Henning Hopf

[2.2]Paracyclophanes in Polymer Chemistry and Materials Science

After a long period as model compounds in basic research [2.2]paracyclophanes are quickly gaining in practical importance. They can be incorporated into numerous polymeric systems in which they either lose (the so-called Parylenes) or retain their layered structure, and they can be used for the construction of unsaturated molecular scaffolds characterized not only by conventional (lateral) pi-electron overlap but also by cofacial pi-electron interactions. Surfaces generated from and with [2.2]paracyclophanes possess interesting biological, photophysical, and optoelectronic properties.

Fast, Ligand‐ and Solvent‐Free Synthesis of 1,4‐Substituted Buta‐1,3‐diynes by Cu‐Catalyzed Homocoupling of Terminal Alkynes in a Ball Mill

A method for the Glaser coupling reaction of alkynes by using a vibration ball mill has been developed. The procedure avoids the use of ligands and solvents during the reaction. Aryl- and alkyl-substituted terminal alkynes undergo homocoupling if coground with KF-Al(2)O(3) and CuI as a milling auxiliary and catalyst. Furthermore, an alternative protocol has been developed incorporating 1,4-diazabicyclo[2.2.2]octane (DABCO) as an additional base allowing the use of KF-Al(2)O(3) with a lower KF loading. Besides Cu salts, the homocoupling of phenylacetylene is also catalyzed by Ni or Co salts, as well as by PdCl(2). TMS-protected phenylacetylene could be directly converted into the homocoupling product after in situ deprotection of the alkyne by fluoride-initiated removal of the trimethylsilyl group.

Energetic assessment of the Suzuki–Miyaura reaction: a curtate life cycle assessment as an easily understandable and applicable tool for reaction optimization

The solvent-less Pd-catalyzed Suzuki–Miyaura cross coupling of different aryl halides with phenylboronic acid and an in situ generated solid base was investigated in order to study the influence of different modes of energy entry. For this reason reactions were performed in two different ball-milling set-ups and also different techniques of microwave irradiation have been applied. Combination of reaction data (yield, selectivity, batch size) with energetic data (line power consumption) was used to generalize the results as a curtate life cycle assessment (cLCA), breaking down the reaction to the reaction step, neglecting up- and downstream processes. Thus cLCA was applied for early-level reaction optimization and as decision guidance for constitutive experiments. Under the applied reaction and boundary conditions applied for the solvent-free Suzuki–Miyaura reaction ball milling is proven to be a more effective tool for energy entry than microwave irradiation.

Learning from Molecules in Distress

From the time we first got an inkling of the geometries and metrics of molecules, the literature of organic chemistry has contained characterizations of molecules as unstable, strained, distorted, sterically hindered, bent, and battered. Such molecules are hardly seen as dull; on the contrary, they are perceived as worthwhile synthetic goals, and their synthesis, or evidence of their fleeting existence, has been acclaimed. What is going on here? Why this obsession with abnormal molecules? Is this molecular science sadistic at its core? Let%s approach these questions, first describing what is normal for molecules, so we can define the deviance chemists perceive. After a digression into the anthropomorphic language chemists generally use and the psychology of creation in science, we will turn to the underlying, more serious concern: “What is the value of contemplating (or creating) deviance within science?” The Denumerable, Flexible Chemical Universe

Unidirectional Molecular Stacking of Tribenzotriquinacenes in the Solid State: A Combined X‐ray and Theoretical Study

A combined X-ray diffraction and theoretical study of the solid-state molecular and crystal structures of tribenzotriquinacene (TBTQ, 2) and its centro-methyl derivative (3) is presented. The molecular structure of the parent hydrocarbon displays C3v symmetry and the three indane wings adopt mutually orthogonal orientations, similar to the case in its previously reported methyl derivative (3). Also similarly to the latter structure, the bowl-shaped molecules of compound 2 form infinite molecular stacks with perfectly axial, face-to-back (convex-concave) packing and with parallel and unidirectional orientation of the stacks. The experimentally determined intra-stack molecular distance is 4.75 Å for compound 2 and 5.95 Å for compound 3. Whereas the molecules of compound 2 show a slight alternating rotation (±6°) about the common axis of each stack, those of compound 3 show perfect translational symmetry within the stacks. We used dispersion-corrected density functional theory to compute the crystal structures of tribenzotriquinacenes 2 and 3. The London dispersion correction was crucial for obtaining an accurate description of the crystallization of both analyzed systems and the calculated results agreed excellently with the experimental measurements. We also obtained reasonable sublimation energies for both compounds. In addition, the geometries and dimerization energies of oligomeric stacks of compound 2 were computed and showed smooth convergence to the properties of the infinite polymeric stack.

