An iterative synthetic approach to nanometre-scale molecular ribbons

Abstract

The hitherto longest molecular ribbons (4-61, composed of several [3.3lmetacyclophane units, are synthesized using a new iterative synthetic strategy, up to a length of seven fourfold-bridged benzene rings, and it is proved by X-ray crystal structural analyses and 1H NMR spectroscopy that these molecules form meander-type folded syn conformations leading to multiple n-staples.Molecular ribbon-structures were proposed years ago, yet only Stoddart and coworkers have found structure-directed syntheses.* Subsequent Diels-Alder reactions led to open-chain and cyclic ribbon structures, which consist of condensed sixmembered ring assemblies.3For some years, we have tried to obtain flexible ribbon structures which could be macrocyclized later on account of their four terminal functional group^.^ So far, we have synthesized only [3.3]metacyclophane structures la-3a with a maximum of four benzene ring units.5

1,1,8,8‐Tetramethyl[8](2,11)teropyrenophane: Half of an Aromatic Belt and a Segment of an (8,8) Single‐Walled Carbon Nanotube

“…The short axis of the ellipse measures 9.08 (the average value of d 3 ) and the long axis measures 12.3 (double the average value of d 4 ). The average of these distances is 10.7 , which is very close to the calculated value for an (8,8) SWCNT (10.86 ). [24] Thus the cross section of the half-belt system corresponds to the more curved half of an ellipse.…”

“…[21] The teropyrene system (36 carbon atoms) in 21 contains more than half of the carbon atoms in the D 6h -symmetric Vögtle belts 3 a (60 carbon atoms) and 3 b (70 carbon atoms). However, as outlined below, its structure (including the benzylic carbon atoms) more closely resembles a substructure of the D 8h -symmetric Vögtle belt 3 c (80 carbon atoms), which is, in turn, a segment of an (8,8) SWCNT.…”

“…[1e] Although interest in the synthesis of aromatic belts, especially cyclacenes, predates the discovery of the fullerenes [2] and SWCNTs [3] by decades, [4] the discovery of SWCNTs and their homology to aromatic belts has engendered more intense synthetic and theoretical interest, not only in cyclacenes [5] and cyclophenacenes, [6] but also in related aromatic/conjugated belts. [7] Of particular interest to us are the Vögtle belts, [8] for example, 3, which are armchair SWCNT segments having a pyrenoid or rylenoid motif, depending upon ones perspective.A recurring theme in the numerous unsuccessful attempts to synthesize aromatic belts is the inability to aromatize partially saturated belt precursors. The interplay of two major energetic factors, strain and aromaticity, is at the heart of the problem.…”

“…It is useful to include the benzylic carbon atoms in the analysis. In the D 8h -symmetric Vögtle belt 3 c (or an (8,8) SWCNT), the bonds corresponding to the two C bridgehead À C benzylic bonds in 21 are parallel. In 21, the observed overall bend (q tot + y 1 + y 2 = 178.58 and 179.08 for molecules A and B, respectively) is indeed very close to 1808.…”

“…In fact, the hypothetical union of two half-belt systems (teropyrene + 2C benzylic ) by fusion of the C bridgehead À C benzylic bonds ( Figure 3) affords an ellipsoidal segment of 3 c or an (8,8) SWCNT. The short axis of the ellipse measures 9.08 (the average value of d 3 ) and the long axis measures 12.3 (double the average value of d 4 ).…”