Novel naphthalene diimides as activatable precursors of bisalkylating agents, by reduction and base catalysis

Article abstract of DOI:10.1021/jo7014328

(Chemical Equation Presented) Mild activation of water-soluble naphthalene diimides (NDIs) as bisalkylating agents has been achieved by base catalysis and by chemical and electrochemical reductions. NDI activation by a single electron reduction represents a novelty in the field of activatable electrophiles. Under mild reduction, induced by S₂O₄^2- in aqueous solution, the resulting NDI radical anion (NDI^?-) undergoes a monomolecular fragmentation to yield a new transient species, where the NDI radical anion is tethered to a quinone methide moiety. The latter still retains electrophilic properties, reacting with amines, thiols, and ethyl vinyl ether. Owing to the NDI recognition properties, these results represent the first step toward selective and bioactivatable cross-linking agents.

Full text of DOI:10.1021/jo7014328

Novel Naphthalene Diimides as Activatable Precursors of
Bisalkylating Agents, by Reduction and Base Catalysis
Marco Di Antonio,† Filippo Doria,† Mariella Mella,† Daniele Merli,‡
Antonella Profumo,‡ and Mauro Freccero*,†
Dipartimento di Chimica Organica, UniVersita` di PaVia, V.le Taramelli 10, 27100 PaVia, Italy, and
Dipartimento di Chimica Generale, UniVersita` di PaVia, V.le Taramelli 12, 27100 PaVia, Italy
mauro.freccero@unipV.it
ReceiVed July 2, 2007
Mild activation of water-soluble naphthalene diimides (NDIs) as bisalkylating agents has been achieved
by base catalysis and by chemical and electrochemical reductions. NDI activation by a single electron
reduction represents a novelty in the field of activatable electrophiles. Under mild reduction, induced by
2-
S2O4 in aqueous solution, the resulting NDI radical anion (NDI•-) undergoes a monomolecular
fragmentation to yield a new transient species, where the NDI radical anion is tethered to a quinone
methide moiety. The latter still retains electrophilic properties, reacting with amines, thiols, and ethyl
vinyl ether. Owing to the NDI recognition properties, these results represent the first step toward selective
and bioactivatable cross-linking agents.
Introduction
or tetraplex structures, and it may contribute to stabilize
complexes designed for gene-specific regulation of protein
expression, as well as therapeutic agents.3 Several naphthalim-
ide-containing compounds have been described as photoinduced
DNA cleaving agents,4 and some of them entered into clinical
trials owing to their strong anticancer activity.5 Selective
covalent modification of nucleotides has been achieved by
tethering these naphthalimides with molecular moieties exhibit-
ing alkylating reactivity.6 On the other hand, several groups7-10
Derivatives of 1,8-naphthalimide, 1,4,5,8-naphthalene tetra-
carboxylic diimides (NDIs), and perylene analogues have been
extensively studied in recent decades owing to their interesting
photophysical properties1 and their applications as molecular
probes with recognition properties toward guanine-rich oligo-
nucleotides by intercalation or end-stacking.2 In many cases,
such binding interactions function to stabilize duplex, triplex,2c
(2) (a) Guelev, V.; Sorey, S.; Hoffman, D. W.; Iverson, B. L. J. Am.
Chem Soc. 2002, 124, 2864-265. (b) Chu, Y.; Sorey, S.; Hoffman, D. W.;
Iverson, B. L. J. Am. Chem Soc. 2007, 129, 1304-1311. (c) Bevers, S.;
Schutte, S.; McLaughlin, L. W. J. Am. Chem Soc. 2000, 122, 5905-5915.
(3) Davis, J. T. Angew. Chem., Int. Ed. 2004, 43, 668-698.
(4) (a) Saito, I.; Takayama, M.; Sakurai, T. J. Am. Chem Soc. 1994,
116, 2653-2654. (b) Matsugo, S.; Kawanishi, S.; Yamamoto, K.; Sugiyama,
H.; Matsuura, T.; Saito, I. Angew. Chem., Int. Ed. Engl. 1991, 30, 1351-
1353.
(5) Bran˜a, M. F.; Castellano, J. M.; Mora´n, M.; Pe´rez de Vega, M. J.;
Qian, X. D.; Romerdahl, C. A.; Keilhauer. G. Eur. J. Med. Chem. 1995,
30, 325-329.
(6) Okamoto, A.; Nakamura, T.; Yoshida, K.; Nakatani, K.; Saito, I. Org.
Lett. 2000, 2, 3249-3251.
* To whom correspondence should be addressed. Telephone: +39 0382
987668. Fax: +39 0382 987323.
† Dipartimento di Chimica Organica.
‡ Dipartimento di Chimica Generale.
(1) (a) Cao, H.; Chang, V.; Hernandez, R.; Heagy, M. D. J. Org. Chem.
2005, 70, 4929-4934. (b) Thalacker, C.; Ro¨ger, C.; Wu¨rthner, F. J. Org.
Chem. 2006, 71, 8098-8105. (c) Wu¨rthner, F.; Ahmed, S.; Thalacker, C.;
Debaerdemaeker, T. Chem.sEur. J. 2002, 8, 4742-4750. (d) Kohl, C.;
Weil, T.; Qu, J.; Mullen, K. Chem.sEur. J. 2004, 10, 5297-531. (e) Nolde,
F.; Qu, J.; Kohl, C.; Pschirer, N. G.; Reither, E.; Mullen, K. Chem.sEur.
J. 2005, 11, 3959-3967. (f) Li, Y.; Wang, N.; Gan, H.; Liu, H.; Li, H.; Li,
Y.; He, X.; Huang, C.; Cui, S.; Wang, S.; Zhu, D. J. Org. Chem. 2005, 70,
9686-9692. (g) Pschirer, N. G.; Kohl, C.; Nolde, F.; Qu, J.; Mullen, K.
