Effective catalysis of imine metathesis by means of fast transiminations between aromatic-aromatic or aromatic-aliphatic amines

Article abstract of DOI:10.1039/c4ob00107a

This paper reports on a quantitative investigation of rates of amine-imine exchange reactions of primary amines with their benzylidene derivatives in organic solvents at room temperature. Exchange reactions involving aromatic-aromatic or aromatic-aliphati

Full text of DOI:10.1039/c4ob00107a

Organic &
Biomolecular Chemistry
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Effective catalysis of imine metathesis by means of
fast transiminations between aromatic–aromatic
or aromatic–aliphatic amines†
Cite this: Org. Biomol. Chem., 2014,
12, 3282
Maria Ciaccia, Silvia Pilati, Roberta Cacciapaglia, Luigi Mandolini and
Stefano Di Stefano*
This paper reports on a quantitative investigation of rates of amine–imine exchange reactions of primary
amines with their benzylidene derivatives in organic solvents at room temperature. Exchange reactions
involving aromatic–aromatic or aromatic–aliphatic amines were in all cases fast enough to allow their use
in the effective catalysis of imine metathesis in the absence of acid and metal catalysis. Transiminations
based on exchange between aromatic and aliphatic amines were retarded both by electron-donating and
electron-withdrawing substituents in the para-position of the benzylidene moiety. This result was inter-
preted as arising from a change in the rate-determining step of the two-step transimination reaction.
Received 14th January 2014,
Accepted 21st March 2014
DOI: 10.1039/c4ob00107a
www.rsc.org/obc
exchange equilibria of imine bonds in organic solvents,5 but it
is unknown whether similar features are displayed by imines
Introduction
The relevance of imines in chemistry and biology has been derived from anilines.
known for a long time,1 but structural effects on rates and
equilibria of important reactions involving the imine bond,
R–CHvN–R′ þ H2N–R″ Ð R–CHvN–R″ þ H2N–R′
ð1Þ
such as transimination and imine metathesis, are still poorly
understood. At variance with the common notion that the reac-
tivity of imines toward nucleophiles is intrinsically low and
becomes quite high only after protonation,1e,2 we recently
reported amine–imine exchange reactions of sterically un-
hindered primary aliphatic amines with their benzylidene
derivatives, eqn (1), to be surprisingly fast at ambient temp-
erature in a variety of non-aqueous solvents in the absence of
acid catalysis.3 These very fast transimination reactions involving
aliphatic amines were utilized in the catalysis of imine meta-
thesis, eqn (2), which occurs rapidly at ambient temperature
under non-acidic conditions in the presence of catalytic
amounts of primary aliphatic amines, as a result of coupled
transimination reactions. These features are of great value for
studies of dynamic covalent chemistry (DCC)4 based on
R–CHvN–R′ þ R″–CHvN–R′′′
Ð R–CHvN–R′′′ þ R″–CHvN–R′
ð2Þ
With a view to filling this gap, we investigated rates and
equilibria of transimination reactions based on aromatic
amines, eqn (3), as well as of a mixed aromatic–aliphatic
system, eqn (4). Here we show that in all cases transimination
reactions are fast enough to allow their use in the effective
catalysis of imine metathesis under mild conditions. We also
show that attempts at enhancing rates by replacing the para-
hydrogen of the benzylidene moiety with electron-donating or
electron-withdrawing substituents were unsuccessful, but
afforded valuable mechanistic insights.
