Approach Matters: The Kinetics of Interfacial Inverse-Electron Demand Diels–Alder Reactions

Article abstract of DOI:10.1002/chem.201703103

Rapid and quantitative click functionalization of surfaces remains an interesting challenge in surface chemistry. In this regard, inverse electron demand Diels–Alder (IEDDA) reactions represent a promising metal-free candidate. Herein, we reveal quantitative surface functionalization within 15 min. Furthermore, we report the comprehensive effects of substrate stereochemistry, surrounding microenvironment and substrate order on the reaction kinetics as obtained by surface-bound mass spectrometry (DART-HRMS).

Full text of DOI:10.1002/chem.201703103

A Journal of
Accepted Article
Title: Approach Matters: The Kinetics of Interfacial Inverse-Electron
Demand Diels-Alder Reactions
Authors: Han Zuilhof, Rickdeb Sen, Digvijay Gahtory, Jorge
Escorihuela, Sidharam P. Pujari, 1 P. Pujari, 1 Pujari, and
Judith Firet
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To be cited as: Chem. Eur. J. 10.1002/chem.201703103
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Approach Matters: The Kinetics of Interfacial Inverse–Electron
Demand Diels–Alder Reactions
Rickdeb Sen,‡,a Digvijay Gahtory,‡,a Jorge Escorihuela,a Judith Firet,a Sidharam P. Pujari,a and Han
Zuilhof*,a,b,c
Abstract: Rapid and quantitative click functionalization of surfaces
remains an interesting challenge in surface chemistry. In this regard,
inverse electron demand Diels Alder (IEDDA) reactions represent a
promising metal-free candidate. Herein, we reveal quantitative
surface functionalization within 15 min. Furthermore, we report the
comprehensive effects of substrate stereochemistry, surrounding
microenvironment and substrate order on the reaction kinetics as
obtained via surface–bound mass spectrometry (DART–HRMS).
but less reactive reactant such as norbornene.[11] In studies of
click reactions at surface, both metal-catalyzed and metal-free a
recurring but hitherto largely unresolved issue is the question
whether it matters which component is better to be surface–bound
or better to be in solution, i.e. in this case norbornene(surf.)
+
tetrazine(soln.) versus tetrazine(surf.) + norbornene(soln.). For example,
for both interfacial strain-promoted azide–alkyne cycloadditions[12]
and surface-bound thiol-ene click reactions[13] the choice of the
surface–bound reaction partner, i.e. overall reaction orientation,
is important, but published results are inconclusive in this regard.
We have recently shown that the microenvironment around
the reactive site on the surface plays an important role in the
kinetics of strain–promoted click reactions, as determined by
highly accurate and facile rate studies using direct analysis in real
time–high resolution mass spectrometry (DART–HRMS).[14]
Those studies allowed and stimulated us to investigate whether
the microenvironment for the interfacial IEDDA using the more
stable norbornene could be optimized to further improve the
reaction rates, and possibly direct the yield of surface–bound
IEDDA reactions towards 100%.
Introduction
The excellent kinetics, high yields, lack of by–products and high
stereoselectivity of click reactions have led to their extensive use
in total synthesis,[1] and biorthogonal[2] and site–specific[3,4]
labelling of biomolecules. The utility of these reactions for surface
modification has blossomed in recent times as evident from a
significant number of reports,[5] specifically their use in
biomolecular attachment and patterning.[6] Crucial for surface
modification are the rate of reactions (for effective
implementation) and complete conversion of the surface–bound
moiety to the product of interest. The latter aspect is of importance,
as surface–bound moieties cannot be removed afterwards by the
standard purifications techniques so central to solution–phase
chemistry (column chromatography, HPLC, etc.).
Scheme 1. a) Overall tetrazine–norbornene IEDDA reaction and b) schematic
depicting the three parameters (in parentheses) under current study.
a)
In these regards, the inverse electron demand Diels–Alder
reaction (IEDDA) between 1,2,4,5–tetrazine and strained
alkenes/alkynes holds great promise.[7] IEDDA reactions have
been extensively studied in solution with particular focus on tuning
the steric and electronic effects of a wide variety of dienes and
dienophiles.[8] Such studies indicated highly interesting features
such as very fast reaction kinetics[8a,9] (among the highest for
metal–free click reactions) and high chemoselectivity.
Surprisingly, this facile reaction has been largely underexplored
for surface functionalization with a few examples in literature
using the highly reactive, but somewhat unstable trans–
cyclooctene (TCO) reacting with tetrazine,[2,10] or a more stable
Tag
Tag
b)
Buried or free ME
Stereochemistry
vs
[Tetrazine (surf.)
Norbornene (soln.)]
[Norbornene (surf.)
Tetrazine (soln.)]
