Fisher carbene complexes

It is well known that metal carbenes can be classified as Fisher and Schrock carbenes. The classification is mainly based on the n electron density distribution on the M = C moiety (Scheme 4.2). On the basis of the n electron density distribution, carbene complexes of the Fisher-type (E) are normally electrophilic at the carbene carbon while carbene complexes of the Schrock-type (F) are nucleophilic at the carbene carbon. Similarly, metal vinylidenes could also be classified into the two types Fisher-type (G) and Schrock-type (H). The majority of isolated metal vinylidenes belong to the Fisher-type. On the basis of the 7t electron density distribution shown in... 

Ethers, sulfides, amines, carbonyl compounds, and imines are among the frequently encountered Lewis bases in the ylide formation from such metal carbene complex. The metal carbene in the ylide formation can be divided into stable Fisher carbene complex and unstable reactive metal carbene intermediates. The reaction of the former is thus stoichiometric and the latter is usually a transition metal complex-catalyzed reaction of a-diazocarbonyl compounds. The decomposition of a-diazocarbonyl compounds with catalytic transition metal complex has been the most widely used approach to generate reactive metal carbenes. For compressive reviews, see Refs 1,1a. 

K. H. Dotz, H. Fisher, P. Hofmann, F. R. Kreissl, U. Schubert and K. Weiss, Transition Metal Carbene Complexes, Verlag Chemie, Deerfield Beach, FL, 1983. 

Enantiopure alkynyl(alkoxy)carbene (37) complexes were produced by formal alkyne insertion into Fisher carbene complexes 39 Reaction of (37) with 1-azadiene gave functionalized 1,4-dihydropyridine (38) with high enantiomeric excess. 

The first carbene complexes 146 were prepared by the Nobel laureate E.O. Fisher [238], At present, several hundred of this kind of compound have been prepared with practically all transition metals [8,239-244]. 

Dotz, K.H. Fisher, H. Hofman, P. Kreissel, F.R. Schubert, U. Weis, K. Transition Metal Carbene Complexes. Verlag Chemie Weinheim, 1984. 

Sugar acetylene reacted with a Fisher chromium carbene complex to give a phenol derivative, a possible intermediate for naturally occurring aryl-C-glycoside antibiotics (O Scheme 34) [55]. 

The intramolecular C-H insertion of chiral diazo compounds has been catalyzed by several metal salts derived from Ag(I), Ni(II), Pd(II), and Co(III) [64, 65]. The use of some stoichiometric chiral Fisher carbene complexes has also been reported to give the corresponding cyclic products with good selectivities [66,67]. 

Alkylation of the Fisher carbene complexes proceeds in high yields under PTC conditions (eq. 194). 

While Fischer carbene omiplexes tend not to react widi unactivated alkenes, many chromium and molybdenum Fisher carbene complexes do mtmocydoprtqianate 1,3-dienes in good yield with high levels of chemo-, regio-, and stereoselectivity... 

Harrity and co-workers described the application of 2-substituted 1-alkynylboronic esters in the Dotz cycloaddition of Fisher chromium carbene complexes, affording in a highly regioselective fashion a novel class of hydroxy-naphthyl boron pinacolates (entry 18, Table 1.3) [217]. These reaction products also provided, upon treatment with ceric ammonium nitrate, the corresponding quinone boronic esters. 

Another synthetic pathway to seven-membered rings is by reacting chromium Fisher carbene complexes with... 

As the Si=Si bond, the Si=C bond is not stable if it is not sterically protected. The electronic stabilization can also be introduced by a 3-center delocalization analogous to that found in Fisher-type metal-carbene complexes. 

Cyclic a-diazocarbonyl compounds (59) and enynones (61) have been used as Rh-and Zn-carbenoid precursors, respectively. Cyclic derivatives (59) have been found to favour intermolecular Rh-catalysed cyclopropanation reactions, relative to the formation of conjugated alkene (60) by intramolecular -hydride elimination as is usually observed in the case of a-alkyl-a-diazocarbonyl compounds this high level of chemoselectivity is reported for the first time. Rh-carbenoids derived from (59) have also promoted cyclo-propenation reactions as well as diverse X-H insertion reactions (i.e., X = C, N, O, S). In parallel, highly functionalized cyclopropylfiirans (62) have been successfiilly prepared from an alkene and an enynone (61) by a cyclization/cyclopropanation sequence conducted in the presence of catalytic amounts of ZnCl2, which is cheap and of low toxicity computations support the probable participation of intermediate Fisher-type Zn(II) carbene complexes (63). 

The reactivity of alkynyl Fisher carbene complexes (82) towards diverse oxime derivatives has been evaluated by experimental and theoretical approaches (Scheme 7). While indanone acetyloxime forms fused pyridine (83), benzophenone acetyloxime forms heterocycles (85), (86), and predominantly (84). 

A combination of a Diels-Alder and a Fisher carbene-cyclopentannulation is described as the last example in this subgroup. Thus, Barluenga and coworkers used a [4+2] cycloaddition of 2-amino-l,3-butadienes 4-115 with a Fischer alkoxy-arylalky-nylcarbene complex 4-116 this is followed by a cyclopenta-annulation reaction with the aromatic ring in 4-116 to give 4-117 (Scheme 4.25) [36]. An extension of this domino process is the reaction of 4-118 with 2equiv. of the alkynyl carbene 4-119 containing an additional C-C-double bond (Table 4.2) [37]. The final product 4-120, which was obtained in high yield, is formed by a second [4+2] cycloaddition of the primarily obtained cyclopenta-annulated intermediate. 

One of typical reactions of the Fisher-type metal carbene is interaction of the electron-deficient carbenic carbon with a pair of non-bonding electrons contributed by a Lewis base (B ) to generate a metal complex-associated ylide or a free ylide. The ylide intermediate thus generated is usually highly reactive and undergoes further reactions to give stable products (Figure 1). 

The imine bond in 2-imidazolines reacts with Fisher carbenes under irradiation (Scheme 144). Low yields were obtained with monosubstituted imidazolines 608a-c due to thermal decomposition of the substrates under the reaction conditions. Optically active 4,4-disubstituted A -imidazolines 608d-e form azapenams (609d, 610e, 611) with (methoxymethylcarbene)chromium complex in good yields and high diastereoselectivity. The electronic nature of RVR clearly influences the sense of diastereoselectivity (609 vs. 610, 611) <1977JOC3586>. 

Complexes Stabilized by One Heteroatom (Fisher-type Carbenes) 218... 

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