A Chiral Polycyclic Aromatic Hydrocarbon Monkey Saddle

Abstract

A contorted polycyclic aromatic hydrocarbon (PAH) in the shape of a monkey saddle has been synthesized in three steps from a readily available truxene precursor. The monkey saddle PAH is consisting of three five‐, seven six‐, and three eight‐membered rings and has been unambiguously characterized by single‐crystal X‐ray diffraction. Owing to the three biaryl axes the monkey saddle PAH is inherently chiral. The inversion of the two enantiomeric structures into each other preferably occurs through a twisting of peripheral rings rather than by a fully planar intermediate, as has been calculated by DFT methods. Enantiomers were separated by chiral HPLC and inversion barriers determined by variable temperature circular dichroism spectroscopy, supporting the twisting mechanism.

An Isosteric Triaza Analogue of a Polycyclic Aromatic Hydrocarbon Monkey Saddle

“…teresting is the comparison of the degree of contortionw ith that of the CH-MS 1. [21] The angle between the unsubstituted rings and the central one is between 41.98 and 48.18.B etween the hexyloxy substituted rings and the central ring the angles are between 29.08 and 36.48.T hese values are very similart o the ones observed for the hydrocarbon monkey saddle 1 (40.6 to 52.98 and 28.1 to 33.68). This extraordinary structural similarity predestines this pair of compounds to studyt he isosterical exchange of one CH unit by nitrogen on molecularp roperties for negatively contortedP AHs.…”

“…Recently we introduced the first monkey saddle shaped PAH 1 (we will abbreviate this compound as CH-MS 1), which is conformationally [1c, 4a, j-l] stable at ambient conditions. [21] This monkey saddle PAHc onsists besides seven hexagons, of three pentagonal and three octagonal rings. Among all possible isosteric C 3 -symmetric triaza monkey saddle PAHs (for detailed analysiso ff rontier molecularo rbitale nergies, see Supporting Information), the largest difference in stabilization of HOMO and LUMO energies is found for the aza monkey saddle 2 (we will abbreviate this compound as aza-MS 2), where the nitrogen is part of the eight membered ring (Figure 1).…”

“…The experimentally determined barrierf or the aza-MS 2 wasw ith E A = 113kJmol À1 , about 10 kJ mol À1 higher than previously reported for CH-MS 1. [21] This prolongs the half-time stabilityf or example, at 70 8C from af ew minutes to several hours. This is very important for the furtheru se of derivativeso fa za-MS 2 as enantiopure chiral reactantt owards the synthesis of Mackay-type cage structures [13] -which is ongoing in our laboratories.…”

“…Circular dichroism UV/Vis spectra of both separatede nantiomersa re mirrorimaged ( Figure 5) showing aCotton effect that is abit less pronounced than was reported for 1. [21] The g-values of 3.41 10 À3 (290 nm), À2.70 10 À3 (343 nm) and À2.52 10 À3 (360 nm) for 2 are in the same order of magnitude than the ones found for the pure hydrocarbon CH-MS 1.B yT D-DFT (BHandHLYP/6-311-G(d,p))i sa ssumed that the red graph of the CD-spectrum is of the enantiomer (S a ,S a ,S a )-2 andt he black one of the opposite enantiomer (R a ,R a ,R a )-2. [23] With enantiopure aza-MS 2 in hand the inversion barrierw as calculated by kinetic measurements of CD spectra at various temperatures (for details, see the Supporting Information).…”

An 80-Carbon Aromatic Saddle Enabled by a Naphthalene-Directed Scholl Reaction

“…5,12 The negative curvature of aromatic saddles is usually induced by embedding seven-or eight-membered rings in a graphitic framework. Compound 1 consisting of 80 sp 2 carbons is a new member of the family of octagon-embedded aromatic saddles, 5,[13][14][15][16][17] which are still less studied than heptagon-embedded aromatic saddles. 18 Scheme 1 Synthesis of 1.…”