Stereoelectronic effect of curved aromatic structures: Favoring the unexpected endo conformation of benzylic-substituted sumanene

Article abstract of DOI:10.1002/anie.201303134

Throwing a curve: The first example of a through-bond stereoelectronic effect for curved aromatic compounds is described for buckybowls, that is, benzylic-substituted sumanenes. Methyl- and hydroxysumanene favor the endo-R conformer because of a difference in the strength, between the conformers, of the hyperconjugation of the benzylic C-H bond with the bowl. Copyright

Full text of DOI:10.1002/anie.201303134

Angewandte
Chemie
Communications
Through-Bond Interactions
S. Higashibayashi,* S. Onogi,
H. K. Srivastava, G. N. Sastry, Y.-T. Wu,
H. Sakurai*
&&&& — &&&&
Stereoelectronic Effect of Curved
Aromatic Structures: Favoring the
Unexpected endo Conformation of
Benzylic-Substituted Sumanene
Throwing a curve: The first example of
a through-bond stereoelectronic effect for
curved aromatic compounds is described
for buckybowls, that is, benzylic-substi-
tuted sumanenes. Methyl- and hydroxy-
sumanene favor the endo-R conformer
because of a difference in the strength,
between the conformers, of the hyper-
À
conjugation of the benzylic C H bond
with the bowl.
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DOI: 10.1002/anie.201303134
Through-Bond Interactions
Stereoelectronic Effect of Curved Aromatic Structures: Favoring the
Unexpected endo Conformation of Benzylic-Substituted Sumanene**
Shuhei Higashibayashi,* Satoru Onogi, Hemant Kumar Srivastava, G. Narahari Sastry,
Yao-Ting Wu, and Hidehiro Sakurai*
Since the discovery of fullerene and carbon nanotubes, curved
aromatic compounds, including bowl-shaped buckybowls,
have elicited much attention in science and industry. One
principal question regarding these curved aromatic com-
pounds revolves around the differences between the concave
face and the convex face. The curvature affects the nature of
the two faces, thus resulting in differences of the through-
space and through-bond effects. The latter would appear as
a stereoelectronic effect between the curved aromatic struc-
ture and a connected functional group. In general, the
stereoelectronic effect is reflected in the conformational
stability and chemical reactivity of a molecule.[1] Therefore, it
is important to understand such conformations or chemical
reactivities as a consequence of stereoelectronic effects. The
interpretation of the stereoelectronic effect of curved aro-
matic compounds would lead to understanding the differ-
ences of the two faces. However, no example of stereoelec-
tronic effects of curved aromatic compounds have been found
and studied to date. We have found the first example of the
stereoelectronic effect of a curved aromatic structure, an
effect which dominates the endo/exo-R conformational sta-
bility of benzylic-substituted sumanenes (1; Figure 1). Herein
we report the experimental observations and the theoretical
investigations including natural bond orbital (NBO) analy-
sis[2] to elucidate the stereoelectronic effect.
Benzylic-substituted sumanenes (1) can adopt two con-
formers, endo-R-1 and exo-R-1, which differ in the direction
of the aromatic bowl shape, concave or convex, with respect
to the substituent R (Figure 2A). The bowl can thermally
Figure 2. A) endo-R and exo-R conformers of benzylic-substituted
sumanenes (1). B) Schematic showing the stereoelectronic effects and
through-space effects between the bowl and the C-H or C-R moieties
of 1.
