Oxidation reactions with Fischer carbene complexes

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). 

Titanium enolates.1 This Fischer carbene converts epoxides into titanium enolates. In the case of cyclohexene oxide, the product is a titanium enolate of cyclohexanone. But the enolates formed by reaction with 1,2-epoxybutane (equation I) or 2,3-epoxy butane differ from those formed from 2-butanone (Equation II). Apparently the reaction with epoxides does not involve rearrangement to the ketone but complexation of the epoxide oxygen to the metal and transfer of hydrogen from the substrate to the methylene group. 

Extensive studies on diastereoselectivity in the reactions of 1,3-dipoles such as nitrile oxides and nitrones have been carried out over the last 10 years. In contrast, very little work was done on the reactions of nitrile imines with chiral alkenes until the end of the 1990s and very few enantiomerically pure nitrile imines were generated. The greatest degree of selectivity so far has been achieved in cycloadditions to the Fischer chromium carbene complexes (201) to give, initially, the pyrazohne complexes 202 and 203 (111,112). These products proved to be rather unstable and were oxidized in situ with pyridine N-oxide to give predominantly the (4R,5S) product 204 in moderate yield (35-73%). 

The metal-free eyclobutane-1,2-dioxime can be generated by oxidative displacement. It is interesting to note that, unlike ketene dimerization, head-to-head dimerization takes place here. The chromium ketenimine complex 20 is prepared by reaction of the Fischer-type chromium carbene complex with alkyl isocyanides.60 A cyclobutane-1,2,3,4-tetraimine 24 has been reported from the reaction of the ketenimine phosphonium ylide 22.61 Bisimine 23 has been proposed as the intermediate in this transformation. 

Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex shown in Fig. 2.4, are often referred to as Fischer carbenes. The other type of alkylidene complexes has the metal ion in a high oxidation state. The tantalum complex is one such example. For both the types of alkylidene complexes direct experimental evidence of the presence of double bonds between the metal and the carbon atom comes from X-ray measurements. Alkylidene complexes are also formed by a-hydride elimination. An interaction between the metal and the a-hydrogen atom of the alkyl group that only weakens the C-H bond but does not break it completely is called an agostic interaction (see Fig. 2.5). An important reaction of alkylidene complexes with alkenes is the formation of a metallocycle. 

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 ). ... 

Compounds with metal-carbon double bonds, which are now known as carbene complexes, were first recognized in 1964 when E. O. Fischer and A. Massbol reported the syntheses of (OC)5M==CR(OR ) (M= Cr, Mo, and W) [13]. This type of carbene complex, which features a low oxidation number of the central metal and heteroatom(s) on the a-carbon, is called a Fischer-type carbene complex and is prepared typically by the following reaction starting from the hexacarbonyls (eq. (1)). 

Reaction of a reduced Philipps catalyst with Fischer-type molybdenum or tungsten carbene or carbyne complexes led to very active bimetallic, heterogeneous olefin metathesis catalysts. Surface metal ions might be involved in bonding interactions with the organometallic complex, possibly leading to heterometallic species on inorganic oxides. ... 

A recent synthesis [29] via phthalidotetralin intermediates is based on the benzannulation [30] of the Fischer chromium carbene complex 22 with the ethynyl phthalide 21. This reaction, which involves incorporation of a CO ligand as well as the carbene carbon in the formation of ring B, is highly regioselective, yielding the product 23. After oxidative removal of chromium... 

Oxidation replaces the carbon-metal double bond of metal-carbene complexes with a carbon-oxygen double bond. A variety of oxidizing agents including pyridine iV-oxide (Cotton and Lukehart, 1971), dimethyl sulfoxide (C. P. Casey, R. L. Boggs, and W. R. Brunsvold, unpublished results, 1974), ceric ion (Casey et al., 1972) and oxygen (Fischer and Riedmuller, 1974) have been employed. These oxidations are normally clean, high-yield reactions... 

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