Mendeleev Commun., 2010, 20, 237–238
in benzylated substrates without debenzylation.17 However,
Br
Br
OH
OH
O-benzylated p-cyanophenol undergoes quantitative debenzyla-
tion even when a poisoned catalyst is used.16 This is due to the
electron-withdrawing effect of the cyano group, which facilitates
hydrogenolysis even in the presence of less active catalysts. The same
probably occurs in the case of 4,5-dibenzyloxyphthalonitrile.5
In the case of nitrile 4, the O-benzyl groups are remote from
the electron-withdrawing phthalonitrile fragment, resulting in lower
reactivity and, in turn, higher sensitivity towards catalyst poisoning.
Notwithstanding the failure of catalytic deprotection of nitrile
4, this compound can be used as a precursor of phthalocyanines
bearing eight 2'-(2''-benzyloxyethoxy)ethoxy groups. Such com-
pounds are expected to be discotic mesogens by analogy with
previously described (2'-benzyloxyethoxy)-substituted phthalo-
cyanines.18 Langmuir–Blodgett layers of these compounds could
be transferred on solid surfaces, and the resulting films demon-
strated remarkable mechanical stability together with a highly
ordered morphology.19 Since nitrile 4 is an open-chained analogue
of crown ethers (podand), phthalocyanines synthesized from
this precursor can be prominent receptors for the binding of
cations and small molecules resulting in formation of functional
supramolecular assemblies.20
+
TsO
O
OCH2Ph
1
2
KOH, DMSO 80%
Br
O
O
O
O
OCH2Ph
OCH2Ph
Br
3
CuCN, DMF 64%
NC
NC
O
O
O
O
OCH2Ph
OCH2Ph
4
Scheme 1
In conclusion, 1,2-dicyano-4,5-bis[2'-(2''-benzyloxyethoxy)-
ethoxy]benzene was synthesized. This compound may act as a
precursor of new functionalized phthalocyanines acting as discotic
mesogens or supramolecular receptors.
debenzylation (Scheme 2). Neither the variation of solvent (EtOAc
or alcohols) and the addition of new catalyst portions, nor
catalytic hydrogen transfer reaction (with cyclohexene as hydro-
gen donor)14 and continuous reaction times (for several days)
afforded cleavage of the protective group. In each case, the
starting compound was recovered pure after the completion of
the process.
In contrast, in the case of dibromide 3, the very rapid absorp-
tion of hydrogen was observed. Even after 5 min, the formation
of a new product was evidenced by TLC and complete con-
version occurred in 1 h. 1H NMR data showed that the resulting
product is a mixture of two compounds: deprotected dibromide
5 and deprotected doubly dehalogenated derivative 6 in a ratio
of 1.6:1 (Scheme 2). The observed reductive dehalogenation is a
common side process in the debenzylation of halogenated sub-
strates, which could be suppressed by the addition of acid in some
cases, but this option was not explored in this study since the syn-
theses of both 56 and 68 have been described in the literature.
The high reactivity of dibromide 3 together with the inertness
of phthalonitrile 4 under debenzylation conditions demonstrate
that the phthalonitrile unit acts as a catalyst poison: its coordi-
nation to Pd inhibits further hydrogenolysis.
An analysis of published data evidences that the behavior
of aromatic nitriles under hydrogenolysis conditions may be
different from a compound to another. Depending on substrate,
solvent or catalyst nature, the nitrile group can be either stable
or partially reduced with the formation of an aminomethyl
group. The latter process can be suppressed by addition of
ethylenediamine (en), acting as a catalytic poison.16 Therefore,
the catalyst Pd/C(en) was proposed for the chemoselective
hydrogenation of aromatic nitriles bearing other reducible groups
(alkenes and alkynes) with retention of the CN group.
This work was supported by ARCUS Alsace (a Russia/Ukraine
project), the Russian Foundation for Basic Research (grant
no. 09-03-93117), the CNRS and the Suprachem European
Research Association.
References
1
2
A. Yu. Tsivadze, Usp. Khim., 2004, 73, 6 (Russ. Chem. Rev., 2004, 73, 5).
I. Beletskaya, V. S. Tyurin, A. Yu. Tsivadze, R. Guilard and C. Stern,
Chem. Rev., 2009, 109, 1659.
3
4
J. A. Faiz, V. Heitz and J.-P. Sauvage, Chem. Soc. Rev., 2009, 38, 422.
J.-P. Sauvage, J.-P. Collin, J. A. Faiz, J. Frey. V. Heitz and C. Tock,
J. Porphyrins Phthalocyanines, 2008, 12, 881.
5
6
7
8
9
B. Cabezón, E. Quesada, S. Esperanza and T. Torres, Eur. J. Org. Chem.,
2000, 2767.
H.-W. Liu, C.-F. Chen, M. Ai, A.-J. Gong, J. Jiang and F. Xi, Tetrahedron:
Asymmetry, 2000, 11, 4915.
H. Liu, Y. Liu, M. Liu, C. Chen and F. Xi, Tetrahedron Lett., 2001, 42,
7083.
C. F. van Nostrum. S. J. Picken, A.-J. Schouten and R. J. M. Nolte,
J. Am. Chem. Soc., 1995, 117, 9957.
M. V. Martínez-Díaz, N. S. Fender, M. S. Rodríguez-Morgade, M. Gómez-
López, F. Diederich, L. Echegoyen, J. F. Stoddart and T. Torres, J. Mater.
Chem., 2002, 12, 2095.
10 W. M. Sharman and J. van Lier, in The Porphyrin Handbook, eds. K. M.
Kadish, K. M. Smith and R. Guilard, Academic Press, Amsterdam,
2003, vol. 15, p. 1.
11 A. V. Ivanov, P. A. Svinareva, I. V. Zhukov, L. G. Tomilova and N. S.
Zefirov, Izv. Akad. Nauk, Ser. Khim., 2003, 1479 (Russ. Chem. Bull.,
Int. Ed., 2003, 52, 1562).
12 S. A. Ahmed and M. Tanaka, J. Org. Chem., 2006, 71, 9884.
13 M. Kohn, J. Am. Chem. Soc., 1951, 73, 480.
14 R. A. W. Johnstone and A. H. Wilby, Chem. Rev., 1985, 85, 129.
15 A. David and M. A. Vannice, J. Catal., 2006, 237, 349.
16 T. Maegawa, Y. Fujita, A. Sakurai, A. Akashi, M. Sato, K. Oono and
H. Sajiki, Chem. Pharm. Bull., 2007, 55, 837.
The O-benzyl protective group can be easily removed under
Pd/C catalyzed hydrogenolysis, though Pd/C(en) can be used
for chemoselective reduction of double and triple C–C bonds
17 H. Sajiki, K. Hottori and K. Hirota, J. Org. Chem., 1998, 63, 7990.
18 E. J. Osburn, L.-K. Chau, S.-Y. Chen, N. Collins, D. F. O’Brien and
N. R. Armstrong, Langmuir, 1996, 12, 4784.
19 P. Smolenyak, R. Peterson, K. Nebesny, M. Tolrker, D. F. O’Brien and
N. R. Armstrong, J. Am. Chem. Soc., 1999, 121, 8628.
20 T. Toupance, V. Ahsen and J. Simon, J. Am. Chem. Soc., 1994, 116,
5352.
X
X
O
O
OH
OH
H2, Pd/C
EtOH
H2, Pd/C
EtOH
3
4
5 (X = Br) + 6 (X = H)
Scheme 2
Received: 18th February 2010; Com. 10/3470
– 238 –