Graham J. Bodwell

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†

and single-walled carbon nanotubes (SWCNTs). Indeed, they can be viewed as the shortest possible open-ended (uncapped) zig-zag and armchair single-walled carbon nanotubes, respectively. Although interest in the synthesis of aromatic belts, especially cyclacenes, predates the discovery of the fullerenes and SWCNTs by decades, the discovery of SWCNTs and their homology to aromatic belts has engendered more intense synthetic and theoretical interest, not only in cyclacenes and cyclophenacenes, but also in related aromatic/conjugated belts. Of particular interest to us are the V gtle belts, for example, 3, which are armchair SWCNT segments having a pyrenoid or rylenoid motif, depending upon one s 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. The generation of a nonplanar aromatic system from a nonor partially aromatic precursor is inherently disfavored by the build-up of strain, but favored by the aromatic stabilization energy (ASE) of the aromatic system being formed. Increasing the distortion from planarity causes the strain energy to become larger and the ASE to become smaller. As a result, strain ultimately becomes dominant and aromatization, by standard methods such as the elimination of water, can become unfavorable. For example, the addition of water (2 equiv) to Schl ter s “double-stranded cycle” has been calculated to be exothermic by 42.2 kcalmol . Therefore, if any approach to aromatic belts is ultimately going to be successful it will not only have to target a relatively stable structural motif, but also rely upon methodology that is capable of generating highly nonplanar aromatic systems under relatively mild conditions. The valence isomerization/dehydrogenation (VID) reaction of [2.2]metacyclophane-1,9-diene (4) to give pyrene 6 is such a method (Scheme 1a), and it has been used for the synthesis of [n](2,7)pyrenophanes containing nonplanar Figure 1. [12]Cyclacene (1), cyclo[12]phenacene (2), and V gtle belts 3a–c.

Povarov reactions involving 3-aminocoumarins: synthesis of 1,2,3,4-tetrahydropyrido[2,3-c]coumarins and pyrido[2,3-c]coumarins.

Condensation of 3-aminocoumarin (5) with 4-nitrobenzaldehyde (8) afforded a 2-azadiene (9), which reacted with various electron-rich alkenes (10 examples) in the presence of Yb(OTf)3 to afford 1,2,3,4-tetrahydropyrido[2,3-c]coumarins. Yields were generally good, but the diastereomeric ratios were highly variable. The products arose through a formal [4 + 2] cycloaddition (inverse electron demand Diels-Alder reaction) followed by tautomerization. As such, these are examples of the Povarov reaction. A range of 1,2,3,4-tetrahydropyrido[2,3-c]coumarins was then synthesized using a three-component version of this reaction, which involves in situ formation of the 2-azadiene component. Some of these products were converted into the corresponding pyrido[2,3-c]coumarins upon treatment with various oxidants, the most effective of which proved to be nitrous gases.

1,7‐Dioxa[7](2,7)pyrenophane: The Pyrene Moiety Is More Bent than That of C70

Themostdistortedpyrenemoietyyetprepared (see picture) has been synthesized by the valence isomerization of a tethered [2.2]metacyclophane-1,9-diene to a 10b,10c-dihydropyrene and in situ dehydrogenation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) at 80 °C. The end-to-end bend of the pyrene moiety exceeds that of the corresponding part of the equator of D5h C70.

Multicomponent synthesis of 6H-dibenzo[b,d]pyran-6-ones and a total synthesis of cannabinol.

A multicomponent domino reaction that affords 6H-dibenzo[b,d]pyran-6-ones is reported. The overall transformation consists of six reactions: Knoevenagel condensation, transesterification, enamine formation, an inverse electron demand Diels-Alder (IEDDA) reaction, 1,2-elimination, and transfer hydrogenation. Both the diene and dienophile for the key inverse electron demand Diels-Alder (IEDDA) step are generated in situ by secondary amine-mediated processes. In most cases, the yields (10-79%) are considerably better than those obtained using a stepwise process. This methodology is employed in a concise total synthesis of cannabinol.

1,8-Pyrenylene−Ethynylene Macrocycles

A concise, highly regioselective synthesis of 1,8-dibromo-4,5-dialkoxypyrenes has been developed and exploited in the synthesis of some 1,8-pyrenylene-ethynylene macrocycles. The (1)H NMR data and NICS calculations indicate that there is little or no macrocyclic ring current. Concentration-dependent UV-visible studies indicate no aggregation at low concentration, but 8b forms dimers with voids suitable for intercalation of small molecules in the solid state.

