Mechanisms of H-D exchange

Fig. 10. The general mechanism of H/D exchange. The right cycle for E = H or SMe. The left cycle holds only for E = H. D.+ = (H20),jD + and RE = reductive elimination.

Two simplest intermediate states of C2 and C3t> symmetry with identical geometrical parameters for H and D in a reaction complex (Figs. 5a,b) can be suggested for the synchronous mechanism of H/D exchange between a water... 

The mechanism of H-D exchange of H-2 in the thiazolium ions (107) was studied by n.m.r. A tetrahedral intermediate, formed by addition of a nucleophile, is proposed. The effects of solvents and size of the nucleophile on the reaction were studied.However, elsewhere it is claimed that the kinetic data demonstrate that the exchange of H at the 2-position of thiazolium ions cannot occur through a tetrahedral intermediate. ... 

Research in the area of small molecule gas-phase H/D exchange in ion traps has focused primarily on two fronts (1) model systems to investigate the mechanisms of H/D exchange and (2) isomer differentiation. 

Mechanism of H/D exchange between methane and acidic OH groups of zeolite that was theoretically justified [147] and confirmed with MAS NMR in situ kinetic measurements... 

Due to the expected noticeable difference in the H chemical shifts of the methyl and methylene groups (eg, 1.0 and 1.5 ppm for the CHj and CH groups of propane), H MAS NMR allows identification of these separate fragments in adsorbed alkanes [136]). This gives us a chance to get into details of the mechanism of H/D exchange for CyC alkanes and therefore to clarify the pathways of these alkanes activation on zeolite catalysts. 

Arzumanov SS, Reshetnikov SI, Stepanov AG, Parmon VN, Freude D. In situ H and C MAS NMR kinetic study of the mechanism of H/D exchange for propane on zeolite HZSM-5. J Phys Chem B 2005 109 19748-57. 

The dependence of the rate of H/D exchange on the electronic properties of the acceptor molecules have been studied extensively 39 40>. Mechanism (4) could... 

NMR has become an indispensable analytical technique for studying rates of H D exchange at specific sites when a protein is deuterated. Such exchange rates provide information on secondary protein structure and are useful in investigating the mechanism of protein folding. 

This general mechanistic scheme is widely accepted, but there are several different pathways possible for each of the three steps. For example, the reductive elimination could proceed via an Sn2 mechanism or via a concerted mechanism involving a three-center transition state. Many research activities have been devoted to the investigation of these detailed questions, which have been reviewed extensively by Stahl [3], while here only a summary is given (1) for the reductive elimination it could be concluded that an Sn2 mechanism is operative (2) the oxidation of RPt" to RPt does not occur by alkyl transfer, but by a two-electron transfer from RPt to Pt and (3) the electrophilic activation of the alkane is the most difficult part to investigate. At present none of proposed pathways shown in Scheme 5 could be discounted also, the experimental observation of H/D exchange in methane can be explained by both mechanisms. 

We have previously shown that the mononuclear zirconium hydride complexes 1 activate, under very mild conditions, the C-H bond of alkanes, including methane [7], The mechanism involves a four center intermediate, as proposed earlier for electrophilic activation of C-H bonds by group 3, 4 and lanthanides d° complexes [8], Given the similarities of the energies of dissociation of C-H and Si-H bonds, it is not surprising at ail that activation of Si-H bonds occurs with 1. Reactions of H/D exchange, followed by in situ IR spectroscopy, reveal that all types of silanes are activated, i.e. primary, secondary and even tertiary silanes [9],... 

SCHEME 11.3 Early examples of H/D exchange between metal hydrides and deuterated hydrocarbons and possible mechanism that involves C—D(H) coordination to the metal center. 

---