There is a mystique surrounding chemical torture. Just how much strain can one subject a poor old carbon atom to? We construct such tortured molecules as cubane and cyclopentyne and trans-fused bicyclo[4.1.0]heptane. Inverted carbons – think of propellanes – are also a fruitful arena for torturing hydrocarbons. Now, Irikura has examined inverted adamantane inv-1.1
The MP2/6-31G(d) optimized geometries of 1 and inv-1 and the transition state separating them are displayed in Figure 1. The inverted structure is a local energy minimum, lying 105 kcal mol-1 above 1.2 The barrier for rearrangement of the inverted adamantane into adamantane, which involved a cleave of a C-C bond, is 17 kcal mol-1, which implies a half-life of 30 ms at 298K and and 2 days at 195 K. The perfluoro isomer has a higher barrier (32 kcal mol-1) and a longer half-life (110 years at 298K).
Table 1. MP2/6-31G(d) optimized geometries of 1, inv-1, and the transition state connecting them.1
So, inv-1 has some kinetic stability. It also has little computed reactivity with water, oxygen, or a second molecule of inv-1. Irikura, however, did not compute reactions that might lead to loss of a hydride from inv-1, which would give a non-classical cation.
As might be expected, the spectroscopic properties of inv-1 are unusual. The C-H vibrational
frequency for the inverted hydrogen is 3490 cm-1 and the C-C-H bend is also 300 cm-1 higher than in 1. The NMR shifts for the inverted methane group are 7.5 ppm for the hydrogen and 21 ppm for the carbon atom.
Irikura ends the article, “Experimental verification (or refutation) of [inv-1] presents a novel synthetic challenge.” Let’s hope someone picks up the gauntlet!
(1) Irikura, K. K., "In-Adamantane, a Small Inside-Out Molecule," J. Org. Chem. 2008, 73, 7906-7908, DOI: 10.1021/jo801806w.
(2) The energies are computed as Eestimate = E[CCSD(T)/6-31G(d)//MP2/6-31G(d)] + E[MP2/aug-cc-pVTZ//MP2/6-31G(d)] – E[MP2/6-31G(d)//MP2/6-31G(d)].