Comparison of CH Activation to Other Alkane Coordination Reactions

Comparison of CH Activation to Other Alkane Coordination Reactions 

Organic Chemistry of Alkanes Coordination chemistry of carbon 

To illustrate the inner-sphere characteristics of the CH activation chemistry, an analogy can be made between CH activation by coordination of an alkane CH bond to a metal center and the known catalysis resulting from coordination of olefins via the CC double bond (note that the nature of the orbitals involved in bonding are quite different). It is well known that coordination of olefins to electrophilic metal centers can activate the olefin to nucleophilic attack and conversion to organometallic, M-C, intermediates. The M-C intermediates thus formed can then be more readily converted to functionalized products than the uncoordinated olefin. An important example of this in oxidation catalysis is the Wacker oxidation of ethylene to acetaldehyde. In this reaction, catalyzed by Pd(II) as shown in Fig. 7.14, ethylene is activated by coordination to the inner-sphere of an electrophilic Pd(II) center. This leads to attack by water and facile formation of an organometallic, palladium alkyl intermediate that is subsequently oxidized to acetaldehyde. The reduced catalyst is reoxidized by Cu(II) to complete the catalytic cycle. The Wacker reaction is very rapid and selective and it is possible to carry out the reaction is aqueous solvents. This is largely possible because of the favorable thermodynamics for coordination of olefins to transition metals that can be competitive with coordination to the water solvent. The reaction is very selective presumably because the bonds of the product (po- 

The useful comparison between the Wacker process and alkane CH activation is that all the coordination steps identified in the Wacker reaction (activation, functionalization and reoxidation) have parallels in catalytic, alkane CH activation and functionalization systems that operate with electrophilic catalysts. Thus, the coordination of the double bond of the olefin to electrophilic Pd(II) followed by cleavage by nucleophilic attack of water can be compared to CH activation of CH4 by an electrophilic substitution (ES) pathway. 

As shown in Fig. 7.14, in the CH Activation by an ES pathway the coordination of the CH bond of the alkane to an electrophilic center followed by loss of a proton can also be described as a nucleophilic attack of sol on the coordinated CH bond, leading to CH cleavage and generation of an intermediate E-CH3 species. Of course, it would be expected that as the CC double bond is considerably more electron rich than an alkane CH bond, more reactive electrophi-lies would be required for similar coordination and cleavage of the CH bond. Given the low energy, T-symmetry and low polarizability of the HOMO of the CH bond of methane, frontier orbital considerations of the interaction between 

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