Angew. Chem., Int. Ed. 2006, 45, 1401-1404.
10.1021/jo7014328 CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/25/2007
8354
J. Org. Chem. 2007, 72, 8354-8360

Naphthalene Diimides as ActiVatable Precursors
SCHEME 1. Generation of QMs from Phenol and Binol
Derivatives
SCHEME 3. Synthesis of 4
QMP, with electron donor/acceptor properties tunable by
reduction (owing its redox properties) and upon base catalysis
(due to its acidity). In fact, it is well-known that naphthalene
diimide derivatives can be easily reduced to a radical anion by
electrochemical means.12
SCHEME 2. Substituent Effects on QM Precursor (QMP)
Reactivity
Results and Discussion
Synthesis. Attempts to elaborate the coupling procedure
between the anhydride 5 and (5-amino-2-hydroxybenzyl)-
trimethyl ammonium iodide, according to published procedure,
were stymied by the thermal instability of the quaternary
ammonium salts. In fact, it is known that quaternary ammonium
salts of Mannich bases easily decompose into free QM and
amine in the presence of electron-donating groups on the
aromatic ring.11 Consequently, at the beginning, the adduct 4
was synthesized by a three-step procedure starting from the
coupling of the anhydride 5 to p-aminophenol, followed by
Mannich reaction and CH3I methylation. Due to solubility
problems of the intermediate imide and also to poor yield in
the Mannich reaction step (mainly due to the formation of both
mono- and bis-CH2NMe2 adducts), the adduct 4 was synthesized
in higher yield by a two-step procedure, starting from the
coupling of the anhydride 5 to the preformed Mannich base
4-amino-2-dimethylaminomethylphenol generated in situ from
its dichloride salt, followed by methylation (Scheme 3).
After the synthesis of 4 was achieved, we began to investigate
the activation of the NDI 4 as bisalkylating agents under mild
conditions. Two different activation protocols have been used:
(i) base catalysis and (ii) a chemical reduction using S2O42-. In
both cases, we performed the reaction of 4 at both 25 and 40
°C in the presence of several nucleophiles (thiols and amines)
in aqueous solutions.
have recently shown that o-hydroxy benzylic alcohols (1),
Mannich base derivatives of phenols (2), and binols (3) are
capable of undergoing mono- and bisalkylation in water and
DNA cross-linking by photoactivation,8,10 base catalysis, and
thermal activation under physiological conditions or in the
presence of fluoride anion (Scheme 1).9
The key intermediates involved in these processes are
transient quinone methides (QMs). The formation of a QM had
previously demonstrated a dependence on the leaving group (Z
or W) attached to the benzylic position of its precursor (QMP)
when studying a model QM in the presence of biological
nucleophiles. Furthermore, the generation of QMs has been
shown to be highly responsive to the presence of electron-
withdrawing and -donating groups (Y, in Scheme 2). Electron-
donating groups greatly facilitate QM generation, while electron-
withdrawing groups strongly suppress its formation.11
In the present work, we report the synthesis and the reactivity
of bisalkylating QMPs directly tethered to the NDI (4 and 4a,
Scheme 3). The latter moiety may act simultaneously as the
molecular recognition structural element and substituent on a
Base Catalytic Activation. Activation by base catalysis takes
advantage of the low pKa (8) of the quaternary ammonium salts
of the Mannich bases, which allows the generation of a reactive
zwitterionic form at pH g 8, which generates alkylating QM
(Scheme 4).
Basic amines such as piperidine, pyrrolidine, and diethylamine
(pKa > 10.7) gave almost quantitative conversion under mild
conditions (25 °C, in 1:1 CH3CN/H2O; reaction condition A;
see Experimental Section). Amines displaying lower basicity
(pKa e 8), such as morpholine and L-proline methyl ester,
require higher temperature (40 °C), carbonate-buffered condi-
tions (pH g 8.5, reaction condition B) and longer reaction times
(7) (a) Wan, P.; Barker, B.; Diao, L.; Fisher, M.; Shi, Y.; Yang, C. Can.
J. Chem. 1996, 74, 465-475. (b) Diao, L.; Cheng, Y.; Wan, P. J. Am.
Chem. Soc. 1995, 117, 5369-5370. (c) Brousmiche, D.; Wan, P. Chem.
Commun. 1998, 491-492.
(8) (a) Chiang, Y. A.; Kresge, J.; Zhu, Y. J. Am. Chem. Soc. 2002, 124,
6349-6356. (b) Chiang, Y. A.; Kresge, J.; Zhu, Y. J. Am. Chem. Soc. 2002,
124, 717-722. (c) Chiang, Y. A.; Kresge, J.; Zhu, Y. J. Am. Chem. Soc.
2001, 123, 8089-8094.
(9) (a) Veldhuyzen, W. F.; Praveen, P.; Rokita, S. E. J. Am. Chem. Soc.
2003, 125, 14005-14013. (b) Veldhuyzen, W. F.; Shallop, A. J.; Jones, R.
A.; Rokita, S. E. J. Am. Chem. Soc. 2001, 123, 11126-11132.
(10) (a) Modica, E.; Zanaletti, R.; Freccero, M.; Mella, M. J. Org. Chem.
2001, 66, 41-52. (b) Richter, S.; Maggi, S.; Colloredo-Mels, S.; Palumbo,
M.; Freccero, M. J. Am. Chem. Soc. 2004, 126, 13973-13979. (c)
Colloredo-Mels, S.; Doria, F.; Verga, D.; Freccero, M. J. Org. Chem. 2006,
71, 3889-3895.