[a]
R. Sen‡, D. Gahtory‡, Dr. J. Escorihuela, J. Firet, Dr. S. P. Pujari,
and Prof. Dr. H. Zuilhof
Laboratory of Organic Chemistry, Wageningen University and
Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands
Prof. Dr. H. Zuilhof
School of Pharmaceutical Sciences and Technology, Tianjin
University, 92 Weijin Road, Tianjin, P.R. China
Prof. Dr. H. Zuilhof
Department of Chemical and Materials Engineering, King Abdulaziz
University, Jeddah, Saudi Arabia
E–mail: han.zuilhof@wur.nl
Given the high signal/noise (S/N) ratio of DART–HRMS, for
example: about two orders of magnitude better than XPS, we are
in this study for the first time able to systematically tune three
aspects relevant for surface–bound organic conversions in detail
(Scheme 1): surface–induced sterics (are the groups buried in a
monolayer, or sticking out above it), stereochemistry (endo/exo–
norbornene, as a means to investigate the effects of approach of
the reactant in solution) and orientation (which reactant is in the
solvent and which one surface–bound).[15] Optimization of these
factors yielded conditions in which the reaction is quantitative and
complete within 15 min, showing the potential of both this
systematic approach and of this reaction.
[b]
[c]
[‡]
These authors contributed equally to this work. Supporting
information for this article is given via a link at the end of the
document
Dedicated to dr. Gerrit Lodder, on the occasion of his 77th birthday.
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Scheme 2. General scheme for the surface modification followed by interfacial IEDDA reaction.
.
by modifying the lengths of the surrounding alkyl chains, so as to
bury it in the monolayer (4 carbon atoms below the surface), or
make it stick out (4 carbon atoms above the surface). The
monolayer composition was confirmed by the N/P ratio (1:4) in
XPS wide scans (Figure S5.1 in the Supporting Information). In
order to obtain a better understanding of surface coverage, a
molecular mechanics study was performed that studied the
average packing energy per chain in dependence of the packing
density.[16] This was performed by creating schematic models of
varying degrees of alkyl monolayer attachment with different
random attachment at the available sites followed by optimization
of the corresponding molecular models.
Results and Discussion
To this end, first the surface of aluminum (Al) slides with its
natural aluminum oxide coating were covalently modified by
phosphonic acid–based monolayers via a well–established
methodology.[5a] For this study, we prepared 3:1 alkyl to amine–
terminated monolayers (M1; see Scheme 2), as previous studies
indicated this to be the optimum between high density of
functionalization and reactivity. In our experience, increasing the
dilution of the reactive sites in the monolayers higher than 33%
leads to an overall decreased reaction kinetics and reaction
yields.[14] The microenvironment of the reacting groups was tuned
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a)
b)
DART gun
M4
M7/M8
F1s
Ionized tag
700
690
680
670
BE (eV)
C1s
m/z 189.016
P2s
N1s
M1
M2/M3
M5/M6
189.0165
C8 H4 O2 F3
F1s
F1s
100
90
80
70
60
50
40
30
20
10
0
M4
M7/M8
750 600 450 300 150
Binding Energy (eV)
0
189.015
189.020
Sample
Figure 1. a) XPS wide range spectra for M1–M8 surfaces showing emergence of F1s signal after the interfacial IEDDA reaction. b) Schematic impression of ionization
of MS-tag (m/z 189.0163) by DART-HRMS.
For more detailed understanding of the randomization process,
see section 7.1-7.9 in the supporting information We found that
a)
1.2
roughly 50% coverage corresponded to the lowest packing
energy per molecule (Supporting Information, section 7.10 and
1.0
0
0.8
0.6
0.4
0.2
0.0
7.11). This means that ideally about half the surface sites
available result in monolayer attachment.
k2 = 1.90 ± 0.09 M-1s-1
-1
-2
-3
-4
Norbornene surfaces (M2/M3) were prepared by coupling 5–
norbornene–2–methanol (exo–/endo–) to M1 surfaces by a
carbamate linkage (Scheme 2 and Figure S4.4, S5.2 and S5.3).
For preparation of tetrazine surfaces (M4), we chose
unsymmetrical tetrazine with benzyl amine and methyl
substitution, again via a carbamate linkage to ensure the same
freedom/buried nature as in M2/M3. As shown by the groups of
Hilderbrand and Chen, unsymmetrical tetrazines provide a better
balance between stability and reactivity than their symmetric
counterparts,[8b,17] while also ensuring enough rigidity on a surface.