Dipartimento di Chimica, Sapienza Università di Roma and Istituto CNR di
Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione,
c/o Dipartimento di Chimica, Sapienza Università di Roma, P.le A. Moro 5, I-00185
Rome, Italy. E-mail: stefano.distefano@uniroma1.it; Fax: +39 06 49913629;
Tel: +39 06 49913057
†Electronic supplementary information (ESI) available: Kinetic equations for
reversible reactions, second order in both directions, kinetic profiles for
ð3Þ
aromatic–aromatic imine metathesis in different solvents and for aromatic–
aliphatic imine metathesis carried out in chloroform with varying mixtures of
amine catalysts. See DOI: 10.1039/c4ob00107a
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Table 1 Rates and equilibrium constants of imine–amine exchange
between N-benzylideneanilines and p-toluidine, eqn (3), at 25 °Ca
Entry
X
Solventb Kc
kf d (M−1 s−1
)
kb d (M−1 s−1
)
t
e (s)
1
2
ð4Þ
1
2
3
4
5
6
H
H
H
CDCl3
CD2Cl2
CD3CN
2.6 1.0 × 10−1
1.9 1.1 × 10−1
3.7 1.3 × 10−1
3.0 1.2 × 10−1
4.5 1.1 × 10−1
7.2 0.94 × 10−1
3.8 × 10−2
5.9 × 10−2
3.4 × 10−2
4.0 × 10−2
2.4 × 10−2
1.3 × 10−2
85
40
70
80
140
240
OCH3 CDCl3
CN
NO2
CDCl3
CDCl3
a Reactants at 60 mM in all cases. The reaction progress was monitored
by 1H NMR spectroscopy. b Water content below 1H NMR detection
limits. c The equilibrium constant was calculated from equilibrium
concentrations of reactants and products. All values are the average of
two to three independent runs. Error limits in the order of 20%. d The
known values of K were introduced into eqn (S1) (see ESI) and the
corresponding kf values were obtained by a nonlinear fitting procedure
to the data points. kb values were calculated as kb = kf/K. e Calculated as
Results and discussion
Transimination
The progress of the exchange reactions of eqn (3) and (4) was
monitored by measuring variations with time of integrated
intensities of suitable 1H NMR signals of reactants or products
or both. Experimental data were treated according to the stan-
dard kinetic equation for reversible reactions, second-order in
both directions,6 that was found to fit to a good precision to
rate data for amine–imine exchange reactions in the aliphatic–
aliphatic systems.3 A typical time-concentration profile is
shown in Fig. 1. A good adherence of data points to the kinetic
equation was obtained in all cases. Rate and equilibrium con-
stants are summarized in Table 1 for the reaction of eqn (3)
and in Table 2 for the reaction of eqn (4).
As shown in Table 1, exchange equilibria involving aromatic
amines are slightly biased towards products in all cases
(K > 1), which is tantamount to saying that the methyl substitu-
ent stabilizes the imine product more than the amine reactant.
We suggest that a hyperconjugative interaction, extended to
the aromatic system of the benzylidene moiety (Fig. 2, a), is
responsible for imine stabilization. In line with this sugges-
tion, the definite tendency of the equilibrium constants to
moderately increase in the presence of electron withdrawing
substituents (Table 1, entries 5 and 6) is a result of increased
hyperconjugative interactions arising from through-resonance
with the substituent (Fig. 2, b, c).
[Pdt]t
= [Pdt]∞/2, where Pdt is either aniline or N-benzylidene-
1
2
p-toluidine.
Table 2 Rates and equilibrium constants of imine–amine exchange
between N-benzylidene-p-toluidines and butylamine, eqn (4), in CDCl3
at 25 °Ca,b
kf (M−1 s−1
)
kb (M−1 s−1
)
c (s)
1
t
2
Entry
X
K
1
2
3
4
H
OCH3
CN
35
0.60
1.7 × 10−2
1.4 × 10−3
2.8 × 10−5
3.3 × 10−6
240
1100
11 900
62 000
23
120
240
3.3 × 10−2
3.4 × 10−3
7.9 × 10−4
NO2
a Reactants at 20 mM in all cases. The reaction progress was monitored
by 1H NMR spectroscopy. b For analysis of rate and equilibrium data
see footnotes c and d in Table 1. c Calculated as [Pdt]t = [Pdt]∞/2,
1
2
where Pdt is either p-toluidine or N-benzylidenebutylamine.
Fig. 2 Hyperconjugative interactions in N-benzylidene-p-toluidines.
