Low-energy-electron-induced hydroamination of an alkene

Full text of DOI:10.1002/anie.200901338

Angewandte
Chemie
DOI: 10.1002/anie.200901338
Electron-Induced Hydroamination
Low-Energy-Electron-Induced Hydroamination of an Alkene**
Thorben Hamann, Esther Bꢀhler, and Petra Swiderek*
The transformation of structurally simple molecules into
functional materials is one of the ultimate challenges of
chemical research. Introducing specific functional groups is
an essential step towards this aim. Amino groups are
particularly useful as they are versatile linkers that can be
used to attach complex molecules.[1] Hydroamination reac-
tions are used in organic synthesis to add ammonia or amines
to unsaturated hydrocarbons. They require specifically tail-
ored catalysts because of the high activation barrier arising
from electrostatic repulsion between the electron lone pair at
the nitrogen atom and the electron-rich double or triple
bond.[2]
Low-energy free electrons provide an alternative
approach to control chemical reactions. Dissociative electron
attachment (DEA) leads to rupture of specific bonds depend-
ing on the energy of the incoming electron. A fine example
demonstrating this selectivity has been provided by recent in-
depth investigations of electron-induced reactions of thymine
in the gas phase.[3] Similarly, DEA can trigger reactions
leading to the modification or functionalization of surfaces.[4,5]
In these cases, the fragments produced by DEA form new
bonds with an initially H-terminated silicon or diamond
surface. Here we show that by proper tuning of the electron
energy (E0), low-energy-electron-induced reactions can also
be used to introduce amino groups to an alkene using
ammonia (NH3) as a starting reagent.
The reaction described herein resembles a hydroamina-
tion except that the electron beam replaces the catalyst of the
organic synthesis (Scheme 1). Ethylene (C2H4) was chosen to
demonstrate the feasibility of our approach. To circumvent
the electrostatic repulsion that prevents the reaction between
neutral C2H4 and NH3, multilayer films condensed on a
cryogenic Au substrate were irradiated with electrons at E0
somewhat above the ionization threshold but not high enough
to produce extensive fragmentation.[6] Ionization above the
first threshold removes an electron from the p orbital of C2H4
(Scheme 1a). The cation interacts attractively with the lone
pair of NH3 that is either leaked into the vacuum chamber
during electron exposure or initially admixed to the C2H4
deposit. Alternatively ionization can also occur from the lone
pair of NH3 (Scheme 1b), and the resulting cation is attracted
towards the electron-rich double bond of C2H4. Intramolec-
ular hydrogen migration and subse-
quent neutralization of the resulting
adduct by thermalized electrons
within the film then produces ami-
noethane (C2H5NH2), which can be
detected by post-irradiation thermal
desorption spectrometry (TDS).
Figure 1 shows the TDS data for
multilayer films formed by condens-
ing a mixture of equal quantities of
C2H4 and NH3 vapor at 32 K. With-
out exposure, the characteristic
desorption peaks of C2H4 at 67 K
and NH3 at 96 K are observed in the
28 amu and 17 amu curves, whereas
the data recorded at 30 amu,
44 amu, and 45 amu show a flat
baseline. After electron exposure
of 4000 mC at E0 = 15 eV new
desorption signals appear at 71 K
and 138 K in the 30 amu curve and
at 138 K in both the 44 amu and
45 amu curves. In addition, a new
signal is observed at 40 K in the
28 amu curve. This latter signal is
ascribed to N2 formed from NH3
upon electron exposure. In multi-
layer films of NH3 this reaction
proceeds without thermal activa-
tion[7] and is ascribed to the dispro-
portionation of NH2 radicals formed
upon electron exposure.[8] Taking
into account that ions in the con-
densed phase are stabilized by typ-
ically 1 to 2 eV relative to the gas
Scheme 1. Proposed
mechanism of the elec-
tron-induced reaction
producing aminoethane
in mixed condensed
films of ethylene and
ammonia for electron
incident energies above
the ionization threshold
of ethylene and ammo-
nia.
phase, NH2 radicals can be produced here by the dissociative
+
ionization of NH3 yielding NH2 (gas-phase threshold: 15–
16 eV[6]) followed by neutralization by thermalized electrons
during exposure. In the present experiment, the H atoms thus
released reduce C2H4 to ethane (C2H6) as evidenced by a new
30 amu desorption signal at 71 K after electron exposure.
