The Chemical Mechanism of Pyridine N-oxide Reduction

The Chemical Mechanism of Pyridine N-oxide Reduction A. The Initial Step 

Equation (8) appears to represent a reaction in which one ligand replaces another. Many examples of such reactions of these complexes have been found with ligands that are not capable of further steps, such as oxygen loss. In general, these reactions can be represented by the chemical equation, 

Both thermodynamic and kinetic issues come into play, as does the involvement of geometrical isomerization when a dithiolate with inequivalent thiolate donors, such as mtp, participates in the reaction the ligand mtp is derived from mtpH2, 2-(mercaptomethyl)thiophenol. 

For purposes of pyridine N-oxide reduction, it suffices to say that many examples of Eq. (14) have been explored, including those in which PR3 is displaced contra-thermodynamically (p/TM may be ca. 2) by PyO. Invoking such a step in the catalytic cycle is therefore perfectly acceptable. Actually, the ligand displacement chemistry is a rich and fascinating area in its own right, and for that reason it will be explored independently in Section V. 

Experiments were carried out using a series of reagents, P(C6H4R)3. A Hammett correlation of kcat against 3a gave the reaction constant Peat = +1-03. The positive value is consistent with the chemistry proposed at this stage, in that the most weakly-coordinated phosphine would be expected to be the most reactive catalyst. 

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