Novel aromatic (or at least potentially aromatic species) just keep on coming! Tobe has prepared this interesting ortho-quinodimethane derivative 1b.1 Ortho-quinodimethane derivatives are rare, due to the high reactivity of the underlying structure 2. Known derivatives of 2 tend to undergo an electrocyclic rearrangement to the cyclobutanobenzene 3. Fusing the benzene rings should minimize this electrocyclization, and the mesityl substituents should minimize dimerization. In fact 1b is stable as a solid or in solution, even in solution exposed to air.

1a: R=H
1b: R=Mes

B3LYP/6-31G(d) computations are in nice agreement with the x-ray structure concerning the important C-C distances. The terminal rings have delocalized bonds, while the interior three rings exhibit bond alternation. The authors claim that 1 should be thought of as ortho-quinodimethane bridged by two benzene rings. There does seem to be a bit of diradical character, though temperature dependent NMR shows no broadening, so the singlet-triplet gap is large, and therefore 1 exhibits small diradical character.

The computed NICS(0) for 1a values are -3.94 for the terminal ring (indicative of an aromatic phenyl), +9.20 for the 5-member ring, and +5.42 for the middle ring. These can be compared to the NICS(0) value of +3.21 for the ring of 2 and +11.51 and +21.17 for the 6 and 5-member rings of 4, respectively. So, 1a expresses some antiaromatic character.



(1) Shimizu, A.; Tobe, Y., "Indeno[2,1-a]fluorene: An Air-Stable ortho-Quinodimethane Derivative," Angew. Chem. Int. Ed. 2011, 50, 6906-6910, DOI: 10.1002/anie.201101950


1a: InChI=1/C20H12/c1-3-7-15-13(5-1)11-19-17(15)9-10-18-16-8-4-2-6-14(16)12-20(18)19/h1-12H

1b: InChI=1/C38H32/c1-21-17-23(3)33(24(4)18-21)35-29-13-9-7-11-27(29)31-15-16-32-28-12-8-10-14-30(28)36(38(32)37(31)35)34-25(5)19-22(2)20-26(34)6/h7-20H,1-6H3

2: InChI=1/C8H8/c1-7-5-3-4-6-8(7)2/h3-6H,1-2H2

3: InChI=1/C8H8/c1-2-4-8-6-5-7(8)3-1/h1-4H,5-6H2

4: InChI=1/C12H8/c1-3-9-7-8-10-4-2-6-12(10)11(9)5-1/h1-8H