ChemComm
Communication
electron-rich aromatic cavity of 1. In fact, the electrostatic
potential surface (ESP) of 3a+ shows that the p-NO2-benzyl ring
(Fig. 1c, left ring) has a lower p-electron density with respect to
the benzylic counterpart (Fig. 1c, right ring).
Interestingly, when benzyl-p-CN-benzylammonium thread
3b+ was used the pseudorotaxane endo-p-CN-benzyl-7b+ was
formed in a 90: 10 ratio with respect to the endo-benzyl-7b+
isomer (Fig. 2c, Table 1). Thus the presence of an EW-substituent
(X = CN) with a Hammett s constant smaller than the nitro group
(sCN = 0.71, sNO = 0.78) lowers the stereoselectivity of the
2
threading process.
In agreement with our prediction, the presence of ED-groups
with a negative Hammett s constant reverses the above observed
preferences for endo-p-X-benzyl-7+ stereoisomers. In fact, when
benzyl-p-Me-benzylammonium thread 3c+ was mixed in a 1 : 1
ratio with calix-wheel 1 in CDCl3 the pseudorotaxane endo-benzyl-
7c+ was favored with respect to the endo-p-Me-benzyl-7c+ one
(sMe = À0.14) with a stereoisomeric ratio of 70 : 30 (Fig. 2d,
Table 1).8 Finally, the presence of the OMe group in 3d+ with a
more negative Hammett spara constant (sOMe = À0.27) increases
the stereoisomeric ratio (85 : 15) in favor of the endo-benzyl-7d+
stereoisomer (Fig. 2e, Table 1).8
The ESP of 3d+ clearly shows that the p-OMe-benzyl ring (Fig. 1d,
left ring) has a higher p-electron density than the benzylic counter-
part (Fig. 1d, right ring) thus justifying its lower aptitude to interact
with the electron-rich aromatic cavity of 1 (Fig. 1e and f).
In conclusion, we have successfully demonstrated that pseudo-
rotaxane orientational stereoisomerism can be effectively controlled
by changing the p-electron density of the thread through EW or ED
aromatic substituent effects. This approach can be extended to
more complex interpenetrated architectures leading to a fine
stereochemical control through the weak p–p interactions.
We thank the Italian MIUR (PRIN 20109Z2XRJ_006) for
financial support and the Centro di Tecnologie Integrate per
Fig. 2 Significant portions of the 1H NMR spectra (400 MHz, CDCl3, 298 K)
of (a) 1; (b) 1:1 mixture of 1 and 3a+; (c) 1:1 mixture of 1 and 3b+; (d) 1:1
mixture of 1 and 3c+; (e) 1:1 mixture of 1 and 3d+. The cavity-shielded ArH
signals of endo-p-X-benzyl and endo-benzyl 7a–d+ isomers are reported in
red and blue, respectively. Signals marked with circles or triangles correspond
to the cavity-shielded ArH protons of the minor stereoisomer. In structure
drawings some groups have been removed for clarity. Signals marked with
,
and E correspond, respectively, to the OMe and ArCH2Ar protons of
the free host 1 and to the ArCH2Ar protons of the complexed host 1.
`
la Salute (Project PONa3_00138), Universita di Salerno, for the
600 MHz NMR instrumental time. Thanks are due to Dr Patrizia
Oliva and Dr Patrizia Iannece for NMR and ESI-MS measure-
ments, respectively.
Notes and references
1 J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,
VCH, New York, 1995.
2 (a) C. A. Hunter and J. K. M. Sanders, J. Am. Chem. Soc., 1990,
112, 5525; (b) L. M. Salonen, M. Ellermann and F. Diederich, Angew.
Chem., Int. Ed., 2011, 50, 4808.
Fig. 3 Side (a) and top (b) view of the optimized structure of the endo-p-
NO2-benzyl-7a+ complex at the B3LYP/6-31G(d,p) level of theory. (c) Side
view of the optimized structure of the endo-benzyl-7a+ complex. A mixed
CPK/wireframe representation was used for clarity.
3 F. Cozzi, M. Cinquini, R. Annunziata and J. S. Siegel, J. Am. Chem.
Soc., 1993, 115, 5330.
4 P. R. Ashton, R. Ballardini, V. Balzani, S. E. Boyd, A. Credi,
Naturally, stabilizing H-bonds were observed between the
+NH2 protons of 3a+ and ethereal-oxygen atoms of 1 with an
average N+Á Á ÁO distance of 3.2 Å (see Fig. S40 left, ESI‡).7
Interestingly, C–HÁÁÁp interactions were also evidenced between
´
´
M. T. Gandolfi, M. Gomez-Lopez, S. Iqbal, D. Philp, J. A. Preece,
L. Prodi, H. G. Ricketts, J. F. Stoddart, M. S. Tolley, M. Venturi,
A. J. P. White and D. J. Williams, Chem. – Eur. J., 1997, 3, 152.
+
the methylene PhCH2 NH2CH2C6H4-p-NO2 protons of 3a+ (Fig. S40
5 R. A. Bissel, E. Cordova, A. E. Kaifer and J. F. Stoddart, Nature, 1994,
´
369, 133.
right, ESI‡) and two Ar rings of 1. Overall comparable interactions
were observed in the optimized structure of the isomeric endo-
benzyl-7a+ complex (Fig. 3c and Fig. S41 and S42, ESI‡).
Again the stereoselectivity observed can be justified on the
basis of the greater aptitude of the p-NO2-substitued aromatic
ring of axle 3a+ to become involved in p–p interactions with the
6 (a) C. Gaeta, F. Troisi and P. Neri, Org. Lett., 2010, 12, 2092. For other
examples of directional threading of dialkylammonium axles with
calixarene macrocycles, see: (b) C. Talotta, C. Gaeta, T. Pierro and
P. Neri, Org. Lett., 2011, 13, 2098; (c) T. Pierro, C. Gaeta, C. Talotta,
A. Casapullo and P. Neri, Org. Lett., 2011, 13, 2650; (d) C. Talotta,
C. Gaeta and P. Neri, Org. Lett., 2012, 14, 3104; (e) C. Gaeta,
C. Talotta, S. Mirra, L. Margarucci, A. Casapullo and P. Neri,
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 9917--9920 | 9919