Complete surface attachment for M4 surfaces was obtained for
this reaction as confirmed by N/P ratios (1.5 ± 0.1) in XPS analysis
(Figure S5.4 in the Supporting Information). After confirmation of
surface attachment of norbornenes and tetrazines, respectively,
we embarked on exploring the IEDDA reaction. For easy analysis
of the reaction progress, we synthesized compounds with
fluorinated tags 3, 4 and 6 that facilitate analysis by both XPS and
DART–HRMS. The reactions were performed for both the
reaction partners under stringently similar conditions (Scheme 2)
and followed by monitoring the F/P ratio in XPS wide spectra
(Figure 1). Interestingly, we found that for the “free” M2, M3 and
M4 systems, a quantitative surface reaction yield was obtained
within 15 min irrespective of the orientation of the reaction (Figure
S5.5 and S5.6 in the Supporting Information). In case of the
“buried” system, the crowded microenvironments around the
k2 = 3.62 ± 0.18 M-1s-1
0
4
8
12
Time (min)
0
5
10
Time (min)
15
20
b)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
k2 = 0.68 ± 0.07 M-1s-1
-1
-2
-3
-4
k2 = 1.56 ± 0.07 M-1s-1
0
5
10
15
Time (min)
0
10
20
30
40
50
60
Time (min)
Figure 2. Normalized DART–HRMS intensity vs time (min) for IEDDA in a “free”
microenvironment: a) exo–Norbornene surfaces (M2) reacting with 3 (red) and
tetrazine surfaces (M4) reacting with 4 (blue). b) endo–Norbornene surfaces
(M3) reacting with 3 (red) and tetrazine surfaces (M4) reacting with 6 (blue).
Inserts: Linear plots of ln [(I – It)/(I – I0)] vs time (min) to obtain the pseudo-
first order constants, and using solute concentrations of 3.0 mM the
subsequently derived 2nd order rate constants (k2).
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Surface group
Reactant in solution
Tetrazine
Free
3.62 ± 0.18 M-1s-1 (exo-)
0.87 ± 0.06 M-1s-1 (exo-)
1.56 ± 0.07 M-1s-1 (endo-)
0.57 ± 0.08 M-1s-1 (endo-)
Norbornene
Tetrazine
Buried
Norbornene
Free
1.90 ± 0.09 M-1s-1 (exo-)
0.58 ± 0.08 M-1s-1 (exo-)
0.68 ± 0.07 M-1s-1 (endo-)
0.61 ± 0.05 M-1s-1 (endo-)
Buried
Table 1. Second-order rate constants k2 (M-1s-1) of the tetrazine–norbornene IEDDA reaction in “free” and “buried” microenvironments.
reactive sites slowed the reaction down, yielding a slightly lower
reaction yield (~80%) after 15 min for “buried” exo–
norbornene(surf.) + tetrazine(soln.) system (Figure S5.10), but >90 %
constant (k) was calculated as the slope of the plot of ln|(퐼푡 −
퐼)/(퐼0 − 퐼)| versus time (t), where I corresponds to the
asymptotic integrated extracted ion chronogram (EIC) intensity at
obtained by exponential decay curve fitting of the data
(Supporting Information, Figure S3.1–S3.8); the second-order
rate constants were derived from there. The highest second-order
reaction rate constant (3.62 M–1s–1 at a solute concentration of 3.0
mM, 30 °C, DCE) was observed for exo–norbornene(surf.) with
tetrazine(soln.) 3 in a “free” microenvironment (Figure 2). Using the
inverse situation, tetrazine on a surface surrounded by lower alkyl
chains reacting with exo–norbornene 4 in solution, afforded a
two–fold slower rate (Figure 2).
conversion was
also observed for
such crowded
microenvironments after 1 h (Figure S5.11). See the supporting
information for a more detailed description of the different
calculations used for reaction quantification using XPS wide and
narrow spectra.
To obtain accurate reaction kinetics, we reacted our samples
for different time intervals (up to 20 min) and followed the signal
intensity of the corresponding MS–tag (m/z 189.016) in DART–
HRMS. Using such high S/N data, the pseudo-first–order rate
a)
b)
16.3 (endo-)
15.2 (exo-)
-17.4
-18.8
16.3 (endo-)
15.2 (exo-)
N2 loss
reaction coordinate
-2.6 (endo-), syn
-4.0 (exo-), syn
0.0
c)
15.1 (endo-)
14. 2(exo-)
-10.2(endo-), anti
-11.7 (exo-), anti
-18.8 (endo-)
-17.4 (exo-)
syn-
anti-
-57.0 (endo-)
-58.0 (exo-)
-18.8
-19.2
-60.0 (endo-)
-61.1 (exo-)
reaction coordinate
reaction coordinate
Figure 3. a) DFT calculation for the full mechanism of the multistep IEDDA reaction between an unsymmetrical tetrazine and exo– and endo–norbornene mimicking
molecules used in our work, b) and c) reaction coordinate diagram for the IEDDA reaction mimicking our reaction condition showing tetrazine and exo–/endo–
norbornene on surface, respectively.
