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Posts Tagged ‘silanes

Oxidative Addition of Non-polar Reagents

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How important are oxidative additions of non-polar reagents? Very. The addition of dihydrogen (H2) is an important step in catalytic hydrogenation reactions. Organometallic C–H activations depend on oxidative additions of C–H bonds. In a fundamental sense, oxidative additions of non-polar organic compounds are commonly used to establish critical metal-carbon bonds. Non-polar oxidative additions get the ball rolling in all kinds of catalytic organometallic reactions. In this post, we’ll examine the mechanisms and important trends associated with non-polar oxidative additions.

Oxidative Additions of H2

Electron-rich metal centers with open coordination sites (or the ability to form them) undergo oxidative additions with dihydrogen gas. The actual addition step is concerted, as we might expect from the dull H2 molecule! However, before the addition step, some interesting gymnastics are going on. The status of the σ complex that forms prior to H–H insertion is an open question—for some reactions it is a transition state, others a discrete intermediate. In either case, the two new hydride ligands end up cis to one another. Subsequent isomerization may occur to give a trans dihydride.

Oxidative addition of dihydrogen to Vaska's complex. Note the cis arrangement of the hydride ligands.

Oxidative addition of dihydrogen to Vaska’s complex. Note the cis arrangement of the hydride ligands.

There’s more to this little reaction than meets the eye. For starters, either pair of trans ligands in the starting complex (L/L or Cl/CO) may “fold back” to form the final octahedral complex. As in associative ligand substitution, the transition state for folding back is basically trigonal bipyramidal. As we saw before, π-acidic ligands love the equatorial sites of the TBP geometry, which are rich in electrons capable of π bonding. As a consequence, π-acidic ligands get folded back preferentially, and tend to end up cis to their trans partners in the starting complex.

Dihydrogen may approach along two distinct trajectories. Placing π-acidic ligands in the equatorial plane of the TBP transition state is favored.

Dihydrogen may approach along two distinct trajectories. Placing π-acidic ligands in the equatorial plane of the TBP transition state is favored.

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Written by Michael Evans

July 4, 2012 at 4:09 pm