Appending aromatic moieties at the para- and meta-position of calixarene phenol rings via p-bromodienone route

Article abstract of DOI:10.1016/j.tetlet.2009.05.032

The silver-mediated nucleophilic substitution on calixarene p-bromodienone derivatives (the 'p-bromodienone route') with activated aromatic substrates allows the introduction of aromatic moieties at the para- or meta-position of calixarene aromatic rings.

Full text of DOI:10.1016/j.tetlet.2009.05.032

Tetrahedron Letters 50 (2009) 4416–4419
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Appending aromatic moieties at the para- and meta-position of calixarene
phenol rings via p-bromodienone route
*
*
Francesco Troisi, Teresa Pierro, Carmine Gaeta , Michele Carratù, Placido Neri
Dipartimento di Chimica, Università di Salerno, Via Ponte don Melillo, I-84088 Fisciano, Salerno, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The silver-mediated nucleophilic substitution on calixarene p-bromodienone derivatives (the ‘p-bromo-
dienone route’) with activated aromatic substrates allows the introduction of aromatic moieties at the
para- or meta-position of calixarene aromatic rings. Less reactive substrates mainly afford C–O para-cou-
pled derivatives, while more activated ones mainly give inherently chiral, C–C meta-coupled products
through a dienone–phenol rearrangement of the intermediate dienone derivative. Examples of C–C
para-coupling and O–C coupling at the endo calixarene oxygen atom were also observed.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 17 April 2009
Revised 6 May 2009
Accepted 12 May 2009
Available online 18 May 2009
Keywords:
Calixarene exo-rim
p-Bromodienone route
Inherently chiral calixarenes
Dienone–phenol rearrangement
Meta-substituted calixarenes
The introduction of functional moieties at the para-position of
calixarene1 aromatic rings (the so-called upper, wide or exo-rim)2
is a general and useful approach for the synthesis of a variety of
calixarene hosts of great interest in supramolecular chemistry.1f,3
Therefore, over the past two decades several strategies to intro-
duce new functionalities at the calixarene exo-rim have been
developed, which include a range of electrophilic aromatic substi-
tutions,4 and the classical ‘Claisen rearrangement route’,5 ‘p-qui-
none-methide route’,6 and ‘p-chloromethylation route’.7
In a different way from all these instances, where no direct
attachment of a nucleophile to the para aromatic carbon can be
obtained, very recently, our group has introduced the ‘p-bromodie-
none route’8 as a novel approach to functionalize calixarene
exo-rim with nucleophiles using calixarene p-bromodienone deriv-
atives (exemplified by 1 in Scheme 1). At the same time, Varma and
co-workers reported a related procedure in which alkoxy groups
are introduced into calixarene exo-rim starting from calixarene spi-
rodienone derivatives.9 In the previous paper we reported that eas-
ily accessible calixarene p-bromodienone derivatives 1 undergo a
silver-mediated nucleophilic substitution and a subsequent re-aro-
matization with a range of different O-nucleophiles (alcohols and
carboxylates) to give p-alkoxy- or p-acyloxy-calixarenes in work-
able yields.8 It was also demonstrated that the reaction can be per-
re-aromatization step. It was also anticipated that in order to ex-
pand the potentiality of p-bromodienone route other nucleophiles
should be tested. Therefore, we decided to investigate the use of
activated aromatic rings as nucleophiles10 and here we wish to re-
port on the results of this study.
In the mechanism proposed for the p-bromodienone route an
aryloxenium cation I (Scheme 1),11 is initially formed upon precip-
itation of AgBr, which can be captured by the present O-nucleo-
philes (alcohols and carboxylates).8 Therefore, it can be expected
that this cation could be intercepted by sufficiently activated aro-
matic substrates by means of an electrophilic aromatic substitu-
tion. Resorcinol 2a was initially chosen as nucleophile due to its
high reactivity toward electrophilic aromatic substitution. The
mixture of exo/endo stereoisomers of 1, obtained directly by oxida-
tion of tripropoxy-p-tert-butylcalix[4]arene with trimethylpheny-
lammonium tribromide,12 was treated with a cold solution of
AgClO4 and resorcinol 2a (Scheme 1) in DME for 2 h at 0 °C in
the presence of Na2CO3 as a base.13 Column chromatography on
silica gel of the reaction mixture afforded derivatives 3a and 4a
in 10% and 30% yield, respectively (Table 1, entry ‘a’).13,14
The structure of 3a was easily assigned by means of spectral
analysis. In particular, the presence of a pseudomolecular ion peak
at m/z 827 in the ESI(+) mass spectrum confirmed the molecular
formula. The Cs symmetry was confirmed by pertinent signals in
the 1H and 13C NMR spectra. In particular, the presence of only
two 2:1 tert-butyl singlets at 0.94 and 1.27 ppm, respectively,
and three signals at 6.99 (d, J = 8.3 Hz, 1H), 6.30 (dd, J = 8.3,
2.4 Hz, 1H), and 6.40 ppm (d, J = 2.4 Hz, 1H), attributable to resor-
cinol ring, were a clear evidence of the displacement of a t-Bu
formed directly on the exo/endo mixture of
1 because the
complication of exo/endo stereoisomerism was overcome in the
* Corresponding authors. Tel.: +39 089969556; fax: +39 089969603 (C.G.); tel.:
+39 089969572; fax: +39 089969603 (P.N.).
E-mail addresses: cgaeta@unisa.it (C. Gaeta), neri@unisa.it (P. Neri).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.05.032