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Hindered
Unhindered
Aggregation
a)
b)
Figure 4. Comparison of surface disposition of IEDDA reactants in “free” ME after molecular dynamics of a) norbornene(surf.) and b) tetrazine(surf.)
.
The IEDDA reaction efficiency can be determined by
comparing the atomic ratio determined by XPS with the theoretical
value of 3:4 (for F/P), which corresponds to the 100% surface
conversion. Interestingly, recently we reported the first 100%-
yielding surface-bound metal-free click reaction (a strain–
promoted cyclooctyne–quinone or SPOCQ reaction), which was
further characterized by a second order rate constant (k) of
3.3 M–1s–1.[14a] While thus displaying virtually the same rate, in that
case, we observed two distinct kinetic regimes: an initial fast
regime followed by a slower, more complex one; the rate constant
refers to the initial well–behaved kinetic regime only. The SPOCQ
reaction was eventually also quantitative, but reaches full
conversion only in 4 h. Here, the IEDDA reaction turned out to be
a significant improvement, reaching an unprecedented complete
conversion within 15 min, while the entire kinetic regime could be
followed using one exponent, i.e. the IEDDA reaction displays
clean kinetics.
In addition, excellent yields were obtained both in ‘free’ and
‘buried’ conditions, and independent of the orientation of the
reaction, thereby showing the scope of this strategy for surface
modifications. Changing the stereochemistry of the immobilized
norbornene to endo– in a “free” microenvironment halved the
reaction rate (Figure S3.3) which is consistent with the rate
differences observed in solution.[9b] Interestingly, also in this case
a kinetic preference (two-fold difference) was observed for
immobilized norbornene reacting with tetrazine in solution than its
reverse, i.e. tetrazine on the surface reacting with endo–
norbornene 6 in solution (Table 1). This result outlines that the
slowing down of reaction rate for tetrazine immobilization occurs
irrespective of norbornene stereochemistry.
(0.87 M-1s–1) was observed for exo–norbornene attached on the
surface reacting with tetrazine while the lowest (0.58 M–1s–1) was
observed for its reverse. Comparing the best and worst possibility,
the rate constant for the exo–norbornene immobilization in “free”
microenvironment was 6.2 fold higher than that for tetrazine
immobilization in a “buried” state. It is of relevance to state that
only with high S/N-techniques like DART–HRMS such relatively
small rate differences can come into view and thus be rationalized.
To this latter aim, we also applied quantum chemical and
molecular mechanics calculations. The reaction between
unsubstituted[18] and substituted tetrazines[19] and various alkenes
has been studied theoretically by Houk, Devaraj and co–workers.
In our case, we modeled the IEDDA reaction between a
substituted tetrazine and exo–/endo–norbornene (substituted so
as to mimic their surface attachment or solution functionalities,
see Figure 3). In line with previous findings, density functional
theory (DFT) calculations (at M06-2X/6–311+G(d,p) level)
revealed that the Diels−Alder cycloaddition, rather than the
subsequent N2 expulsion, is the rate–determining step in this
reaction. In accordance with experimental results, exo–
norbornene has a lower activation barrier than endo–norbornene
(Figure 3a). Also, our calculations show that the reaction
corresponding to norbornene(surf.) (exo– and endo–) and
tetrazine(soln.) had a lower energy barrier (by ~1 kcal mol–1) than
the reverse orientation (Figure 3b and c). This energy difference
is in line with the observed rate difference for the interfacial IEDDA
in the “free” microenvironment.
A visual representation of the orientation of immobilized groups
was obtained by performing sequential molecular dynamics and
molecular mechanics optimizations of the IEDDA cycloadducts on
aluminum oxide surface. The modelling was performed on large
supercells obtained by attachment of the norbornene or tetrazine
moieties in a random pattern followed by subsequent energy
minimization (see section 7.13–7.14 in the Supporting
Information). Next, a series of molecular dynamics runs at 773.0
K were performed to ‘shake up’ the conformations of the chains,
and get truly random orientations of the surface-bound moieties.
Finally, those geometries were optimized by molecular mechanics,
Finally, we wanted to know the kinetic effects of doing the
reaction ‘above’ the monolayer versus ‘within’ the monolayer.
