Oxidative Addition of Dihydrogen

Reaction coordinate diagrams and orbital interactions that occur during the oxidative addition of to transition metal complexes. (Orbital interactions from Hall, M. B. et al. Chem. Rev. 2000,100,353.) 

The stereochemistry of the metal center resulting from the addition of H has been studied, and the resulting data have revealed several principles. First, the kinetic product from addition of H has a cis disposition of the two hydride ligands, and the trans products are formed by isomerization of the initial cis products. Second, the addition of hydrogen to a square-planar complex can occur with high selectivity along one of the two bond axes due to electronic effects (Equation 6.7).  

Examples of Oxidative Addition of Hjto a Singie Metal Center 

One of the most significant oxidative additions of dihydrogen occurs to the complex RhQ(PPh3)3 (Equation 6.4) - which is Wilkinson s hydrogenation catalyst. Kinetic studies - of this oxidative addition reaction show that addition of can occur to the 

A third example of the oxidative addition of Hj is provided in Equation 6.6. This addition is also reversible. In this case, addition occurs to a d metal center to form a product containing a d metal center.  

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Figure 2.75 Oxidative addition of dihydrogen to Vaska s compound.

As briefly discussed in section 1.1, and shown in Figure 1, the accepted mechanism for the catalytic cycle of hydrogenation of C02 to formic add starts with the insertion of C02 into a metal-hydride bond. Then, there are two possible continuations. The first possibility is the reductive elimination of formic acid followed by the oxidative addition of dihydrogen molecule to the metal center. The second possible path goes through the a-bond metathesis of a metal formate complex with a dihydrogen molecule. In this section, we will review theoretical investigations on each of these elementary processes, with the exception of oxidative addition of H2 to the metal center, which has already been discussed in many reviews. 

Oxidative addition of dihydrogen commonly involves transformation of a d8 square planar metal complex into a d6 octahedral metal complex, or similar transformations involving d2 — d°, d10 —> d8 etc. The oxidative addition of... 

Ruthenium has a sufficient number of d-electrons to undergo oxidative addition of dihydrogen, which could then be quickly followed by reductive... 

The results in the table suggest that, if we neglect changes in the concentrations of the inactive species caused by the changes in the ligand, the rate-determining step is the oxidative addition of dihydrogen. 

This means that the less stable intermediate alkene complex reacts faster in the subsequent reactions. The next step in the hydrogenation sequence involves the oxidative addition of dihydrogen to the alkene complex (Figure 4.10). 

There are a few exceptions amongst the cationic complexes that also undergo oxidative addition of dihydrogen prior to alkene complexation. Alkylphosphines, raising the electron density on the rhodium cation, have been shown to belong to these exceptions, which seems logical [16] electron-rich phosphine complexes can undergo oxidative addition of H2 before the alkene coordinates to the rhodium metal. 

In Fig. 2.10 ignore the ligand dissociation step and assume the oxidative addition of dihydrogen to be a rapid reversible equilibrium followed by the rate-determining step of alkene insertion. What is the expected rate expression ... 

Ans. Not by oxidative addition of dihydrogen. Polarizations of bonds, including the C-H of methane by direct interaction between the substrate and metal ion. 

Assuming that only the reactions shown in Fig. 5.1 operate for the hydroformylation of propylene to n-butyraldehyde with 5.1 as the catalyst, and oxidative addition of dihydrogen is the rate-determining step, what should be the rate expression What is the implicit assumption ... 

Note that although the conversion of 7.11 to 7.12 assumes anti-Markovnikov addition, the Markovnikov product also gives butane. Conversion of 7.9 to 7.11 could also take place by prior coordination of alkene followed by the oxidative addition of dihydrogen. Indeed this parallel pathway for the formation of 7.11 does operate. Like the equilibrium shown between RhClL3, 7.9, and the dimer [RhClL L, there is an equilibrium between 7.9 and the alkene coordinated complex RhCl(alkene)L2. 

Oxidative addition of dihydrogen to 9.19 and 9.20 produces the intermediates 9.21 and 9.22, respectively. Insertion of the alkene into the Rh-H bonds produces diastereomers 9.23 and 9.24. It is important to note that the coordination site vacated by the hydride ligand, circled for easy identification, is taken up by the O atom of the carbonyl functionality of the acetamido group, and a solvent molecule occupies the original position of the O atom. 

The overall enantioselectivity of the catalytic process obviously depends on the relative speed with which the left and right catalytic cycles of Fig. 9.3 operate. Oxidative addition of dihydrogen is found to be the rate-determining step. Therefore, the relative rates of conversion of 9.19 to 9.21 on the one hand and 9.20 to 9.22 on the other determine which enantiomer of the organic product would be formed preferentially. The reaction between 9.28 and a-acetamido methyl cinamate has been monitored by multinuclear NMR, and both 9.19 and 9.20 have been identified. Depending on the stereochemistry of the chiral phosphine, one of these two diastereomers is preferentially formed. 

NMR data indicate the ratio of the concentrations of the major and the minor isomer to be approximately 10 1. Since the concentration of one of the isomers is almost ten times the other, if the rate constants for oxidative additions of dihydrogen are approximately the same, the major diastereomer should undergo conversion to the dihydride ten times faster. This, however, is not the mechanism for enantioselection. The mechanism of enantioselection is the much larger rate constant (—600 times) for the reaction between the minor isomer and dihydrogen ... 

Figure 9.5 Free energy diagram for the oxidative addition of dihydrogen to the two diastereomers of [Rh(R,R DIPAMP)(S)]+, where S = methyl-(Z)-a-acetamidocinamate. The major and minor refer to intermediates 9.19 and 9.20, one of which has higher equilibrium concentration than the other. The one with higher equilibrium concentration is called the major, and the other is called the minor isomer.

Assuming the free energy values shown in Fig. 9.5 are measured at 20°C, calculate the concentration ratio of 9.19 to 9.20 and the ratio of the rate constants for the oxidative addition of dihydrogen. 

In general, oxidative addition reactions occur at mononuclear complexes as discussed above. Oxidative addition of dihydrogen often occurs at binuclear complexes. The reaction of dicobalt octacarbonyl may illustrate this ... 

For metals in less than their highest oxidation state, this may proceed Figure 4.22 Hydrogenolysis of via oxidative addition of dihydrogen followed by reductive elimination a-alkyl ligands... 

There seems no reason why any of the mechanisms discussed in Sections 3.4-3.6 cannot function in the conversion of alkynes to alkenes. The alkene route of hydrogenation is frequently encountered because alkynes complex more strongly to transition metals than alkenes and their complexes are formed preferentially in competition with the oxidative addition of dihydrogen. Internal alkynes coordinate to bis(arylimino)acenaphthene complexes of palladium and the fricoordinate species activate molecular hydrogen. Transfer of both atoms of hydrogen forms... 

Several pentacoordinate monohydrido complexes can add solvents are nnsnitable since the hydrido complexes produced an additional, small, nentral ligand to form complexes of the may reduce them with formation of HCl and contaminate type [RhHX2L2L ]. The dihydrido complexes are conveniently the product with monohydrido complexes (see Scheme 16). prepared by the oxidative addition of dihydrogen to rhodium(I) A characteristic stoichiometric reaction of many dihydrides is... 

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