Electrophiles metal carbene synthesis

There is a variety of reagents that undergo carbene-related transformations that do not fit into the categories of nucleophilic and electrophilic metal carbenes described earlier. Those that are the most versatile for organic synthesis, like the Tebbe... 

A review gives an overview over the contribution of organometalllc catalysis to Industry, another In the series on transition metals In organic synthesis covers CO reactions, hydrogenations and oxidations. The role of zirconium catalysts In hydro- and carboalumination of alkenes and alkynes, cyclopropanatlons and diene oligomerisations has been dlscussed, and an account deals with the role of electrophilic metal carbenes In cyclopropanatlons and other synthetic applications. 

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. 

Alternatively, alkoxycarbene complexes are formed upon alcoholysis of strongly electrophilic acyloxycarbene complexes 16 [26] generated by in situ acylation of tetraalkylammo-nium acyl metalates 15. These compounds are obtained from lithium precursors 11 and can be stored in a refrigerator for months. This is the method of choice for the synthesis of chiral metal carbenes 17 bearing terpene or sugar auxiliaries (Scheme 9). 

The chemistry of transition metal carbene complexes has been examined with an eye to applications in organic synthesis ever since their discovery by Fischer in 1964, and the growth in the number of useful applications has been exponential with tirne. " There are two types of transition metal carbene complexes those which have electrophilic carbene carbons and which are typified by the pentacarbonylchro-mium complex (1), and those which have nucleophilic carbene carbons and which are typified by the biscyclopentadienyltitanium complex (2). Complexes (1) and (2) are often referred to as carbene and alkylidene complexes, respectively. This review will be limited to the chemistry of electrophilic carbene complexes of the Fischer type. The chemistry of the nucleophilic alkylidene complexes will be covered in Chapter 9.3, this volume. ... 

Electrophilic aromatic substitution as a route to differentially substituted products is well established. The often forcing conditions, the incompatibility of this process with acid-sensitive functional groups, and the need for mild and selective syntheses have been the driving forces in the search for new methods of synthesis. A large range of methods has been developed over the past 20 years they include the trimerization of alkynes, the directed orfho-metallation, the benzannellation via metal carbenes, and transition metal-catalyzed carbon-carbon and carbon-heteroatom bond formation. Aromatic C-H activation, while still in its beginning stages, is another area of promise. 

These complexes, synthesized by E.O. Fischer in 1964, form a category called Fischer carbenes. They were the first successful approach to the synthesis and stabilization of metal-carbene complexes. They gave a rich chemistry (see Part V) because of their stability and ease of access. The presence of carbonyl ligands on the metal and of electronegative heteroatoms on the carbenic carbon makes this type of carbene electrophilic. i ... 

Complexes of nucleophilic carbenes are expected to react, like ylids, with electrophiles whereas complexes of electrophilic carbenes are expected to react, like carbocations, with nucleophiles and bases. All the complexes of terminal carbenes have in common the reactions with olefins, although their nature also varies. The principles of these reactions are detailed here, and application in catalysis and organic synthesis, are exposed in Parts IV and V respectively. Reactions of metal-carbene complexes leading to metal-carbyne complexes are mentioned in section 2. 

Cyclopropylcarbene complexes of the type L M=C(XR )R2 (X = O, S R1 = alkyl, aryl R2 = cyclopropyl) having a stabilizing heteroalkyl (XR1) group on the electrophilic carbene ligand (Scheme 3) have found widespread application in organic synthesis. These so-called Fischer carbene complexes are best known via their group 6 transition metal carbonyl complexes (CO)5M=(OR )R2(M = Cr, Mo, W)132. Much less abundant are the Schrock-type cyclopropylcarbene complexes L M=CR R2 where no heteroatom is bound to the carbene carbon atom133. 

The synthesis of equation (14) illustrates another useful reaction of Fischer carbenes, the abstraction of a proton to the metal by a base such as an organolithium reagent. The resulting negative charge can be delocalized onto the metal, and is therefore stabilized. The anion can be alkylated by carbon electrophiles as shown. 

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