Therefore we studied the reaction in a “buried” state: surfaces
were prepared with either norbornene or tetrazine moieties bound
to the surface that are surrounded by long alkyl chains, and used
for IEDDA reaction (Scheme 2). The rate differences again
amounted to about two folds in favor of norbornene immobilization
(Table 1). The highest reaction rate in this microenvironment
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to indicate the most stable orientations of reactive groups with
respect to the surface (see section 7.15–7.19 in the Supporting
Information and two movies with rotating 3D structures of the
surface). Representative geometries, from a much large set, are
shown in Figure 4. For surface–bound norbornenes (Figure 4a),
the surface-bound molecules prefer to be apart (no specific
attractions; significant steric repulsions) with the double bond
(highlighted in green) facing outwards. In contrast, tetrazines(surf.)
in a “free” microenvironment showed a higher propensity to
clump/cluster together due to additional stabilization attributable
to – stacking (Figure 4b). These orientations and clustering (or
lack thereof) will influence the reactivity and angle of approach of
any reacting solute.
available by combining techniques like XPS and DART-MS to
further improve surface modification procedures for a wide range
of applications.
Experimental Section
Materials. Unless otherwise specified, all chemicals were used as
received
without
further
purification.
Octylphosphonic
acid,
hexadecylphosphonic acid, 1,1'–carbonyldiimidazole (CDI), exo–5–
norbornene–2–methanol, hydrochloric acid, methanol, hexane, acetone,
dichloromethane (CH2Cl2), 1,2–dichloroethane (DCE) and 2–propanol
were purchased from Sigma–Aldrich. 12–Aminododecylphosphonic acid
hydrochloride salt and 6–aminohexylphosphonic acid hydrochloride salt
were purchased from SiKÉMIA. Aluminium pieces (99.5% purity, mirror
polished, Staalmarkt Beuningen BV) were cut using mechanical cutter into
exactly 2 × 1 cm dimension. For surface modification reactions, the
samples were loaded onto a specially constructed PTFE wafer holder able
to hold up to 16 samples at a time thus ensuring rigorous reproducibility
between samples.
a)
b)
X–ray Photoelectron Spectroscopy (XPS) Measurements. The XPS
analysis of surfaces was performed using a JPS–9200 photoelectron
spectrometer (JEOL, Japan). Survey and high–resolution spectra were
obtained under UHV conditions using monochromatic Al Kα X–ray
radiation at 12 kV and 20 mA, and an analyzer pass energy of 50 eV for
wide scans and 10 eV for narrow scans. The emitted electrons were
collected at 10° from the surface normal (take–off angle relative to the
surface normal 10°). All XPS spectra were evaluated by using Casa XPS
software (version 2.3.15). Survey spectra were corrected with linear
background before fitting, whereas high–resolution spectra were corrected
with linear background. Atomic area ratios were determined after a
baseline correction and normalizing the peak area ratios by the
corresponding atomic sensitivity factors (1.00 for C1s, 1.80 for N1s, 2.93
for O1s, 4.43 for F1s, 1.18 for P2s and 0.75 for Al2s).
Figure 5. DFT calculation of approach of a) Norbornene(surf.) with tetrazine(soln.)
and b) vice–versa, clearly showing different angles of approach to surface
groups.
Based on the TS geometry obtained by DFT calculations we
deduce that the approach of tetrazine(soln.) moieties preferentially
occurs in a “top–down” manner or perpendicular to the surface
(Figure 5a). In contrast, the approach of the incoming
norbornene(soln.) should be “side–ways” or parallel to the surface
(Figure 5b), and the reactant in solution thus encounters a lot
more steric hindrance along the reaction path. In addition, the
aggregation of surface-bound tetrazines also lowers the available
number of tetrazines in statistical terms by masking one or both
the available faces. Both these factors will likely contribute to the
overall slower kinetics of tetrazine(surf.) + norbornene(soln.) reaction.
These findings point to the overall relevance of choosing prior to
immobilization which agent is surface-bound and which comes in
from solution.
DART–HRMS Measurements. Analysis of the modified mica surfaces
were performed using a DART–SVP ion source (Ion–Sense, Saugus, MA,
USA) coupled to a Q–Exactive orbitrap high–resolution mass spectrometer
(Thermo Fisher Scientific, San Jose, CA, USA), mounted on a motorized
rail travelling at 0.2 mm/s. Thermo Scientific Xcalibur software
(V2.1.0.1139) was used for data acquisition and processing. The
measurements were performed in negative mode at 450 °C using a scan
range of m/z 188.6–189.4, a mass resolution of 70000 (FWHM) at a scan
rate of 1 Hz. The ion trap was tuned with 0.1 mg/mL methanol solution of
quinine (m/z 323.41 in negative mode) and optimized. The DART source
was positioned 6.1 cm on the horizontal scale, 7 cm on the vertical scale
with an angle of 45°, such that it is around 1 mm above the surface (Fig.