invert through a flat transition state and the bowl inversion
energies of sumanenes are around 20 kcalmolÀ1 [3]
. Therefore,
the relative stability of endo-R-1 versus exo-R-1 is thermo-
dynamically determined by the through-bond stereoelec-
tronic effect or the through-space effects (such as steric
repulsion, CH–p interaction), depending on the relative
configuration between the bowl and the substituent R (Fig-
ure 2B).[4] The exo-R conformer of cis-tris(trimethylsilyl)-
sumanene (2; Figure 1) was reported to be much more stable
than the endo-R conformer because only the exo-R confor-
mer was observed in the 1H NMR spectrum.[5] We also
confirmed that trimethysilylsumanene (1b) exists only in the
exo-R conformation (Table 1). These results are reasonable,
judging from the steric repulsion of the substituent by the
nearby aromatic hydrogen atoms, as clearly seen in the
optimized structure, shown in Figure 3A, from the DFT
calculations [M06-2x/6-311 ++ G(d,p)]. Thus, we presumed
that repulsive steric hindrance governs the thermodynamic
stability between endo-R-1 and exo-R-1 in general, and
therefore resulting in exo-R-1 being more stable than endo-R-
1 even with substituents other than R = Si(CH3)3. However, in
Figure 1. Benzylic-substituted sumanenes (1) and cis-tris(trimethyl-
silyl)sumanene (2).
[*] Prof. Dr. S. Higashibayashi, S. Onogi, Prof. Dr. H. Sakurai
Institute for Molecular Science and The Graduate University for
Advanced Studies, Myodaiji, Okazaki 444-8787 (Japan)
E-mail: higashi@ims.ac.jp
hsakurai@ims.ac.jp
Dr. H. K. Srivastava, Dr. G. N. Sastry
Molecular Modeling Group, Indian Institute of Chemical Technol-
ogy, Tarnaka, Hyderabad 500607 (India)
Prof. Dr. Y.-T. Wu
Department of Chemistry, National Cheng Kung University
No.1 Ta-Hsueh Rd. 70101 Tainan (Taiwan)
[**] This work was supported by the MEXT and JST, ACT-C. We thank
Sachiko Nakano for her technical contribution.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303134.
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Table 1: Experimental and calculated data for endo-R-1 and exo-R-1.
À
Compound
R
Experimental
Calculated
NBO analysis of C H bond
endo-R/exo-R[a]
Conformer[b]
DE[c]
Conformer[d]
DE[e]
Conformer[f]
DENBO
[g]
1a
1b
1c
1d
1e
1 f
H
–
–
–
–
–
–
2.0
À1.4
À2.3
À4.6
À2.1
–
À0.5
4.4
À1.1
À0.9
4.2
Si(CH3)2
CH3
OH
2,6-(CH3)2C6H3
C6H5
0:100
81:19
90:10
82:18
49:51
exo-R
exo-R
endo-R
endo-R
endo-R
endo-R
exo-R
endo-R
endo-R
exo-R
endo-R
endo-R
endo-R
endo-R
endo/exo-R
À0.9
À1.3
À0.9
0.0
À0.3
Experimental endo-R-1/exo-R-1 ratio from 1H NMR spectra and the energy difference, DE, between endo-R-1 and exo-R-1. Calculated energy difference,
À
DE, between endo-R-1 and exo-R-1 [M06-2x/6-311+ +G(d,p)]. The difference DENBO of hyperconjugation energies for the exo C H bond in endo-R-
À
1 and the endo C H bond in exo-R-1 as determined by NBO analysis of 1 [M06-2x/6-311G(d,p)]; the favored conformers in each case are
indicated.[a] 1H NMR ratio in CDCl3 (296 K). [b] Observed more-stable conformer. [c] DE=E(endo-R)ÀE(exo-R)=ÀRTlnK kcalmolÀ1, K: equilibrium
constant between endo-R-1 and exo-R-1. [d] More-stable conformer from calculations. [e] DE=E(endo-R)ÀE(exo-R) kcalmolÀ1. [f] More-stable
conformer in NBO analysis. [g] DENBO =ENBO(exo C H bond of endo-R-1)-ENBO(endo C H bond of exo-R-1) kcalmolÀ1, ENBO: hyperconjugation energy of
À
À
À
exo/endo C H bond of endo-R-1 or exo-R-1 by NBO analysis.
lations [M06-2x/6-311 ++ G(d,p)] (Figures 3B–D) indicate
that no steric factors favor the endo-R stability in 1c, 1d, and
1e, however CH–p interactions between the substituent and
the sumanene core could exist in 1e, thus favoring the endo-R
conformation.[7] M06-2x/6-311 ++ G(d,p) calculations repro-
duced the relative conformational stability of 1a–e, albeit
with an overestimation of the endo-R stability in 1e and 1 f
(Table 1). These experimental and calculated results strongly
indicate the existence of through-bond stereoelectronic
effects in 1c and 1d, effect which favor the endo-R
conformation.
