Triphenylamine-based rhombimine macrocycles with solution interconvertable conformation

Article abstract of DOI:10.1039/c004999a

Three imine macrocycles having rhomboidal shape were synthesized in good yields by [2+2] cyclocondensation reaction between equimolar quantities of (R,R)-1,2-diaminocyclohexane and 4,4′-bisformyl triphenylamine derivatives. The macrocycles structure was a

Full text of DOI:10.1039/c004999a

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Triphenylamine-based rhombimine macrocycles with solution interconvertable
conformation†
Mircea Grigoras,* Loredana Vacareanu, Teofilia Ivan and Gabriela Liliana Ailiesei
Received 12th April 2010, Accepted 18th May 2010
First published as an Advance Article on the web 8th June 2010
DOI: 10.1039/c004999a
Three imine macrocycles having rhomboidal shape were synthesized in good yields by [2+2]
cyclocondensation reaction between equimolar quantities of (R,R)-1,2-diaminocyclohexane and
4,4¢-bisformyl triphenylamine derivatives. The macrocycles structure was assigned by electrospray
ionization mass spectrometry (ESI-MS), 1H-NMR, and elemental analysis. UV, FTIR spectroscopy,
and TG measurements were also used to characterize and prove the structure of these compounds. A
conformational modification arising out of the rotation of triphenylamine group around the flexible
cyclohexane-N bonds was observed in solution by 1H-NMR and UV spectroscopy for all three
macrocycles.
of the diamine and dialdehyde geometry.2c,2e (1R,2R)-trans-1,2-
Introduction
Diaminocyclohexane is a rigid molecule with ortho NH2 functions
The shape-persistent imine macrocycles1 derived from aromatic
occupying equatorial positions and having in plane-projected
dialdehydes condensed with aliphatic2–6 or aromatic diamines7
have attracted much attention in the past years due to their
exotic shapes, academic interest, and potential applications in
supramolecular chemistry. It has also gained interest in the
materials science field as host–guest complexes, tubular channels,
porous organic solids, nanocapsules, and nanoreactors, etc.1b,8
They are built using condensation reaction between rigid diamines
and p-conjugated dialdehydes and have specific two-dimensional
(2D) shapes: rhombus, triangle, rectangle, etc. The most frequently
encountered shapes of imine macrocycles are rhomb formed by
[2+2] cyclocondensation or triangle obtained by [3+3] cyclocon-
densation. Unlike arylene-ethynylene macrocycles,8 imine homo-
logues can be obtained in a single step method with an almost
quantitative yield. The common strategy used for the obtaining of
imine macrocycles is based on one-pot synthesis, using Schiff base
condensation reaction between aldehyde and amine compounds.
The reaction is reversible, highly selective, and the intermediate
reaction products are dynamically interchangeable. The composi-
tion of the library is determined by the thermodynamic stability
of the library members.9 Therefore, under thermodynamically
controlled conditions and dynamic combinatorial chemistry, the
imine condensation proceeds to completion, generating with
almost quantitative yield, the most stable mixture of potential
macrocycles.10 The reversibility of the imine condensation allows
it to self-correct the eventual errors in the bond-forming step and
finally, to obtain with high efficiency, the most thermodynamically
stable products. Formation of macrocyclics is favored over linear
oligomerization if diamine, dialdehyde, or both reactants have
structures favoring the formation of cyclic products.2h The size
and shape of the macrocycle can be controlled by the selection
angle between the two C–NH2 close to 60◦. With linear dialdehyde,
i.e. terephthaldehyde or its derivatives, chiral trianglimines are
obtained as main product by [3+3] cyclocondensation,.2a,2d,2e3,4,5a,5c
When dialdehyde has a bent structure with an angle between
aldehyde groups close to 120◦, then rhombimines are obtained by
[2+2] cyclocondensation,.2c,4c,4f,5b6 Therefore, the shape of macro-
cycle can be anticipated from dialdehyde and diamine geometry.
Imine macrocycles with more complex structures and aesthetically
three-dimensional (3D) shapes have been also reported by Severin
et al.10 and Warmuth et al.11 by thermodynamically controlled
condensation reactions from multicomponent mixtures in a single
step.
The purpose of this paper is to present our results about
synthesis and the characterization of rhombimine macrocycles
obtained by cyclocondensation of 4,4¢-diformyl triphenylamine
derivatives with (R,R)-1,2-diaminocyclohexane. In solution these
macrocycles showed a conformational modification induced by
=
rotation of triphenylamine group around two Cyclohexyl-N C-
Aryl bonds. The isomerization was evidenced by NMR and UV
spectroscopy.
