Fischer-type carbenes carbene synthesis

Fischer-type carbene complexes in the synthesis of furan, pyrrole, 5//-furanone, and 5//-pyrrolone derivatives 98YGK413. 

Scheme 5 Synthesis of the first Fischer-type carbene complex...

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. 

Particularly stable are coordinatively saturated, 18-electron carbene complexes of the type (CO)5M=C(X)R (M W, Cr X OR, NR2 R H, alkyl, aryl). These complexes are often referred to as Fischer-type carbene complexes, in honor of E. O. Fischer, who prepared these compounds for the first time in 1964 [61]. Since then these compounds have attracted broad interest, and many hundreds of heteroatom-substituted carbene complexes have been synthesized. Thereby valuable new insights were gained into the nature of the carbon-metal double bond. These complexes are also becoming increasingly important for organic synthesis, both as reagents and as catalysts. 

Heteroatom-substituted (Fischer-type) carbene complexes are mostly used as stoichiometric reagents. For this reason only carbene complexes of reasonably cheap metals, such as chromium, molybdenum, tungsten, or iron have found broad application in organic synthesis. 

In addition to reactions characteristic of carbonyl compounds, Fischer-type carbene complexes undergo a series of transformations which are unique to this class of compounds. These include olefin metathesis [206,265-267] (for the use as metathesis catalysts, see Section 3.2.5.3), alkyne insertion, benzannulation and other types of cyclization reaction. Generally, in most of these reactions electron-rich substrates (e.g. ynamines, enol ethers) react more readily than electron-poor compounds. Because many preparations with this type of complex take place under mild conditions, Fischer-type carbene complexes are being increasingly used for the synthesis [268-272] and modification [103,140,148,273] of sensitive natural products. 

Few examples of the preparation of six-membered heteroaromatic compounds using Fischer-type carbene complexes have been reported [224,251,381]. One intriguing pyridine synthesis, reported by de Meijere, is sketched in Figure 2.35. In this sequence a (2-aminovinyl)carbene complex first rearranges to yield a complexed 1 -azadiene, which undergoes intermolecular Diels-Alder reaction with phenylacetylene. Elimination of ethanol from the initially formed adduct leads to the final pyridine. 

A major breakthrough in the synthesis of transition metal methylene and methylidyne complexes has been achieved by Stone and his group it originates from the simple idea that M=C double bonds in Fischer-type carbenes and M=C triple bonds in carbyne systems should add to low valent metal complexes as do C=C and C=C linkages, respec-... 

Reaction of Electrondeficient Olefins with Donor-Carbene-Equivalents One interesting application of Fischer-type carbene complexes in organic synthesis is their addition to acceptor olefins affording methoxy substituted cyclopropanes 65 (Eq. 20). 

After their discovery by E. O. Fischer and A. Maasbdl in 1964 [1], a large number of carbene complexes with various transition metals such as Cr, Mo, W, Mn, and Fe were prepared [2]. Their synthetic applications in organometallic and organic chemistry increased rapidly, especially with respect to annelation reactions (Ddtz reaction). Among all known Fischer carbene complexes there is no example featuring a diazo functionality. In this contribution we describe the synthesis of a new class of Fischer-type carbene complexes with a diazomethylsilyl substituent in a-position to the carbene carbon-atom. 

The first carbene compound to be well characterized was prepared in 1966 and was one of many Fischer-Type Carbene Complexes to be reported (see equation 7). Fischer carbenes are characterized by heteroatom substituents at the carbene carbon, stabilization by a low-valent metal center, and a partial positive charge at the carbene carbon. In contrast, Schrock-Type Carbene Complexes, or alkylidenes," that have alkyl substituents, are found on metal centers in higher oxidation states, and are nucleophihc at carbon. Many Fischer carbenes are known for chromium, whereas chromium alkylidenes are much less common. Monohalocarbenes of chromium, for example, (OC)5Cr=C(F)NEt2, have also been extensively investigated." Two carbene reactions of note for their application to organic synthesis are the cycloaddition of alkenes with carbene complexes and the reaction of aromatic carbenes with aUcynes to yield complexed naphthols (the Dotz reaction ). ... 

A very rapid growth of novel and synthetically usefiil reactions of Fischer-type carbene complexes see Fischer-type Carbene Complexes) has occurred over the last decade. Reactions leading to cychc and polycyclic compounds of high complexity and reactions wherein chirahty has been transferred from the starting material to the product are of particular importance. A number of recent reviews on the chemistry of Fischer carbene complexes and their application in organic synthesis are avaUable. ... 

The cyclic carbene complex (23) formed in equation 10.722 is analogous to the original Fischer-type carbene complex. The ring, however, offers an added complexity that has utility in the realm of organic synthesis. 

Nitrogen often appears attached directly to carbon in Fischer-type carbene complexes. Equations 10.823 and 10.924 provide two examples of N-substituted carbene complex synthesis. The first procedure involves attack by the amide on one of the carbonyls (analogous to equation 10.6) followed by alkylation. 

Among the many reactions of Fischer-type metal-carbene complexes (see Chap. 9), the most remarkable and most applied to organic synthesis is the Dbtz reaction. " This complex reaction involves an unsaturated Fischer-type carbene complex with an alkyne yielding polycyclic hydroquinones. The principle of the scheme, the mechanism and an example are represented p. 522. 

In this chapter, we present an overview of organometallic transformations of Fischer-type carbene complexes relevant to organic synthesis with a specific focus on chromium carbenes bearing multiple functionalities. 

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