(12) (a) Heywang, G.; Born, L.; Fitzky, H.-G.; Hassel, T.; Hocker, J.;
Mu¨ller, H-K.; Pittel, B.; Roth, S. Angew. Chem., Int. Ed. 2004, 43, 668-
698. (b) Zhong, C. J.; Kwan, W. S. V.; Miller, L. Chem. Mater. 1992, 4,
1423-1428.
(11) Weinert, E. E.; Dondi, R.; Colloredo-Mels, S.; Frankenfield, K. N.;
Mitchell, C. H.; Freccero, M.; Rokita, S. E. J. Am. Chem. Soc. 2006, 128,
11940-11947.
J. Org. Chem, Vol. 72, No. 22, 2007 8355

Di Antonio et al.
2-
TABLE 1. Reactivity of 4 by Base Catalysis and S2O4 Reduction
SCHEME 4. Base Catalytic and Reductive Activation of 4
as Bisalkylating Agent through the Generation of QM-4 and
QM-4•-
(Table 1). 4 is much less reactive in the absence of basic
nucleophiles, affording the diol 6 in low yield only in the
presence of carbonate buffer at pH 8.5. In the presence of aniline
(pKa 4.6), adduct 16 formation was detected at 40 °C after 24
h, only under buffered conditions. In the absence of good
nitrogen nucleophiles, the reaction was sluggish, and in addition
to unreacted starting material, an uncharacterized oligomeric
product was formed. Monoalkylated adducts were not detected
by HPLC under the reaction condition described, and therefore,
if they formed at all, their yields were very low (i.e., <5%).
The results described above suggest that the activation of 4
as bisalkylating agent at rt is efficient when the phenol is mainly
deprotonated. This suggests that, similar to the prototype
quaternary ammonium salt 2a, also for the QMP 4, the
zwitterionic form has to be populated to lower the barrier for
the QM generation.
Reductive Activation. The second activation procedure
should switch the electron properties of the NDI moiety from
strongly withdrawing to donating by monoelectronic reduction,
with formation of the NDI radical anion. Very interestingly,
the activation of 4 as bisalkylating agent became even more
facile under chemical reduction by dithionite anion (S2O42-),
both in the absence and in the presence of nucleophiles, yielding
the adducts 6-16, after incubation at 25 °C, followed by workup
with O2. Oxygen oxidizes the adduct generated as radical anions
(6•--16•-) to the stable adducts (6-16), also quenching the
excess of dithionite anion (Scheme 4).
a Reaction time/h, T/°C, CH3CN/H2O ) 1:1, [4] ) 5 × 10-3 M, [HNu]
) 5 × 10-2 M. b Reaction time/h, T/°C, CH3CN/H2O ) 1:1, in the presence
of Na2S2O4 10-2 M, [4] ) 5 × 10-4 M, [HNu] ) 5 × 10-3 M. Reaction
yields (%) are reported in parentheses. c Condition B, carbonate buffered
(pH g 8.5), as reported in the Experimental Section. d Undetected by HPLC.
e Condition A, not buffered.
TABLE 2. Redox Properties of the NDI Derivatives 4 and 4a
DMF
Bu4NBF4 0.1 M
CH3CN/H2O ) 1:1
Bu4NBF4 0.1 M
a
b
a
b
NDI
E°1/2
E°1/2
E°1/2
E°1/2
4
4a
-0.40
-0.46
-0.85
-0.96
-0.32
-0.44
e-0.65
e-0.85
a First wave potential (NDI/NDI•-). b Second wave potential (NDI•-
/
NDI2-). E1/2 values in volts were taken as the center of the anodic and
cathodic peak potentials vs Ag/AgCl/KCl (4 M KCl saturated with AgCl).
The conversions into adducts 6-16 were obtained at lower
temperature and shorter reaction time in comparison to the
thermal incubations in the absence of dithionite. Data in Table
1 show that the reductive condition activates the bisalkylation
process by diimide 4. Such an activation is effective also with
less basic nucleophiles such as L-proline methyl ester and
aniline. Thus the diimide 4 does not react with aniline in aqueous
acetonitrile at 40 °C for 1 day, but it undergoes bisalkylation
under S2O42- reducing conditions. 4a, unlike 4, is stable under
cyclic voltammetry (Table 2). Both the amine 4a and its
quaternary ammonium salt 4 exhibit two one-electron reduction
peaks. In DMF, using Bu4NBF4, compound 4a, used as
dichloride salt, showed two redox couples at -0.46 and -0.96
V (vs Ag/AgCl/KCl). Although the first wave is perfectly
reversible (in the sense that the peak separation for the reductions
is exactly 60 mV), the second reduction is slightly less reversible
(peak separation 120 mV). Apparent E° values in Table 2 were
taken as the center of the anodic and cathodic peak potentials
and are consistent with those reported for other aromatic
2-
both base catalysis and S2O4 reduction.
The reactivity as bisalkylating agent of the NDI 4 is related
to its favorable redox properties, which were characterized by
8356 J. Org. Chem., Vol. 72, No. 22, 2007

Naphthalene Diimides as ActiVatable Precursors
SCHEME 5. Reactivity of QM-4 and QM-4•- as
Heterodiene in a [4 + 2] Cycloaddition
FIGURE 1. Generation from 0 to 20 min (a) and reactivity from the
following 2 h (b) of 4•- monitored by UV-visible spectroscopy.