Results and discussion
By reacting 4,4¢-bisformyl triphenylamine (1a), 4,4¢-bisformyl 4¢¢-
bromo triphenylamine (1b) or 4,4¢,4¢¢-trisformyl triphenylamine
derivatives (1c) with (1R,2R)-trans-1,2-diaminocyclohexane (2),
we prepared three triphenylamine-based imine macrocycles
(3a–c) having rhomb shape (Scheme 1). The reaction is a [2+2]
cyclocondensation.
Rhombimine 3a was previously synthesized by Gawronski
et al.2g and reported without a detailed research of its spectral
data.
Condensation of 1a–c with 2 was carried out in CH2Cl2 at room
temperature or under reflux, yielding the three rhombimines in
nearly quantitative yield without the use of dehydrating conditions
and without any external template. It is noteworthy that the [2+2]
“P. Poni” Institute of Macromolecular Chemistry, 41A Gr. Ghica Voda Alley,
700487-Iasi, Romania. E-mail: grim@icmpp.ro; Fax: +40-0232-211299;
Tel: +40-0232-217454
† Electronic supplementary information (ESI) available: Experimental
section, ESI-MS spectra, TG measurements, FTIR spectra, in time-
variable NMR spectra, syn and anti isomers. See DOI: 10.1039/c004999a
3638 | Org. Biomol. Chem., 2010, 8, 3638–3643
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Scheme 1 Synthesis of functionalized triphenylamine-based rhombimine macrocycles by [2+2] cyclocondensation of 4,4¢-bisformyl triphenylamine
derivatives (1a–c) and (R,R)-1,2-diaminocyclohexane (2).
macrocycle is the main product (observed by ESI-MS) of the crude
product of all condensation reactions between 1 and 2 at equimolar
feed ratio (see ESI†). The rhombimine shape of macrocycles 3 was
anticipated because (1R,2R)-trans-1,2-diaminocyclohexane (2) is
a rigid molecule with equatorial positioned ortho NH2 functions
and the angle between the two cyclohexyl-N bonds is close of 60◦.
Dialdehydes 1a, 1b and 1c have the dihedral angle between OHC–
C6H4–N–C6H4-CHO bonds close to 120◦. Therefore, free-strain
macrocycles (3a–c) of rhomboidal shape should be obtained as
the dominant product by [2+2] cyclocondensation and all analysis
methods have confirmed this assumption.
(DMF, THF or DMAc), and insoluble in methanol and ethanol.
Macrocycles retained solvent molecules and even after 24 h of
vacuum drying at 50 ◦C, the presence of solvent and other
1
low molecular compounds was observed by TGA or H-NMR
spectroscopy. The differences between calculated and founded
values given by elemental analysis can be explained by retention
of the solvent in macrocyclic cavity. The thermal stability of
macrocycles was studied by TGA measurements in a nitrogen
atmosphere. They showed a high thermal stability, up to 340 ◦C,
with an onset degradation temperature at 348 ◦C (3a), 322 ◦C
(3b), and 300 ◦C (3c), respectively, and a high melting temperature
(325 ◦C for 3a) (see the ESI†). The high thermal stability is due to
the imine structure and the absence of end groups. The weight
loss (1–10%) observed in the temperature range 80–200 ◦C is
caused by the release of the solvent molecules that left their host.
The loss of solvent was also observed from the first heating scan
Characterization of imine macrocycles
All three macrocycles were highly soluble in halogenated solvents;
CH2Cl2, CHCl3 or CCl4 and less soluble in polar solvents
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 3638–3643 | 3639
©

View Article Online
of DSC studies by an endothermic effect. In the second DSC
heating scan, this endothermic effect was absent. The DSC runs
showed no◦ glass transitions occurred in the temperature range
-40/+300 C. The structure of the compounds was assigned on
the basis of ESI-MS, FTIR, 1H-NMR and UV spectroscopy. X-ray
data also showed that all the compounds are crystalline but single
crystals suitable for exact structure determination could not be
obtained.
(DMF) and 1.69 ppm came to solvents used in different steps. By
maintaining 3a in solution, its proton NMR spectrum evidenced
significant modifications, mostly observed for the imine, aromatic,
=
and –CH–N protons region (Fig. 3).
