Tetraphenylene 1 has a saddle-shape. The barrier for interconverting the two mirror image saddles has been estimated to range from about 5 kcal mol-1 to as much as 220 kcal mol-1. These estimates were made either experimentally, by placing a substituent on the ring and measuring the energy needed to racemize the compound or by fairly primitive computation (CNDO).

Bau, Wong and co-workers have prepared the dideutero and dimethyl derivations 2 and 3.1 The optical activity of 2 turns out to be far too small to be useful (and the effort expended to determine if 2 is enantiopure is truly heroic). 3 proved to have significant optical activity and so could be used to determine if enantiopure 3 racemized under heating. Amazingly, there was no measurable loss of optical activity upon heating at 550 °C for 4 hours. Rather, heating at higher temperature lead to decomposition and no noticeable racemization. To corroborate this very high barrier for racemization, they optimized the structure of 1 in its ground state saddle geometry 1s and in its planar form 1p, presumably the transition state for racemization. These two geometries (B3LYP/6-31G(d,p) are shown in Figure 1. The barrier is an astounding 135.8 kcal mol-1, consistent with the experiment.




They apparently did not perform a frequency computation to confirm that 1p is a true transition state. In fact, Müllen, Klärner, Roth and co-workers demonstrated that the transition state for the ring flip of 1 is not planar.2 They located a C1 TS using MM2 that is some 66 kcal mol-1 above the tub-shaped ground state.

I have just published a follow up study on 1 and the related benzannulated cyclooctatetraenes.3 The true transition state for the ring flip of 1 has D4 symmetry (1ts) and is shown in Figure 1. The barrier for ring flip through 1ts is 76.5 kcal mol-1 at B3LYP/6-31G(d,p) (78.6 kcal mol-1 at MP2/6-31G(d,p)). This barrier is too large to be overcome by heating, and so Bau and Wong are correct in concluding that decomposition proceeds racemization.




Figure 1. B3LYP/6-31G(d,p) structures of 1,1 1p,1 and 1ts.3


(1) Huang, H.; Stewart, T.; Gutmann, M.; Ohhara, T.; Niimura, N.; Li, Y.-X.; Wen, J.-F.; Bau, R.; Wong, H. N. C., “To Flip or Not To Flip? Assessing the Inversion Barrier of the Tetraphenylene Framework with Enantiopure 2,15-Dideuteriotetraphenylene and 2,7-Dimethyltetraphenylene,” J. Org. Chem. 2009, 74, 359-369, DOI: 10.1021/jo802061p.

(2) Müllen, K.; Heinz, W.; Klärner, F.-G.; Roth, W. R.; Kindermann, I.; Adamczk, O.; Wette, M.; Lex, J., “Inversionsbarrieren ortho,ortho’-verbücketer Biphenyle,” Chem. Ber. 1990, 123, 2349-2371.

(3) Bachrach, S. M., “Tetraphenylene Ring Flip Revisited,” J. Org. Chem. 2009, DOI: 10.1021/jo900413d.


1: InChI=1/C24H16/c1-2-10-18-17(9-1)19-11-3-4-13-21(19)23-15-7-8-16-24(23)22-14-6-5-12-20(18)22/h1-16H/b19-17-,20-18-,23-21-,24-22-

2: InChI=1/C24H16/c1-2-10-18-17(9-1)19-11-3-4-13-21(19)23-15-7-8-16-24(23)22-14-6-5-12-20(18)22/h1-16H/b19-17-,20-18-,23-21-,24-22-/i1D,3D

3: InChI=1/C26H20/c1-17-11-13-23-24-14-12-18(2)16-26(24)22-10-6-4-8-20(22)19-7-3-5-9-21(19)25(23)15-17/h3-16H,1-2H3/b20-19-,24-23-,25-21-,26-22-