Hexahelicenophanes and Their Racemization

Meier, Herbert; Schwertel, Manfred; Schollmeyer, Dieter

doi:10.1002/(sici)1521-3773(19980817)37:15<2110::aid-anie2110>3.0.co;2-k

Cited by 19 publications

(9 citation statements)

References 4 publications

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“…81 The work of Meier and coworkers indicated that the macrocyclization can proceed in low to moderate yields (18-63%), but the photocyclization was more difficult (32-37% yield). 82,83 To conclude, the chiral tether strategy did not seem to improve the photocyclization yields to afford carbohelicenes, but it provided in one case, a very high diastereoselectivity (a single diastereoisomer detected).…”

Section: Asymmetric Photosynthesis With Chiral Inducers and Auxiliariesmentioning

confidence: 91%

One hundred years of helicene chemistry. Part 2: stereoselective syntheses and chiral separations of carbohelicenes

2013

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Carbohelicenes generally incorporate a helical, distorted, conjugated, polyaromatic system with ortho-fused benzenoid rings, which is a fundamental molecular characteristic of this class of compounds. They have been described as "molecules in distress" due to their distortion. The generation of a chiral helicity in helicenes was observed because of a severe intramolecular steric strain. Helicity is a molecular necessity in the higher series of carbohelicenes, when at some point, a helical pitch occurs when a second coil is formed. The most interesting properties resulting from such molecular distortion are the very high chiroptical and circular dichroism values. For instance, the resolution of some helicene racemates by "hand picking" of a few homochiral single enantiomeric crystals allowed for a measurement of their optical rotation. Due to that intrinsic chirality spanned over a large polyaromatic template, preliminary results clearly established the efficiency of carbohelicenes to induce asymmetry and chirality in organic synthesis and in supramolecular chemistry. Additionally, they have some potential uses in several fields: materials science, nanoscience, chemical biology and supramolecular chemistry. It has encouraged many attempts to develop new asymmetric syntheses of carbohelicenes, as well as some chiral separations of enantiomers and diastereoisomers. This review is thus dedicated to carbohelicene chirality. It gathered a substantial collection of data, and a comprehensive review on the preparations of enantioenriched helicenes, either from an asymmetric synthesis or from a chiral separation. Utilizations of non-racemic helicenes and their applications will be treated in the following review (Part 3), and will not be the subject of this manuscript.

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Section: Asymmetric Photosynthesis With Chiral Inducers and Auxiliariesmentioning

confidence: 91%

One hundred years of helicene chemistry. Part 2: stereoselective syntheses and chiral separations of carbohelicenes

2013

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“…1,n-Dioxa[n](4,13) [6]helicenophanes 155b-c were synthesized using a Type III approach, which culminated with the Mallory reaction of macrocycles 154b-c (Scheme 34). 101 Interestingly, the lower homolog 154a did not afford helicenophane 155a, but rather a dimer resembling 64 (Scheme 13). This enabled the determination of the racemization barriers (at 200 °C).…”

Section: Scheme 31mentioning

confidence: 99%

Cyclophanes containing large polycyclic aromatic hydrocarbons

2015

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Cyclophanes have been firmly entrenched as a distinct class of compounds for well over half a century. The two main factors that have kept this field of chemistry going so strongly for such a long time are tremendous structural diversity and the interesting behaviour that is often observed. Although a very large number cyclophanes has been reported, only a very small proportion of them contain polycyclic aromatic systems that can be thought of as "large", i.e. with ≥4 rings. This Review puts the spotlight on such cyclophanes, illuminating both the chemistry that was used to synthesize them and what was learned from studying them. Context for the main body is provided by the careful consideration of the anatomy of a cyclophane and the classification of general synthetic approaches. The subsequent sections cover eleven different PAHs and are organized primarily according to increasing size of the aromatic system, starting with pyrene (C16, the only large polycyclic aromatic system to have been incorporated into numerous cyclophanes) and ending with hexabenzo[bc,ef,hi,kl,no,qr]coronene (C42).

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“…Compounds (I) and (II) were prepared according to the literature (Meier et al, 1998) and recrystallized from chloroform.…”

Section: Methodsmentioning

confidence: 99%

“…The steric energy, particularly that of (II), raises the energy of the ground states. In the transition states of the racemization of (I) and (II), the bridges are rotated, the C 2 symmetry of the molecules in solution being transformed to a C s symmetry (Meier et al, 1998), and the steric interactions between the hexahelicene moiety and the bridge are lowered. Thus, we conclude that the decreasing racemization barrier in going from [6]helicene to (I) to (II) is due to the increase of the steric energy in the ground states in this series whereas the transition states are less affected.…”

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confidence: 99%

Two strained hexahelicenophanes

Meier¹,

Schwertel²,

Schollmeyer³

2000

Acta Crystallogr C

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The crystal structures of the [6]helicenes 4,13-(1,10-decamethylenedioxy)hexahelicene, C 36 H 34 O 2 , (I), and 4,13-(1,8octamethylenedioxy)hexahelicene, C 34 H 30 O 2 , (II), show strong steric interactions between the terminal benzene rings and the polymethylenedioxy chains. The shortest ring A and F distances amount to 2.941 (3) and 2.902 (3) A Ê , respectively. The increased steric energy of the ground state is responsible for a signi®cantly lower racemization barrier of (I) and (II) in comparison to the unsubstituted [6]helicene. CommentRecently, we succeeded in the preparation of hexahelicenophanes (Meier et al., 1998) that have polymethylenedioxy chains between positions 4 and 13 in the terminal benzene rings A and F. It turned out that the racemization barriers ÁG À of (I) and (II) at 473 K in 1,2,4-trichlorobenzene are 8.6 and 19.0 kJ mol À1 , respectively, lower than in the parent compound [6]helicene. This surprising result must be due to the special geometrical effects of the bridges.

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Hexahelicenophanes and Their Racemization

Cited by 19 publications

References 4 publications

One hundred years of helicene chemistry. Part 2: stereoselective syntheses and chiral separations of carbohelicenes

One hundred years of helicene chemistry. Part 2: stereoselective syntheses and chiral separations of carbohelicenes

Cyclophanes containing large polycyclic aromatic hydrocarbons

Two strained hexahelicenophanes

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