S4.1). The distance from the surface to the ceramic tube is minimized by
placing them at the edge of the moving rail so that maximum of the (4–
trifluoro)methyl benzoate ion (m/z 189.016 respectively) would enter the
MS.[14]
Conclusions
Computational Procedures. All of the DFT calculations reported herein
were carried out using Gaussian09.[20] All geometries were fully optimized
using the M06–2X functional[21] and the 6–311+G (d,p) basis set, which
has been found to give relatively accurate energetics for
cycloadditions.[18,22] Analytical frequencies were calculated at this level in
all cases, and the nature of the stationary points was determined in each
case according to the proper number of imaginary frequencies. The
intrinsic reaction coordinate (IRC) path was traced to check the energy
profiles connecting each transition state to the two associated minima of
the proposed mechanism.[23] Initially, a Monte Carlo conformational search
using conformer distribution option available in Spartan’14 was used
(Wavefunction, Inc., Irvine, CA, USA, 2014.). With this option, a search
We have achieved an expeditious (within 15 min) and
quantitative surface functionalization using inverse electron
demand Diels–Alder reactions. The reaction displays clean
pseudo-first order kinetics over the full conversion range. We
found that the approach of the solution reactant towards the
surface–bound counterpart plays an important role in the course
of the reaction, supporting the significance of the orientation in
surface-bound reactions. Such detailed insights into surface-
bound organic reactions are both required and becoming
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without constraints was performed for every structure. The torsion angles
were randomly varied and the obtained structures were fully optimized
using the MMFF force field. Thus, different minima of energy within an
energy gap of 10 kcal·mol–1 were generated. These structures were
analyzed and ordered considering the relative energy, being the repeated
geometries eliminated. In all cases, molecules with the lowest energy and
an energy gap of 4.0 kcal·mol–1 were selected and studied quantum
chemically.
MHz, CDCl3), δ 29.6, 38.1, 41.6, 43.72, 45.0, 69.6, 122.3, 125.4, 130.0,
133.7, 134.52, 136.1, 137.0, 165.4.
Synthesis of ((1S,2S,4S)–bicyclo[2.2.1]hept–5–en–2–yl)methanol, 5.
Commercially available
mixture
of
endo–
and
exo–
5–
norbornenecarboxylic acid, was subject to column chromatography
(heptane : EtOAc = 1: 4) to obtain the endo–norbornene carboxylic acid
only. (1S,2S,4S)–bicyclo[2.2.1]hept–5–ene–2–carboxylic acid (206.6 mg,
1.5 mmol) was mixed with LiAlH4 (113.5 mg, 3.0 mmol) in 20mL dry ether
for 2 h. The reaction was quenched with water and the organic product
was extracted into ether followed by recovery of the product by evaporating
the solvent under reduced pressure. Yield= 0.15 g (81%). 1H–NMR (400
MHz, CDCl3 ) δ 6.15 (dd, J = 5.7, 3.0 Hz, 1H), 5.97 (dd, J = 5.7, 2.9 Hz,
1H), 3.40 (dd, J = 10.4, 6.5 Hz, 1H), 3.26 (dd, J = 10.4, 8.9 Hz, 1H), 2.93
(s, 1H), 2.81 (s, 1H), 2.29 (ddt, J = 9.2, 6.6, 4.5 Hz, 1H), 1.82 (ddd, J =
11.6, 9.2, 3.8 Hz, 1H), 1.48 – 1.41 (m, 1H), 1.27 (d, J = 8.2 Hz, 1H), 0.55
– 0.49 (m, 1H).
Synthesis of 3–methyl–6–phenyl methylamine–1,2,4,5–tetrazine,1.
Metal–catalyzed tetrazine synthesis described elsewhere [24] was applied
to synthesize 3–methyl–6–phenyl methylamine–1,2,4,5–tetrazine. Briefly,
4–(aminomethyl)benzonitrile hydrochloride (1.50 g, 8.9 mmol), Ni(OTf)2
(1.59 g, 4.45 mmol), and acetonitrile (4.7 mL, 89 mmol) were added in a
250 mL round flask, followed adding 60% NH2NH2·H2O (30 mL), under N2
flow. The mixture was stirred at 60 °C for 16 h and allowed to cool to room
temperature. Sodium nitrite (14.6 g, 212 mmol) in 25 mL of water was
slowly added to the reaction followed by 5M HCl until gas evolution ceased
(pH 3), in an ice bath. The product was extracted into ethyl acetate and
purified by silica column chromatography (MeOH:DCM = 1:9) as a red solid.
