Oxidation reaction with carbene complexes

The majority of gold(I) carbene complexes are pure organometallic compounds and the are out of the scope of this work. Some halide or triphenylphosphine carbene complexes are known and they will be considered here. Nucleophilic addition of alcohols or amines to gold-coordinated isocyanides is one of the best-established methods to obtain gold carbene derivatives. The reaction of H[Au(CN)2] with propene oxide and estirene oxide yields (cyano)carbene complexes (380) avoiding the intermediate step.2257 A cyclic carbene compound is obtained by reaction of a dinuclear isocyanide with amine (Scheme 32).2258... 

Alkenes and alkynes coordinate to transition metals and undergo a variety of reactions, and are very important substrates for transition metal-catalysed reactions. Their reactions with halides, cyclization via carbene complexes, hydrogenation, and oxidative reactions with Pd(II) are treated in Sections 3.2, 8.2, 10.1 and 11.1, respectively. The many other reactions of alkenes and alkynes as main reactants are treated in this chapter. 

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 ). ... 

Studies on these carbene complexes, especially those of the Schrock type, have attracted special interest in connection with the mechanism of catalytic olefin metathesis reactions. The formation of metallacyclobutane intermediate from the oxidative cycloaddition reaction between carbene complex and olefin was found to be an important key step in the catalytic cycle (eq. (5)). 

Redox reactions. With Pdl2 complexed to 3 kinetic resolution of secondary henzyhc and allylic alcohols can be carried out via enantioselective oxidation (O2, CS2CO3, PhMe, 80°). The acetoxydiiodo-Rh carbene complex of 3 is a catalyst for asymmetric reduction of aroylacetic esters with Ph2SiH2- ... 

Organometallic complexes of copper, silver, and gold are ideal precursors for carbene complexes along with some C- and N-coordinated species. Their reactivity pattern, in particular in oxidative addition reactions, was the most comprehensively studied. 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

The electrophilic carbene carbon atom of Fischer carbene complexes is usually stabilised through 7i-donation of an alkoxy or amino substituent. This type of electronic stabilisation renders carbene complexes thermostable nevertheless, they have to be stored and handled under inert gas in order to avoid oxidative decomposition. In a typical benzannulation protocol, the carbene complex is reacted with a 10% excess of the alkyne at a temperature between 45 and 60 °C in an ethereal solvent. On the other hand, the non-stabilised and highly electrophilic diphenylcarbene pentacarbonylchromium complex needs to be stored and handled at temperatures below -20 °C, which allows one to carry out benzannulation reactions at room temperature [34]. Recently, the first syntheses of tricyclic carbene complexes derived from diazo precursors have been performed and applied to benzannulation [35a,b]. The reaction of the non-planar dibenzocycloheptenylidene complex 28 with 1-hexyne afforded the Cr(CO)3-coordinated tetracyclic benzannulation product 29 in a completely regio- and diastereoselective way [35c] (Scheme 18). 

The use of a stereogenic carbon centre allowed an efficient asymmetric induction in the benzannulation reaction towards axial-chiral intermediates in the synthesis of configurationally stable ring-C-functionalised derivatives of al-locolchicinoids [51]. The benzannulation of carbene complex 52 with 1-pen-tyne followed by oxidative demetalation afforded a single diastereomer 53 (Scheme 33). 

The fact that pentacarbonyl carbene complexes react with enynes in a chemo-selective and regiospecific way at the alkyne functionality was successfully applied in the total synthesis of vitamins of the Kj and K2 series [58]. Oxidation of the intermediate tricarbonyl(dihydrovitamin K) chromium complexes with silver oxide afforded the desired naphthoquinone-based vitamin K compounds 65. Compared to customary strategies, the benzannulation reaction proved to be superior as it avoids conditions favouring (E)/(Z)-isomerisation within the allylic side chain. The basic representative vitamin K3 (menadione) 66 was synthesised in a straightforward manner from pentacarbonyl carbene complex 1 and propyne (Scheme 38). 

Diazoalkanes are u.seful is precursors to ruthenium and osmium alkylidene porphyrin complexes, and have also been investigated in iron porphyrin chemistry. In an attempt to prepare iron porphyrin carbene complexes containing an oxygen atom on the /(-carbon atom of the carbene, the reaction of the diazoketone PhC(0)C(Ni)CH3 with Fe(TpCIPP) was undertaken. A low spin, diamagnetic carbene complex formulated as Fe(TpCIPP)(=C(CH3)C(0)Ph) was identified by U V-visible and fI NMR spectroscopy and elemental analysis. Addition of CF3CO2H to this rapidly produced the protonated N-alkyl porphyrin, and Bit oxidation in the presence of sodium dithionitc gave the iron(II) N-alkyl porphyrin, both reactions evidence for Fe-to-N migration processes. ... 

Reaction of unsaturated chromium and tungsten carbene complexes 407 with piperidazine provides amino-carbene complexes 408 and 409. Although various solvents such as CH2C12 benzene, ether, and THF can be used in the reaction, the yields of the desired products 410 are 32-59%, and considerable amounts of by-products 409 are formed. The carbene complexes 408 are rather stable and can be stored in a refrigerator. Oxidation of both complexes with iodoso-benzene affords oxo derivative 410 in 70% (M = Cr) and 41% (M =W) yields (Scheme 64) <1994CL777>. 

---