For the elucidation of the stereoelectronic factors, we
conducted NBO analysis of 1 [M06-2x/6-311G(d,p)]. After
thorough analysis of the output results, we found that
À
hyperconjugation of the benzylic methine C H bond of
1 can be a factor controlling the conformation of 1 (Figure 2B,
Table 1).[1c] The C H bond possesses conjugation energy
À
through hyperconjugation as a donor orbital to acceptor
orbitals, mainly p* orbitals of the aromatic bowl. The
strengths of the conjugation energies ENBO of the exo and
À
endo C H bonds are different because of the difference of the
acceptor p* orbitals in the concave and convex faces of the
bowl. In the nonsubstituted 1a (R = H), the conjugation
À
energy, ENBO, of the exo C H bond of the methylene is
11.1 kcalmolÀ1 and that of the endo C H bond is 10.6 kcal
À
À1
À
mol . Therefore, the exo C H bond has stronger hyper-
À1
À
conjugation (DENBO = 0.5 kcalmol ) than the endo C H
À
bond (Table 1). Similarly, the exo C H bond of the endo-R
conformer of 1c (CH3) and 1d (OH) exhibits stronger
Figure 3. The endo-R and exo-R conformers of 1b, 1c, 1d, and 1e as
determined by DFT calculations [M06-2x/6-311+ +G(d,p)].
hyperconjugation (DENBO = 1.1 and 0.9 kcalmolÀ1) than the
À
endo C H bond of the exo-R conformer. Therefore, this
effect favors the endo-R conformer of 1c and 1d, and is
consistent with the observed and calculated endo-R stabilities.
spite our presumption, we recently discovered that methyl-
sumanene (1c), hydroxysumanene (1d), and (2,6-dimethyl-
phenyl)sumanene (1e)[6] favor the endo-R conformation over
the exo-R conformation, and phenylsumanene (1 f) shows an
almost even ratio (Table 1). The endo-R conformer was more
stable than the exo-R for 1c (CH3), 1d (OH), and 1e [2,6-
(CH3)2C6H3] with DE = 0.9, 1.3, and 0.9 kcalmolÀ1, respec-
tively. The optimized structures obtained from DFT calcu-
We also considered other stereoelectronic effects from the
NBO analysis, such as the conjugation of the orbitals of the
OH group of 1d. However, the conjugation energy of the
À
lone-pair orbital or the O H bond orbital of the OH group of
1d as a donor orbital is almost equal in the endo-R and exo-R
conformers, and does not support the endo-R stability. While
the steric repulsion to favor the exo-R conformer in 1b [R =
Si(CH3)3] and the CH–p interaction to favor the endo-R
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À
conjugation of the benzylic exo/endo C
H bonds with the bowl in the benzylic-
substituted sumanenes. The hyperconju-
gation is a dominant factor for deter-
mining the endo-R conformational sta-
bility of methylsumanene and hydroxy-
sumanene. Our finding reveals the
potential through-bond stereoelectronic
effect of curved aromatic compounds, an
effect that results from the differences in
the concave and convex faces. This
finding helps to further our understand-
ing of the nature of curved aromatic
compounds and to control properties
such as conformation, chemical reactiv-
ity, and interaction.
Figure 4. The energy levels of the p* orbitals (LUMO, LUMO+1, LUMO+2) of the endo-R and
exo-R conformers of 1a–f as determined by DFT calculations [M06-2x/6-311+ +G(d,p)]. Values
in eV vs. LUMO of 1a.
