H/D scrambling

Many mechanistic implications have been discussed, but we will concentrate here only on the most important structures in the context of dihydrogen-cation complexes. Deuterium-labeled methane and methyl cations were employed to examine the scrambling and dissociation mechanisms. The protonated ethane decomposition yields the ethyl cation and dihydrogen. Under the assumption that the extra proton is associated with one carbon only, a kinetic model was devised to explain the experimental findings, such as H/D scrambling. ... 

Attention, however, has been drawn to the problem of H/D scrambling at the liquid-liquid interface.167 In the spectrum of a solution obtained by extraction of cis-3,5-dimethylpiperidine from a dilute solution in cyclohexane with D2S04, the triplets due to 112 show a clear additional coupling... 

The nitrosyl complex [Rh(NO)(PPh3)3] (36) catalyzes the hydrogenation of both 1-hexene and cyclohexene in dichloromethane as solvent and was also found to add deuterium to cyclohexene without H/D scrambling.138 A further study extended the range of substrates hydrogenated to internal alkenes, to conjugated and non-conjugated dienes, activated alkenes and terminal and internal alkynes.139... 

C—H activation of the solvent however, upon addition of a strong donor (PMes) dihydrogen elimination is induced, affording TpPt(Me)(PMe3) (377). Definitive explanations for this observation, the facility of H/D scrambling and reluctance toward methane elimination have yet to be established, but are the subject of on-going experimental124 and theoretical studies.125... 

A propensity for H/D scrambling with solvent, in preference to methane elimination, has also been noted for the thermolysis (55—70 °C) of the related TpPtMe(H)2 (347) in 1 1 Cf,Hf,/CHyOD, which affords the fully deuterated 347-d5 over 6 h, with no evidence for loss of methane.124... 

Experiments with deuterated species show extensive H/D scrambling, which implies longer-lived intermediates in the reaction. Calculations at the B3LYP/6-311+G(2d,p) level suggest that the reaction pathway involves bond formation between the cycloheptatrienylidene dication and acetylene, followed by isomerization and hydrogen loss (Scheme 9). The authors of this study note the possibility of similar chemistry generating polycyclic aromatic compounds in interstellar space. 

H NMR studies gave insights into the stabilization of nanoclusters of iridium in imidazolium-type ionic liquids.30 It has been clearly established by H/D scrambling under a D2 atmosphere that NHCs are involved in this stabilization. 

Catalytic H/D scrambling of mixtures of H2 and D2 often takes place via intramolecular heterolysis of H2, as will be discussed further below. A recent example was proposed to involve cleavage of H2/D2 and proton transfer to NO ligands (Scheme 3) (54). 

Reductive elimination of H2 occurs when [Co(H)2 P(OR)3 4]+ salts are treated with CO or P(OR )3 and the reaction is not measurably reversible, p(H2) = 1 atm (equation 68).383 This contrasts with that found for isoelectronic [Fe(H)2 P(OR)3 4]. Also, the hydrogen exchange reaction (69) occurs without H-D scrambling and both reactions have been interpreted in terms of a dissociative [Co P(OR)3 4]+ intermediate.383... 

Upmacis, Poliakoff and Turner(lS) observed HD exchange for mixtures of Cr(CO)4(H2)2 and D2 but interestingly not for mixtures ol Cr(CO)5(H2) and D2. Kubas et al (lx) observe a similar phenomenon where HD is produced from reacting W(CO)3(PR3)2(H2) with D2 either in solution or in the solid state. In addition other cases of H—D scrambling occur readily with metal hydride complexes(U) as in the case of Cp ScH or Cp2ZrH where Cp =... 

The 4a- and 4j8-amino-steroids (90), (91), and (92) were prepared from 5a- and 5j8-cholestan-4-one by classical procedures, and their mass spectral fragmentations were discussed. The loss of a primary radical in the main fragmentations explained the low rate constant of these reactions and the important intramolecular H-D scrambling observed with a deuteriated derivative. 

In this case the starting ether has no methyl to exchange so that decomposition of the salt should give DME-dg (and dg, reflecting isotopic purity) exclusively. However in the observed isotope distributions (Table 5), dg is the smallest component (3.5%) and dg is absent. The data indicate extensive H-D scrambling even at the low (ca. 1%) conversions. Tetrahydropyran (0.4%), formed presumably via either a Stevens rearrangement or a carbenoid route was also observed. 

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... 

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