Hydrogen oxidative addition, </8 species

On the basis of the above results, Krische and co-workers proposed the mechanism depicted in (Scheme 12) (32). The catalytic cycle begins with oxidative addition of the Rh complex to enyne 25, forming the rhodacyclopentene intermediate B-I. Homoljdic hydrogen activation via either (a) hydrogen oxidative addition or (b) a-bond metathesis to form vinyl rhodium species B-II is expected. 

Several mechanisms have been proposed for the platinum-catalyzed homogeneous hydrosilylation reaction. The most commonly invoked mechanism, proposed by Chalk and Harrod in 1965, consists of elementary steps similar to homogeneous hydrogenation, oxidative addition, migratory insertion, and reductive elimination (226). However, this mechanism fails to describe the indnction period or the presence of colloidal species at the end of the reaction. Lewis proposed an alternative mechanism based on the intermediacy of colloids that were detected by transmission electron microscopy after evaporation of catalsdically active solutions (227,228). 

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

The event, oxidative addition of a Ni(0) species upon dienes and aldehydes activated by coordination with Lewis acids to provide oxanickellacycles 45, has proven to take place quite generally, and many variations making the best use of the intermediate 45 have been developed. The key issue of the reactions discussed in Sect. 3 is a regioselective and stereoselective hydrogen delivery... 

As reported in Scheme 1 the process involves a series of steps. The alkylpalladium species 1 forms through oxidative addition of the aromatic iodide to palladium(O) followed by noibomene insertion (4-7). The ready generation of complex 2 (8-11) from 1 is due to the unfavourable stereochemistry preventing P-hydrogen elimination from 1 (12). Complex 2 further reacts with alkyl halides RX to form palladium(IV) complex 3 (13-15). Migration of the R group to the... 

The observation of extremely facile formation of an acetyl complex and the finding that oxidative addition is the rate-determining step are almost certainly related to the high selectivity observed in the reaction. Thus, the extremely short lifetime of any CH3—Rh species makes it unlikely that it would be reacted off to methane in the presence of hydrogen (and/or metal hydrides). 

The 2,3-substituted indols are formed via a palladium-catalyzed coupling reaction of aryl halide, o-alkenylphenyl isocyanide, and amine (Equation (122)).481 Oxidative addition of an aryl halide, insertion of both the isonitrile and alkene moieties of o-alkenylphenyl isocyanide, and 1,3-hydrogen migration may form a 7r-allylpalladium species, which is then attacked by an amine to afford an indol. 

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