Participation in through-resonance with the electron withdrawing sub-
stituents is shown in structure (b) and (c).
As to rates of forward reactions, kf values are not only
remarkably independent of the solvent (Table 1, entries 1–3),7
but also insensitive to the nature of para substituents. A vir-
tually identical picture emerges from inspection of backward
rate constants kb, when the modest thermodynamic handicap
suffered by N-benzylidene-p-toluidines bearing NO2 and CN
substituents is taken into account.
Fig. 1 Time-concentration profile of N-(p-methoxybenzylidene)butyl-
amine production from reaction between 20 mM N-(p-methoxybenzyli-
dene)aniline and 20 mM butylamine, in CD3Cl, 25 °C. The curve is a plot
of eqn (S1) (see ESI†) with data from Table 2, entry 2.
Amine–imine exchange is a two-step process, proceeding
via a high energy aminal intermediate (Fig. 3).3 There are two
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Fig. 3 Qualitative energy diagram for transimination reactions of
Table 1.
Fig. 5 Hammett plots for the second-order rate constants of the
forward (kf) and backward (kb) transiminations between butylamine and
N-benzyilidene-p-toluidines of eqn (4). Data from Table 2.
Fig. 6 Hyperconjugation with α C–H and α C–C bonds in N-benzyl-
idenebutylamines.
imine nitrogen (Fig. 6), is more important than in N-benzyl-
Fig. 4 Hammett plot for the equilibrium constants of transimination
reactions between butylamine and N-benzyilidene-p-toluidines (eqn idene-p-toluidines (Fig. 2).
(4)). Data from Table 2.
Table 2 shows that forward (kf) and backward (kb) transimi-
nation rates are strongly retarded both by the electron-releas-
ing OCH3 and electron-withdrawing NO2 and CN. Nonlinear
Hammett plots are well documented,9 but those in which the
plot is concave-down are less common. According to Leffler
and Grunwald,10 a change in the rate-determining step with
otherwise constant mechanism can cause a Hammett plot to
be concave-down. Accordingly, we interpret the break in the
Hammett plots in Fig. 5, featuring concave-down shapes, as
arising from a change in the rate-determining step of the two-
step transimination reaction (Fig. 3). It is likely that the large
difference in basicity–nucleophilicity between butylamine and
p-toluidine causes an unbalance between bond-making and
bond-breaking processes. Although a detailed description of
TS1 and TS2 in terms of fractional charges on the four atoms
undergoing bond changes is outside the scope of this work,
there is no doubt that in terms of bond-making and bond-
breaking processes the roles of the two nitrogen atoms in TS1
are reversed in TS2. If the uneven charge distribution is
accompanied by a change of electron density on the benzyl-
idene carbon of TS1, there will be by necessity a variation of
the electron density of opposite sign in TS2. As a consequence,
substituents will have opposite effects on the chemical poten-
bond-making and two bond-breaking processes occurring sim-
ultaneously in the formation of TS1 from reactants, as well as
of TS2 from products. While the C–N bond between the amine
nitrogen is formed and the CvN double bond is transformed
in a C–N single bond, a proton is transferred from the amine
to the imine nitrogen.8 Thus, the lack of sensitivity to the
solvent and electronic effects of transiminations involving aro-
matic amines, eqn (3), would suggest that the nature of the
four-membered transition states TS1 and TS2 is substantially
apolar, on account of a good balance between bond-making
and bond-breaking processes.
At variance with the behavior of aromatic–aromatic trans-
iminations, rates and equilibrium constants of the mixed
aromatic–aliphatic transiminations listed in Table 2 and
graphically displayed in the form of Hammett plots in Fig. 4
and 5 are markedly dependent on electronic effects. Trans-
imination equilibria are more shifted towards products and K
values are strongly enhanced by electron-withdrawing substitu-
ents, most likely because hyperconjugation in N-benzylidene-
butylamines, occurring with C–H and C–C bonds α to the
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tial of the two transition states, and in any case the transition
state destabilized by the substituent, independent of the elec-
tronic nature of the substituent itself, will become the rate-
limiting transition state. Interestingly, an analogous concave-
down Hammett plot, reported a long time ago for the reaction
between butylamine and substituted benzaldehydes,11 was
rationalized using a similar argument.