Synthesis of ((1S,2S,4S)–bicyclo[2.2.1]hept–5–en–2–yl)methyl 4–
(trifluoromethyl)benzoate, 6. 4–(Trifluoromethyl)benzoic acid (0.43 g,
2.25 mmol), DMAP (0.024 g, 0.19 mmol), 4 mL DMF, DIC (0.3 mL, 1.93
mmol) and ((1S,2S,4S)–bicyclo[2.2.1]hept–5–en–2–yl)methanol, 5, (0.19
mL, 1.61 mmol), were successively added in a 100 mL rounded flask, and
allowed to react for 24 h. DMF was removed using a rotary evaporator
under reduced pressure. The mixture was extracted with EtOAc and
organic phase was washed by brine, dried by sodium sulfate. The product
was purified using silica column chromatography (EtOAc : Heptane = 1: 4)
1
Yield: 0.56 g (27%). H–NMR (400MHz, DMSO-d6), δ 2.99 (3H, s), 4.15
(2H, s), 7.71 (2H, d, J=8Hz), 8.49 (2H, d, J=8Hz). 13C–NMR (101MHz,
DMSO–d6), δ 20.8, 42.3, 127.6, 129.5, 131.8, 139.2, 162.3, 167.2.
Synthesis of 3–methyl–6–phenyl methanol–1,2,4,5–tetrazine 2. 3–
methyl–6–phenyl methanol–1,2,4,5–tetrazine was synthesized as per
protocol described elsewhere.[24] Briefly, 4–cyanobenzylachohol (0.51 g,
3.75 mmol), Ni(OTf)2 (0.67 g, 1.88 mmol) and acetonitrile (1.97 mL, 37.6
mmol) were added in a 100 mL round flask, followed adding 60%
NH2NH2·H2O (12 mL), under N2 flow. The mixture stirred at 60 °C for 24 h
and allowed to cool to room temperature. Sodium nitrite (6.35 g, 92.0
mmol) in 14 mL water was slowly added to the reaction followed by careful
addition of 5 M HCl until gas evolution ceased. (pH 3), in an ice bath with
stirring. The product was extracted with ethyl acetate and organic phase
was dried by sodium sulfate. The product was purified using silica column
chromatography (EtOAc : Heptane = 1:2.8) as a red solid. Yield = 0.33 g
(41 %). 1H–NMR (400 MHz, CDCl3), δ 3.08 (3H, s), 4.82 (2H, d), 7.57 (2H,
d, J=8Hz), 8.55 (2H, d, J=8Hz). 13C–NMR (101 MHz, CDCl3), δ 21.1, 64.7,
127.4, 128.1, 130.9, 145.6, 163.9, 167.2.
1
as white solid. Yield = 0.14 g. (33%) H–NMR (400 MHz, CDCl3), δ 8.17
(d, J = 7.6 Hz, 2H), 7.72 (d, J = 7.5 Hz, 2H), 6.22 (s, 1H), 6.01 (s, 1H), 4.21
– 4.07 (m, 1H), 3.96 (t, J = 9.9 Hz, 1H), 2.98 (s, 1H), 2.87 (s, 1H), 2.57 (s,
1H), 1.93 (t, J = 10.1 Hz, 1H), 1.51 (d, J = 7.9 Hz, 1H), 1.32 (d, J = 7.8 Hz,
1H), 0.67 (d, J = 10.2 Hz, 1H). 13C–NMR (100 MHz, CDCl3), δ 165.30,
137.81, 134.50, 134.18, 133.78, 132.07, 129.94, 125.35, 125.01, 125.01,
122.30, 68.91, 49.44, 43.99, 42.24, 37.89, 29.69, 28.98.
Preparation of phosphonic acid monolayers. 2×1 cm Al slides were
sonicated in hexane for 15 min followed by wiping with lint–free cotton
swabs (Texwipe, NC, USA) to remove the polymer protection layer on top
and remove any residual glue. The surfaces were chemically activated by
immersion in 1:1 (v/v) 37% HCl–MeOH mixture for 5 min, followed by
washing with copious amounts of water and 2–propanol. The activated
surfaces were then immersed into N2 filled vials of solution of
octylphosphonic acid (1.5 mM) and 12–aminododecylphosphonic acid
Synthesis
of
4-(6-methyl-1,2,4,5-tetrazin-3-yl)benzyl
4-
(trifluoromethyl)benzoate, 3. 4–(Trifluoromethyl)benzoic acid (0.27 g,
1.38 mmol), DMAP (0.01 g, 0.123 mmol), 2.5 mL DMF, DIC (0.18 mL, 1.18
mmol) and compound 2 (0.21 g, 0.98 mmol) were successively added and
allowed to react at room temperature for 17 h. DMF was evaporated by
rotatory evaporation under reduced pressure. The mixture was extracted
with EtOAc and organic phase was washed with brine, dried with sodium
sulfate and concentrated by rotatory evaporation to yield an oil, which was
purified using silica column chromatography (EtOAc : Heptane = 1: 4) to
hydrochloride salt (0.5 mM) for “free” ME (m
= 7, n = 3);
hexadecylphosphonic acid (1.5 mM) and 6–aminohexylphosphonic acid
hydrochloride salt (0.5 mM) for “buried” ME (m = 1, n = 11) mixture in 2–
propanol, heated to 50 °C for 5 min, and then left undisturbed for 5 h at
room temperature to obtain self–assembled mixed monolayers. Then
surfaces were taken out and sonicated successively for 5 min with 2–
propanol, acetone and CH2Cl2. The surfaces were finally cleaned with
CH2Cl2, air dried and stored under N2 atmosphere. From static water
contact angle (SCA) measurements, it was found that the reaction was
complete after 5 h, yielding monolayers with 28–30% C as determined by
XPS. Substantially longer reaction times (16 h) contributed to the formation
of undesirable multilayers (42–44% C1s in XPS wide scans).