TABLE 3. Activation Free Energy in the Gas Phase and in
Aqueous Solution at the R(U)B3LYP/6-31+G(d,p) Level of Theory
for the Generation of the QMs Starting from 18, Its Anion 18-, and
Its Radical Anion 18•-
b
QMP
∆Ea
∆Ggas
∆Gaq
18
37.0
32.1
46.5c
41.1d
41.4e
f
18-
3.7
3.8
1.0
1.4
8.5d
8.5e
17.0c
6.3d
6.7e
18•-
imides.12b Compounds 4, with quaternary ammonium groups,
had reduction potentials at slightly more positive potentials,
-0.40 and -0.85 V, but both reduction processes were strongly
irreversible, with dependence on the sweeping rate (peak
separation >180 and 300 mV at 200 mV/s). Although in
aqueous solution both substrates are more easily reduced (Table
2), the compounds 4a and 4 behave similarly to that described
for the DMF solution. The process is chemically reversible for
the amine 4a and chemically irreversible for the quaternary
ammonium 4. In fact, after a few redox cycles, the amine 4a
was recovered unreacted, unlike the quaternary ammonium 4
which was completely consumed, with diol 6 as the only
detectable adduct (20%, yield). After a few redox cycles in a
1:1 acetonitrile/water solution containing morpholine (10-2 M),
the quaternary ammonium 4 was converted into the adduct 10
(yield >80%).
a Activation electronic energies in kcal/mol. b Activation free energies
in the gas phase. c Optimized in aqueous solution at R(U)B3LYP/6-
31+G(d,p) using PCM (UA0 radii) solvation model. d Single-point calcula-
tion at B3LYP/6-31+G(d,p) on gas-phase geometries (UA0 radii). e Single-
point calculation at B3LYP/6-31+G(d,p) on gas-phase geometries (UAHF
radii). f We failed to locate the TS-18•- in aqueous solution.
thus we decided to corroborate the generation of a similar
electrophilic intermediate by chemical reduction, producing
further independent evidence. To this end, we managed to
achieve the activation of 4 as bisalkylating agent by coulom-
bometric reduction at E ) -0.5V [vs Ag/AgCl/KCl (4 M KCl
saturated with AgCl)]. In fact, under these conditions, the salt
4 was efficiently consumed, affording the diol 6 as the only
detectable adduct in aqueous acetonitrile after O2 workup. A
similar experiment performed with morpholine (10-2 M) af-
forded only the adduct 10 in quantitative yield. The electro-
chemical reduction was also monitored as a function of time
by UV-visible spectroscopy, which revealed two sequential
transient species on the time scale of a few hours. The reduction
of 4 in 1:1 CH3CN/H2O bleached the reactant absorbance (360
and 378 nm), generating a new species with maximum absor-
bance centered at 458 nm (Figure 1a). Such a transient was
assigned to the radical anion of the diimide 4 (4•-) on the basis
of (i) its rapid quenching in the presence of O2, (ii) the
spectroscopic similarity to NDI radical anions, bearing cationic
Diels-Alder Reactivity of the Transient Electrophile. In
order to clarify the nature of the electrophilic intermediate
involved in the alkylation process, we ran two set of experiments
in the presence of EVE (ethyl vinyl ether), under basic catalysis
and reductive activation. 4 gave the adduct 17 as the main
adducts under (i) basic conditions (pH 9, CH3CN/H2O) at 40
2-
°C and (ii) in the presence of S2O4 at rt, pH 7, followed by
oxygen quenching (Scheme 5).
The formation of the adducts 6-17 by base catalysis is
rationalized by the generation of a transient electrophilic QM
(QM-4) from the reactive zwitterionic form, followed by
nucleophile (Scheme 4) or EVE trapping in a hetero-Diels-
Alder [4 + 2] cycloaddition (Scheme 5). The formation of the
same adducts 6-17 by reduction, following oxygen quenching,
reveals that it is possible to switch the electron properties of
the NDI as a substituent to a QMP, from an electron-
withdrawing to -donating group simply by monoelectronic
reduction. In this way, it has been possible to generate a QM
directly tethered to a NDI radical anion (QM-4•-), which retains
electrophilicity at the exocyclic methylene group and heterodiene
reactivity as the prototype o-QM.
+
substituents to the imide nitrogen atom [such as -(CH2)nNMe3
(λmax 449 nm)],12b generated by electrochemical reduction and
by intramolecular photoinduced single electron transfer.13 The
radical anion 4•- decays with a first-order kinetic (k1 ) 8.1 ×
10-4 s-1, r2 ) 0.99), in the absence of O2, to generate another
species with absorbance centered at 412, 542, and 590 nm
(Figure 1b).
This UV-vis spectrum displays the features of both the
radical anion NDI•- (542 and 590 nm) and the ortho-QM (410
nm).10a This species decays within a few hours under aqueous
Detection of the Transients by UV-Vis Spectroscopy. The
(13) Abraham, B.; McMasters, S.; Mullan, M. A.; Kelly, L. A. J. Am.
Chem. Soc. 2004, 126, 4293-4300.
QMgenerationbybasecatalysishadbeenpreviouslyrationalized,10a
J. Org. Chem, Vol. 72, No. 22, 2007 8357

Di Antonio et al.
SCHEME 6. Generation of QM-18 and QM-18•- through the TSs TS-18, TS-18-, and TS-18•- (Bond Lengths are in
Angstroms; Data in Parentheses are for Full R(U)B3LYP/6-31+G(d,p) Optimization in Aqueous Solution)
acetonitrile, in the absence of O2, and it is rapidly quenched by
O2, morpholine, and EVE. The electrolyzed solution of 4 in
the presence of morpholine (10-3 M) and EVE (10-2 M), after
30 min at rt and workup with O2, gave the adducts 10 and 17,
respectively, in good yields (g85%), with no hydration adduct
6 detected. Therefore, we assigned the absorbance of this species
to a QM tethered to a NDI•- (QM-4•-), displaying benzylating
reactivity. Similarly, the NDI 4a can be easily reduced to its
radical anion (4a•-) with λmax ) 473 nm, which unlike 4•- is
stable in the presence of nucleophiles in aqueous acetonitrile.
