To Flip or Not To Flip? Assessing the Inversion Barrier of the Tetraphenylene Framework with Enantiopure 2,15-Dideuteriotetraphenylene and 2,7-Dimethyltetraphenylene

Abstract

Two chiral tetraphenylenes, 2,15-dideuteriotetraphenylene (7) and 2,7-dimethyltetraphenylene (15) were synthesized and resolved to address the tetraphenylene inversion barrier problem. Neutron diffraction investigation of enantiopure 7 showed that the molecule retained its chirality integrity during its synthesis from enantiopure precursors and therefore rules out the possibility of the tetraphenylene framework possessing a low-energy barrier to inversion. Thermal study on 15 and tetraphenylene 1 further revealed that their inversion barriers were not overcome up to 600 degrees C, at which temperature these compounds underwent skeletal contraction into triphenylene with activation energies of 62.8 and 58.2 kcal/mol, respectively. This result is supported by computational studies which yielded an inversion barrier of 135 kcal/mol for tetraphenylene as a consequence of the peri-hydrogen repulsions at its planar conformation.

Synthesis of tetraphenylene derivatives and their recent advances

“…Later, Eisch examined various nickel complexes for this reaction, and found that the ligands attached to the metal center of nickel complexes played a crucial role in the oxidative insertion into the strained C-C bond of 5 [15]. For example, complex Ni(COD) 2 was unreactive towards 5, whereas Ni(Et 3 P) 4 completely even at 0 • C. Importantly, a dibenzonickelole intermediate (9) was proposed to be involved in the reaction mechanism (Scheme 5). Then, 9 lost Et 3 P and dimerized to a dinuclear nickel complex (10) (R = Et), whose structure was determined by an X-ray diffraction study.…”

“…However, it is unfortunate that mononuclear 9 could not be isolated for structural characterization. The likely existence of 9 was nonetheless proved by its reactivity with deuterium hydrochloride (DCl), LiAlH 4 , and CO, leading to the respective desired products. Notably, the structures of 9 and 10 lend support to a unified mechanistic scheme for interpreting the nickel-catalyzed Reppe tetramerization reaction of acetylenes to COT [16].…”

“…In addition, Pt(PEt 3 ) 3 and Pd(PEt 3 ) 3 cleaved the C-C bond of 5 to give a metal 2,2-biphenyl complex that is also capable of catalyzing the formation of 1 from 5 via the Pt(IV) intermediate (16) (Scheme 7). The resting-state species in the catalytic cycle was the same complex whether Pt(PEt 3 ) 4 or 16 was used as the catalyst. Of note, intermediates 15 and 16 in the catalytic cycle have been identified and characterized by X-ray crystallographic analysis.…”

“…Therefore, the central COT of 1 is 'non-aromatic'. Although the magnitude of inversion barrier had been controversial, with various estimations in the early years, the saddle-shaped geometry of 1 was found to be very stable, which was confirmed by neutron diffraction study and thermal experiments [4,5]. With this geometric rigidity, studies of 1 and its derivatives have led to their application in asymmetric catalysis, liquid crystalline materials, molecular devices, organic light-emitting diodes and others.…”

Catalytic Enantioselective Synthesis of Chiral Tetraphenylenes: Consecutive Inter‐ and Intramolecular Cycloadditions of Two Triynes

“…To this end, Wong and co-workers have comprehensively studied the asymmetric synthesis and the use of chiral poly(hydroxy)tetraphenylenes. [8] The only example of an enantioselective synthesis is the sparteine-mediated lithiation of 2,2'-dibromobiphenyl derivatives and subsequent coupling using an excess amount of CuBr 2 , for which moderate enantioselectivity was achieved. [9] Herein we disclose a new [2+2+2] cycloaddition-based strategy for the synthesis of substituted tetraphenylenes, and its application to asymmetric synthesis.…”