1
obtain a red solid. Yield = 0.22 g (57 %). H–NMR (400 MHz, CDCl3), δ
3.11 (3H, s), 5.51 (2H, s), 7.67 (2H, d, J=8Hz), 7.73 (2H, d, J=8Hz), 8.22
(2H, d, J=8Hz), 8.63 (2H, d, J=8Hz).13C–NMR (100 MHz, CDCl3), δ 21.2,
66.5, 125.5, 128.3, 128.4, 128.7, 130.2, 131.9, 132.5, 133.1, 140.2, 163.8,
165.1, 167.4.
Synthesis of ((1R,2S,4R)–bicyclo[2.2.1]hept–5–en–2–yl)methyl 4–
(trifluoromethyl)benzoate, 4. 4–(Trifluoromethyl)benzoic acid (0.43 g,
2.25 mmol), DMAP (0.024 g, 0.190 mmol), 4 mL DMF, DIC (0.3 mL, 1.93
mmol) and ((1R,2S,4R)–bicyclo[2.2.1]hept–5–en–2–yl)methanol (0.19 mL,
1.61 mmol), were successively added in a 100 mL rounded flask and
allowed to react for 16 h. DMF was removed using a rotary evaporator
under reduced pressure. The mixture was extracted with EtOAc and
organic phase was washed by brine, dried by sodium sulfate. The product
was purified using silica column chromatography (EtOAc : Heptane = 1: 4)
as white solid. Yield = 0.35 g (69%). 1H–NMR (400 MHz, CDCl3), δ 1.23–
1.40(4H, m), 1.88 (1H, m), 2.80(1H, s), 2.87(1H, s), 4.25(1H, m), 4.43(1H,
m), 6.11(2H, m), 7.70 (2H, d, J=8Hz), 8.16(2H, d, J=8Hz). 13C–NMR (100
Preparation of exo–norbornene terminated monolayers (M2). Amino–
terminated monolayers (M1) were stirred with 50 mM 1,1'–
carbonyldiimidazole (CDI) solution in water for 1 h to yield acyl imidazole
activated surfaces. These surfaces were immediately reacted with 50 mM
exo–5–norbornene–2–methanol solution in DCE. This resulted in
carbamate bond formation and covalent tethering of the exo–norbornene
on the surface via carbamate bond in 16 h. The samples were sonicated
and washed with copious amounts of CH2Cl2, dried and stored under
nitrogen atmosphere.
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10.1002/chem.201703103
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Preparation of endo–norbornene terminated monolayers (M3).
Amino–terminated monolayers (M1) were stirred with 50 mM aqueous
solution of 1,1'–carbonyldiimidazole (CDI) for 1 h to yield acyl imidazole–
activated surfaces. These surfaces were immediately reacted with 50 mM
endo–5–norbornene–2–methanol solution in DCE. This resulted in
covalent tethering of the endo–norbornene on the surface via carbamate
bond in 16 h. The samples were sonicated and washed with copious
amounts of CH2Cl2, dried and stored under nitrogen atmosphere.
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Acknowledgements
The authors thank the Netherlands Organization for Scientific
Research for funding (ECHO project number 712.012.006); Prof.
Floris van Delft and Medea Kosian for stimulating discussions.
Keywords: cycloaddition, inverse electron demand Diels–Alder
reaction, mass spectrometry, surface chemistry, reaction rate.
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Entry for the Table of Contents (Please choose
one layout)
Layout 1:
FULL PAPER
Approach Matters: For click
reactions on a surface, the angle of
approach matters greatly. We show
for tightly monitored Inverse Electron
Demand Diels-Alder reactions that
fast and quantitative landings are
feasible.
Rickdeb Sen,‡ Digvijay Gahtory,‡ Jorge
Escorihuela, Judith Firet, Sidharam P.
Pujari, and Han Zuilhof*
Page No. – Page No.
Approach Matters: The Kinetics of
Interfacial Inverse–Electron Demand
Diels–Alder Reactions
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