Such an evidence is in accord with the lack of reactivity of 4a
in preparative experiments.
catalysis and (ii) mild reduction. The involvement of QM as
key electrophile of the bisalkylating process has been supported
by product distribution analysis with nucleophiles and EVE and
other methods, including UV-vis spectroscopy, electrochemical
reduction, and computational modeling. The creation of a QMP
requiring activation by single electron reduction represents a
novelty in the field of the mild generation of QMs. In addition,
owing to the NDI recognition properties, these results suggest
bioapplications of 4 and its derivatives as triggerable and
selective cross-linking agents toward guanine-rich oligonucle-
otides.14
Computational Evaluation of the Basic and Reductive
Activation Processes. The final proof that NDIs tethered to
quaternary ammonium salts of a Mannich bases are activatable
QMP by both base catalysis and monoelectronic reduction was
given by a computational investigation at the R(U)B3LYP/6-
31+G(d,p) level of theory both in the gas phase and in aqueous
solution (by PCM solvation model) on the model imide 18
(Scheme 6).
The activation free energies computed both in the gas phase
and in aqueous solution (Table 3) suggest that the generation
of an alkylating QM (QM-18) becomes a much easier process,
passing from the protonated quaternary ammonium salt 18 to
its zwitterionic form 18-. The catalytic effect of the base is
massive both in the gas phase and in solution (>30 kcal/mol).
The reduction of the naphthalimide moiety to its radical anion
18•-, which generates the QM tethered to the imide moiety QM-
18•-, also induces a similar activation, lowering the barrier in
the gas phase by 35 kcal/mol. Geometry optimization in the
solvent bulk of the TS-18 and TS-18•- suggests a slightly lower
catalytic effect since the activation free energy in aqueous
solution is reduced by 29.5 kcal/mol. We were unable to
optimize TS-18- in aqueous bulk; therefore, for this TS, we
only compute the solvation by single-point calculation on
B3LYP/6-31+G(d,p) gas-phase geometry.
Experimental Section
N,N′-Bis[3-(dimethylamino)methyl-4-hydroxyphenyl]-1,4,5,8-
naphthalenetetracarboxylic diimide (4a): 2.0 g of 4-amino-2-
[(dimethylamino)methyl]phenol dihydrochloride, prepared accord-
ing published procedure,15 (8.4 mmol) and 1.07 g (4 mmol) of
1,4,5,8-tetracarboxylic dianhydride were suspended in 20 mL of a
9:1 mixture of dioxane/DMF. TEA (1 mL) was added to the
suspension, and the mixture was allowed to reflux with vigorous
stirring for 20 h under nitrogen. The reaction mixture was cooled
and poured into 30 mL of water. The resulting suspension was
filtered and washed first with water and then twice with anhydrous
EtOH. The N,N′-bis[3-(dimethylamino)methyl-4-hydroxy]-1,4,5,8-
naphthalenetetracarboxylic diimide was obtained as a pale green
1
solid: mp > 350 °C; H NMR (DMSO-d6) δ 2.25 (s, 12H, Me2),
3.60 (s, 4H, CH2), 6.90 (d, 2H, J ) 7.4 Hz), 7.20 (d, 2H, J ) 7.4
Hz), 7.25 (s, 2H), 8.70 (s, 4H), 9.70 (s, 2H); 13C NMR (DMSO-
d6) δ 41.87, 54.43, 116.05, 116.68, 126.24, 126.64, 126.90, 130.55,
131.66, 132.93, 156.74, 163.03. Anal. Calcd for C32H28N4O6: C,
68.07; H, 5.00; N, 9.92; O, 17.00. Found: C, 68.14; H, 4.99; N,
9.90.
N,N′-Bis[3-(trimethylamino)methyl-4-hydroxyphenyl]-1,4,5,8-
naphthalenetetracarboxylic diimide iodide (4): 2.0 g (3.54 mmol)
of diimide 4a was suspended in CH3CN (50 mL), and CH3I (1.2 g,
8.5 mmol) was added. This suspension, refluxing under nitrogen,
turned a dark red color in a few minutes. After 3 h, the reaction
was chilled and Et2O (50 mL) was added with formation of red
crystals. The suspension was filtered and washed twice with CH3-
Conclusion
In conclusion, we have described the activatable bisalkylating
properties of the NDI moiety tethered to a quaternary ammonium
salt of a Mannich base, exploring two protocols: (i) classic base
(14) Reactivity of 4 and its duplex-quadruplex selectivity is under
investigation.
(15) Werbel, M. J. Med. Chem. 1983, 26, 1258-1267.
8358 J. Org. Chem., Vol. 72, No. 22, 2007

Naphthalene Diimides as ActiVatable Precursors
CN to give 4 (2.5 g, 3 mmol, 84% yield) as a red-brownish solid:
mp > 350 °C; H NMR (DMSO-d6) δ 3.1 (s, 18H), 4.5 (s, 4H),
131.3, 158.8, 163.2. Anal. Calcd for C40H44N4O6: C, 70.99; H,
6.55; N, 8.28; O, 14.18. Found: C, 71.09; H, 6.53; N, 8.21.
Adduct 10: Yellow needles; mp > 350 °C; H NMR (DMSO-
d6) δ 3.70 (s, 20H), 6.90 (d, 2H, J ) 8.6 Hz), 7.20 (d, 2H, J ) 8.6
Hz), 7.25 (s, 2H), 8.75 (s, 4H); 13C NMR (DMSO-d6) δ 52.8, 58.0,
66.1, 115.4, 122.8, 126.3, 126.6, 127.0, 128.5, 129.7, 130.4, 156.3,
163.1. Anal. Calcd for C36H32N4O8: C, 66.66; H, 4.97; N, 8.64;
O, 19.73. Found: C, 66.73; H, 5.00; N, 8.59.
