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Synlett 2011(4): 539-542  
DOI: 10.1055/s-0030-1259539
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Enantioselective Synthesis of Planar Chiral Paracyclophanes with Short ansa Chains and Structure of Strained Dioxa[7]paracyclophane

Tatsuya Arakia, Daiki Hojoa, Keiichi Noguchib, Ken Tanaka*a
a Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
Fax: +81(42)3887037; e-Mail: tanaka-k@cc.tuat.ac.jp;
b Instrumentation Analysis Center, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
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Publication History

Received 27 November 2010
Publication Date:
08 February 2011 (online)

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Abstract

The first enantioselective synthesis of planar chiral [7] and [8]paracyclophanes has been achieved by the cationic rhodium(I)-(S)-H8-BINAP complex catalyzed [2+2+2] cycloaddition. Planar chiral [9]paracyclophanes were also synthesized by the same method. The first X-ray crystallographic analysis of the strained dioxa[7]paracyclophane revealed the significant deformation of the benzene ring from planarity.

Key words

alkynes - paracyclophanes - planar chirality - rhodium - [2+2+2] cycloaddition

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10

Lowering the catalyst loading to 2 mol% resulted in poor conversion (at least <50%) of 1a and 2a under the same reaction conditions of Table  [¹] .

11

Racemization of [9]paracyclophane 3ca was not observed at all in a DCE solution at 80 ˚C for 6 h.

12

The corresponding meta- and ortho-cyclophanes were also generated as minor byproducts.

13

Employing a more diluted reaction conditions (0.01-0.005 M) did not further improve the yield of 3aa.

16

CCDC 801251 [(R)-(+)-3aa] contains the supplementary crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

18

Although the significant deformation of the benzene ring from planarity was observed in the strained [7]paracyclophane 3aa, the ¹H NMR chemical shifts of the aromatic protons of 3aa appears in the standard aromatic region (δ = 8.28-7.56 ppm). Furthermore, isomerization of 3aa to the corresponding Dewar or prismane isomer was not observed in both solution and solid states at room temper-ature under visible light. These observations indicate that 3aa still possesses stable aromatic structure.