Isotope scrambling eliminations

The thermodynamic stabilities of phenonium ions relative to the parent have been determined in the gas phase by measuring the position of the equilibrium between (46) and (47)7 The results followed a Yukawa-Tsuno relationship with a p value of -12.6 and an r+ value of 0.62, the general behaviour being more like benzenium ions than benzyl cations, with tt-delocalization less effective than in benzyl cations. A theoretical study of the elimination of molecular H2 from the benzenium ion C6H7+ shows that the barrier to this process appears to be very small.The gas-phase Friedel-Crafts alkylation reaction of CF3C6L6+ (L = H or D) with C2L4 is accompanied by isotopic scrambling, which has been used to elucidate the mechanism of this process. A theoretical calculation shows that the lifetime of triplet phenyl cation must be very short. ... 

In addition to the indirect evidence of O-complexes as intermediates in metal-mediated reductive elimination/oxidative addition transformations from isotopic scrambling, KIEs for C—H reductive elimination reactions lend credence to the intermediacy of coordinated alkane intermediates.20 Assuming coordinated alkane intermediates, the equilibria and rates of reductive elimination and oxidative addition can be described with four rate constants that correspond to reversible C—H reductive coupling/oxidative addition and dissociation/association of the hydrocarbon substrate (Scheme 11.18). Several groups have reported that the rates of C—H reductive elimination of alkanes of perprotio versus perdeuterio variants yield inverse KIEs (i.e., kyjki) < l).20... 

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). 

The photochemistry of liquid and solid ethylene at 184.9 run shows that the ratio of the bee radical to the molecular decomposition is about 0.03 in the liquid at — 160°C (14). The same work indicates that there was isotopic scrambling in the various unreacted dideuterioethylenes. This scrambling was assumed to involve the relaxation of excited ethylene to ethylidene and may be followed by the molecular elimination of H2 ... 

When the deprotonation occurs in a reversible step prior to leaving group departure, isotope scrambling from the solvent to the site of deprotonation will occur. With a carbonyl containing reactant, the a-hydrogens will become deuterated at a rate faster than elimination if the reaction is performed in a deuterated protic solvent. 4-Methoxy-2-butanone is an example of a reactant that shows all the elimination attributes discussed here (Eq. 10.73). Note, as we mentioned above, these eliminations can occur with poor leaving groups— in this case methoxide. Such reactions are called EIcBr, where the R indicates a reversible first step. 

Cp Ir(H)2(CO)] is probably an intermediate under these conditions. Study of the reductive elimation of hydrocarbons from [Cp Rh(H)(R)L] revealed that the ease of elimination is influenced by the nature of the phosphine, L, in terms of both steric and electronic factors.A lack of isotope scrambling in frozen CD4 matrices supported the mechanism of simple H2 elimination during irradition of [CpIr(H)2(CO)]. ° On the other hand, carbonyl replacement by PPh3 in [CpRh(CO)2] showed a l iear relation between quantum efficiency and phosphine concentrations. This suggests that an associative process dominates with this nucleophile. 

Parkin and Bercaw reported that Cp 2W(Me)(H) eliminates methane to form Cp (ri5,ri1-C5Me4CH2)WH.26 For the mixed isotopomer, Cp 2W(CH3)(D), H/D scrambling to give Cp 2W(CH2D)(H) is competitive with the methane elimination process (Scheme 11.7). Although the authors point out that the H/D exchange process could occur by pathways other than formation of a methane-coordinated intermediate, the observation of an inverse kinetic isotope effect (KIE) for the methane reductive elimination (see bottom of Scheme 11.7) provides additional support for the reversible formation of coordinated alkane (see below for a more detailed discussion of KIEs for reductive elimination of C—H bonds). Furthermore, at relatively low concentrations, heating a mixture of Cp 2W(CH3)(H) and Cp 2W(CD3)(D) produces only CH4 and CD4 with no observation of H/D crossover, which is consistent with intramolecular C—H(D) processes. Similar results have been obtained for... 

For the thermolysis of tra s -[PdEt2(PR3)2] the results are very different The reactions are retarded only slightly by added phosphine, and no H-D scrambling occurs, suggesting that the /1-H elimination is irreversible in this case. Thus the decomposition of tra 5-[Pd(CH2CD3)2(PR3)2] evolved only a 1 1 mixture of CH2=CD2 and CD3CH2D, and showed a moderate isotope effect (%/fco =... 

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