Imine metathesis
As shown in a previous paper,3 fast transimination rates can
be utilized in the generation of efficient dynamic systems
based on imine metathesis catalyzed by primary amines under
mild conditions. By way of illustration, let us consider the two
transimination reactions (a) and (b) in Scheme 1, whose
common feature is that the amine reactant in one reaction is a
product in the other, and vice versa. Thus, if aniline is added
to a mixture of A and B, it will react with A to give a mixture of
C and p-toluidine. The latter, in turn, will react with B to give a
mixture of D and aniline. In conclusion, aniline is consumed
in step (a) and restored in step (b), while the opposite holds
for p-toluidine. Thus, as the reactions proceed, C and D build
up at the expenses of A and B, until the metathetic equilibrium
(c) is established as a result of coupled transimination reac-
tions. Imine metathesis is catalyzed by either amine or, more
properly, the mixture of two amines formed in the course of
the reaction.
Experimental data fully consistent with expectations are
shown in Fig. 7. Starting from a mixture of imines A and B
(60 mM each) in CD2Cl2, the metathetic equilibrium (c) was
reached in less than 100 min at 25 °C (curve a) in the presence
of a 1 : 1 mixture of aniline and p-toluidine (9.0 mM each).
Since the equilibrium constant turned out to be very close to
1, the reaction was predicted3,12 and found to obey first-order
time dependence, as shown by the close adherence of data
Fig. 7 Reaction progress as a function of time for amine-catalyzed (a)
and background (b) metathesis between 60 mM N-(p-methoxybenzyl-
idene)p-toluidine and 60 mM N-benzylideneaniline in CD2Cl2 at 25 °C.
Curve (a) is a plot of a first-order equation, curve (b) is a guide to the
eye.
1
points to a first-order equation (t = 14 min). In the absence of
2
added amines the reaction was much slower (curve b). The
time-concentration profile has the sigmoid shape of the logis-
tic curve of autocatalytic reactions. Similar results were
obtained in CDCl3 and CD3OD (see ESI, Fig. S1 and S2†).
A clue to the mechanism of the seemingly spontaneous
metathesis in the control experiment was found in the 1H
NMR spectra of the reaction mixture. Tiny signals of the alde-
hydic hydrogens of benzaldehyde (δ = 10.04) and p-anisalde-
hyde (δ = 9.90), missing at the early stages but clearly visible in
the course of the reaction, revealed the occurrence of slow
hydrolytic processes, eqn (5), caused by reaction with trace
amounts of adventitious water.13 These observations support
the hypothesis that metathesis in the control reaction is actu-
ally catalyzed by the liberated amines. Since the amount of lib-
erated amines increases with increasing reaction time, the
concentration-time profile has the same shape of canonical
autocatalytic processes, in which the catalyst is a product of
the main reaction, rather than of an accidental side-reaction.
ð5Þ
ð6Þ
Analogous results were obtained for the metathesis reaction
in the aromatic–aliphatic case (Fig. 8). The reaction of N-p-
(methoxybenzylidene)p-toluidine with N-benzylidenebutyl-
amine (60 mM each) in CDCl3 at 25 °C, eqn (6), was strongly
accelerated by an equimolar mixture of p-toluidine and butyl-
Scheme 1 Transimination reactions (a) and (b) and imine metathesis (c)
catalyzed by primary amines.
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carried out in the NMR tube in the thermostated probe of the
spectrometer as previously reported.3
Materials
All reagents and solvents were purchased of the highest com-
mercial quality and were used without further purification,
unless otherwise stated. Deuterated halogen solvents were
flashed through basic alumina immediately prior to use,
while deuterated acetonitrile and methanol were first dried
over activated molecular sieves (4 Å) and then flashed through
basic alumina immediately prior to use. Amines were freshly
distilled before use.