Pentalenes—From Highly Reactive Antiaromatics to Substrates for Material Science

Antimatter: Once studied primarily for their antiaromatic properties, pentalenes are rapidly becoming important π-systems for novel electronic materials. Recent developments in this area are summarized.

MP2 and DFT Calculations on Circulenes and an Attempt to Prepare the Second Lowest Benzolog, [4]Circulene

MP2 and DFT calculations have been carried out for [n]circulenes for n=3 to 20 in order to predict the strain energy and topology of these cyclically condensed aromatic systems. To synthesise [4]circulene (2), 1,5,7,8-tetrakis(bromomethyl)biphenylene (14) was prepared from the corresponding tetramethyl derivative (8) and subjected to various dehalogenation reactions; all attempts to obtain [2.2]biphenylenophane (7) as a precursor for 2 by this route failed. Treatment of 14 with sodium sulfide furnished the thiaphanes 16 and 17, thermal and photochemical desulfurization of which also failed to provide 7. In a second approach [2.2]paracyclophane was converted to the pseudo-geminal dithiol 23, which was subsequently bridged to the thiaphanes 22 and 24. On flash vacuum pyrolysis at 800 degrees C these were converted exclusively into phenanthrene (30). An approach to dehydrochlorinate the commercial product PARYLENE C to the tetrahydro[4]circulene 7 led only to polymerisation. The X-ray structures of the intermediates 8, 14, 17, 23, 24, 26, and 35 are reported.

Tribenzotriquinacene: A Versatile Synthesis and C3-Chiral Platforms†

Molecules belonging to point group C3 have recently attracted much interest owing to their applications in asymmetric catalysis and chiral recognition. Nonetheless, the number of C3-chiral molecules is still limited compared to the numerous C2-chiral systems, and more entries to C3-chiral molecules are needed. One way to obtain functional C3-chiral molecules, which is the most common in the synthesis of tripodal ligands, is to append three enantiopure handles to an otherwise achiral platform. The second approach, which is much more common in supramolecular chemistry, is to start with a C3-chiral platform from the very beginning and to extend it with achiral recognition units. The molecular bowl tribenzotriquinacene (1; Scheme 1) constitutes an excellent platform for the second strategy owing to its rigidity and configurational stability. However, the preparation of C3chiral derivatives is severely hampered by the lack of regioselectivity. Herein we wish to report a C3-specific entry to this class of compounds, thereby providing a new access to this novel family of C3-chiral molecules. The work is based on a new and versatile synthesis of the parent hydrocarbon 1 and its previously only poorly accessible ortho derivatives. The latter furthermore provide a highly anticipated entry to extended carbon networks. Our synthesis of tribenzotriquinacene 1 starts off with the benzylidene propanedione 2 (Scheme 1), which can be easily obtained by Knoevenagel condensation. Reduction to the diastereomeric diols 3 had already been reported by Olah et al. (32 % yield), and we improved this step by developing an optimized Luche procedure (93% yield). 11] Olah s motivation for accessing diols 3 was their study under superacidic conditions (FSO3H/SO2ClF, 80 8C), whereby he observed a cyclodehydrated intermediate, presumably of form 4. While working with diols 3, we found that isomerizations and cyclodehydrations took place even under mildly acidic conditions (cat. p-toluenesulfonic acid in CH2Cl2, RT) and hypothesized that such cyclizations might eventually lead to tribenzotriquinacene (1). Application of Kuck s cyclodehydration conditions (H3PO4, chlorobenzene, 130 8C, 20 h) to diols 3 indeed gave tribenzotriquinacene in 28% yield. Switching to polyphosphoric acid (PPA) as dehydrating agent increased the yield to 32 %, making 1 available in gram quantities for the first time. Other acids were also tested, but did not prove effective (acetic acid, trifluoroacetic acid, methanesulfonic acid, Eaton s reagent, trifluoromethanesulfonic acid (TfOH), Tf2O, H2SO4). The reaction presumably proceeds through a series of intramolecular Friedel–Crafts alkylations with carbocation intermediates, which is supported by the fact that the yield did not depend on the diastereomer of 3 being used. A reaction mechanism that also explains the formation of the dihydroindenoindene byproduct 5 is proposed in the Supporting Information. The synthesis is higher-yielding than Kuck s synthesis of the parent hydrocarbon (over three steps: 19% vs. 5%). Moreover, as we will show below, it allows the planned introduction of aromatic substituents by varying the easily available benzaldehyde and dibenzoylmethane components of the Knoevenagel adduct. Functionalization of the aromatic rings in tribenzotriquinacene has largely been limited to the outer rim positions, as these are easily accessible by electrophilic aromatic substitution. 13] Ortho functionalization of tribenzotriquinacenes is rare and limited in scope. One example is known in which Scheme 1. The synthesis of tribenzotriquinacene (1).