F. Troisi et al. / Tetrahedron Letters 50 (2009) 4416–4419
4417
But
But
But
But
But
But
AgClO4
But
Br
But
+
O
O
O
O
O
O
O
O
AgBr
Pr
Pr
Prn
Prn
Prn
Prn
(I)
1
HO
OH
2a
But
But
But
But
Ar
But
But
But
Ar
Ag+
De-tert-Butylation
O
Prn
O
O
O
O
O
O
OH
Pr
Prn
Pr
Prn
Prn
(II)
3a
Hd
Ha
Ag+
Dienone-Phenol
Rearrangement
e
HO
c
OH
Hb
Ar =
f
f
But
But
But
But
But
But
But
But
g
g
H
Ar
H
Ar
O
O
O
O
OH
Pr
O
OH
O
Prn
Prn
Pr
Prn
Prn
4a
Scheme 1. Proposed mechanism for the formation of derivatives 3a and 4a via p-bromodienone route.
group by the aromatic moiety.13 The linkage at the 4-position of
resorcinol ring was readily deduced by the presence of the two
ortho- and one meta-coupled signals. As expected, 3a was very
likely formed by the silver-mediated initial formation of aryloxeni-
um cation I (Scheme 1), which was captured by resorcinol nucleo-
phile through an electrophilic aromatic substitution to give CÀC
coupled dienone intermediate II. The latter then underwent de-
tert-butylation to give re-aromatized derivative 3a (Scheme 1).
Surprisingly, derivative 4a showed an 1H NMR spectrum in
DMSO-d6 (Fig. 1) consistent with an asymmetric calix[4]arene
structure.13 In fact, four t-Bu singlets were present at 0.88, 0.89,
1.15, and 1.31 ppm, while the diastereotopic ArCH2Ar protons ap-
peared as eight partially overlapped doublets (Fig. 1).13 In addition,
the aromatic protons of calixarene skeleton gave six doublets with
a typical meta-coupling (2.3 Hz) at 6.42 (1H), 6.52 (1H), 6.64 (1H),
6.67 (1H), and 7.23 ppm (2H, overlapped), and one singlet at
7.24 ppm attributable to the isolated ArH proton of the substituted
phenol ring (Fig. 1).13 Two OH singlets relative to resorcinol ring,
were observed at low field (9.02 and 9.25 ppm), while the calixa-
rene phenolic OH group was present at 6.35 ppm.13 The 4-substitu-
tion of resorcinol ring was proved by an ortho J-coupling of
aromatic H(a) proton (Scheme 1) at 6.83 ppm (d, J = 8.1 Hz, 1H)
with the vicinal H(b) proton at 6.29 ppm (dd, J = 8.1 and 2.1 Hz,
1H) (COSY-45), which in turn was also meta-coupled with H(d)
proton at 6.38 ppm (J = 2.1 Hz, 1H). These data were compatible
with an asymmetric calix[4]arene structure in which a resorcinol
and a t-Bu group were, respectively, meta- and para-linked to the
calixarene phenol ring, or vice versa. In order to establish which
of the two groups was meta-linked a 2D-NOESY study (250 MHz,
C6D6, 298 K) was performed.13 Diagnostic cross-peaks between
the t-Bu(f) group (see Scheme 1) at 1.37 ppm and the isolated
ArH(g) singlet at 7.51 ppm indicated their spatial proximity, only
possible if the t-Bu and resorcinol groups were para- and meta-
linked, respectively.
The formation of 4a can be easily explained by assuming that
intermediate dienone II undergoes a typical dienone–phenol rear-
rangement15 to give meta-substituted calix[4]arene derivative 4a
(Scheme 1). Clearly, the rearrangement of dienone II occurs by
1,2-migration of resorcinol moiety, in accordance with the higher
migratory aptitude of aromatic groups with respect to alkyl
ones.15,16 It is worth mentioning here that, to the best of our
knowledge, this represents the first example of dienone–phenol
rearrangement in calixarene chemistry. The asymmetric structure
of 4a coupled to its three-dimensional nature makes it inherently
chiral and, consequently, it should be formed as a racemic mixture.
The racemic nature of 4a was evidenced by the doubling of several
resonances in its 1H NMR spectrum upon addition of Pirkle’s re-
agent [(S)-(+)-(9-antryl)-2,2,2-trifluoroethanol] (Fig. 2).
The reaction of exo/endo mixture of 1 was extended to resor-
cinol dimethyl ether 2b (Table 1, entry ‘b’), which afforded meta-
substituted, inherently chiral calix[4]arene 4b in very low yield
(5%) probably because this substrate was less activated, with re-
spect to resorcinol 2a, toward the electrophilic attack by aryloxeni-
um cation I (Scheme 1). The reaction with particularly activated

