Communications
DOI: 10.1002/anie.201101406
Propeller Structures
Enantioseparation and Electronic Properties of a Propeller-Shaped
Triarylborane
Hiroshi Ito,* Takanori Abe, and Kazuhiko Saigo*
Triarylboranes are a series of the most major propeller-
shaped compounds, and their conformations and conforma-
tional dynamics have been the center of interest since the
early stage of the study of propeller-like molecules.[1] Tri-
arylboranes can exist in a chiral form with a propeller shape in
the solid state,[2] while they show a rapid exchange between
their enantiomers in a solution, which prevents the isolation
of nonracemic forms; in spite of the inherent chirality, the
separation of enantiomers of propeller-like chiral triarylbor-
anes has never been achieved because of the low energy
barrier that separates the (P)- and (M)-forms, although
Okada et al. reported the synthesis, enantioseparation, and
stereoisomerization of particular helicene-shaped chiral tri-
arylboranes.[3] Mislow and co-workers have studied the
molecular dynamics of a series of propeller-shaped molecules
with a boron, nitrogen, or carbon atom as the pivot to show
that the flipping of the propellers proceeds through a two-ring
flip mechanism.[4] In recent years, many novel features of
triarylboranes have been revealed that stem from the
electron-accepting properties and the pp–p* conjugation of
the boron atom, in which its vacant orbital plays a central
role.[5] These observations strongly suggest that the electronic
properties of the central boron atom, as well as the molecular
structural features, would also affect the molecular dynamics
of propeller-shaped triarylboranes. Herein, we report the first
enantioseparation of a propeller-shaped triarylborane and its
electronic properties and molecular dynamics arising from the
central boron atom.
cap of the ethynyl groups to make the steric repulsions more
efficient and to allow possible future elongation of the blades.
On the basis of these considerations, we targeted the
propeller-shaped triarylborane 1 with the expectation that a
two-ring flipping motion would be sterically suppressed by
the 1,3-diethynylphenyl groups (Scheme 1).
Scheme 1. Design and schematic illustration of a propeller-shaped
triarylborane preventing two-ring flip isomerization.
Oligo(1,3-diethynylphenylene)s are known to form inter-
twined supramolecular complexes and foldamers because of
their rigid and restricted structures,[6] and we speculated that
three 1,3-diethynylphenyl groups (blades) in a triarylborane
would lead to effective steric repulsion between the blades. In
our design, we chose a trimethylsilyl (TMS) group as the end
The propeller molecule 1 was synthesized in two steps
(Scheme 2). The bromide 2, a blade moiety, was obtained by
the modified Hart coupling[7] of m-dichlorobenzene with
trimethylsilylethynylmagnesium bromide and subsequent
quenching with bromine. The bromide 2 was lithiated and
then treated with trifluoroborane ether complex to afford the
propeller compound 1 in moderate yield. The latter reaction
proceeded at room temperature; therefore, the steric hin-
drance around the boron atom of 1 is considered to be less
than that of trimesitylborane, a typical propeller-shaped
[*] Dr. H. Ito,[+] T. Abe
Department of Chemistry and Biotechnology
Graduate School of Engineering, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (81)6-6605-2560
E-mail: h-ito@sci.osaka-cu.ac.jp
Dr. K. Saigo
School of Environmental Science and Engineering
Kochi University of Technology
Miyanokuchi, Tosayamada-cho, Kami-shi, Kochi 782-8502 (Japan)
E-mail: saigo.kazuhiko@kochi-tech.ac.jp
[+] Current address: Department of Chemistry
Graduate School of Science, Osaka City University
Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)
Scheme 2. Synthesis of propeller-shaped triarylborane 1. a) 1) nBuLi,
THF, À788C, 2) trimethylsilylethynylmagnesium bromide, À788C to
RT, 3) Br2, RT; b) 1) nBuLi, Et2O, À788C, 2) BF3·OEt2, À788C to RT.
Supporting information for this article is available on the WWW
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7144 –7147