Received: April 15, 2013
Published online: && &&, &&&&
Keywords: conformation analysis ·
.
conformer in 1e [R = 2,6-(CH3)2C6H3][7] are the controlling
factors in determining the stable conformation, NBO analysis
of 1b and 1e indicates that the hyperconjugation of the endo
density functional calculations · hyperconjugation ·
through-bond interactions · through-space interactions
À
À
C H bond is stronger than that of the exo C H bond to favor
the exo-R conformer (Table 1), and is opposite to what is
indicated by NBO analysis for 1a, 1c, and 1d.[8] In 1b, NBO
analysis indicates that the conjugation between the C-Si
s* orbital and p orbital also favors the exo-R conformer
(2.2 kcalmolÀ1).
[1] a) G. Mehta, J. Chandrasekhar, Chem. Rev. 1999, 99, 1437 – 1467;
b) C. L. Perrin, Tetrahedron 1995, 51, 11901 – 11935; c) D. S.
Ribeiro, R. Rittner, J. Org. Chem. 2003, 68, 6780 – 6787, and
references therein.
[2] A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88, 899 –
926.
[3] a) S. Higashibayashi, R. Tsuruoka, Y. Soujanya, U. Purushotham,
G. N. Sastry, S. Seki, T. Ishikawa, S. Toyota, H. Sakurai, Bull.
Chem. Soc. Jpn. 2012, 85, 450 – 467; b) R. Tsuruoka, S. Higashi-
bayashi, T. Ishikawa, S. Toyota, H. Sakurai, Chem. Lett. 2010, 39,
646 – 647; c) S. Higashibayashi, H. Sakurai, J. Am. Chem. Soc.
2008, 130, 8592 – 8593; d) T. Amaya, H. Sakane, T. Muneishi, T.
Hirao, Chem. Commun. 2008, 765 – 767.
[4] Conformational analysis of 1-alkyl-1,2-dihydrocorannulenes has
been reported. Only strain energy and steric repulsion were
discussed: A. Sygula, R. Sygula, F. R. Fronczek, P. W. Rabideau, J.
Org. Chem. 2002, 67, 6487 – 6492.
[5] H. Sakurai, T. Daiko, H. Sakane, T. Amaya, T. Hirao, J. Am.
Chem. Soc. 2005, 127, 11580– 11581. See also Refs. [3a] and [3b].
[6] J.-J. Chen, S. Onogi, Y.-C. Hsieh, C.-C. Hsiao, S. Higashibayashi,
H. Sakurai, Y.-T. Wu, Adv. Synth. Catal. 2012, 354, 1551 – 1558.
[7] The stronger CH–p interaction to favor the endo-R-1e was
analyzed by the CH–p distance and AIM (atoms in molecules)
analysis (see the Supporting Information).
It is also noteworthy that the difference in the stereoelec-
tronic effects between the endo-R and exo-R conformers of
1 are reflected in the energy levels of the acceptor p* orbitals.
Figure 4 shows the energy levels of LUMO, LUMO + 1, and
LUMO + 2 orbitals of the endo-R and exo-R conformers of
1a–f obtained by DFT calculations. In an overall trend, these
p* orbitals of the endo-R conformers of 1c, 1d, and 1 f are
higher in energy than those of the exo-R conformer as a result
of a stronger interaction with the donor C-H orbital, whereas
those of the exo-R conformers of 1b and 1e are higher than
those of the endo-R conformers. These results are consistent
with those of the NBO analysis (Table 1).
In conclusion, as the first example of the through-bond
stereoelectronic effect of curved aromatic compounds, meth-
ylsumanene and hydroxysumanene were found to favor the
unexpected endo-R conformation. Natural bond orbital
(NBO) analysis indicates that the stereoelectronic effect of
the aromatic bowl is different for the concave and convex
faces, and is reflected in the different strengths of hyper-
[8] The reason for the reverse strength of hyperconjugation of 1b and
À
1e can be explained by the angle changes between the C H bond
and the p face (see the Supporting Information).
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