DEA probably does not contribute to this reaction as it has
been reported to be efficient only at lower E0.[9,10]
[*] T. Hamann, E. Bꢀhler, Prof. Dr. P. Swiderek
Institute of Applied and Physical Chemistry
Universitꢁt Bremen, Fachbereich 2 (Chemie/Biologie)
Leobener Strasse, 28359 Bremen (Germany)
Fax: (+49)421-218-4918
The relative intensities of the desorption peaks at 138 K
for 45 amu, 44 amu, and 30 amu of 20:20:100 (Figure 1)
reproduce well the mass spectrum of C2H5NH2 obtained also
with electron impact (EI) ionization at 70 eV.[6] This shows
that electron exposure at E0 = 15 eValso drives formation of a
stable addition product of NH3 and C2H4, namely C2H5NH2.
Formation of its isomer dimethylamine ((CH3)2NH) is
excluded because the mass spectrum of this compound
shows only a very small signal at 30 amu.[6] The absolute
E-mail: swiderek@uni-bremen.de
Homepage: http://www.iapc.uni-bremen.de/swiderek/
[**] P.S. thanks Sven Doye for an inspiring GDCh lecture on hydro-
amination. This work was funded by the DFG and has profited from
exchange within COST action ECCL.
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Figure 2. Thermal desorption spectra of multilayer films of ethylene
and ammonia without electron exposure (0 mC) and after an exposure
of 4000 mC. The films were deposited from the gas phase at 32 K by
first condensing ethylene and then an equal amount of ammonia. The
exposure was performed simultaneously with the ammonia deposition.
The overall thickness corresponds to 15–20 monolayers. The electron
incident energy was 15 eV. Signals above 200 K are attributed to
desorption from the sample holder.
Figure 1. Thermal desorption spectra of multilayer films of a 1:1
mixture of ethylene and ammonia without electron exposure (0 mC)
and after an exposure of 4000 mC. The films were deposited from the
gas phase at a thickness corresponding to 15–20 monolayers and
exposed at 32 K. The electron incident energy was 15 eV. Signals above
200 K are attributed to desorption from the sample holder.
quantity of C2H5NH2 can not be determined from the present
data because the absolute ionization cross section for
C2H5NH2 is not available. An ongoing study will address
this question by comparison with samples of known compo-
sition as described earlier.[11]
that reported previously.[5] This process is obviously not
efficient enough to lead to a noticeable yield of C2H5NH2.
Only at E0 above the ionization threshold reported as
10.07 eV and 10.50 eV for gaseous NH3 and C2H4,[6] respec-
tively, does formation of C2H5NH2 commence. Its rate
increases with increasing E0, which is characteristic of an
ionization-driven process.[12] This finding supports the pro-
posed reaction mechanism. Gas-phase cross sections for E0 =
15 eV[12] further suggest that ionization of C2H4 (cross section
of 0.661 ꢀ2) is probably somewhat preferred over ionization
of NH3 (0.379 ꢀ2).
The same reaction is observed when NH3 is dosed onto a
condensed film of C2H4 during electron exposure but leads to
a smaller quantity of products (Figure 2). Mixing of this
layered film during desorption thus does not efficiently
counterbalance the smaller degree of direct contact between
NH3 and C2H4 during electron exposure as compared to a film
produced from deposition of a gas mixture. This suggests that
the actual reaction occurs mainly on a short time scale during
electron exposure and not during the thermal desorption step.
