A facile synthesis of functionalized 7,8-diaza[5]helicenes through an oxidative ring-closure of 1,1’-binaphthalene-2,2’-diamines (BINAMs)

Summary A facile and moderately functional-group-tolerant synthetic method for the preparation of 7,8-diaza[5]helicenes has been developed. It comprises of an oxidative ring-closing process of 1,1’-binaphthalene-2,2’-diamine (BINAM) derivatives with a chlorine-containing oxidant (t-BuOCl) in the presence of a base (2,6-lutidine). In addition the basic physicochemical properties of newly synthesized compounds have been investigated.


General remarks
All reactions were carried out under an atmosphere of nitrogen unless otherwise noted. Melting points were determined on a Stanford Research Systems MPA100 OptiMelt Automated Melting Point System. 1 H and 13 C NMR spectra were recorded on a JEOL JMTC-400/54/SS spectrometer ( 1 H NMR, 400 MHz; 13 C NMR, 100 MHz) using tetramethylsilane as an internal standard. Infrared spectra were acquired on a SHIMADZU IRAffinity-1 FT-IR Spectrometer. Mass spectra were obtained on a JEOL JMS-DX303HF mass spectrometer. High-resolution mass spectra were obtained on a JEOL JMS-DX303HF mass spectrometer. UV/vis spectra were recorded on a Shimadzu UV-2550 spectrophotometer.
Preparation of 6,6'-di-n-butyl-1,1'-binaphthalene-2,2'-diamine (1f) Biaryldiamine 1f was prepared by modified cross-coupling method S7 from biaryldiamine 1g as follows (Scheme S1): THF was degassed through freeze-pump-thaw cycling for three times before used. To a round-bottomed flask (50 mL) equipped with a magnetic stir bar, were added biaryldiamine 1g (1.326 g, 3.0 mmol), Pd(OAc) 2 (13.4 mg, 0.06 mmol), and SPhos (49.2 mg, 0.12 mmol) under the air. The tube was capped with a rubber septum, evacuated, and then refilled with N 2 gas for three times. THF (2 mL) and 0.5 M THF solution of n-BuZnBr (14.4 mL, 7.2 mmol) were added to the tube through the septum, and the mixture was stirred under N 2 atmosphere at room temperature for 12 h. To the reaction mixture, was added saturated aqueous NH 4 Cl solution (10 mL), and the resulting mixture was extracted with EtOAc (20 mL × 3). The organic extract was dried over Na 2 SO 4 and concentrated under vacuum to give the crude product. Purification by flash column chromatography on silica gel (eluent: hexane/EtOAc 8:2) gave biaryldiamine 1f as pale brown solid (1.051 g, 88% Scheme S1. Preparation of 1f.

Optimization studies of reaction conditions
A typical procedure for the optimization studies using 1a as substrate To a two-necked reaction tube (20 mL, entry 2 in Table S1) or a two-necked round-bottomed flask (50 mL, entries 1 and 3-11 in Table S1, Table S2, and Table S3) equipped with a magnetic stir bar, was added 1,1'-binaphthalene-2,2'-diamine (1a) (0.2 mmol) under the air. The vessel was capped with a rubber septum and evacuated and refilled with N 2 gas for three times, and an appropriate solvent was added through the septum. To the mixture, were added an additive and an appropriate oxidant under a stream of N 2 gas at the indicated temperature. The resulting solution was stirred for indicated time before quenched with aqueous Na 2 S 2 O 3 solution (1.0 M, 20 mL), and the resulting mixture was extracted with S4 CH 2 Cl 2 (20 mL × 3). The combined organic extracts were dried over Na 2 SO 4 and concentrated under vacuum to give the crude product. The yields of products were calculated by the integration of 1 H NMR signals of the crude product. Separation by flash column chromatography on silica gel gave product 2a. [188-55-6] Spectroscopic data were in good agreement with those previously reported. S8 The spectroscopic data are also available in our previous paper. S9   S5 Table S2. The effect of chlorine-containing oxidants.

UV-vis and emission spectra
CH 2 Cl 2 (fluorescence spectroscopic grade) was purged with N 2 for 30 min before the measurements. UV-vis and emission spectra of diazahelicenes 2 and cinnoline 4 were measured at room temperature using CH 2 Cl 2 solutions (1.0  10 -5 M).

Cyclic voltammetry
Cyclic voltammetry experiments were conducted at room temperature with CH 2 Cl 2 solutions of diazahelicenes 2 and cinnoline 4 (5.0  10 -4 M) containing 0.1 M tetrabutylammonium hexafluorophosphate as a supporting electrolyte in a cell equipped with a Pt as the working electrode (scanning rate: 100 m/V). A Pt wire and an Ag wire were applied as the counter and the reference electrode, respectively. All the potentials were corrected against the Fc/Fc + (Fc = ferrocene) couple and the values of LUMO levels were calculated with the equation S1.