10.1002/anie.201904673
Angewandte Chemie International Edition
COMMUNICATION
ppm) (Figures 3). For the benzene ring, the signals for C(13)–H
(δ 7.31 ppm) and C(14)–H (δ 6.71 ppm) resonate at ca. 0.8 and
0.2 ppm lower field, respectively, than the analogous protons in
11 (δ 6.48 ppm) and 25 (δ 6.51 ppm) (Figure 4).[19] In contrast,
the signals for C(16)–H (δ 5.56 ppm) and C(17)–H (δ 4.97 ppm)
are observed at substantially higher field than the corresponding
protons in 11 (δ 6.48 ppm) and even 25 (δ 5.66 ppm). C(10)–H
(δ 6.28 ppm) is also observed at significantly higher field than its
counterpart in 25 (δ 6.80 ppm).
developing shorter synthetic pathways and achieving two-
directional contractive annulation are underway.
6.48
H
6.71 (6.82) 7.31 (7.49)
H
H
2.96–2.74 (2.61)
4.32 (4.37)
3.07
H
H
H
H
H
H
3.88 (3.85)
2.96–2.74 (2.89)
H
H
H
3.21 (3.31)
H
2.96–2.74 (2.63)
11
21
5.56 (5.64) 4.97 (5.06)
6.51
H
6.80
6.28 (6.30) 7.00 (7.14)
3.16
H
H 2.95
Figure 5. (A) Normalized UV-Vis absorption spectra of 21 measured in
different organic solvents, and TD-M06-2X/Def2TZVP calculated absorption
H
H
H
H
5.66
7.75
H
H
7.04
(7.18)
H 3.11
H
spectrum of (red bar graph). (B) Contour plots (isovalue
= 0.03) and
H
21
3.85
eigenvalues (in eV) of frontier molecular orbitals of 21 (M06-2X/Def2TZVP).
6.93 (7.10)
H
H
7.10
25
7.46 H
7.32 (7.51) (7.22)
Figure 4. Experimental (blue) and calculated (red) (M06-2X/6-311+G(2d,p))
chemical shifts for cyclophanes 11, 21 and 25.
Acknowledgements
The Natural Sciences and Engineering Research Council
(NSERC) of Canada is gratefully acknowledged for financial
support of this research (G.J.B, Y.Z.).
The absorption spectrum of naphthalenophane 21 was
measured in four solvents, ranging from nonpolar (cyclohexane)
to polar (acetonitrile) and they show nearly identical features,
indicating a negligible solvatochromic effect (Figure 5A). There
are two prominent peaks at ca. 225 and 255 nm, and a weak,
broad band envelope ranging from 300 to 375 nm. TD-DFT
calculations predict that the lowest-energy absorption band is
due mainly to the HOMO to LUMO transition, while the peak at
250 nm is due to HOMO–1 to LUMO and HOMO to LUMO+1
transitions (see Supporting Information). The naphthalene unit
makes the major contribution to the frontier molecular orbitals
(Figure 5B), and the calculated HOMO-LUMO gap is 6.40 eV.
The lowest-energy band of 21 is red-shifted compared to those
of [2.2]paracyclophane (11) (λmax (CH2Cl2) = 287, 305 nm)[20] and
naphthalenophane 25 (λmax (95% EtOH) = 300, 310 nm).[21] The
red shift in going from 25 (less distorted naphthalene system) to
21 (more distorted naphthalene system) is consistent with the
continual red shift observed in the [n]paracyclophanes as n
becomes smaller.[22] The distortion of the naphthalene system in
21 may also be responsible for the weakly emissive nature of 21
(λmax = 405 nm, φ = 0.036, cf. 0.23 for naphthalene,[23] see
Supporting Information).
Keywords: cyclophanes • strain • naphthalene • nonplanar
aromatic compounds
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In conclusion, the new contractive annulation strategy for the
synthesis of small, strained cyclophanes has been successfully
applied to the conversion of [2.2]paracyclophane (11) (SE=25.6
[7]
[8]
D. J. Cram, N. L. Allinger, J. Am. Chem. Soc. 1955, 77, 6289–6294.
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kcal/mol)
into
[2](6,1)naphthaleno[1]paracyclophane
(21)
[9]
W.-M. Shu, J.-R. Ma, K.-L. Zheng, H.-Y. Sun, M. Wang, Y. Yang, A.-X.
Wu, Tetrahedron 2014, 70, 9321–9329.
(SE=44.1 kcal/mol) using a 9-step synthetic pathway. Structural
distortions in 21 are spread around the molecule, but are more
pronounced in the vicinity of the one-carbon bridge. Efforts
aimed at applying this strategy to other small cyclophanes,
[10] T. Honda, H. Ishige, H. Nagase, J. Chem. Soc., Perkin Trans. 1 1994,
3305–3310.
[11] The M06-2X functional was chosen for the DFT and TD-DFT
calculations because of its reliable performance in computing ground-
and excited state geometries and energies. See: (a) Y. Zhao, D. G.
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