Isotope effects, kinetic inverse

The existence of tr-complex intermediates in C-H activation chemistry has been suggested to explain inverse kinetic isotope effects in reductive elimination processes whereby alkanes are formed from alkyl metal hydrides (Scheme 3).9... 

There is an inverse kinetic isotope effect, k /k-Q = 0.45 0.1 (for decomposition of PtH2(Mc3P)2) compared with PtD2(Me3P)2) in THF at 21 °C. This supports the previous prediction of nearly complete H-H formation in a late transition state. 

INVERSE KINETIC ISOTOPE EFFECT STERIC ISOTOPE EFFECT Isotope effects on Aiax/fCn,... 

Kinetic experiments with the acids HX and DX, performed under pseudo-first-order conditions, have revealed inverse kinetic isotope effects, shown in... 

Kaiser, E. W and T. J. Wallington, Comment on Inverse Kinetic Isotope Effect in the Reaction of Atomic Chlorine with C2H4 and C2D4, J. Phys. Chem. A, 102, 6054-6055 (1998). 

Participation of the hydride-formyl equilibrium in (16) is also plausible in light of an apparent inverse kinetic deuterium isotope effect for the catalytic process. Use of deuterium gas instead of hydrogen (cf. Expts. 6 and 4 in Table II) causes an increased rate, with kH/k = 0.73 (37). The existence of an isotope effect implies that hydrogen atom transfer occurs before or during the rate-determining step, and an inverse kinetic isotope effect may be possible in the case of a highly endothermic, product-like transition state (73). On the other hand, Bell has concluded that inverse kinetic isotope... 

As seen from Fig. 10, the non-exponential drop in the intensity of fluorescence for Nh-spontaneous deactivation, for which a normal isotopic effect (td > ih) is observed. Note that the possibility of the abnormal isotopic effect for electron tunneling reactions follows directly from the theoretical concepts set out in Chap. 3, Sect. 6. The mean values of the parameter / obtained from experiments with various concentrations of CC14 proved to be p = (0.240 0.010) M 1 for Nh d8 and j = (0.205 0.010) M l for Nh. As the effect of the nuclear motion on W R) must be reflected more in the value of ve than in that of ae it seems natural to connect the difference observed between the values of P for Nh and those for Nh-d8 with the change in the parameter ve. At the value of ae 1 A typical of tunneling reactions, the difference observed in the values of P corresponds to an approximately 2.5-fold increase of ve upon naphthalene deuteration. With an increase in temperature from 77 to 140 K, the parameter / remained virtually unchanged, although the time, t, for spontaneous deactivation was markedly reduced. Thus, tunneling reaction (14) proceeds via a non-activated mechanism. 

The kinetics and the products of bromination of several substituted stilbenes with Bu4N+Br3 have been investigated in aprotic solvents at different temperatures and concentrations. Stilbenes bearing electron-withdrawing or moderately electron-donating substituents gave stereospecifically the anti addition products the reaction followed a second-order rate law and inverse kinetic isotope effect Ap/Ap = 0.85 ( 0.05) was... 

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... 

SCHEME 11.19 Two scenarios that lead to inverse kinetic isotope effect for overall C—H reductive elimination (HE = inverse isotope effect NIE = normal isotope effect). 

It is interesting to point out that significant inverse kinetic isotope effects (Rd/Rh = 1.3-3.3) for the initial rates of a number of alkane conver-... 

Mori et obtained an inverse kinetic isotope effect for the hydrogenation... 

In summary, it is apparent that an inverse kinetic isotope effect has now been demonstrated conclusively for a variety of catalysts. Most investigators now consider the... 

Basallote has shown that protonation of the hydride in eq 35 results in an Fe(Ti -H2) complex [55]. The kinetics (monitored electrochemically) showed an inverse kinetic isotope effect consistent with the mechanism shown [56] — normal kinetic isotope effects are expected for metal protonation. Again (eq 18, Section 2.3.1.1) [36], no correlation between the rate of protonation and the apparent p (HX) in THF [38] was observed, possibly due to homoconjugate or ion pairing interactions. 

Unfortunately, the two situations (a single elementary step with a late transition state and a rapidly maintained thermodynamic equilibrium) that can give inverse kinetic isotope effects are kinetically indistinguishable. 

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). 

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