Oxygen-containing carbenes

Schollkopf and co-workers have synthesized a number of cyclopropanone acetals by the addition of various sulfur- and oxygen-containing carbenes to ketene diethylacetals (Table 3).26>27> Similarly, cyclopropanone dithioacetals may be prepared by the addition of the Ws-thiomethyl and Ws-thiobenzylcarbenes 12a, b to olefins.29) However, cyclopropanone acetal formation by this method requires double bonds with considerable electron enrichment and the yields are generally low. With unsubstituted olefins such as cyclohexene, the carbenes 12 a, b tend to form dimeric and trimeric products such as 13 and 14, instead of the double bond addition products. 

Several other oxygen-containing carbenes are known, such as carbenes 220 through 225." ... 

The increased stability of ruthenium carbene complexes towards oxygen-containing compounds might be because later transition metals, having more d-electrons, are softer and hence react better with soft bases, e.g. olefins. The early transition metals, on the other hand, having few d-electrons, are generally harder and react preferentially with hard bases, such as water or carbonyl compounds. 

Table 4.7. Preparation of five-membered, oxygen-containing heterocycles by intramolecular carbene 1,5-C-H insertion.

If compound 8 is irradiated (X > 300 nm) in the presence of diazoalkanes, the product pattern shifts dramatically for the diazoacetates and -malonates which have oxygen-containing substituents capable of coordination. At temperatures of about -40 to -20°C (tetrahydrofuran), photolysis predominantly yields mononuclear metallacycles 12 which are not yet accessible by any other means (135, 136). An X-ray diffraction study revealed the strict planarity of the cyclic system (136). The bidentate ligands that wind around the metal arise from metal-mediated carbonyla-tion of the respective carbenes (Scheme 6). 

Another bis-carbene bearing an oxygen-containing functional group was synthesised by Diez-Barra et al. [188] employing a hydroxyethyl functional group on the backside of... 

The intramolecular O—H insertion reaction of a carbene has been successful in the synthesis of the oxygen-containing heterocycle 216 in quantitative yield (85JOC5223). Carbene generated by photolysis of diazo precursor 217 in protic solvents gives 2-phenyl-2,5-dihydroisobenzofuran competitively with insertion into the O —H bonds of the solvent (90JOC2325). 

The final product is a carbene (alkylidene) complex. Since the carbene C atom has an oxygen-containing substituent, it is an example of a Fischer carbene (see Section 22.19). The structures of two reaction products are shown above. 

The importance of 0x0 ligands has been previously suspected by the numerous examples of oxygen containing activators. These reactions clearly demonstrated the crucial role of an 0x0 hgand. This activity could well be based on the double bond character of the M=0 Unkage. This may well facilitate carbene formation via a sequence of metal hydride reactions involving internal oxidative addition and reductive elimination. 

Several experiments tend to confirm our finding that the stable high-valent metathesis catalyst is an oxo-carbene. First, Basset et found that oxygen-containing compounds (O2 or alcohols)... 

J-Oxygen-functionalised sp3 organolithium compounds react with alkenyl-carbene complexes to generate the corresponding cyclic carbene complexes in a formal [3+3] process (see Sect. 2.8.1). In those cases where the organolithium derivative contains a double bond in an appropriate position, tricyclic ether derivatives are the only products isolated. These compounds derive from an intramolecular cyclopropanation of the corresponding cyclic carbene complex intermediate [89] (Scheme 83). 

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. ... 

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