Carbenes preparation

These transition metal carbenes, prepared in 66-97% yield from amino acid esters, are cleaved by acid hyrolysis (CF3CO2H, 20°, 80% yield 80% AcOH M = W, BBr3, -25°). ... 

Allylzinc halides, with polymer-supported imines, 2, 398 [(7t-Allyl)zirconoxyl]carbenes, preparation, 4, 921 a-Activation, metal fluoroalkyls and fluoroaryls, 1, 735 Altemaric acid, via Alder-ene reactions, 10, 595 Alternative solvents, for green organometallic chemistry fluorous media, 12, 844 ionic liquids, 12, 846... 

Manganese carbenes, preparation, 5, 825 Manganese carbonyl complexes with 776-arene complexes, 5, 787 cyclopentadienyl complexes, 5, 783 with hydrocarbon ligands, complexes, 5, 776 synthesis and characteristics, 5, 761 Manganese carbonyl halides, applications and reactivity,... 

When each stereoisomeric reactant forms a different stereoisomeric product the reaction is known as stereospecific reaction. For example, the addition of CBr2 (dibromo-carbene, prepared from bromoform and base) to ds-2-butene gives cis-2,3-dimethyl-l,l-dibromocyclopropane (1.32), whereas addition of CBr2 to the trans-isomer exclusively yields the trans-cyclopropane 1.33. 

Fig. 3.23. Fischer-type carbenes prepared from [Ir(K -Tpp )(ri -isoprene)] and described in Ref. 133.

B.i. Diazomethane. Important precursors for the preparation of carbenes and related molecules are diazoalkanes. Although diazomethane is not used in carbene preparations, it has other synthetic applications There are four convenient preparations of diazomethane, which is a highly reactive gas. 

Dithioacetals are recently employed as a carbene synton. Titanium carbenes prepared in situ by reduction of CP2T1CI2 with Mg in the presence of P(OEt)3 followed by addition of dithioacetals are effective reagents for olehnation of carbonyl compounds (Eq. 3.38) [152]. 

The second column (Generation/Matrix or Solvent) briefly desribes the generation of the carbene. For a detailed description of the carbene preparation, we refer to the original papers. Also the matrix medium, in which the EPR spectrum was measured, is specified. Furthermore, the method (MethodIT) used to determine the magnetic parameters is given, the sample temperature is stated in K units. 

Phenylketene generated from phenylacetyl chloride with triethylamine gives [2 + 2] imine cycloaddition with catalysis by the N-heterocyclic carbene prepared by partial electrochemical reduction of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4) as solvent gives iraws-p-lactam formation (Eqn (4.92)). The zwitterionic intermediate formed in the reaction is stabiHzed by the highly ionic solvent, which also allows isomerization to form the trans product. The use of such N-heterocyclic carbenes in the catalysis of [2 + 2] cycloadditions of ketenes has been reviewed. ... 

A brief review has also appeared in which various carbene preparative methods are considered. A frequent competing reaction is carbene insertion into an allylic or other C—bond. After generation of cyclopentylcarbene (39X however, it is the intramolecular C—H insertion which generates the cyclopropane ring this has been investigated in detail. ... 

Carbenes and carbenoids constitute a class of reactive intermediates traditionally intimately associated with the synthesis of cyclopropanes, and more recently with products derived from C-H insertion processes. The reactions of diazoacetic acid esters with aromatic hydrocarbons such as toluene to give cyclopropanes with the liberation of N2, date back to the seminal experiments by Buchner and Curtius reported in 1885 [11]. Much later, in 1942, Meerwein reported that carbenes generated from diazomethane undergo insertion into C-H bonds [12], Seminal experiments that had significant impact for the utilization of such reactions in synthesis were performed by Stork, who demonstrated that carbenes prepared by photolysis of diazoketones participated in stereospecific intramolecular cycloadditions with al-kenes to form bicyclic cyclopropanes (Equations 1 and 2) [13]. Julia reported that intramolecular C-H insertion reactions of a chiral substrate 5 including a stereogenic methine center proceeded stereospecifically with retention of configuration (Equation 3) [14]. 

---