General procedure for the preparation of the imines
Fig. 8 Reaction progress as a function of time for amine-catalyzed (a)
and background (b) metathesis between 60 mM N-(p-methoxybenzyl-
idene)p-toluidine and N-benzylidenebutylamine in CDCl3 at 25 °C. The
curves are guides to the eye.
According to a literature procedure,16 an equimolar solution of
amine and aldehyde in benzene was refluxed for 4 h in Dean–
Stark apparatus. The solution was then concentrated
under vacuum and the resulting imine was distilled twice
under reduced pressure, in order to eliminate any trace of the
amine (9.0 mM each), the equilibrium being reached in less
than 2 h (curve a). When a single amine (18 mM), either
p-toluidine or butylamine, was used as a catalyst, similar
equilibration rates were observed (see ESI Fig. S3†), because in
either case the reaction was eventually catalyzed by the same
mixture of amines. The sigmoid shape of the time-concen-
tration profile of the control reaction (curve b) was again
dictated by the liberation of minute amounts of amines pro-
duced by reaction of imines with adventitious water in the
nominally anhydrous CDCl3. Consistently, trace amounts of
benzaldehyde and p-anisaldehyde were detected in the course
of reaction as tiny 1H NMR signals in the aldehydic proton
region.
1
corresponding amine. H NMR data of isolated imines were in
agreement with literature data.16,17
Kinetics
All kinetic experiments were run in duplicate or triplicate.
Kinetic runs were carried out on equimolar solutions of the
substrates (20 mM in aliphatic amine–aromatic amine trans-
iminations, 60 mM in all other experiments). The calculated
amount of substrates was added to 600 μL of the deuterated
solvent in the thermostated NMR tube (T = 25 °C). In imine
metathesis experiments, the calculated volume of a 0.10 M
solution of the amine catalyst was added to the reaction
mixture. Spectra were recorded at time intervals. The time
elapsed from the reaction start to recording of the first 1H
NMR spectrum was typically 3–4 minutes. This time interval
was necessary to allow thermal equilibration in the probe of
the spectrometer after addition of the reactant, and shimming
for field optimization. Product formation was monitored by
measuring the area of the chosen 1H NMR signal as a function
of time. For transiminations, experimental data were fitted to a
standard kinetic equation for reversible reactions, second-
order in both directions (eqn (S1), see ESI†).6 Data of experi-
ments on imine metathesis between aromatic imines were
fitted to a first-order kinetic equation.
Conclusions
In this work we have shown that amine–imine exchange reac-
tions based on aromatic–aromatic and aromatic–aliphatic
amines occur smoothly at ambient temperature in organic
solvents under non-acidic conditions. When combined with
our previous results based on amine–imine exchange reactions
of the aliphatic–aliphatic type,3 the present work shows that
imine metathesis catalyzed by substoichiometric amounts
of primary amines of any kind has the potential for the
implementation of effective dynamic combinatorial libraries
based on exchange of imine bonds in organic solvents in the
absence of proton14 or metal catalysis.15
Acknowledgements
Thanks are due to the Ministero dell’Istruzione, dell’Università
e della Ricerca (MIUR, PRIN 2010CX2TLM) and the Consiglio
Nazionale delle Ricerche (CNR) for financial support. This
Experimental section
Instruments and general methods
1H–NMR spectra were recorded on either a 200 or a 300 MHz work was also partially supported by the Dipartimento di
spectrometer. The spectra were internally referenced to the Chimica, Sapienza Università di Roma through the Supporting
residual proton solvent signal. Kinetic experiments were Research Initiative 2013.
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8 The possible involvement of a 6-membered cyclic transition
state in which an additional amine molecule acts as a
proton relay agent is ruled out by the fact that the reaction
was found to be strictly first-order in the amine concen-
tration (see ref. 3).
9 T. H. Lowry and K. S. Richardson, Mechanism and Theory in
Organic Chemisrty, Harper and Row Publisher, New York,
3rd edn, 1987, p. 149.
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