Thus, new signals appeared for imine (8.11–8.13 ppm), aromatic
=
(7.42–7.44 ppm), and –CH–N diamine (3.35 ppm) protons, sug-
gesting the presence of two non-equivalent triphenylamine groups.
The intensities of the new signals integrals are correlated between
them. This behavior was observed for all three rhombimines.
Having in mind the reversibility of imine condensation reaction,
the first thing done was to check if an expansion or hydrolysis
of macrocycle is responsible for NMR spectrum changes. The
ESI-MS spectrum of 3a was unchanged after evaporation of
NMR chloroform solution, showing the same molecular ion peak
at 759.28 Da and fragmentation peak at 380.13 Da. Therefore,
the modifications observed by NMR spectroscopy could only be
assigned to an isomerization or hydrolysis process.
The FTIR spectra of macrocycles 3a and 3b does not show
-1
=
any absorption for aldehyde C O (1680 cm ) and amine N–
H (3390 and 3315 cm-1) stretches, while 3c shows the presence
of C O absorption band (1695 cm ). The characteristic peaks
-1
=
=
corresponding to the stretching vibration of the nCH N bond
-1
(1634 cm -1) and nC C bond (1597 and 1507 cm ) are observed for
=
all compounds. The absorption bands localized between 697 and
837 cm-1 (n C–H aromatic from benzene rings), 1268–1286 cm-1
(the stretching vibration of tertiary amine) and 2856–2930 cm-1
(C–H aliphatic from cyclohexane) are also evident in 3a, 3b and
3c.
It was concluded that the MS and NMR data support structures
of [2+2] macrocycles for the reaction products. On the other hand,
the NMR spectrum of macrocycle is, as a rule, an overlap of
spectra arising from several conformers. The structure of these
isomers can be intuited by considering that:
(i) the cyclohexyl rings are in the chair conformation, which is
the most stable one, and
(ii) the fact that a conformation of a macrocycle can be changed
to the other via the concerted rotation of triphenylamine group
around flexible cyclohexane-N bonds.
The ESI-MS spectrum of 3a (macrocycle solution in
chloroform–methanol = 3/1) presented in Fig. 1 shows signals
with the molecular ion peak (M+H+) at 759.28 Da (3a++1) and
a fragmentation ion peak at 380.13 Da. That is consistent with
the chemical structure proposed in Scheme 1. For 3b, molecular
ion peak is at 917.09 Da (3b++1), 839.2 Da (3a+ +1-Br) and
fragmentation peak at 458.06 Da. The peak at 478.06 Da could be
assigned to the linear [1+1] compound or to ions (3b+2Na+)/2.
The imine linkage in Schiff base compounds could adopt E or
Z configuration, therefore the four imine linkages in macrocycles
3a–c would exist as all-E, all-Z, or mixture of E-Z configuration.
However, the E isomer has lower energy when compared to the Z
isomer and it is accepted that all-E isomer is the product obtained
due to its higher thermodynamic stability. In addition, all-E isomer
can exist in many conformations, according to the orientation of
the lone electrons pair located at imine nitrogens as syn or anti
rotamers. The lowest energy was observed for the structures where
the lone electrons pair is oriented outside (exo) of the macrocycle,
=
as syn conformer where imine and N–CH– (from cyclohexane)
hydrogens are in syn position.2a
As normally expected all imine bonds have E configuration.
One can conclude that the connection bridges between the two
cyclohexyl rings of a macrocycle may be in one of three forms due
to relative orientation of imine bond: A (syn-syn), B (anti-anti) and
C (anti-syn), as well as in the forms A¢, B¢ or C¢, as mirror images of
A, B and C, respectively. MM2+ molecular modeling of fragments
and macrocycles indicated a preference for the fragments in order:
A > C>B, and also for [2+2] macrocycles in order: A–A¢ > C–
C¢ > B–B¢ (see ESI†). Therefore, twelve macrocycle conformations
would result by combination of fragments A (A¢), B (B¢) and C
(C¢) but only A–A¢ and B–B¢ forms have centers of symmetry.
Fig. 1 ESI-MS for 3a.
The ESI-MS spectrum of the crude product 3c displayed a major
signal with the molecular ion peak at 815.36 Da (3c+ +1).
Macrocycle 3a was purified by dissolving in DMF at high
temperature, but the solution immediately became gel-like by
cooling. The macrocycle was then separated by filtration and
drying. The gel structure results due to inclusion of solvent
as guest in macrocyle’s cavity. Tanaka et al.12 have recently
reported obtaining a gel when the macrocyle synthesized by
[3+3] cyclocondensation of trans-1,2-diaminocyclohexane with
azobenzene-4,4¢-dicarbaldehyde was dissolved in benzene.
