D₂-Symmetric Dimer of 1,1‘-Binaphthyl and Its Chiral π-Conjugated Carbodianion

Abstract

The synthesis of a chiral, nonracemic π-conjugated system with D 2 point group of symmetry and the corresponding chiral carbodianion is described. Cu(II)-mediated oxidation of (R)-2,2′-dilithio-1,1′-binaphthyl (prepared from (R)(+)-2,2′-dibromo-1,1′-binaphthyl) gives the corresponding dimer, (R)(+)-3, which is a chiral π-conjugated derivative of o-tetraphenylene, a saddle shaped molecule. When partially resolved 2,2′dibromo-1,1′-binaphthyl is used, amplification of the enantiomeric excess (ee) is observed in the coupling product (3). The dihedral angles between the naphthalene moieties in (R)(+)-3 are about 70 °, as determined by the X-ray crystallography. However, CD and UV spectra, and cyclic voltammetric data of (R)(+)-3 are consistent with small, but far from negligible, conjugation (or homoconjugation) between the naphthalene moieties. The melting points of (R)(+)-3 and its racemate differ by more than 200 °C. Reaction of (R)(+)-3 with Li or Na in tetrahydrofuran (THF) gives the corresponding chiral carbodianions 3 2-,2M + (M ) Li and Na). NMR spectroscopy indicates two non-equivalent naphthalene moieties. UV-vis, ESR, NMR, and electrochemical studies show almost complete disproportionation of the radical anion to the carbodianion in THF with Na + as counterions. The free energy barrier for racemization in carbodianion 3 2-,2Na + is lowered by at least 25 kcal/mol compared to that in (R)(+)-3. However, carbodianions 3 2-,2M + (M ) Li and Na) remain configurationally stable at room temperature. Solid 3 2-,2Na + is a diamagnetic insulator.

Chiral Carbon‐rich Macrocycles and Cyclophanes

“…Rajca and coworkers have elaborated on the core structure of the cyclic orthotetraphenylene via the synthesis of a helical 1,1 0 -binaphthyl dimer as outlined in Scheme 6.19 [51]. This work represents the first example of a non-racemic macrocycle provided in this discussion.…”

Synthesis of tetraphenylene derivatives and their recent advances

“…The structure of 66 was determined by X-ray crystallographic analysis [45]. This method was based on the configurationally stability of 2,2 -dilithio-1,1 -binaphthyl and amplification of enantiomeric excess was observed in the coupling product (Scheme 20).…”

Asymmetric Synthesis of Chiral Tetraphenylenes

“…[2] The twisted biaryl moieties are associated with four chiral axes with the R/S pairs related by mirror planes. [3] Therefore, tetraphenylene is an achiral molecule ( Figure 1). …”

“…In spite of the large arylaryl torsional angles, chiral tetraphenylenes possess significant p conjugation, as indicated by their chiroptical properties. [3,4] More importantly, these molecules with chiral pconjugated systems have extraordinary high barriers for racemization ( Figure 2) [3,5,6] compared to those of [n]helicenes or typical hindered biaryls such as BINOL (1,1'-binaphthalene-2,2'-diol). [7,8] Owing to the unique structure and chiral properties of tetraphenylenes, a wide range of potential applications may be expected.…”

“…[3] In the case of configurationally fluxional biaryls, asymmetric synthesis by oxidative homocoupling of 2,2'-dilithiobiaryls in the presence of (À)-sparteine provides tetraphenylenes in good yields and moderate enantioselectivities. [4,15] Racemic and achiral tetraphenylenes are also efficiently prepared by the metal-mediated and metal-catalyzed CÀC bond cleavage of biphenylenes or by pyrolysis of oligophenylenes.…”