Reductive elimination isotopic labeling

The acetylene-insertion reaction presumably occurs by the following mechanistic sequence (1) insertion of Pd(0) into the SCB, (2) regioselective yy -silylpalladation of the acetylenic compounds to provide seven-membered l-pallada-4-silacyclic intermediate, and (3) reductive elimination of Pd(0) to afford silacyclohexene. Alternatively, /3-hydride elimination would open the palladacycle, generating a vinylpalladium hydride species that would undergo reductive elimination to yield the ring-opened allylvinylsilane. Isotopic labeling studies provided evidence in support of this mechanistic hypothesis (Scheme 47). 

Other rhodium complex also catalyzed the addition of the C-H bond in aldehyde to olefins [115-117]. The use of paraformaldehyde results in the formation of aldehydes [115]. Marder et al. proposed the reaction mechanism of CpRh(eth-ylene)2-catalyzed addition of C-H bond in aldehyde to ethylene by the use of isotope-labeling experiments [117]. They suggested that insertion of ethylene to the Rh-H bond must take place rapidly and reversibly, and this equilibrium must be established significantly faster than either aldehyde reductive elimination or product formation (Scheme 4). 

Solution thermolysis of [PtIV(Me)2H(K3-Tp )] also induces reductive elimination of methane. C-D bond activation occurs after methane elimination in benzene-, to yield [PtIV(Me)(C6D5)D(k 3-Tp )] (Fig. 2.128), that upon a second reductive elimina-tion/oxidative addition reaction forms isotopically labeled methane and [PtIV(C6D5)2D(K3-Tp )]. [Pt(Me)(NCCD3)(K2-Tp )] has been obtained upon elimination of methane from [PtIV(Me)2H(K3-Tp )]. The a-methane complex [Ptn(Me)(K2-Tp )(CH4)] has been indicated... 

In the literature [25, 26a,c-g], inverse kinetic isotope effects for the reductive elimination of alkanes from metal centers, which is the miaoscopic reverse of alkane activation by oxidative addition, have been explained by the presence of an a alkane intermediate. Recently, thermolysis of the diastereomerically pure complexes (R5),(5R)-[2,2-dimethylcyclopropyl) (Cp )-(PMe3)lrH] and (/ / ),(5 5)-[2,2-dimethylcyclopropyl)(Cp )(PMe3)IrH] (see Scheme VI.5) in CaDs has been shown [26h] to result in its interconversion to the other diastereomer. The analogous reaction of the deuterium-labeled complexes resulted additionally in scrambling of the deuterium from the a-position of the dimethylcyclopropyl ring to the metal hydride position. Diastereomer interconversion and isotopic scrambling occurred at similar rates and have been discussed in terms of a common intermediate mechanism involving a metal alkane complex (Scheme VI.5). 

One of the earliest examples of the use of isotopic labeling involved the Cp Rh(PMe3)(CH2CH3)H system. The introduction of H at the metal hydride site revealed H/D exchange during the reductive elimination of ethane, but at a rate faster than overall ethane loss. In order to determine if the exchange process occurred via a /3-hydride elimination... 

The catalytic system consisting of [Pd(OAc)2] and P(t-Bu)2Me was applied to the arylation of a primary alkyl tosylate with 9-phenyl-9-BBN (Equation 5.7) [11]. An isotope-labeling study revealed that the substitution reaction proceeded with an inversion of configuration at the stereogenic carbon center as the net result of an inversion during the oxidative addition step and a retention during the subsequent reductive elimination step (Equation 5.8). 

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