=
Their imine protons (–N CH–) are magnetically equivalent and,
consequently, they give singlet NMR signals. On the other hand,
the other isomers have plane symmetry and the imine protons
will give two singlet NMR signals because they are two by two
magnetically equivalent. Since there being a singlet signal for the
all imine protons, the spectrum in Fig. 2 must belong to one of
the isomers A–A¢ or B–B¢. Being placed at the lowest field in
comparison with other imine signals, this signal (7.91 ppm) may
be assigned to the isomer A–A¢ where the imine protons are placed
1
The H-NMR spectrum of 3a fresh solution in CDCl3 is well
resolved (Fig. 2). It illustrated signals assigned to the proposed
structure in Scheme 1, with only a singlet signal at 7.91 ppm for
the four imine protons suggesting a highly symmetric macrocycle.
Some signals positioned at 7.26 (CHCl3), 2.96, and 2.88 ppm
3640 | Org. Biomol. Chem., 2010, 8, 3638–3643
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Fig. 2 1H-NMR spectrum (CDCl3) of 3a after purification from DMF.
Fig. 3 1H-NMR spectrum (CDCl3, 25 ◦C) of 3a, (presented in Fig. 2) registered again after several days.
inside of the macrocycle, therefore feeling an aromatic ring current
shielding which is stronger than in other isomers. An interesting
observation is that the spectrum presented in Fig. 2 changes when
the symmetrical macrocycle is maintained in solution; the signal
at 7.91 ppm decreases while new imine proton signals appear
between 8.1 and 8.25 ppm (Fig. 3). This fact might be explained by
assuming that the macrocycle A-A¢, changes to other isomer or to
intermediate isomers by rotation of triphenylamine group around
cyclohexyl–N single bonds.
Therefore, the changes observed in the NMR spectrum of 3a
can be assigned to an isomerization (interconversion) process that
takes place in solution (Fig. 4). Interconversion takes place via
the concerted rotation of triphenylamine group around flexible
cyclohexane-N bonds. It was observed from NMR studies that the
rotamer inversion is enhanced by increasing of the temperature
Fig. 4 Modification of the macrocycle conformation in solution.
and solvent polarity. Furthermore, using 2D NOE analysis, we
investigated the 3D conformational structure of 3a. The NOE
correlations were observed between the imine proton (signal at
=
7.91 ppm), the –CH–N (signal at 3.23 ppm), and imine ortho
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 3638–3643 | 3641
©

View Article Online
Fig. 5 Reference spectrum (A) and the 1H-1H NOE difference spectrum (B) for irradiation (1) on the imine proton resonance in the symmetrical isomer.
There are also marked the NOE effects for the irradiation (2) on imine proton resonance in another isomer.
Fig. 6 In time-evolution of UV spectra (CHCl3) of 3.
phenyl (signal at 7.34 ppm) protons. The NOE difference spectra
were recorded for irradiation on the imine proton resonances of
the symmetrical isomer and of an unsymmetrical one (Fig. 5).
As expected, one observes magnetic polarization transfer to the
UV absorption spectra of 3 showed important changes in the
shape and positions of the absorption maxima of the CHCl3
solution (Fig. 6). Thus, 3 in all-syn conformation presented three
absorption maxima (in CHCl3 solution), the first two being
characteristic of the p–p* transitions in triphenylamine group
(240–250 and 336 nm). The third absorption (356 nm) can be
assigned to n-p* conjugation between the lone electron pair of the
imine nitrogen conjugated with triphenylamine moiety. After a few
minutes, the solutions of macrocycles showed a new and broader
absorption band above 400 nm. In time this absorption band was
bathocromically shifted and its intensity gradually increased in
correlation with the disappearance of the absorption at 356 nm.
A stationary state was reached after 24 h. We assume that, in
CHCl3 solution, the all-syn macrocycles 3 spontaneously isomerize
to thermodynamically more stable rotamers, having better n-p*
electronic conjugation between imine and triphenylamine bridge.
This behavior could be explained considering the sterical structure
of the Schiff base compounds. It is recognized that the imine group
itself is nonplanar with adjacent groups of aldehyde and amine.