Nonplanar aromatic compounds. Part 10: A strategy for the synthesis of aromatic belts—all wrapped up or down the tubes?* , **

A strategy for the synthesis of cyclophenacene-type aromatic belts (or armchair nanotube segments) that relies upon a valence isomerization/dehydrogenation reaction is described, and progress toward achieving this goal is presented.

Interplay of π-Electron Delocalization and Strain in [n](2,7)Pyrenophanes

The geometries of a series of [n](2,7)pyrenophanes (n = 6-12) were optimized at the B3LYP/6-311G** DFT level. The X-ray crystal structures determined for the [9](2,7)- and [10](2,7)pyrenophanes agreed excellently with the computed structures. The degree of nonplanarity of the pyrene moiety depends on the number of CH2 groups in the aliphatic bridge and, as analyzed theoretically, influences the strain energy and the extent of pi-electron delocalization in the pyrene fragment. Various indices, e.g., the relative aromatic stabilization energies (DeltaASE), magnetic susceptibility exaltations (Lambda), nucleus-independent chemical shifts (NICS), and the harmonic oscillator model of aromaticity (HOMA) were used to quantify the change in aromatic character of the pyrene fragment. DeltaASE and relative Lambda values (with respect to planar pyrene) were evaluated by homodesmotic equations comparing the bent pyrene unit with its bent quinoid dimethylene-substituted analog. The bend angle, alpha, DeltaASE, and Lambda were linearly related. The aromaticity decreases smoothly and regularly over a wide range of bending, but the magnitude of the change is not large. The differences between planar pyrene (alpha = 0 degrees) and the most distorted pyrene unit (alpha = 39.7 degrees in [6](2,7)pyrenophane) are only 15.8 kcal/mol (DeltaASE) and 18.8 cgs-ppm (Lambda). Also, the geometry-based HOMA descriptor changes by only 0.07 unit. The local NICS descriptors of aromatic character also correlate very well with the global indices of aromaticity. In line with the known reactivity of pyrenophanes, the variations of NICS(1), a measure of pi-electron delocalization, were largest for the outer, biphenyl-type rings. The strain energies of the pyrene fragments were much larger and varied more than those evaluated for the bridge. Both strain energies were interrelated (correlation coefficient R = 0.979) and depend on the bend angle, alpha.

Synthesis of 6H-dibenzo[b,d]pyran-6-ones using the inverse electron demand Diels-Alder reaction.

A set of coumarin-fused electron-deficient 1,3-dienes was synthesized, which differ in the nature of the electron-withdrawing group (EWG) at the terminus of the diene unit and (when EWG = CO(2)Me) the nature and position of substituents. These dienes reacted with the enamine derived from cyclopentanone and pyrrolidine to afford the corresponding cyclopenteno-fused 6H-dibenzo[b,d]pyran-6-ones, most likely via a domino inverse electron demand Diels-Alder (IEDDA)/elimination/transfer hydrogenation sequence. The parent diene (EWG = CO(2)Me, no substituents) was reacted with a range of electron-rich dienophiles (mostly enamines) to afford the corresponding 6H-dibenzo[b,d]pyran-6-ones or their nondehydrogenated precursors, which were aromatized upon treatment with a suitable oxidant. The enamines could either be synthesized prior to the reaction or generated in situ. The syntheses of 30 dibenzopyranones are reported.

Carbon nanotubes: Growth potential

Could carbon nanotubes of a single chirality be grown from the bottom up using a common organic reaction?

Non-planar aromatic compounds. Part 4: Fine tuning the degree of bend in the pyrene moiety of [7](2,7)pyrenophanes by modifying the nature of the bridge

Abstract Structures for several mono-, di-, tri- and tetraoxa[7](2,7)pyrenophanes were calculated at the AM1 level of theory and the bend angle ( θ ) of the pyrene system was found to be affected by both the number and the position(s) of the oxygen atom(s). In varying the type of atom X at the 4 position, θ was predicted to decrease with the C–X bond length. Attempted synthesis of 1,4,7-trioxa[7](2,7)pyrenophane gave traces of the target compound as the minor component of a mixture with its recovered precursor. 1 H NMR evidence supports the prediction that it possesses a more bent pyrene unit than that of the current record holder. 4-Oxa[7](2,7)pyrenophane was prepared readily in 11% overall yield over ten steps and its crystallographically measured bend angle is 102.9°.