrock-type carbene complexes usually consist of a transition metal in a high oxidation state. 

Dorwald FZ (1999) Metal carbenes in organic synthesis. Wiley-VCH, Weinheim... 

The subsequent insertion of the alkyne into the metal-carbene bond affords the (r]1 r]3)-vinylcarbene complex D, which may exist either as a (Z)- or an ( )-metallatriene. This intermediate maybe considered as a branching point in the benzannulation reaction as three diverging routes starting from this point have been explored. 

The inherent plane of chirality in the metal carbene-modified cyclophane 45 was also tested in the benzannulation reaction as a source for stereoselectivity [48]. The racemic pentacarbonyl(4-[2.2]metacyclophanyl(methoxy)carbene)-chromium 45 reacts with 3,3-dimethyl-1-butyne to give a single diastereomer of naphthalenophane complex 46 in 50% yield the sterically less demanding 3-hexyne affords a 2 1 mixture of two diastereomers (Scheme 30). These moderate diastereomeric ratios indicate that [2.2]metacyclophanes do not serve as efficient chiral tools in the benzannulation reaction. 

Scheme 37 Transmetalation of chromium to nickel in a metal carbene-mediated cyclisation reaction (L=cod, MeCN, alkyne)...

Casey CP (1981) Metal-carbene complexes. In Jones M Jr, Moss RA (eds) Reactive intermediates. Wiley, New York, p 135... 

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