4418
F. Troisi et al. / Tetrahedron Letters 50 (2009) 4416–4419
Table 1
Coupling products (yield, %) of p-bromo-dienone 1 with activated aromatic substrates
2a–l
But
But
But
But
But
But
But
Br
Ar
1.4 1.2 1.0 ppm
9.4 9.2 9.0 ppm
2a-l
DME
O
O
Na2CO3, AgClO4
O
O
O
O
OH
O
0 °C
Prn
Prn
Prn
Prn
Prn
Prn
74
72
70
68
66
64
62
1
3
Figure 1. Significant portions of the 1H NMR spectrum of derivative 4a (DMSO-d6,
400 MHz, 298 K).
But
But
But
But
Ar
+
O
O
O
OH
Prn
Prn
Prn
1.26 1.24 1.22
ppm 1.15
1.10
1.05
1.00
0.95
ppm
4
2
3 (Ar-)
4 (Ar-)
HO
OH
HO
OH
HO
OH
(30)
(10)
(a)
MeO
N
OMe
MeO
OMe
1.18
1.16
ppm
1.10
1.05
1.00
0.95
ppm
(5)
(b)
(c)
(d)
—
—
—
Figure 2. Portion of the 1H NMR spectra (CDCl3, 400 MHz, 298 K) of 4a in the
presence (top) or in the absence (bottom) of Pirkle’s reagent.
(20)
N
aromatic nucleophiles, such as N-methylpyrrole 2c and 2,6-
dimethylphenol 2d afforded both meta-substituted products 4c
and 4d in 20% yield (Table 1, entries ‘c and ‘d’).13
OH
OH
(20)
In all these instances the preferential 1,2-migration of the aro-
matic moiety was confirmed by detailed NOESY studies.13 Thus,
it appears clear that through the p-bromodienone route with
highly activated substrates is possible to obtain meta-substituted
calix[4]arenes,17 which are of special interest because of the inher-
ent chirality of calix[4]arene skeleton exploitable for enantiodis-
crimination processes.18 Interestingly, the analogous reaction
with highly activated phloroglucinol (1,3,5-trihydroxybenzene)
2e unexpectedly afforded, in addition to meta-substituted calix[4]-
arene 4e (10%; Table 1, entry ‘e’), partial-cone derivative 5e in 20%
yield (Fig. 3),13 in which the phloroglucinol moiety is linked to the
calixarene oxygen atom at the endo-rim (also called lower or nar-
row rim). As demonstrated by chemical shift arguments and
molecular modeling, the phloroglucinol moiety is self-filling the
calix cavity in 5e (Fig. 3). Evidently, this product is formed by elec-
trophilic attack of the oxygen atom of aryloxenium cation I to
phloroglucinol ring, after that a ‘through-the-annulus’ ring inver-
sion of dienone moiety has occurred. It is worth mentioning here
HO
HO
OH
But
HO
OH
(10)
(e)a
(f)
—
OH
OH
But
(35)
(20)
—
O
OBn
Me
OBn
Me
(g)
(h)
—
—
O
O
HO
HO
(35)
OH
OH
(20)
(20)
(i)
(j)
—
O
O
HO
HO
HO
(10)
OH
OH
OH
But
But
But
(5)
HO
(k)
(l)b
—
OH
OH
OH
OH
HO
O
OH
But
OH
OH
OH
HO
OH
(5)
O
O
O
+
HO
OH
O
OH
4l (20)
4l' (15)
a
In addition, derivative 5e (Fig. 3) was also isolated in 20% yield.
This reaction was conducted in the absence of Na2CO3.
Figure 3. Chemical drawing of partial-cone 5e (left) and its lowest MM3-energy
structure (CHCl3, GB/SA implicit model solvent) (right).
b

