Synthesis and Properties of Cyclooctatetraene Congeners Tobe Lab. Kazuya Fujita 1

Contents Introduction Aromaticity- Hückel’s Rule and NICS(0) value Structure and physical properties of cyclooctatetraene and its congeners Biradical character Twisted cyclooctatetraene congeners Synthesis and physical properties My work and results Summary 2

Hückel’s Rule and NICS(0) Value Hückel’s Rule: 3 aromatic ( 芳香族 ) non-aromatic ( 非芳香族 ) anti-aromatic ( 反芳香族 ) number of  electrons NICS(0) Value negative positive zero ( N = 0, 1, 2, …) 4N + 2 4N planar non-planar a rule to distinguish whether cyclic conjugated polyenes will have aromatic property on the basis of stabilization energy  -conjugated cyclic compounds NICS(0) value: a value by theoretical calculations to estimate aromatic property larger absolute value means stronger aromaticity/anti-aromaticity structure

Hückel’s Rule and NICS(0) Value 4 aromatic ( 芳香族 ) non-aromatic ( 非芳香族 ) anti-aromatic ( 反芳香族 ) number of  electrons NICS(0) Value negative positive zero ( N = 0, 1, 2, …) 4N + 2 4N planar non-planar benzene NICS(0): aromatic compound is stabilized by resonance energy planar has 6  electrons aromatic cyclic conjugated polyenes Calculatied by HF/6-31G*//B3LYP/6-31G* resonance ( 共鳴 ): two forms are not under equilibrate ( 平衡 ) the true structure is the mixture of two forms all bond lengths are equal (1.39 Å)

Cyclooctatetraene (COT) has 8  electrons alternating single and double bonds non planar tub-shaped structure 5 non-aromatic == 1.34 Å 1.48 Å ° NICS(0): +1.9 tub-to-tub inversion in solution flexible  conjugated skeleton equilibrate ( 平衡 ), not resonance ( 共鳴 )

COT Congeners and NICS(0) Value 6 anti-aromatic Nishinaga, T. et al. J. Am. Chem. Soc. 2010, 132, NICS(0): Crystal structure of 1 NICS(0): +1.9 has 8  electrons non-planar non-aromatic Planarized COT by introducing substituents

Biradical Character Ethylene 7 o-Quinodimethane cleavage of  bond (double bond) leads to loss of bonding energy open shell structure is stabilized by forming benzene ring closed shellopen shell closed shellopen shell resonance ( 共鳴 ) Formal [4+4] dimerization of o-quinodimethane Boekelheide, V. et al. J. Am. Chem. Soc. 1981, 103, open shell structure contributes to resonance hybride and has diradical character

Dimerization of Indeno[2,1-a]fluorenes dimerization and formation of COT ring undergo one step more  conjugated system having COT ring can be synthesized 8 new synthetic method of COT congeners

Crystal Structure and NICS(0) Value of Dimer 5 9 An ORTEP drawing of Dimer 5. Single crystals of 5 suitable for X-ray analysis were obtained from benzene/methanol. C1 C2 C3 C4 C1’ C1-C1’-C4’-C4 C2-C3-C3’-C2’ 29.2 ° 38.6 ° torsion angle C2’ C3’ C4’ COT ring is twisted NICS(0): decreased anti-aromaticity Calculatied by HF/6-31G*//B3LYP/6-31G* steric repulsion

My Work 10 Tobe, Y. et al. Angew. Chem. Int. Ed. 2013, 52, 4184.

Structures of Dimer 7 and 8’ 11 Optimized structure by DFT (B3LYP/6-31G * ) HOMOLUMO HOMO- LUMO gap torsion angle red torsion angle orange NICS(0) eV-2.81 eV2.08 eV42.7 °33.6 ° ’-5.10 eV-2.30 eV2.80 eV4.5 °5.5 °+12.2

Synthesis of dibromodihydroindenofluorene Mataka, S. et al. New. J. Chem. 2003, 27, 1377.

Attempted Synthesis of Dimer 7 13

Reaction Mechanism of Dimerization 14 Calculated at RB3LYP/6-31G(d) + LanL2DZ (for Br) (TS1, INT2) Calculated at UB3LYP/6-31G(d) + LanL2DZ (for Br) (INT1, TS2) by Dr. Nobusue, S

Reaction Mechanism of Dimerization 15 Calculated at RB3LYP/6-31G(d) + LanL2DZ (for Br) Structure of TS1

Energy Diagram of Dimerization 16 5 INT2 INT1 TS1 TS2 3 Relative energy / kcal mol -1 Calculated at RB3LYP/6-31G(d) + LanL2DZ (for Br) (TS1, INT2); Calculated at UB3LYP/6-31G(d) + LanL2DZ (for Br) (INT1, TS2) by Dr. Nobusue, S reaction coordinate = Ph TS1

Energy Diagram of Dimerization 17 5 INT2 INT1 TS1 TS2 3 Relative energy / kcal mol -1 Calculated at RB3LYP/6-31G(d) + LanL2DZ (for Br) (TS1, INT2); Calculated at UB3LYP/6-31G(d) + LanL2DZ (for Br) (INT1, TS2) by Dr. Nobusue, S reaction coordinate = Ph TS1 higher activation barrier

Synthetic Scheme of Dimer 8 and 8’ 18 † Steven, G. N. et al. PCT Int. Appl. US 03/022898, A1, ‡ Lai, Y-H. et al. J. Chem. Soc. Perkin Trans , 7, 1377.

Summary Non-planar tub-shaped COT exhibits non-aromaticity and planarized COT exhibits anti-aromaticity. Twisted COT congener was formed by dimerization of indeno[2,1- a]fluorenes and its twisted COT ring exhibits decreased anti- aromaticity. Synthesis of Ph-substituted indenofluorene dimer was attempted, but unexpected reaction occurred. 19