Symmetrically tetrasubstituted [2.2]paracyclophanes: their systematization and regioselective synthesis of several types of bis-bifunctional derivatives by double electrophilic substitution.

The possible number of chiral and achiral tetrasubstituted [2.2]paracyclophanes possessing different types of symmetry (C(2), C(i), C(s), C(2v), C(2h)) is evaluated and a unified independent trivial naming descriptor system is introduced. The reactivity and regioselectivity of the electrophilic substitution of the chiral pseudo-meta- and achiral pseudo-para-disubstituted [2.2]paracyclophanes are investigated in an approach suggested to be general for the synthesis of bis-bifunctional [2.2]paracyclophanes. The mono- and diacylation of chiral pseudo-meta-dihydroxy[2.2]paracyclophane 14 with acetylchloride occur ortho-regioselectively to produce tri- 22, 23 and symmetrically 21 tetrasubstituted acyl derivatives. The same reaction with benzoylchloride is neither regio-, nor chemoselective, and gives rise to a mixture of ortho-/para-, mono-/diacylated compounds 27-31. The double acylation of pseudo-meta-dimethoxy[2.2]paracyclophane 18 is completely para-regioselective. Electrophilic substitution of pseudo-meta-bis(methoxycarbonyl)[2.2]paracyclophane 20 regioselectively generates the pseudo-gem-substitution pattern. Formylation of this substrate produces the monocarbonyl derivatives 35 only, whereas the Fe-catalyzed bromination may be directed towards mono- 36 or disubstitution 37 products chemoselectively by varying the reactions conditions. The diacylation and dibromination reactions of the respective achiral diphenol 12 and bis(methoxycarbonyl) 40 derivatives of the pseudo-para-structure retain regioselectivities which are characteristic for their pseudo-meta-regioisomers. Imino ligands 26, 25, and 39, which were obtained from monoacyl- 22 and diacyldihydroxy[2.2]paracyclophanes 21, 38, are tested as chiral ligands in stereoselective Et(2)Zn addition to benzaldehyde producing 1-phenylpropanol with ee values up to 76 %.

Synthesis of planar chiral [2.2]paracyclophanes by biotransformations: kinetic resolution of 4-formyl-[2.2]paracyclophane by asymmetric reduction

Abstract The synthesis of enantiomerically pure (S)-4-formyl-[2.2]paracyclophane 1 (>99% ee) and (R)-4-hydroxymethyl-[2.2]paracyclophane 2 (>78% ee) was achieved by bioreduction of (RS)- 1 with a yield of 49 and 34% respectively. From several microorganisms screened only a strain of the yeast Saccharomyces cerevisiae (DSM 11285) showed a stereospecific reduction of this planar chiral substrate (E>100). Despite the high enantiomeric ratio, it is necessary to maintain the conversion at almost 50% in order to obtain a high enantiomeric excess of both substrate and product of the reduction reaction. Tween 80 together with methanol was found to be the most suitable cosolvent mixture which enhances the solubility of the substrate and does not effect the biocatalyst. For the calculation of E the enantiomeric excesses of substrate and product were measured at various conversions by chiral gas chromatography. Commercially available alcohol dehydrogenases such as HLADH, YADH and TBADH were tested for the desired reaction too, but found to be completely inactive.