1
1
7.10 (d, 2H, J ) 8.6 Hz), 7.4 (d, 2H, J ) 8.6 Hz), 7.50 (s, 2H),
8.80 (s, 4H), 10.9 (s, 2H); 13C NMR (DMSO-d6) δ 52.04, 62.81,
114.84, 116.43, 126.34, 126.63, 126.97, 130.53, 132.60, 134.82,
157.35, 163.07. Anal. Calcd for C34H34I2N4O6: C, 48.13; H, 4.04;
I, 29.91; N, 6.60; O, 11.31. Found: C, 48.18; H, 4.02; I, 29.80; N,
6.63.
Activation by Base Catalysis. General Procedure. Base
Catalysis General Method: The bisalkylating properties of the
diimide 4 have been activated by base catalysis in the presence of
several nucleophiles (amines and thiols), according the following
procedures:
Adduct 11: Characterized as hydrochloride (11‚2HCl); green
needles; mp > 350 °C; 1H NMR (DMSO-d6) δ 2.75 (s, 18H), 4.30
(s, 4H), 7.20 (d, 2H, J ) 8.6 Hz), 7.4 (d, 2H, J ) 8.6 Hz), 7.5 (s,
2H), 8.8 (s, 4H), 10.0 (s, 4H), 10.8 (s, 2H); 13C NMR (DMSO-d6)
δ 25.1, 41.9, 54.6, 115.8, 116.0, 126.3, 126.6, 126.9, 130.9, 131.7,
132.9, 156.7, 163.0. Anal. Calcd for C36H38Cl2N4O6: C, 62.34; H,
5.52; Cl, 10.22; N, 8.08; O, 13.84. Found: C, 62.29; H, 5.61; Cl,
10.19; N, 8.06.
Procedure A (For nucleophile with pKa > 8): To a solution of
4 (50 mL, CH3CN/H2O ) 1:1, 5 × 10-3 M) oxygen-purged with
a N2 flux was added the nucleophile in order to reach a 5 × 10-2
M concentration. This solution was allowed to stand at 25 or
40 °C and after ∼0.5 h, a pale yellow solid begins to form. After
a few hours, the solution was concentrated under reduced pressure
and the resulting suspension was cooled at 4 °C for 12 h. The solid
was filtered and washed with cold anhydrous EtOH to provide the
bisalkylated adducts 6-9 and 11-13 in good yield (60-85%).
Procedure B (For nucleophile with pKa < 8): To 50 mL of a
solution of 5 × 10-3 M of the diimide 4 in CH3CN/H2O (1:1)
buffered to pH 8.5 with a Na2CO3/NaHCO3 buffer, outgassed with
a N2 flux, was added the nucleophile in order to reach a
concentration of 5 × 10-2 M. This solution was heated at 40 °C
for 12 h. After this time, the CH3CN was evaporated under vacuum
and the suspension was kept at 4 °C for 12 h. The solid was filtered
and washed twice with cold water and anhydrous EtOH to provide
the bisalkylated adducts 10 and 14-17 in good yield (55-70%).
Activation by S2O42- Reduction. General Procedure. In order
to trap the nucleophiles by reductive activation of 4, a procedure
similar to the base catalytic method was used. However, lower
concentration of substrate is required in order to avoid π-stacking
between 4 and its radical ion.
1
Adduct 12: Yellow needles; mp > 350 °C; H NMR (DMSO-
d6) δ 1.75 (s, 12H), 3.5 (m, 8H), 3.85 (s, 4H), 6.80 (d, 2H, J ) 8.8
Hz), 7.25 (d, 2H, J ) 8.8 Hz), 7.25 (s, 2H), 8.8 (s, 4H); 13C NMR
(DMSO-d6) δ 25.5, 30.7, 53.3, 59.5, 115.5, 122.7, 126.3, 126.5,
126.90, 126.94, 127.0, 130.4, 156.3, 163.7. Anal. Calcd for
C38H36N4O6: C, 70.79; H, 5.63; N, 8.69; O, 14.89. Found: C, 70.73;
H, 5.65; N, 8.70.
1
Adduct 13: Yellow needles; mp > 350 °C; H NMR (DMSO-
d6) δ 2.00 (m, 8H), 3.10 (m, 4H), 3.5 (m, 4H), 4.3 (s, 4H), 7.1 (d,
2H, J ) 8.6 Hz), 7.35 (d, 2H, J ) 8,6 Hz), 7.50 (s, 2H), 8.7 (s,
4H); 13C NMR (DMSO-d6) δ 22.5, 51.5, 52.9, 116.0, 117.7, 126.3,
126.6, 126.9, 130.6, 131.4, 132.5, 156.4, 163.0. Anal. Calcd for
C36H32N4O6: C, 70.12; H, 5.23; N, 9.09; O, 15.57. Found: C, 70.09;
H, 5.25; N, 9.07.
1
Adduct 14: Yellow needles; mp > 350 °C; H NMR (DMSO-
d6) δ 2.0 (m, 4H), 2.1 (m, 2H), 2.4 (m, 2H), 3.2 (m, 2H), 3.5 (m,
2H), 3.7 (m, 2H), 3.9 (s, 6H), 4.1 (s, 4H), 7.1 (d, 2H, J ) 8.65
Hz), 7.35 (d, 2H, J ) 8.65 Hz), 7.50 (s, 2H), 8.7 (s, 4H); 13C NMR
(DMSO-d6) δ 23.2, 29.4, 52.0, 52.6, 57.6, 65.0, 116.1, 117.2, 126.1,
126.4, 126.8, 130.7, 131.9, 132.6, 156.4, 163.0, 173.5. Anal. Calcd
for C40H36N4O10: C, 65.57; H, 4.95; N, 7.65; O, 21.84. Found: C,
65.53; H, 4.99; N, 7.61.
