Angewandte
Chemie
DOI: 10.1002/anie.201410752
Medium-Ring Compounds
Structure and Reactivity of an Isolable Seven-Membered-Ring
trans-Alkene**
Brisa Hurlocker, Chunhua Hu, and K. A. Woerpel*
Abstract: The isolation and characterization of a trans-
oxasilacycloheptene is reported. Single-crystal X-ray crystallo-
graphic studies indicate a high level of strain and deviation
from ideal geometry. Reactions with several electrophiles
one-flask, two-step synthesis involved the formation of vinyl-
silacyclopropane 2 by silylene transfer to diene 1 followed by
diastereoselective addition to benzaldehyde.[11] Previously,
the structure of alkene 3 was inferred only from spectroscopic
data, and all attempts at purification proved unsuccessful. The
alkene was highly reactive, undergoing thermal rearrange-
ment with a half-life of twelve hours at 228C. Herein, we have
developed a procedure for isolating crystalline alkene 3. The
reaction mixture was filtered through Celite with hexanes to
remove residual silver salts, and the filtrate was concentrated
and crystallized at 08C. The purified compound could be
stored at low temperatures, enabling preliminary studies of its
reactivity. Structural analysis by X-ray crystallography con-
firmed the structure of 3 and provided insight into the
structural consequences of strain on the olefin (Figure 1).[12]
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demonstrated the nucleophilicity of the C C double bond,
affording oxasilacycloheptane and tetrahydrofuran products
as single diastereomers.
S
even-membered-ring trans-alkenes are uncommon syn-
thetic targets as a consequence of their challenging synthesis
and instability under atmospheric conditions.[1,2] Few exam-
ples of these compounds have been synthesized, and trapping
experiments[3–6] or low-temperature spectroscopy[7] have been
used to verify the formation of these compounds. Two
examples, however, have been isolated and characterized by
X-ray crystallography.[8,9] These seven-membered-ring trans-
alkenes incorporated one or more silicon atoms[8,9] that
relieve strain due to longer bonds to these atoms.[10] In this
Communication, we report the isolation of a strained seven-
membered-ring trans-alkene. Although the ring contains one
silicon atom, X-ray crystallographic analysis indicates that the
distortion of the ring approaches the distortion calculated for
trans-cycloheptene.
The seven-membered-ring trans-alkene 3 was synthesized
in 72% overall yield on a multigram scale (Scheme 1). This
Figure 1. Molecular structure of trans-oxasilacycloheptene 3. Thermal
ellipsoids set at 50% probability. Hydrogen atoms, except those on the
ring, have been removed for clarity. Selected bond lengths [ꢀ] and
angles [8]: C1-C2 1.498(2), C2-C3 1.335(2), C3-C4 1.484(2), C4-C5
1.582(2), C5-O1 1.418(2), O1-Si1 1.663(1), Si1-C1 1.927(2); Si1-C1-C2
96.2(1), C4-C3-C2 116.2(1).
Scheme 1. Synthesis of trans-oxasilacycloheptene 3.
The amount of strain in alkene 3 can be assessed by
examining the deformation of the sp2-hybridized centers of
[*] B. Hurlocker, Dr. C. Hu, Prof. K. A. Woerpel
Department of Chemistry, New York University
100 Washington Square East, New York, NY 10003 (USA)
E-mail: kwoerpel@nyu.edu
[13]
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the C C double bond (Figure 2). This deformation can be
quantified by several different parameters. The misalignment
of the p orbitals that results from constraining a trans double
bond in a medium-sized ring is measured by both the
torsion angle (q) and the twisting angle (f; Figure 2).[14] The
[**] This research was supported by the United States National Science
Foundation (CHE-1362709) and is based on work supported by the
National Science Foundation Graduate Research Fellowship under
Grant No. DGE-1342536 to support BH. We thank the NYU
Molecular Design Institute for the purchase of the Bruker SMART
APEX II diffractometer.
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C-C C-C torsion angle (q) of 3 is 126.18, which is a 53.98
distortion away from the ideal torsion angle (180.08). The
average twisting angle (f) is 23.38 away from coplanarity. To
regain some of the orbital overlap lost by twisting, the sp2-
hybridized carbons in alkene 3 undergo rehybridization.[15]
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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