Markownikoff and anti-Markownikoff hydroamination with palladium catalysts immobilized in thin films of silica supported ionic liquids

Article abstract of DOI:10.1039/b605567b

The concept of immobilising organometallic complexes in a thin film of supported ionic liquids was utilised to synthesise novel bi-functional catalysts combining soft Lewis acidic and strong Bronsted acidic functions; the materials showed exceptional catalytic activity for the addition of aniline to styrene, providing the Markownikoff product under kinetically controlled conditions and mainly the anti-Markownikoff product in the thermodynamic regime. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b605567b

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Markownikoff and anti-Markownikoff hydroamination with palladium
catalysts immobilized in thin films of silica supported ionic liquids{
Oriol Jimenez, Thomas E. Mu¨ller,* Carsten Sievers, Andreas Spirkl and Johannes A. Lercher
Received (in Cambridge, UK) 19th April 2006, Accepted 12th May 2006
First published as an Advance Article on the web 6th June 2006
DOI: 10.1039/b605567b
As test reaction, the addition of aniline to styrene was
investigated (Eq. 1). The reaction can, in principle, provide the
Markownikoff product N-(1-phenylethyl)aniline (1) and the anti-
Markownikoff product N-(2-phenylethyl)aniline (2).
The concept of immobilising organometallic complexes in a
thin film of supported ionic liquids was utilised to synthesise
novel bi-functional catalysts combining soft Lewis acidic and
strong Brønsted acidic functions; the materials showed excep-
tional catalytic activity for the addition of aniline to styrene,
providing the Markownikoff product under kinetically con-
trolled conditions and mainly the anti-Markownikoff product
in the thermodynamic regime.
ð1Þ
In batch experiments performed at low temperatures (125 uC),
the Markownikoff addition product 1 was the only product apart
from some oligomerization products of styrene. The initial
catalytic activity increased linearly with the Pd loading (Fig. 2)
The direct addition of amines to weakly or non-activated alkenes
(hydroamination) is an important target reaction. For industrial
applications, however, the known catalysts provide insufficient
activity and long-term stability. Thus, a new generation of—
preferentially solid—catalysts is required. Bi-functional catalysts
combining soft Lewis acidic function (activation of the alkene) and
strong Brønsted acidic function (acceleration of the rate determin-
ing step (r.d.s.)) were reported to provide high catalytic activities.1
A new concept, immobilisation of homogeneous catalysts in a
supported film of ionic liquid,2 allows joining both functions in one
material and tailoring systematically its performance in catalysis.
As Lewis acid function, the palladium complex
[Pd(DPPF)(CF3CO2)2], prepared in-situ from Pd(CF3CO2)2 and
1,19-bis(diphenylphosphino)ferrocene (DPPF), was chosen.
Trifluoromethanesulfonic acid (TfOH) provided the Brønsted acid
function. Complex and acid (Pd2+/H+ 5 1/10) were immobilised in
a thin film of imidazolium based ionic liquids{ on a silica support
(Fig. 1).{ Systematic increase in the length of the alkyl chain
provided a series of catalysts with decreasing polarity of the ionic
liquid phase.
corresponding to first order in Pd complex. However, a minimum
21
loading of 0.011 mmolPd2+ gCat
seemed necessary and no
conversion was observed at lower loadings or without palladium.§
This suggests that, in each case, an equal amount of the Pd
complex was irreversibly adsorbed on the silica surface and did not
contribute to the catalytic activity.
Particularly noteworthy is that the initial catalytic activity was
strongly dependent on the choice of the ionic liquid. The highest
catalytic activity was observed for the EMIm{ based catalyst
(1.47 mmol (gCat h)21 at a Pd loading of 0.044 mmolPd2+ gCat
21
,
corresponding to a turnover frequency of 24 mol (molPd2+ h)21),
whereas BMIm{ and HM2Im{ provided lower catalytic activities
(0.89 and 0.43 mmol (gCat h)21, respectively).
Thus, the highly polar environment of the EMIm containing
catalyst is concluded to be particularly favourable. This is
attributed to the higher solubility of the reactants in EMIm as
the catalyst phase, or to an intrinsically higher rate of reaction in
the more polar ionic liquid.
Fig. 1 Concept for the immobilisation of homogeneous catalysts for
application in fixed reactors, R 5 H, Me; Alkyl 5 C2H5, C4H9, C6H13.
Lehrstuhl II fu¨r Technische Chemie, Technische Universita¨t Mu¨nchen,
Lichtenbergstrasse 4, 85747 Garching, Germany.
