Kinetics carbene catalysis

The insertion of an NHC (84) into a non-acidic CH bond has been studied in depth.64 The mechanism, kinetics, and catalysis by an amide base (K-HMDS) were all investigated. The carbene was found to be stable to dimerization and inserted slowly into a methyl CH bond of toluenes to give aminals (85). The rate of aminal formation was strongly dependent on the para-substituent (Hammett p value of 4.8 0.3). The rate-determining step is CH bond cleavage, which occurs with a late transition state. 

Very recently, Rovis et al. [20] reported the application of combined enamine and carbene catalysis in the diastereo- and enantioselective synthesis of functionalized cyclopentanones (Scheme 43.10). The authors proposed that the secondary amine catalyst was capable of epimerizmg the a-position of the intermediate aldehyde to form an equilibrium between two diastereomers. Then the chiral triazohum catalyst preferred cyclization with only one of these diastereomers to the final product. The second step of this reaction could be considered analogous to a dynamic kinetic resolution because the j0rgensen-Hayashi amine catalyst would be able to interconvert the two diastereomers. 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

Others have investigated the kinetics of amination reactions mediated by catalyst systems employing the new electron-rich monodentate ligands. In particular, Hartwig has shown that for catalysis by a 1 1 palladium to Xn tert-butyl)phosphine system, a mechanism in which oxidative addition of aryl chlorides follows coordination of base to the palladium competes with the standard nonanionic pathway. Finally, Caddick, Cloke, and coworkers have studied amination reactions of aryl chlorides performed by palladium complexes of N-heterocyclic carbene ligands. They found the rate to be limited by the oxidative addition step, which occurs first through the dissociation of an NHC ligand. 

McGuinness DS, Saendig N, Yates BF, CaveU KJ (2001) Kinetic and density functional studies on alkyl-carbene eUmination from Pd(II) heterocycUc carbene complexes a new type of reductive eUmination with clear impUcations for catalysis. J Am Chem Soc 123 4029 1040... 

Transition metal salts or complexes are known to catalyze effectively the cyclopropanation of olefins with diazoalkanes. Asymmetric synthesis with chiral copper catalysts (Nozaki et ai, 1966, 1968 Noyori et al., 1969 Moser, 1969), as well as a detailed kinetic study (Salomon and Kochi, 1973), has suggested the intervention of copper-carbene complexes as reactive intermediates. Recently synthesis of crysanthemic acid (CCXXXIV) (R = H) with high optical yield (60-70 %) has been achieved by applying this asymmetric catalysis (Aratani et al., 1975). The camphorglyoxime-cobalt(I) complex is also effective for the enantioselective reaction (Tatsuno et al., 1974). 

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