Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this?
The mechanism, as elaborated in the previous two posts, involves a one-electron transfer from a sodium atom to form the radical anion, which is then protonated in a second step, and this is again reduced to form a pentadienyl anion in the penultimate step.[1] The question now becomes why does this anion protonate to give predominantly the less stable diene product? The answer involves the actual structure of this anion. A calculation at the ωB97XD/6-311+G(d,p)/SCRF=acetonitrile level for the ion pair comprising the cyclohexadienyl anion and a Na(NH3)3+ counterion is shown below.
Structure of the cyclohexadienyl ion pair. Click for 3D.
From this, it appears that the sodium cation is η2 coordinated to each of two relatively localised double bonds (1.37Å), resulting in the negative charge accumulating on just the one carbon (red arrow), this being the carbon that then exclusively receives a final proton. The highest energy (-0.115 au) natural bond orbital (NBO) also emerges as being located on this carbon (the next two highest energy NBOs only come in at -0.303 au, and reside on each of the localised alkene bonds).
The highest energy NBO orbital. Click for 3D.
The molecular electrostatic potential in effect integrates over all the electrons (not just those in the highest orbital), resulting in a function that measures the attractiveness of any point to a proton (red). It too shows that the most attractive region (red) for a proton is again on this carbon.
Molecular electrostatic potential. Click for 3D.
There is even evidence from crystal structures that this sort of motif is possible. Thus the dianion of 1,4-diphenylbenzene (with two Na(thf)3+ counter-ions) reveals[2] this type of coordination. The buckling seen in the above mono-anion is inhibited by the presence of cations on both sides of the di-anion, but the pattern of short/long bonds seen above also manifests in the crystal structure.
Crystal model. Click for 3D.
So the take home message is that the counter-ion (solvated sodium cations) in the Birch reduction of benzene itself may coordinate to the anionic intermediates in the reductive process, and the resulting geometry of this ion-pair determines the eventual product of protonation.
References
- H.E. Zimmerman, and P.A. Wang, "Regioselectivity of the Birch reduction", Journal of the American Chemical Society, vol. 112, pp. 1280-1281, 1990. https://doi.org/10.1021/ja00159a078
- J.H. Noordik, H.M. Doesburg, and P.A.J. Prick, "Structures of the sodium–<i>p</i>-terphenyl ion pairs: disodium terphenylide–tetrahydrofuran (1/6) and disodium diterphenylide terphenyl–1,2-dimethoxyethane (1/6)", Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, vol. 37, pp. 1659-1663, 1981. https://doi.org/10.1107/s0567740881006833