Fischer aminocarbene complexes

ELECTROCHEMISTRY OF FISCHER AMINOCARBENE COMPLEXES EFFECTS OF STRUCTURE ON REDOX PROPERTIES, ELECTRON DISTRIBUTION, AND REACTION MECHANISMS... 

Non-heteroatom-stabilised Fischer carbene complexes also react with alkenes to give mixtures of olefin metathesis products and cyclopropane derivatives which are frequently the minor reaction products [19]. Furthermore, non-heteroatom-stabilised vinylcarbene complexes, generated in situ by reaction of an alkoxy- or aminocarbene complex with an alkyne, are able to react with different types of alkenes in an intramolecular or intermolecular process to produce bicyclic compounds containing a cyclopropane ring [20]. 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

The Dotz benzannulation reaction, based on the alkyne cycloaddition to chromium carbene complexes, is the most important application of Fischer carbene complexes. Among the various Fischer carbene complexes, alkoxy and aminocarbene complexes of chromium undergo a novel inter- and intramolecular tandem alkyne insertion/ carbene annotation sequence to give 9H-carbazoles and nf/-benzo[fl]carbazoles. 

Fischer-type carbenes can also be modified via transition metal catalyzed reactions. Fischer chromium aminocarbene complexes can be used as nucleophiles in palladium-catalyzed allyUc substitution reactions with aUylic acetates and carbonates, alFording the corresponding allyl-substituted aminocarbenes. For example, reaction of the Uthiated carbene (15) gives (16) in good yield (Scheme 25). ... 

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. 

Metal-carbene complexes are stable in dilute aqueous acid, but strongly basic aminocarbene complexes react with HCl or HBr in ether at -40° to give products derived from protonation of the carbon-metal bond (Fischer et ai, 1973b). Alkoxy-substituted carbene complexes react with mixtures of HCl... 

The application of electrochemistry to the investigation of new organometallic molecules is illustrated in this contribution on two groups of recently synthesized Fischer-type aminocarbene complexes of chromium, tungsten, and iron involving about 40 derivatives. All of them represent molecules with two separated redox-active centers where the extent of their interaction (mutual influence) is given by the electron delocalization of the bridging unit, by the distance between the centers and/or by sterical effects. 

Electrochemical measurements permit also a quantitative comparison of two types of Fischer carbene complexes with different stabilizing heteroatom the aminocarbene complexes of Cr and W (data from Fig. 48.1 Table 48.1) and some analogous alkoxycarbene complexes of Cr and W (data published in Reference 26), whose reduction potentials were measured by cyclic voltammetry under similar conditions (in nonaqueous acetonitrile vs SCE). 

Herein we report on their reactions with a variety of nucleophiles and the transformation of the p.-carbyne groups into the Fischer type carbene ligands. Our attention will be focused on the methods that lead to aminocarbenes and to the electronic and steric factors which stabilize this currently unknown class of complexes. 

The tungsten alkoisycarbene complexes 40-41 ( Fischer-type metallocarbenes. Fig. 6.8) are known to possess a marked electrophilic character at the carbemc carbon. Facile aminolysis reaction occurs with primary and secondary amines, affording stable aminocarbenes (Scheme 6.10) [74]. 

---