Fischer complexes

Alkynyl complexes Fischer EO, Kalder HJ (1977) J Organomet Chem 131 57... 

The use of group 6 heteroatom stabilized carbene complexes (Fischer carbene complexes) in organic synthesis is relatively recent and, in spite of that, it has already produced impressive synthetic results [1]. These versatile organometallic reagents have an extensive chemistry, and they are probably one of the few systems that undergo cycloadditions of almost any kind. For instance, [1+2], [2+2], [3+2], [3+3], [4+1], [4+2], [4+3], [6+3]... 

A further synthetic approach to carbon-metal double bonds is based on the acid-catalyzed abstraction of alkoxy groups from a-alkoxyalkyl complexes [436 -439] (Figure 3.11). These carbene complex precursors can be prepared from alk-oxycarbene complexes (Fischer-type carbene complexes) either by reduction with borohydrides or alanates [23,55,63,104,439-445] or by addition of organolithium compounds (nucleophilic addition to the carbene carbon atom) [391,446-452]. 

Low-valent, 18-electron carbene complexes (Fischer-type) are electronically and coordinatively saturated and must usually be activated to become efficient metathesis catalysts. This activation can be effected by thermal or photolytic [781,782] removal of one ligand, e.g. carbon monoxide, or by treatment with a Lewis acid (Table 3.15) [783]. 

The subject of this book has been organized in three main sections preparation and applications of heteroatom-substituted carbene complexes (Fischer-type carbenes), non-heteroatom-substituted carbene complexes, and acceptor-substituted carbene complexes. In each section the different types of reaction have been ordered either according to the mechanism or according to the type of product. In addition to a selection of illustrative examples, several experimental procedures have been included. These were chosen taking into account safety, availability of starting materials, relevance of the products, and general interest. 

The reactions of carbyne complexes will be discussed, albeit more briefly, along the lines developed for alkylidene complexes, to emphasize the parallels. Once again, as with alkylidenes, early generalizations suggested two classes of complexes, Fischer-type and Schrock-type, which have subsequently been recognized as extremes in a tunable reactivity continuum. Furthermore, within the chemistry of tungsten, Fischer-type... 

Another important route for the preparation of 1-organo-l-organooxycyclopropanes is the reaction of alkenes with transition-metal carbonylcarbene complexes (Fischer complexes). ... 

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. 

Some particular stoichiometric reactions of stable carbenes complexes (Fischer type) of ylides and of alkylidene complexes are also considered. 

Owing to their physicochemical properties and reactivities, carbene complexes may conveniently be divided into (1) Fischer-type coordination compounds and (2) Schrock-type complexes. Fischer-type complexes encompass 18 compounds of... 

In a large group of complexes (Fischer carbenes) one of the substituents on the carbene contains a heteroatom X such as nitrogen, oxygen or sulphur which itself... 

Ammonia reacts with metal-carbene complexes under mild conditions to give amino-substituted carbene complexes (Fischer and Klabunde, 1967). A detailed investigation of the reaction of cyclohexylamine with (CO)s-... 

The conjugate addition of nucleophiles to a, -unsaturated carbene complexes has been observed in several cases. Dimethylamine was found to add to acetylenic carbene complexes (Fischer and Kreissl, 1972). Phenyllithium... 

Other interesting reactions of nucleophiles with metal-carbene complexes include the reaction of 1-aminoethanol which leads to imino-substituted carbene complexes (Fischer and Knauss, 1971a) and the reaction of isonitriles which leads to the unusual complex XV (Aumann and Fischer, 1968a). 

The reaction of sulfur-substituted carbene complexes with HBr at —30° gives sulfide complexes (Fischer and Kreis, 1973). 

Treatment of alkoxycarbene complexes with strong Lewis acids such as BBr3 leads to the formation of an interesting new class of organometallic compounds, the carbyne complexes (Fischer ef al., 1973d). 

Cr(CO)3-coordinated hydroquinone from vinylic alkoxy pentacarbonyl chromium carbene complex (Fischer carbene) and alkynes. 

The reduction of a metal salt by aluminium and aluminium trichloride in the presence of an aromatic hydrocarbon, which is illustrated in Table XIII, provides the most general method known for the preparation of bis-ff-arene complexes. It was first used by Fischer and Hafner for the synthesis of bis-w-arene chromium complexes (Fischer s reducing Friedel-Crafts proceditfe). 

Fischer Tropsch—The SASOL Complex. Fischer-Tropsch synthesis is the best-known indirect liquefaction method. It was developed in 1925 by German chemists Franz Fischer and Hans Tropsch. In this process, synthesis gas is reacted over an iron or cobalt catalyst at 350 psi and 330°C to produce largely aliphatic hydrocarbons, including gasoline, jet fuel, diesel oil, middle distillates, heavy oil, and waxes. The Germans used this technology during World War II to produce nearly 15,000 barrels/day of military fuels. 

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