Electrochemical synthesis of heterodehydro[7]helicenes

Abstract

Dehydrohelicenes are some of the most attractive chiroptical materials with unique helical chirality. However, to our knowledge, there are no prior reports on their direct construction by asymmetric methods. In this work, sequential synthesis of aza-oxa-dehydro[7]helicenes via the electrochemical oxidative hetero-coupling of 3-hydoxycarbazoles and 2-naphthols followed by dehydrative cyclization and intramolecular C–C bond formation has been realized. In addition, an efficient enantioselective synthesis through chiral vanadium-catalyzed hetero-coupling and electrochemical oxidative transformations afforded heterodehydro[7]helicene without any racemization. The obtained dehydro[7]helicenes showed intense blue-colored circularly polarized luminescence (|glum| ≈ 2.5 × 10−3 at 433 nm). Thermodynamic and kinetic studies of the racemization barrier of heterodehydro[7]helicenes indicated significant chiral stability with ΔG‡> 140 kJ mol−1.

Electrochemical Synthesis of Hetero[7]helicenes Containing Pyrrole and Furan Rings via an Oxidative Heterocoupling and Dehydrative Cyclization Sequence

“…2‐Naphthol 2 c possessing a (pinacolato)boryl (Bpin) group at the 7‐position, which enables further transformations, was tolerated to afford 3 ac – 3 ec in 50–70% yields. Under the present re‐optimized electrochemical conditions, 7‐methoxy‐2‐naphthol derivative 2 e (containing EDG at 7‐position) afforded the corresponding dehydro[7]helicene 5 ea in 61% yield within 3 h. Although the yield dropped a little bit compared to our previous conditions, [11] we avoided here using BF 3 ⋅ OEt 2 and compensated for their role by increasing the amount of 2 e (4.0 equiv.) to get 5 ea in good yield and higher Faradic efficiency (55% FE).…”

“…Under the optimized conditions, neither homocoupling products nor diol 4 aa was observed. Although our previously optimized conditions for dehydrohelicenes, [11] can afford 3 aa in a comparable yield of 82% with 24%FE, we had to use an excess of BF 3 ⋅ OEt 2 (50 equiv.) (entry 2).…”

“…Our previous conditions [FTO(+)/FTO(−), constant current=3 mA ( J =1.2 mA/cm 2 ), 1 a / 2 a =1/1, BF 3 ⋅ OEt 2 (50 equiv. ), Bu 4 NPF 6 (0.1 M), CH 2 Cl 2 , rt, 6 h] [11] …”

“…Our previous conditions [FTO(+)/FTO(À ), constant current = 3 mA (J = 1.2 mA/ cm 2 ), 1 a/2 a = 1/1, BF 3 • OEt 2 (50 equiv. ), Bu 4 NPF 6 (0.1 M), CH 2 Cl 2 , rt, 6 h] [11] 82 (24) 0 À 30 °C no reaction [a] Electrolysis conditions: Pt anode, Pt cathode, constant current = 1 mA (J = 0.51 mA/cm 2 ), 1 a/2 a = 1/4, Bu 4 NPF 6 (0.1 M), CH 2 Cl 2 , rt, 1.5 h. [b] Determined by 1 H NMR spectroscopy using 1,3,5-trimethoxybenzene as the internal standard. [c] Faradic efficiency in parentheses.…”

“…[9] In light of our previous interest in developing synthetic methods using electrochemistry, [10] we have established a sequential electrochemical protocol to synthesize dehydro [7]helicenes from simple arenols (Figure 1A). [11] This approach features a high yield under mild conditions. However, some limitations still remain making these previous conditions less sustainable, such as the necessity of an excess amount of the acidic additive BF 3 • OEt 2 (50 equiv.…”