D/H exchange of alkanes

The D/H exchange occurred mainly after complete consumption of the alkene, and no D-incorporated alkane was detected, which indicated that the coordinated NHC was easily displaced by the alkene and that these carbenes were less strongly bounded to the metal surface than was seen with mononuclear metal compounds [28]. These results strongly suggested that the imidazolium cations reacted with the nanoparticle surface preferentially as aggregates of the type [(DAI) (X) ] [(DAI) e (X)J" (where DAI is the 1,3-dialkylimidazolium cation and X the anion), rather than as isolated imidazolium cations. 

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],... 

The catalytic properties of this silica-supported tantalum hydrides are noteworthy. First, H/D exchange in D2/CH4 mixture is fast (0.2 mol/mol/s at 150 °C), which shows that these systems readily cleave and reform the C-H bonds of alkanes (Scheme 36(a)). Second, it also converts alkanes into its lower homologs and ultimately methane in the presence of H2 (hydrogeno lysis) at relatively low temperatures (150 °C). " The key step of carbon-carbon bond cleavage probably corresponds to an a-alkyl transfer on a Ta(m) intermediate followed by successive hydrogenolysis steps (Schemes 36(b) and 37). In the case of cycloalkanes, hydrogenolysis yields smaller cycloalkanes, but deactivation is very fast. This phenomenon has been associated with the rapid formation of cyclopentane and, thereby, with the formation of cyclopentadienyl derivatives videsupra Scheme 35), which are inactive for the hydrogenolysis of alkanes. 

The D/H exchange only occurs after the complete consumption of the alkene and no D-incorporated alkane was detected, indicating that the coordinated NHC carbene is easily displaced by the alkene and that these carbenes are less strongly bound to the metal surface than is observed in monuclear metal compounds [114]. 

Shilov reported some of the earliest evidence that transition metal complexes could selectively cleave the C-H bonds of alkanes in a catalytic fashion. Shilov showed that H/D exchange would occur between alkanes and deuterated acid in the presence of platinum complexes (Equation 18.5 and Table 18.1). In addition, Shilov showed that the oxidation of alkanes occurred in the presence of a platinum(II) catalyst, although a platinum(IV) complex was needed as the oxidant. These reactions led to a mixture of alkyl halides formed from the halide of the Pt(IV) oxidant (Equation 18.6) and trifluoroacetate from the trifluoroacetic acid solvent. The cost of platinum(IV) as an oxidant makes this reaction impractical. However, these results provided hope that selective alkane functionalization could be developed because H/D exchange occurred faster at primary C-H bonds than at secondary C-H bonds (Table 18.1), and some selectivity for oxidations of primary C-H bonds over secondary C-H bonds was observed. As noted in Chapter 6, these results motivated a large number of groups to seek transition metal complexes that would insert into, or by other means selectively cleave, the C-H bond of alkanes and create products from this bond cleavage that could be observed directly. 

There is ample evidence that the reductive elimination of alkanes (and the reverse) is a not single-step process, but involves a o-alkane complex as the intermediate. Thus, looking at the kinetics, reductive elimination and oxidative addition do not correspond to the elementary steps. These terms were introduced at a point in time when o-alkane complexes were unknown, and therefore new terms have been introduced by Jones to describe the mechanism and the kinetics of the reaction [5], The reaction of the o-alkane complex to the hydride-alkyl metal complex is called reductive cleavage and its reverse is called oxidative coupling. The second part of the scheme involves the association of alkane and metal and the dissociation of the o-alkane complex to unsaturated metal and free alkane. The intermediacy of o-alkane complexes can be seen for instance from the intramolecular exchange of isotopes in D-M-CH3 to the more stable H-M-CH2D prior to loss of CH3D. 

It is not exactly known how large a Pt ensemble must be which can catalyze the multiple H/D exchange with D2 of alkanes such as cyclopentane, but it stands to reason that at least two adjacent Pt atoms are required (probably more). It follows that a catalyst which has its Pt atoms predominantly isolated from each other should NOT show this product pattern, but give a product distribution typical of stepwise exchange. Such a product should follow the binomial law i.e. no predominant peak at C5H5D5 the concentrations of the CsHio- iD products at low exchange should show a monotonous decrease with x. 

In aqueous acetic acid, the disproportionation of the platinum still occurs quite rapidly, and it can be suppressed further by adding mineral acid. Hydrochloric acid is often used, but this has a disadvantage in that the exchange rate is inversely proportional to the chloride ion concentration. Perchloric acid has been found to be more satisfactory (55). The platinum(II) catalyst most used is sodium or potassium tetrachloropla-tinate(II). An aromatic compound added to the reaction mixture also inhibits disproportionation of the platinum(II) complex—benzene, pyrene, and other aromatics have been used. A comparative study of the effect of various aromatics on the H—D exchange in alkanes has been carried out (55). Even under optimum conditions, the disproportionation [Eq. (4)] still takes place, and the catalytic platinum(II) is slowly removed from the reaction mixture. To get useful rates of exchange in alkanes, temperatures of 100° to 120°C have to be used, and the disproportionation rate increases with temperature. 

Fig. 5. Rate of H—D exchange versus ionization potential of alkanes and aromatic compounds 1 = methane 2 = ethane 3 = propane 4 = n-butane 5 = n-pentane 6 = n-hexane 7 = cyclopentane 8 = cyclohexane 9 = benzene 10 = naphthalene 11 = phenanthrene 12 = 2,2-dimethylbutane (see text) 13 = 1,1-dimethylpropy I benzene (see text) 14 = 2-methylpropane 15 = 2-methylbutane 16 = 2,2-dimethylpropane 17 = 2-methylpentane 18 = 3-methylpentane 19 = 2,3-dimethylbutane 20 = 2,2-dimethylbutane.

Fig. 25. Two possible pathways proposed by Sommer and co-workers explaining the observed H/D exchange of the alkanes. Pathway 1 the carbenium ion Rh (nondeuterated) is formed by protolytic cleavage of a C-H bond (via a carbonium ion intermediate). Pathway 2 the olefin R-is formed by acid-base bifunctional dehydrogenation. (Reprinted with permission from Sommer et al. (130a). Copyright 1995 American Chemical Society.)...

The stereochemistry of hydrogenation, the long recognized predominantly cis addition103-105 (see Section IV.A.3) is also consistent with the stepwise addition of the two H atoms via the half-hydrogenated intermediate. H-D exchange reaction of alkanes is also interpreted with the involvement of the surface alkyl intermediate106,107. 

Under the same conditions, several types of hydrocarbon are also converted to fully deuterated compounds. The results are summarized in Table 1. Cydooctene was also transformed into fully deuterated cydooctene without a skeletal rearrangement. As shown in entries 2 and 3, saturated hydrocarbons have also been transformed into fully deuterated compounds. As described above, an interaction between allylic C-H bonds and palladium hydride induces the H-D exchange reaction for alkenes. H-D exchange in alkanes, however, cannot be explained in this way. Direct C-H activation without assistance from any functional group may be a route to the formation of fully deuterated alkanes. 

H/D exchange in alkanes is catalyzed by a number of heterogeneous catalysts, such as Ni/alumina. ... 

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