=
nearest protons, namely to the hexyl protons –CH–N and
to the aromatic protons in ortho position relative to the imine
substitution. NOE effects between other protons were not clear
due to the signal overlapping. In addition, due to the rather long
acquisition times (3–5 h), the spectra became complicated due to
the spontaneous isomerization. For instance, the starting spectrum
for Fig. 5 was that of the symmetrical isomer, but significant signals
from unsymmetrical isomers are already present at the end of the
experiment.
Dibromine- and dialdehyde-functionalized rhombimines (3b
and 3c) were synthesized by a similar protocol as 3a, reacting 2 with
1b and 1c, respectively, and recrystalized from tetrahydrofuran.
The 1H- NMR spectra are similar with that of 3a, having all-syn
structure. Therefore, a syn conformation can be assigned for 3b
and 3c. The signal at 9.81 ppm is assigned to protons from CHO
groups of 3c. In solution, a rotamer inversion was also observed
for both macrocycles.
=
The amine ◦substituent at –N C is out of the imine group plane
=
with 40–55 , while the aromatic ring linked at –CH is out of
3642 | Org. Biomol. Chem., 2010, 8, 3638–3643
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
plane with 10◦, but in the opposite sense.13 This arrangement
allows lone electron pair of nitrogen to be in conjugation with
the amine substituent (cyclohexyl) rather the p-conjugated imine
functionality in symmetrical isomer. By rotation of triphenylamine
groups around cyclohexane-N simple linkages, the conjugation
of lone electron pair of imine nitrogen with triphenylamine
group is possible having an effect on electronic absorption
maximum (bathocromic shifting). Moreover, this rotation allows
apparition of a transanular electronic interaction between the
cofacial benzene rings of triphenylamine. The conjugation length
increased also via the through space interactions, similar with [2,2]
paracyclophane.
Gawronski et al.2g have stated that rhombimine 3a is flattened
around atom N that leads to puckering of the molecule about
the imine fragments and is quite unstable in solution, making its
crystallization and study difficult. In our opinion, macrocycles 3
are enough stable to be purified but in solution they changed their
3-D structure toward a more favorable conformation.
Reduction of 3a with NaBH4 in a methanol–THF mixture gave
rhombamine 4a in almost quantitative yield. The NMR spectrum
showed the disappearance of the imine proton signals (7.9–8.2
ppm) and the appearance of new signals at 3.5–3.9 ppm (–CH2–)
and 2.3 ppm (–NH–). Rhombamine 4a shows absorptions only
at 250 and 310 nm, the absorption associated to imine group
being absent. Furthermore, the UV and1H-NMR spectra were
unchanged in time.
Acknowledgements
We thank Dr V. Barboiu for NMR spectra and helpful comments.
This work was supported by CNCSIS –UEFISCSU, project
number 649/2009 PNII – IDEI 993/2008
References
1 For reviews see: (a) N. E. Borisova, M. D. Reshetova and Y. A.
Ustynyuk, Chem. Rev., 2007, 107, 46–79; (b) M. J. MacLachlan, Pure
Appl. Chem., 2006, 78, 873–878.
2 (a) J. Gawronski, H. Kolbon, M. Kwit and A. Katrusiak, J. Org. Chem.,
2000, 65, 5768–5773; (b) M. Kwit and J. Gawronski, Tetrahedron:
Asymmetry, 2003, 14, 1303–1308; (c) J. Gawronski, M. Brzostowska,
M. Kwit, A. Plutecka and U. Rychlewska, J. Org. Chem., 2005, 70,
10147–10150; (d) M. Kwit, P. Skowronek, H. Kolbon and J. Gawronski,
Chirality, 2005, 17, S93–S100; (e) J. Gawronski, K. Gawronska, J.
Grajewski, M. Kwit, A. Plutecka and U. Rychlewska, Chem.–Eur. J.,
2006, 12, 1807–1817; (f) M. Kaik and M. Gawronski, Org. Lett., 2006,
8, 2921–2924; (g) J. Gawronski, M. Kwit, J. Grajewski, J. Gajewy and
A. Dlugokinska, Tetrahedron: Asymmetry, 2007, 18, 2632–2637; (h) P.
Skowronek and J. Gawronski, Org. Lett., 2008, 10, 4755–4758.
3 (a) M. Chadim, M. Budesinsky, J. Hodacova, J. Zavada and P. C. Junk,
Tetrahedron: Asymmetry, 2001, 12, 127–133; (b) J. Hodackova and M.
Budesinsky, Org. Lett., 2007, 9, 5641–5643.
