Cyclophane



A cyclophane is a strain. Despite this, cyclophane structures are not unknown to biomolecules.
Basic cyclophane types are [n]metacyclophanes (I) in scheme 1, [n]paracyclophanes (II) and [n,n']cyclophanes (III). the prefixes meta and para correspond to the usual arene substitution patterns and n refers to the number of atoms making up the bridge.

Structure

Paracyclophanes adopt the benzene to 39 pm [1] [2].

In organic reactions [6]cyclophane tends to react as a diene derivative and not as an aromat. With chlorine the 1,2-addition product forms.

Yet the diamagnetic ring current criterion for aromaticity this cyclophane is still aromatic.

One particular research field in cyclophanes involves probing just how close atoms can get above the center of an aromatic nucleus [3]. In so-called in-cyclophanes with part of the molecule forced to point inwards one of the closest hydrogen to arene distances experimentally determined is just 168 picometer.


A non-bonding nitrogen to arene distance of 244 pm is recorded for a pyridinophane and in the totally weird superphane the two benzene rings are separated by a mere 262 pm. Another representative of this group are in-methylcyclophanes.

Synthetic methods

[6]paracyclophane can be synthesized [4] [5] in the laboratory by a rearrangement reaction of the bicyclopropenyl copound 7.


Metaparacyclophanes constitute another class of cyclophans like the [14][14]metaparacyclophane [6] in scheme 4 [7] featuring a in-situ alkene 4.


Naturally occurring cyclophanes

Despite carrying strain, the cyclophane motif does exist in nature. One example of a metacyclophane is cavicularin.

Haouamine A is a paracyclophane found in a certain species of tunicate. Because of its potential application as an anticancer Diels-Alder reaction in the crucial step with expulsion of carbon dioxide (scheme 5) [8].

In this compound the deviation from planarity is 13° for the benzene ring and 17° for the bridgehead carbons [9]. An alternative cyclophane formation strategy in scheme 6 [10] was developed based on aromatization of the ring well after the formation of the bridge.


[n,n]Paracyclophanes

A well exploited member of the [n,n]paracyclophane family is [2,2]paracyclophane. One method for its preparation is by a 1,6-Hofmann elimination [11]:


The [2.2]paracyclophane-1,9-diene has been applied in Grubb's second generation catalyst [12]:

The driving force for ring-opening and polymerization is strain relief. The reaction is believed to be a living polymerization due to the lack of competing reactions.

Because the two benzene rings are in close proximity this cyclophane type also serves as guinea pig for photochemical dimerization reactions as illustrated by this example [13]:

The product formed has an octahedrane skeleton. Interestingly when the LUMO.

References

  1. ^ Synthesis and molecular structure of (Z)-[6]Paracycloph-3-enes Yoshito Tobe, Kenichi Ueda, Teruhisa Kaneda, Kiyomi Kakiuchi, Yoshinobu Odaira, Yasushi Kai, Nobutami Kasai J. Am. Chem. Soc.; 1987; 109(4); 1136-1144. Abstract
  2. ^ J. Hunger, C. Wolff, W. Tochtermann, E-M. Peters, K.Peters, H.G. von Schering Chem. Ber., 119, 2698 (1986)
  3. ^ Molecular Iron Maidens: Ultrashort Nonbonded Contacts in Cyclophanes and Other Crowded Molecules Robert A. Pascal, Jr Eur. J. Org. Chem. 2004, 3763-3771 doi:10.1002/ejoc.200400183
  4. ^ [6]Paracyclophane Vinayak V. Kane, Anthony D. Wolf, Maitland Jones, , Jr. J. Am. Chem. Soc.; 1974; 96(8); 2643-2644. Abstract
  5. ^ Interconversion of [6]paracyclophane and 1,4-hexamethylene(Dewar benzene) Seetha L. Kammula, Linda D. Iroff, Maitland Jones, , Jr. J. W. Van Straten, W. H. De Wolf, F. Bickelhaupt J. Am. Chem. Soc.; 1977; 99(17); 5815-5815. Abstract
  6. ^ [14][14]Metaparacyclophane: First Example of an [m][n]MetaparacyclophaneChunmei Wei, Kai-For Mo, and Tze-Lock Chan J. Org. Chem.; 2003; 68(7) pp 2948 - 2951; (Note) Abstract
  7. ^ Scheme 4. Reaction scheme: with para-ring in place ring closure of meta part by palladium on carbon
  8. ^ Total Synthesis of (±)-Haouamine A Phil S. Baran and Noah Z. Burns J. Am. Chem. Soc.; 2006; ASAP Web Release Date: 04-Mar-2006; Abstract The authors mark the biosynthetic origin as mysterious
  9. ^ Synthesis of the 3-Aza-[7]-paracyclophane Core of Haouamine A and B Peter Wipf and Markus Furegati Org. Lett.; 2006; 8(9) pp 1901 - 1904; (Letter) Abstract
  10. ^ Scheme 6. Reaction scheme: step I dimethyl sulfate. Ns = Nosylate
  11. ^ Organic Syntheses, Coll. Vol. 5, p.883 (1973); Vol. 42, p.83 (1962) Link.
  12. ^ Soluble Poly(p-phenylenevinylene)s through Ring-Opening Metathesis Polymerization Chin-Yang Yu and Michael L. Turner Angew. Chem. Int. Ed. 2006, 45, 7797 –7800 doi:10.1002/anie.200602863
  13. ^ Photoreaction of a 2,11-Diaza[3.3]paracyclophane Derivative: Formation of Octahedrane by Photochemical Dimerization of Benzene Hideki Okamoto, Kyosuke Satake, Hiroyuki Ishida, and Masaru Kimura J. Am. Chem. Soc.; 2006; 128(51) pp 16508 - 16509; (Communication) doi:10.1021/ja067350r
 
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