Regiochemical Control in Triptycene Formation-An Exercise in Subtle Balancing Multiple Factors

Lamm J-H, Vishnevskiy Y, Ziemann E, Neumann B, Stammler H-G, Mitzel NW. Regiochemical Control in Triptycene Formation-An Exercise in Subtle Balancing Multiple Factors. ChemistryOpen. 2018;7(1):111-114.Reactions between 1,8-dichloroanthracenes with substituents in position 10 and ortho-chloroaryne afford mixtures of 1,8,13- (syn) and 1,8,16-trichlorotriptycenes (anti). The syn/anti ratio is dependent on these substituents. Electropositive substituents like SiMe3 and GeMe3 lead to preferred formation of the syn-isomer, whereas CMe3 groups exclusively afford the anti-isomer. Different quantum chemical calculations including location of transition states give conflicting results, but indicate the importance of dispersion forces for an at least qualitative prediction of results. The syn-trichlorotriptycenes with SiMe3 and GeMe3 substituents were characterized by using NMR spectroscopy, mass spectrometry, and X-ray diffraction experiments. Triptycene represents one of a few rigid organic frameworks of D3h symmetry without any (Lewis-basic) heteroatoms. It was first synthesized by Bartlett et al. in 1942 using a multi-step procedure starting from anthracene and p-benzoquinone.[1] In 1956, Wittig and Ludwig reported a more efficient access to triptycene in one step from anthracene by reacting it with in situ-formed benzyne.[2] The symmetry and rigidity of triptycene have inspired a plethora of applications in fundamental and applied chemical research.[3-5] Substituted triptycenes are widely used, for example, as building blocks for fluorescent or non-fluorescent organic macromolecules, polymers, and liquid crystals,[3, 6] as rigid spacers in several Pd complexes used for cross coupling reactions,[7] as devices in molecular machines,[8] in crystal engineering processes,[9, 10] and as a basis for the design of highly porous organic materials with numerous applications.[11] Although the chemistry of triptycenes and their functionalization is generally in an advanced state, the 1,8,13-trisubstitution motif remains a challenge for synthesis. However, exactly this pattern is interesting to introduce three functionalities oriented in the same direction. We try to make use of such 1,8,13-trisubstituted triptycenes (also called syn-triptycenes) as rigid organic frameworks for constructing directed polydentate Lewis acids,[12, 13] but many other applications might be envisioned. syn-Triptycenes can be obtained through Diels–Alder reactions of 1,8-disubstituted anthracenes with ortho-functionalized arynes, a protocol introduced by Rogers and Averill in 1986.[14] The drawback of this method is that the corresponding anti-trisubstituted 1,8,16-isomer is always formed as the main product when, for example, Cl-functionalized anthracenes and arynes are used.[12, 14] In 2010, we reported attempts to increase the syn/anti ratio by making use of the steric interference of the (bulky) anthracene substituent at C-10 with the chlorine atom of the chloroaryne (Scheme 1). We expected this strategy to provide an increased formation of the syn-isomer. However, the steric influence of the C-10 substituent turned out to be minimal, whereas the electronic properties are dominant;[12] of all substituents tested, the biggest R=C(CH3)3 led to the formation of 100 % anti-isomer, despite the formation of an extremely deformed product by mutual repulsion of the Cl and R substituents, as indicated in Scheme 1 b