Carbenes, generation isolation

Carbenes, generated by several methods, are reactive intermediates and used for further reactions without isolation. Carbenes can also be stabilized by coordination to some transition metals and can be isolated as carbene complexes which have formal metal-to-carbon double bonds. They are classified, based on the reactivity of the carbene, as electrophilic heteroatom-stabilized carbenes (Fischer type), and nucleophilic methylene or alkylidene carbenes (Schrock type). 

Tellur oformaldehyde is the only representative of this class of tellurium compounds that has been isolated. Tclluroformaldehyde was prepared by passing carbene, generated by the decomposition of a diazomethane/diethyl ether mixture below 500°, over tellurium... 

The principle underlying the use of organomercury compounds for carbene generation (entry 6, Scheme 8.1) is again the a-elimination mechanism. The carbon-mercury bond is much more covalent than the C-Li bond, however, so that the merciuy systems are generally stable at room temperature and easily isolated. They then decompose to the carbene when heated in solution with an appropriate alkene. The decomposition appears to be a reversible unimolecular reaction, and... 

Free carbenes generated using this method can also be generated in situ in Schlenk glassware, either in the presence of a metal precursor or with the subsequent addition thereof. However, this approach requires the use of a strong base each time, and additional experimental time for the deprotonation reaction. Where a glovebox is available, the use of isolated pure free carbenes is more convenient. 

Yet another kind of alkene addition is the reaction of a carbene with an alkene to yield a cyclopropane. A carbene, R2C , is a neutral molecule containing a divalent carbon with only six electrons in its valence shell. It is therefore highly reactive and is generated only as a reaction intermediate, rather than as an isolable molecule. Because they re electron-deficient, carbenes behave as electrophiles and react with nucieophiiic C=C bonds. The reaction occurs in a single step without intermediates. 

Dichlorocarbene, generated by the action of 50 % potassium hydroxide on chloroform, adds to ethyl 1 W-azepine-l-carboxylate to furnish the all /rntu-trishomoazepine 12 in 35% yield280 (see Houben-Weyl, Vol. E 19b, p 1523). Subsequently, and as a result of a careful and detailed study of the addition of dichlorocarbene generated by the thermal decomposition of phenyl(trichloromethyl)mercury, it was deduced that carbene addition takes place sequentially in the order C4 —C5, C2—C3 and C6 — Cl. The intermediary mono- 10 and bis(dichlorocar-bene) 11 adducts have been isolated and characterized. 

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). 

A plausible pathway is that the aromatisation of the cyclohexadienone 92 by a proton shift is accelerated in the presence of Ac20 under formation of acetate 93. The simultaneously generated acetic acid then cleaves the acetate to form the free phenol 94 (Scheme 44). This effect was observed for the first time during studies towards the total synthesis of the lipid-alternating and anti-atherosclerotic furochromone khellin 99 [64].The furanyl carbene chromium complex 96 was supposed to react with alkoxyalkyne 95 in a benzannulation reaction to give the densely substituted benzofuran derivative 97 (Scheme 45). Upon warming the reaction mixture in tetrahydrofuran to 65 °C the reaction was completed in 4 h, but only a dimerisation product could be isolated. This... 

Van der Schaaf et al. described a synthesis of the 14-electron complex [RuHCl(PPr13)2] (32) from [RuCl2(COD)]A.,PPr31,isopropanol,and abase.Compound 32 is a suitable precursor for ruthenium carbene complex 33, as outlined in Scheme 10. Although 32 was isolated and structurally characterized, it may also be generated in situ for the preparation of the carbene complex 33 [18]. 

Kinetic studies using 1,9-decadiene and 1,5-hexadiene in comparison widi catalyst 14 and catalyst 12 demonstrate an order-of-magnitude difference in their rates of polymerization, widi 14 being the faster of the two.12 Furdier, this study shows diat different products are produced when die two catalysts are reacted widi 1,5-hexadiene. Catalyst 14 generates principally lineal" polymer with the small amount of cyclics normally observed in step condensation chemistry, while 12 produces only small amounts of linear oligomers widi die major product being cyclics such as 1,5-cyclooctadiene.12 Catalyst 12, a late transition metal benzylidene (carbene), has vastly different steric and electronic factors compared to catalyst 14, an early transition metal alkylidene. Since die results were observed after extended reaction time periods and no catalyst quenching or kinetic product isolation was performed, this anomaly is attributed to mechanistic differences between diese two catalysts under identical reaction conditions. 

These complexes can be isolated in some cases in others they are generated in situ from appropriate precursors, of which diazo compounds are among the most important. These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, palladium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Ethyl a-diazoacetate reacts with styrene in the presence of bis(ferrocenyl) bis(imine), for example, to give ethyl 2-phenylcyclopropane-l-carboxylate. Optically active complexes have... 

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