To 20 mL of an oxygen-purged solution of the diimide 4 (5 ×
10-4 M) was added the nucleophile in order to reach a concentration
of 5 × 10-3 M and 2 mL of a 0.1 M Na2S2O4 solution. The resulting
violet solution was allowed to stand at 40 °C for 4 h. After this
time, the solution was chilled and treated with an air flux for 10
min in order to consume the unreacted S2O42-. The CH3CN was
evaporated under vacuum, and the product precipitated as a pale
yellow solid.
1
Adduct 15: Yellow needles; mp > 350 °C; H NMR (DMSO-
d6) δ 2.00 (s, 18H), 4.30 (s, 4H), 7.20 (d, 2H, J ) 8.8 Hz), 7.4 (d,
2H, J ) 8.8 Hz), 7.5 (s, 2H), 8.8 (s, 4H), 10.8 (s, 2H); 13C NMR
(DMSO-d6) δ 30.2, 45.3, 60.3, 115.8, 116.0, 126.3, 126.6, 126.9,
130.9, 131.7, 132.9, 156.7, 163.0. Anal. Calcd for C36H34N2O6S2:
C, 66.03; H, 5.23; N, 4.28; O, 14.66; S, 9.79. Found: C, 66.09; H,
5.25; N, 4.23; S, 9.73.
Adduct 6: Pale yellow needles; mp > 350 °C; 1H NMR (DMSO-
d6) δ 4.40 (s, 4H), 7.20 (d, 2H, J ) 8.6 Hz), 7.4 (d, 2H, J ) 8.6
Hz), 7.5 (s, 2H), 8.8 (s, 4H), 10.8 (s, 2H); 13C NMR (DMSO-d6)
δ 64.6, 115.9, 117.2, 126.2, 126.9, 127.7, 131.0, 131.4, 132.9, 156.7,
163.1. Anal. Calcd for C28H18N2O8: C, 65.88; H, 3.55; N, 5.49;
O, 25.07. Found: C, 65.77; H, 3.58; N, 5.45.
Adduct 16: Characterized as hydrochloride (16‚2HCl); pale
green needles; mp > 350 °C; 1H NMR (DMSO-d6) δ 2.00 (s, 18H),
4.30 (s, 4H), 6.8 (m, 6H), 7.10 (m, 4H), 7.20 (d, 2H, J ) 8.7 Hz),
7.4 (d, 2H, J ) 8.7 Hz), 7.5 (s, 2H), 8.8 (s, 4H), 10.0 (s, 2H), 10.8
(s, 4H); 13C NMR (DMSO-d6) δ 55.3, 115.0, 115.8, 116.0, 118.4,
126.3, 126.6, 126.9, 129.3, 130.6, 131.0, 132.8, 147.0, 157.0, 163.1.
Anal. Calcd for C40H30Cl2N4O6: C, 65.49; H, 4.12; Cl, 9.67; N,
7.64; O, 13.09. Found: C, 65.43; H, 4.16; Cl, 9.69; N, 7.62.
Adduct 17: Yellow solid; mp > 350 °C; 1H NMR (DMSO-d6)
δ 1.3 (t, 6H), 2.00 (m, 4H), 2.70 (m, 2H), 3.0 (m, 2H), 3.7 (m,
2H), 4.0 (m, 2H), 5.4 (dd, 2H, J ) 2.7 Hz), 7.1 (d, 2H, J ) 8.6
Hz), 7.35 (d, 2H, J ) 8.6 Hz), 7.50 (s, 2H), 8.70 (s, 4H); 13C NMR
(DMSO-d6) δ 15.1, 22.5, 51.5, 52.9, 116.0, 117.7, 126.3, 126.6,
126.9, 130.6, 131.4, 132.5, 156.4, 163.0. Anal. Calcd for
C36H30N2O8: C, 69.89; H, 4.89; N, 4.53; O, 20.69. Found: C, 69.94;
H, 4.90; N, 4.49.
Electrochemical Measurements. Cyclic voltammetry (CV) was
performed using an Amel 433/W polarographic analyzer equipped
with a standard three-electrode cell with a platinum disk electrode
as working electrode (2.0 mm diameter), a platinum wire as
auxiliary electrode, and a Ag/AgCl/KCl (4 M KCl saturated with
AgCl) reference electrode. All potentials in the text are reported
versus Ag/AgCl/KCl (4 M KCl saturated with AgCl).
1
Adduct 7: Yellow needles; mp > 350 °C; H NMR (DMSO-
d6) δ 1.10 (t, 12H), 2.75 (q, 8H), 3.70 (s, 4H), 6.90 (d, 2H, J ) 9.2
Hz), 7.25 (d, 2H, J ) 9.2 Hz), 7.25 (s, 2H), 8.6 (s, 4H); 13C NMR
(DMSO-d6) δ 11.2, 45.9, 54.8, 115.5, 123.2, 126.1, 126.6, 127.0,
128.4, 128.8, 130.39, 157.5, 163.1. Anal. Calcd for C36H36N4O6:
C, 69.66; H, 5.85; N, 9.03; O, 15.47. Found: C, 69.69; H, 5.84; N,
9.10.