F. Troisi et al. / Tetrahedron Letters 50 (2009) 4416–4419
4419
pp 1369–1454; (f) Calixarenes in the Nanoworld; Vicens, J., Harrowfield, J., Eds.;
Springer: Dordrecht, 2007.
2. Gutsche, C. D. Calixarenes, an Introduction; Royal Society of Chemistry:
Cambridge, UK, 2008. pp 25–26 and following chapters.
that this kind of O–C bond formation has been already observed in
the chemistry of aryloxenium cations.19
In a second series of experiments we decided to examine the
additional group of phenolic nucleophiles 2f–i (Table 1). Thus, it
was evidenced that phenols para-substituted with an alkyl (2f
and 2h), alkoxy (2g), and hydroxy (2i) group only gave CÀO cou-
pled p-aryloxy derivatives 3f–i in 20–35% yield (Table 1). In these
instances the nucleophilicity of OH group appears to be higher
with respect to that of these aromatic rings which are less acti-
vated toward electrophilic aromatic substitution. A border line sit-
uation was obtained by the simple phenol 2j which afforded both
CÀO coupled para-substituted calix[4]arene 3j (20% yield) and
CÀC coupled meta-substituted derivative 4j (10% yield; Table 1).
Thus, it can be generalized that the p-bromodienone route with
less activated substrates bearing a single OH group mainly affords
CÀO coupled, para-substituted derivatives through a nucleophilic
attack of phenolic oxygen atom to aryloxenium cation I, in analogy
with what observed with aliphatic alcohols.8 Catechol 2k afforded
a second example of CÀC coupled para-substituted derivative 3k
albeit in very low yield (5%; Table 1). Finally, pyrogallol 2l gave
mainly two isomeric CÀC coupled meta-substituted calix[4]arenes
4l and 4l’ (in 20% and 10% yield, respectively; Table 1) besides to a
small amount (5%) of para-substituted derivatives 3l CÀO coupled
at the central OH group. The formation of the latter derivative
could be ascribed to the higher acidity of the central OH group
due to H-bonding stabilization by the ortho ones.
In conclusion, we have demonstrated that the p-bromodienone
route, previously applied to alcohols and carboxylates, can be ex-
tended to activated aromaticsubstrates. Depending onthe reactivity
of the substrate, aromatic moieties can be introduced at the para- or
meta-position of calixarene aromatic ring, mainly through CÀO or
CÀC coupling, respectively. In a few instances, examples of CÀC cou-
pling at the para-position and OÀC coupling at thecalixarene oxygen
atom were also observed. The p-bromodienone route with highly
activated aromatic substrates provides a practicable method for
the synthesis of meta-substituted inherently chiral calix[4]arene
derivatives, which can find interesting applications in enantiodis-
criminationprocesses. In addition, theintroductionof aromaticmoi-
eties at the calixarene exo-rim represents a novel procedure to
obtain calix[4]arenes with enlarged aromatic central cavity, which
could show enhanced recognition properties with respect to the na-
tive macrocycle. The potentiality of p-bromodienone route can be
further expanded by using other N-, S-, and C-nucleophiles. Work
along these lines is currently underway in our laboratory.
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Supplementary data
Supplementary data (synthetic details, 1D and 2D 1H and 13C
NMR data) associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2009.05.032.
References and notes
1. For comprehensive reviews on calixarenes see: (a) Böhmer, V. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 713–745; (b) Ikeda, A.; Shinkai, S. Chem. Rev. 1997, 97,
1713–1734; (c) Gutsche, C. D. Calixarenes Revisited; Royal Society of Chemistry:
Cambridge, 1998; (d) Calixarenes 2001; Asfari, Z., Böhmer, V., Harrowfield, J.,
Vicens, J., Eds.; Kluwer: Dordrecht, 2001; (e) Böhmer, V. In The Chemistry
of Phenols; Rappoport, Z., Ed.; Wiley: Chichester, UK, 2003; Chapter 19,