Hydrogen/deuterium scrambling

When perdeutero-MMA was copolymerized with perhydro-MMA in the presence of AIBN and a CCT catalyst under conditions that favor formation of MMA-dimer, it was observed that the product had undergone hydrogen/deuterium scrambling. Unreacted monomers in the reaction mixture were also scrambled. 

The cationic rhenium complexes [Re(NO)2(PR3)2] (R = Cy, Pr) [50] indeed show great potential for hydrolytic activation of dihydrogen. When [Re(NO)2(PR3)2l is treated with a mixture of H2 and D2 in toluene or chlorobenzene, hydrogen-deuterium scrambling is observed, and HD can be traced in the NMR spectrum of the reaction mixture [51]. The proposed mechanism for this catalytic exchange is illustrated in Scheme 4. 

Ferguson, P.L., Pan, J., Wilson, D.J., et al. (2007) Hydrogen/deuterium scrambling during quadrupole time-of-flight MS/MS analysis of a zinc-binding protein domain. Ana Chem, 79 (1), 153—160. 

However, despite the fact that the catalytic hydrogenation of double or triple bonds is a well-known and widely used reaction, several authors reported that when deuteration of unsaturated fatty acids or seed oils is earried out with deuterium instead of hydrogen, deuterium scrambling along the aliphatic chain and incorporation of a number of deuterium atoms higher than those required by the saturation of the double bonds can sometimes occur. [37] This extensive incorporation of deuterium atoms was explained in terms of... 

An N-benzyl-N-methyl carbonate in EtOH was hydrogenated on 10% Pd/C under 60 psi hydrogen for 20 hours. NMR analysis indicated that no deuterium scrambling had occurred during debenzylation (Scheme 4.81).328... 

Figure 29.24. Deuterium scrambling in indene via unstable intermediates IVfl and IV6 a series of [1,5] hydrogen shifts.

Figure 25.2 Thermal desorption spectrum of hydrogen deu-teride (HD, mass 3) from a Pd(210) surface that had received a simultaneous exposure of hydrogen, Hj, and deuterium, Dj, at 40 K. The ft states represent atomically adsorbed hydrogen (deuterium), while the y states are due to the molecular species. Apparently, practically no isotopic scrambling occurs in the y states (absence of HD), while the exchange is complete in the atomic f states. After Schmidt et al. [9,10].

A large number of papers related to this subject has been published since then, which can be classified into three types of mechanistic approaches (i) studies of initial product distribution, (ii) hydrogen deuterium exchange studies and (iii) theoretical calculations. In this review we will limit ourselves essentially to H/D isotope exchange studies between the acidic catalyst and small alkanes. In most cases, it is possible to observe inter-molecular H/D exchange in the absence of side reactions such as intramolecular atom scrambling, isomerisation or cracking. 

A useful compilation of 220 MHz n.m.r. data of substituted bicyclo[3,2,l]octan-6-ones has been published. The bicyclo[3,3,0]octyl cation appears to be implicated in the isomerization of bicyclo[3,2,l]oct-2-ene over SiOj-PaOj at 250 °C as deduced from the scrambling of label in [3- C]-tagged material and the distribution of label in the product, namely bicyclo[3,3,0]oct-2-ene. It has been found that 2,4-diphenyl-bicyclo[3,2,l]octa-2,6-diene (630) and its 6,7-dihydro-derivative undergo base-catalysed hydrogen-deuterium exchange at C-4 at essentially the same rate. Hence, the phenyl groups result in the total elimination of charge delocalization from C-4 to the C-6-C-7 double bond that is found in the unsubstituted bicyclo-octadiene. The... 

The lower KIE for the overall cycle indicates the existence of another step that has influence on the rate of the reaction. Isotope scrambling was not observed, which points to the irreversibility of the hydrogen/deuterium transfer reaction. Overall, the experimental results point to a C-H activation in which an irreversible proton abstraction is rate determining. 

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