The production of C2H5NH2 was investigated at different
E0 to show that ionization in fact drives the reaction. TDS
curves at 45 amu of multilayer films of a 1:1 mixture of C2H4
and NH3 after an exposure of 4000 mC at different E0 show
that the amount of produced C2H5NH2 is lower at E0 = 12 eV
than at 15 eV (Figure 3). At 9 eVand 5 eV, this product is not
observed. These values of E0 were chosen because they lie in
the range of energies where DEA to NH3 leading to
dissociation of an NH bond was reported.[9,10] We initially
assumed that the reactive fragments formed in these process-
es could add to a hydrocarbon in a radical reaction similar to
A series of experiments aimed at inducing the formation
of C2H5NH2 in mixtures of C2H6 and NH3 so far did not clearly
yield the desired product. The ionization energy of C2H6
(11.52 eV in the gas phase[6]) is also considerably lower than
E0 = 15 eV used in these experiments, and fragmentation to
+
yield C2H4 occurs at about 0.5 eV above the ionization
threshold.[6] This would in principle yield the same inter-
mediate as that assumed in the present reaction mechanism
(Scheme 1). In ongoing experiments to be reported elsewhere
we thus vary film thickness and electron exposure in order to
give conclusive evidence for or against this reaction. It must
be noted that cations with the stoichiometry H+(C2H5NH2)-
(NH3)n (n = 0-4) have been observed by electron-stimulated
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Angewandte
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delivering currents of the order of a few mAcmÀ2 to the sample with
an estimated energy resolution of 0.5–1 eV was used for electron
exposure at the same temperature. The film composition was
monitored by TDS at a heating rate of 1 KsÀ1 using a Quadrupole
Mass Spectrometer (QMS) residual gas analyzer (Stanford, 200 amu)
with EI ionization at 70 eV. TDS curves were recorded simulta-
neously for five different masses.
For film deposition, the gas was leaked into the TDS chamber by
means of a gas-handling manifold consisting of precision leak valves
and a small calibrated volume where the absolute pressure was
measured with a capacitance manometer. The pressure drop in the
manifold was used to measure the gas quantity. The amount of gas
required for monolayer formation was estimated from the thickness
dependence of the TDS curves. Prior to each deposition, the substrate
was cleaned by resistive heating of two thin tantalum ribbons spot-
welded to the thicker gold substrate.
C2H4, NH3, and C2H6 were purchased from Air Liquide at stated
purities of 99.95%, 99.98%, and 99.95%, respectively, and were used
as received.
Received: March 10, 2009
Published online: May 14, 2009
Keywords: alkenes · ammonia · electron-induced reactions ·
hydroaminations · thermal desorption spectrometry
.
Figure 3. Thermal desorption spectra of multilayer films of a 1:1
mixture of ethylene and ammonia recorded at 45 amu without electron
exposure (0 mC) and after an exposure of 4000 mC at different E0. The
films were deposited from the gas phase at 32 K at a thickness
corresponding to 15–20 monolayers and exposed at the same temper-
ature.
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In conclusion, we have demonstrated a new strategy to
control chemical synthesis by exposure to low-energy elec-
trons and used it to synthesize C2H5NH2 from C2H4 and NH3.
The direct reaction is exothermic according to standard
thermodynamic data[6] but hindered by the electrostatic
repulsion between the neutral reaction partners. Our
approach relies on the electrostatic attraction caused by the
soft ionization of one of the reaction partners. It is thus
complementary to the strategy of controlling chemical
reactions by inducing DEA. So far, we have concentrated at
irradiation using an electron energy roughly 4 to 5 eV above
the ionization threshold of both reaction partners. This is
sufficient to induce, to some extent, dissociation and thus
concurrent reactions. Future work will aim at investigating
whether further decrease of the energy can make the reaction
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Experimental Section
The experiments were performed in an UHV setup[14] with a base
pressure of 10À10 Torr. Molecular films were deposited from the gas
phase onto a polycrystalline gold substrate (area 2.8 cm2) held at 32 K
by means of a closed-cycle helium cryostat. A commercial flood gun
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