Exchange Reactions deuterium-hydrides

Kohn and Taylor (40) also studied the influence of illumination on the hydrogen-deuterium exchange reaction using specimens of barium, calcium, lithium, and sodium hydrides. If the specimens were annealed in the hydrogen atmosphere, the photocatalytic effect on these specimens was positive. And if the specimens of the same hydrides were preliminarily calcined in vacuum, the irradiation of these specimens retarded the reaction. 

Deuterium labelling can, in principle, provide mechanistic information on hydro-formylation reactions, but complications may arise if scrambling of the D-label bet veen a metal hydride and D2 is fast. The isotopic exchange reaction shown in Eq. (9) was probed by rapid scan HP IR spectroscopy (1.3 scans s" ) where 21 is the bidentate pyrrolyl-based phosphorus amidite ligand illustrated [8]. 

Important mechanistic information on the hydroformylation reaction can be obtained by deuterium labeling studies. The outcome of these labeling studies, however, will be influenced if fast exchange between rhodium hydrides (or deuterides)... 

Analogous parahydrogen conversion and deuterium exchange reactions, catalyzed by NH2, have been observed in liquid ammonia (Wilmarth and Dayton, 61). The kinetics are of the same form as those of the OH -cat-alyzed reaction in water and the mechanism is open to similar interpretations. The NH2 -catalyzed reaction is much faster, its rate constant at —50° being 10 times that of the OH -catalyzed reaction at 100°. The assumption of equal frequency factors for the two reactions leads to a calculated activation energy for the NH2 -catalyzed reaction of about 10 kcal. This low value has been attributed to the much greater base strength of NH2 relative to OH . The results provide some support for the hydride ion mechanism. 

The reverse reaction was shown to be faster than the forward one, leading to the incorporation of deuterium into the alkyl hydride. With Cpf Zr(H)R, however, no exchange of deuterium with the hydride position occurred with D2, and the first step was proposed to be a facile and reversible metal-to-ring hydrogen transfer. This was then followed by oxidative addition of H2 to the intermediate 14-electron system followed by elimination of the hydrocarbon (91) ... 

The power to catalyse certain other reactions, in which dissociation of the molecules of hydrogen or of some other gas must occur at some stage of the reaction, is more certain evidence of chemisorption. One instance, the exchange reaction in which HD, deuterium hydride, is formed from a mixture of hydrogen and deuterium, has already been mentioned. Another is the immensely important synthesis of ammonia the reaction... 

It is interesting to note that C-H activation on ruthenium NHC complexes is not limited to intramolecular protons located in the N-sidechain of the carbene, but occurs inter-molecularly as well. Leimer et al. reacted [MesIRuH PCyj] with toluene-dg at ambient temperature and observed a rapid H/D exchange reaction involving the four hydride hydrogen atoms on ruthenium, the methyl protons of the mesityl substituents of the carbene ligand and the deuterium atoms on the meta positions of toluene-dg. The ortho-, para- and methyl-deuterium atoms of the solvent did not participate [145]. 

Studies of hydrogen-deuterium exchange reactions of some of these iridium hydrides show that two mechanisms are apparently exhibited, one possibly involving seven- or eight-coordinate intermediates. 

The activation energy for stereoisomerization of metal carbonyl hydrides varies considerably. Complexes possessing the coordination numbers 6 and 4 are generally inert, while compounds which have coordination numbers 5 or higher than 6 are usually stereochemically labile. Because of crucial importance in catalytic processes, the isotopic H-D exchange between hydrido complexes and hydroxyl solvents, deuterium, and olefins is of particular interest. Often, exchange reactions with solvents such as D2O or EtOD are catalyzed by acids and bases. 

It is found that the experimental data is explained by supposing that the metal surface covers itself with a layer of activated molecules of deuterium similar to the situation shown in Figure 16.29. The advantage of studying such exchange reactions is that the stabilities of hydrides and deuterides are equal, and hence attention may be focused exclusively on the surface conditions. 

The mechanism of the addition of the olefin to the hydride and its subsequent reduction is not yet known however, there is some evidence which is suggestive. If a deuterium/hydrogen mixture is used during the reduction of 1-pentene there is essentially quantitative formation of the di-deuteroparaffin and non-deuterated analogue. It follows that the Hz (or D2) molecule has been added to only one olefin and there are no exchange reactions. Further, it has been shown that the addition takes place by a cis mechanism. On the basis of these observations it is postulated that both the hydrogens add simultaneously to the olefin [47], e.g. 

Proton Abstraction. Although the exopolyhedral hydrogens of nido and arachno boranes are generally considered hydridic, the bridge hydrogens are acidic as first demonstrated by titration of and deuterium exchange (71). Some typical reactions are... 

The first step of the reduction by cobalt(II) chloride and NaBH4 involves the production of cobalt hydride species which is capable of exchanging hydrogen ligands with the medium. The second step is a hydrometallation reaction followed by a reductive cleavage of the carbon-cobalt bond. The hydrocobaltation seems to be reversible, as indicated by deuterium label incorporation93. 

At this point it is worthy of mention that solutions of these alkenyl-hydrido isomers react with hydrogen, at room temperature, to yield styrene and the starting [lrH2(NCMe)3(P Pr3)]BF4 complex. Deuterium treatment of the alkenyl-hydrido isomers shows an easy H/D hydride exchange, which suggests that the reaction with hydrogen is more favorable than C—H reductive elimination. Therefore, the hydrogenahon is dominated by an iridium(lll) species, and most probably iridium(l) species are not involved under catalytic conditions. 

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