Second Step of The Birch Reduction With Regiochemistry Giving Unconjugated Cyclohexadienes
The second step of the Birch reduction affording unconjugated cyclohexadienes also poses mechanistic questions. Thus as shown in the figure below there are three resonance structures B, C and D for the carbanion. Simple Hückel computations lead, as noted in the first entry of the table below, to equal electron densities at the three atoms 1, 3 and 5. However, in contrast to densities the Hückel computation is less naïve about bond orders, and bonds 2-3 and 5-6 will be shortened as shown in the first entry of the table. With bond orders modifying simple exchange integrals in a Mulliken-Wheland-Mann computation it was shown that electron density at the central atom 1 become largest. More modern RHF computations lead to the same result.
Electron introduction to benzene and 3 resonance structures for the carbanion of the second step, and central protonation to give the unconjugated diene:
Five carbons of the cyclohexadienyl anion.
| Approximation | Density Atom 3 | Density Atom 2 | Density Atom 1 | Bond Order 2-3 | Bond Order 1-2 |
|---|---|---|---|---|---|
| Hückel (1st Approx) | 0.333 | 0.00 | 0.333 | 0.788 | 0.578 |
| 2nd Approx | 0.317 | 0.00 | 0.365 | 0.802 | 0.564 |
| 3rd Approx | 0.316 | 0.00 | 0.368 | 0.802 | 0.562 |
There are known precedents for central anion protonation. Thus conjugated enolates as C=C-C=C-O- have been known for some time as kinetically protonating in the center of the enolate system to afford the β,γ-unsaturated carbonyl compound under conditions where the anion, and not the enol, is the species protonated.
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