4 (a) N. Kuhnert and A. M. Lopez-Periago, Tetrahedron Lett., 2002, 43,
3329–3332; (b) N. Kuhnert, C. Strassnig and A. M. Lopez-Periago,
Tetrahedron: Asymmetry, 2002, 13, 123–128; (c) N. Kuhnert, G. M.
Rossignolo and A. Lopez-Periago, Org. Biomol. Chem., 2003, 1, 1157–
1170; (d) N. Kuhnert, N. Burzlaff, C. Patel and A. Lopez-Periago, Org.
Biomol. Chem., 2005, 3, 1911–1921; (e) N. Kuhnert, A. Lopez-Periago
and G. M. Rossignolo, Org. Biomol. Chem., 2005, 3, 524–537; (f) N.
Kuhnert, C. Patel and F. Jami, Tetrahedron Lett., 2005, 46, 7575–7579.
5 (a) J. Gao and A. E. Martell, Org. Biomol. Chem., 2003, 1, 2795–2800;
(b) J. Gao and A. E. Martell, Org. Biomol. Chem., 2003, 1, 2801–2806;
(c) J. Gao, J. H. Reibenspies, R. A. Zingaro, F. R. Wooley, A. E. Martell
and A. Clearfield, Inorg. Chem., 2005, 44, 232–241.
Conclusions
In this study, three imine macrocycles having rhomb shape
were synthesized by [2+2] cyclocondensation of (R,R)-1,2-
diaminocyclohexane with 4,4¢ bis formyltripenylamine, 4,4¢ bis-
formyl 4¢¢-bromo triphenylamine, and 4,4¢,4¢¢-trisformyl tripheny-
lamine. They were characterized by a combination of ESI-MS, 1H-
NMR, FTIR, UV, and thermal methods. Imine linkages are in E
configuration and all-syn conformation, concluding all macrocy-
cles in this conformation have low energy and highly symmetrical
structure. In solution, with rotation of a triphenylamine group
around cyclohexane–nitrogen bonds, macrocycle may interconvert
to a more stable conformational isomer with enhanced n-p*
conjugation between triphenylamine and imine linkages and p
stacking interactions between triphenylamine groups. In solution,
the rotation is monitored by changes in the NMR and UV
spectra. All three imine macrocycles are functional compounds
and they could be used in chemical or electrochemical oxidative
polymerization (3a), metal-catalyzed polycondensation of Suzuki,
Heck or Stille type (3b) and Schiff base polycondensation with
diamines (3c) to obtain polyrhombimines.
6 S. Srimurugan, B. Viswanathan, T. K. Varadarajan and B. Varghese,
Org. Biomol. Chem., 2006, 4, 3044–3047.
7 (a) B. N. Boden, J. K. H. Hui and M. J. MacLachlan, J. Org. Chem.,
2008, 73, 8069–8072; (b) A. J. Gallant, J. K. H. Hui, F. E. Zahariev,
Y. A. Wang and M. J. MacLachlan, J. Org. Chem., 2005, 70, 7936–7946;
(c) C. Ma, A. Lo, A. Abdolmaleki and M. J. MacLachlan, Org. Lett.,
2004, 6, 3841–3844; (d) A. J. Gallant and M. J. MacLachlan, Angew.
Chem., Int. Ed., 2003, 42, 5307–5310.
8 D. Zhao and J. S. Moore, Chem. Commun., 2003, 807–818and references
therein.
9 P. T. Corbett, J. Leclaire, L. Vial, K. R. West, J. L. Wietor, J. K. M.
Sanders and S. Otto, Chem. Rev., 2006, 106, 3652–3711.
10 B. Icli, N. Christinat, J. Tonnemann, C. Schuttler, R. Scopelliti and
K. Severin, J. Am. Chem. Soc., 2009, 131, 3154–3155and references
therein.
11 (a) X. Liu, Y. Liu, G. Li and R. Warmuth, Angew. Chem., Int. Ed.,
2006, 45, 901–906; (b) Y. Liu, X. Liu and R. Warmuth, Chem.–Eur. J.,
2007, 13, 8953–8959; (c) D. Xu and R. Warmuth, J. Am. Chem. Soc.,
2008, 130, 7520–7521.
12 K. Tanaka, S. Fukuoka, H. Miyanishi and H. Takahashi, Tetrahedron
Lett., 2010, 51, 2693.
13 H. B. Bu¨rgi and J. D. Dunitz, J. Chem. Soc. D, 1969, 472–473.
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 3638–3643 | 3643
©