Cyclopentannulated Dihydrotetraazapentacenes

Abstract

The transition-metal-catalyzed cyclization of bissilylethynylated N,N'-dihydrotetraazapentacene (TIPS-TAP-H₂ ) into bissilylated cyclopenta[fg,qr]pentacenes is reported. Depending on the catalyst either none, one or two silyl groups migrate and change their positions in the formed five-membered rings. The optoelectronic properties are quite similar, whereas the packing motifs differ dramatically. Control experiments and quantum chemical calculations were performed to investigate the mechanism of the reaction and the selectivity of the silyl shift.

Keywords: aromaticity; azaarenes; catalysis; cycloisomerization; cyclopentannulation; silyl shift.

Figure 1

a) Structure of TIPS‐TAP‐H₂ ; b) Cu‐catalyzed cycloisomerization of ethynyl‐substituted tetrahydroquinoline; c) transition metal catalyzed cycloisomerization of 2‐propargylpyridines; d) different modes of cyclopentannulation of dihydroazaacenes; e) cyclopentannulated pentacenes.

Scheme 1

Synthetic route towards cyclopentannulated tetraazapentacenes 1–4. Conditions: i) IPrAuNTf₂ (10 mol%), DCE, 80 °C, 16 h; ii) PdCl₂ (10 mol%), MeCN/CHCl₃ 1 : 1, 80 °C, 18 h; iii) TBAF (10 equiv.), THF, 60 °C, 5 h.

Figure 2

Normalized absorption spectra (full lines) and emission spectra (dotted lines) of 1–4 in DCM.

Figure 3

X‐ray structures (top) and packing motifs (bottom) from left to right: 1, 2, 3 and 4. Silylethynyl substituents were reduced in size and solvent molecules omitted for clarity (except for 4).

Figure 4

Top: Calculated NICS(1) values of 3 (left) and 3²⁺ (right). (Gaussian 16, geometry optimization: B3LYP/def2‐SVP/, B3LYP/def2‐TZVP; NICS calculations: GIAO‐method B3LYP/def2‐TZVP Bottom: AICD plot of 3 (left) and 3²⁺ (right). (TMS groups were used instead of TIPS; the red arrows indicate the ring current flow magnetic field, isovalue=0.02; magnetic field vector is orientated out of plane; AICD: CSGT‐method B3LYP/def2‐TZVP IOP(10/93=1)).

Scheme 2

Control experiments to prove the suggested mechanism. Conditions: i) IPrAuNTf₂ (10 mol%), DCE, 80 °C, 16 h; ii) PdCl₂ (10 mol%), MeCN/CHCl₃ 1 : 1, 80 °C, 18 h; iii) IPrAuNTf₂ (10 mol%), DCE, 80 °C, 21 d; iv) PdCl₂ (10 mol%), MeCN/CHCl₃ 1 : 1, 80 °C, 21 d.

Scheme 3

a) Proposed mechanism to explain the observed regioisomer 1; b) reaction profile with calculated Gibbs energies (Q‐Chem 5.2.2, wB97X‐D3/pc‐1).

Scheme 4

Control experiments for the silyl group shifts.

Scheme 5

a) Silyl shift with protonation: calculated Gibbs energies (top)/reaction profile (bottom); b) silyl shift without protonation: calculated Gibbs energies (Orca 4.10, PBE−D3/def2‐SV(P), def2‐TZVPP effective‐core‐potentials on Gold and def2‐TZVPP on the alkyne moiety).