1
Adduct 8: Yellow needles; mp > 350 °C; H NMR (DMSO-
d6) δ 0.91 (t, 12H), 1.10 (t, 12H), 1.30 (q, 8H), 1.5 (m, 8H), 2.75
(m, 8H), 3.75 (s, 4H), 6.75 (d, 2H, J ) 8.8 Hz), 7.1 (d, 2H, J )
8.8 Hz), 7.15 (s, 2H), 8.7 (s, 4H); 13C NMR (DMSO-d6) δ 10.9,
13.8, 20.0, 28.2, 46.4, 52.0, 55.4, 115.5, 123.2, 126.06, 126.5, 127.0,
128.4, 128.9, 130.4, 157.5, 163.1. Anal. Calcd for C40H44N4O6: C,
70.99; H, 6.55; N, 8.28; O, 14.18. Found: C, 71.02; H, 6.58; N,
8.25.
Adduct 9: Pale yellow needles; mp > 350 °C; 1H NMR (CDCl3)
δ 2.1 (d, 12H), 2.5 (m, 2H), 3.75 (s, 4H), 6.9 (s, 2H), 7.0 (d, 2H,
J ) 8.6 Hz), 7.2 (d, 2H, J ) 8.6 Hz), 8.8 (s, 4H); 13C NMR (CDCl3)
δ 30.8, 44.5, 62.5, 115.5, 117.0, 122.6, 125.1, 127.0, 127.9, 128.4,
J. Org. Chem, Vol. 72, No. 22, 2007 8359

Di Antonio et al.
In degassed DMF using Bu4NBF4 (0.1 M), with a sweep rate )
100 mV s-1, although the processes for both the compounds were
not perfectly reversible, 4a showed a more reversible behavior with
a peak separation of 70 ( 10 and 120 ( 15 mV for the first and
the second process, respectively (Supporting Information). A similar
behavior has been found also in 0.1 M Bu4NBF4, CH3CN/H2O )
1:1, at 100 mV s-1; in this case, ill-defined second reduction,
especially for 4, was obtained. As expected, less negative peak
potentials have been found in CH3CN:H2O, with respect to DMF.
Coulometric reductions were performed on a BAS 100B/W
Version 2.3 and a BAS C3 cell stand with a 200 mL glass cell,
using a carbon sponge working electrode, a platinum wire as
auxiliary electrode set in a separated quartz tube, and a Ag/AgCl/
KCl (4 M KCl saturated with AgCl) reference electrode in degassed
CH3CN/H2O ) 1:1 solutions. Reduction was performed for the
amine 4a and its ammonium quaternary salt 4 at -600 and -500
mV, respectively. The formation of the anion radicals and their
decay have been monitored as a function of time, recording the
UV-vis spectra on a HP-8452A DAD spectrophotometer with a
661-500-QX Quarz Tauchsonde with 10 mm path length, HELL-
MA.
anion energies, and in our reactive system, a zwitterionic character
is present in both the reactants 18- and 18•- and related TSs (TS-
18- and TS-18•-). Thermal contributions (δG) to activation free
energy (∆G; see Supporting Information) were computed from
B3LYP/6-31G(d) structures and harmonic frequencies, by using
the harmonic oscillator approximation and the standard expressions
for an ideal gas in the canonical ensemble at 298.15 K and 1 atm.
The optimization of the stationary points in the solvent bulk was
calculated via the self-consistent reaction field (SCRF) method using
PCM18 as implemented in the D.02 version of Gaussian 03. The
geometry optimizations in water solution for TS-18 and TS-18•-
have been quite difficult; therefore, to solve the problem, the “loose”
convergence criteria on geometry optimization were adopted. We
were unable to optimize TS-18- in aqueous bulk; therefore, for
this TS, we compute the solvation by single-point calculation on
B3LYP/6-31+G(d,p) gas-phase geometry. The cavity is composed
by interlocking spheres centered on non-hydrogen atoms with UA0
radii. Such a model includes the nonelectrostatic terms (cavitation,
dispersion, and repulsion energy) in addition to the classical
electrostatic contribution.
Methods and Computation Details. All calculations were
carried out using revision D.02 of the Gaussian 03 program
package.16 The geometric structures of the reactants (18, 18-, and
18•-) and the transition states (TS-18, TS-18-, and TS-18•-) located
were fully optimized in the gas phase using the hybrid density
functional B3LYP17 with the 6-31+G(d,p) basis set. It is known
that diffuse functions are mandatory for a reliable evaluation of
Acknowledgment. Financial support from Pavia University
(“Fondo FAR 2005”) and “Consorzio CINMPIS” is gratefully
acknowledged. We also thank CICAIA (Modena University)
and CINECA (Bologna University) for computer facilities.
Supporting Information Available: 1H NMR and 13C NMR
for the NDIs 4, 4a, and the adducts 6-17, electrochemical data
(Figures S1 and S2), Cartesian coordinates of the reactants (18,
18-, and 18•-), and TSs (TS-18- and TS-18•-) at the R(U)B3LYP/
6-31+G(d,p) level (gas phase and aqueous solution). This material
is available free of charge via Internet at http://pubs.acs.org.
(16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, revision D.02; Gaussian, Inc.: Wallingford, CT, 2004.
JO7014328
(17) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 1372-1377. (b) Schmider,
H. L.; Becke, A. D. J. Chem. Phys. 1998, 108, 9624-9631.
(18) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995-2001.
For the original COSMO model, see: (b) Klamt, A.; Schu¨u¨rmann J. Chem.
Soc., Perkins Trans. 1993, 799-805. (c) Klamt, A.; Jonas, V.; Burger, T.;
Lohrenz, J. C. W. J. Phys. Chem. A 1998, 102, 5074-5085. For a more
comprehensive treatment of solvation models, see: (d) Cramer, C. J.;
Truhlar, D. G. Chem. ReV. 1999, 99, 2161-2200.
8360 J. Org. Chem., Vol. 72, No. 22, 2007