E-mail: thomas.mueller@ch.tum.de; Fax: +49 89 28913544;
Tel: +49 89 28912827
{ Electronic supplementary information (ESI) available: experimental
section, catalyst composition. See DOI: 10.1039/b605567b
Fig. 2 Catalytic activity of the Lewis/Brønsted bi-functional catalysts in
the addition of aniline to styrene (slurry phase, 125 uC).
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The absorption constant of aniline from heptane into the
supported ionic liquid phase decreased slightly in the sequence
In the kinetic regime, the selectivity to 1 was 100% at ¡ 170 uC
(for all catalysts) and decreased steadily at higher temperatures.
Interestingly, the relative product concentration was strongly
dependent on the choice of the ionic liquid. At 220 uC, e.g., the
selectivity to 1 was 67, 73 and 87% for the EMIm, BMIm and
HM2Im based catalysts, respectively.
EMIm . BMIm . HM2Im (0.235, 0.221 and 0.204 mmol g21
,
respectively). Only part of the absorbed aniline was physically
dissolved in the supported ionic liquid (0.037, 0.018 and
0 mmol g21, respectively), while the remainder was either
protonated, or bound to the Pd centre (aniline/Pd 5 2/1). In
contrast, no significant absorption of styrene was measured.
However, it was reported that co-absorption of aniline and styrene
leads to enhanced styrene uptake.3
The activity of the three catalyst series for formation of 1 was
similar in the kinetic regime, decreasing slightly in the sequence
EMIm . BMIm . HM2Im. In contrast, for formation of 2 the
EMIm based catalyst was 1.7 (4.3) times more active than the
BMIm (HM2Im) based catalyst (220 uC). Steady state conversion
of 65% (EMIm based catalyst, 0.044 mmolPd2+ gCat21, 240 uC)
corresponds to an integral reaction rate of 8.4 mmol (gCat h)21 and
The temperature dependence of the catalytic activity was
explored in a fixed bed reactor (Fig. 3). Above 150 uC, the
conversion increased exponentially with temperature, attaining a
maximum of 35% (EMIm based catalyst, 0.044 mmolPd2+ gCat21) at
approximately 240 uC (kinetic regime). At temperatures higher
than 240 uC, the conversion decreased as the thermodynamic limit
of the (slightly exothermic) reaction was encountered. In the
kinetic regime, the main product was 1, while significant amounts
a turnover frequency of 199 mol (molPd2+ h)21
.
Based on the observations given above, and considering the
current literature on hydroamination,4,5 we conclude that two
different mechanisms are operative (Scheme 1). The
Markownikoff product 1 is probably formed via coordination of
the olefinic p-system of styrene to the palladium centre, which
renders it susceptible to a nucleophilic attack of the lone electron
pair of the aniline nitrogen atom.4 Subsequent protolytic cleavage
of the metal–carbon bond is rate determining. The formation of
the anti-Markownikoff product 2 occurs via intermediate forma-
tion of a palladium hydride, insertion of the olefinic double bond
of styrene and nucleophilic attack (r.d.s.) of the lone electron pair
of the aniline nitrogen atom at the a-carbon atom.5
of
2 were formed under thermodynamic control. In the
thermodynamic regime, the ratio of products 1 to 2 was
approximately equal for all catalysts (0.77(3) : 1, 0.75(1) : 1
and 0.65(4) : 1 for the EMIm, BMIm and HM2Im based
catalysts, respectively) and nearly independent of the temperature.
Assuming activity coefficients close to one, the difference in
thermodynamic stability of the two products was calculated to
DrGu # 1.4(4) kJ mol21 (2 being the more stable product).
In the case of the Markownikoff product, the more polar ionic
liquid is concluded to provide an intrinsically higher rate of
reaction, which is related to stabilisation of a polar transition state
associated with the rate determining step.6 In the case of the anti-
Markownikoff product, the higher aniline concentration in the
ionic liquid phase with higher polarity is speculated to lead to
higher turnover frequencies in the rate determining step.
Fig. 3 Temperature dependence with EMIm based catalyst (fixed bed
reactor, 0.044 mmolPd2+ gCat21, top) and comparison of the yield in 1
(middle) and 2 (bottom) with catalysts differing in the supported ionic
liquid.
Scheme 1 Mechanisms proposed for the formation of 1 (top) and 2
(bottom), X 5 CF3SO32, CF3CO22, r.d.s. = rate determining step.
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2 See, e.g., T. Welton, Coord. Chem. Rev., 2004, 248, 2459;
S. Breitenlechner, M. Fleck, T. E. Mu¨ller and A. Suppan, J. Mol.
Catal. A: Chem., 2004, 214, 175; A. Riisager, P. Wasserscheid, R. Hal
and R. Fehrmann, J. Catal., 2003, 219, 452; C. P. Mehnert, R. A. Cook,
N. C. Dispenziere and M. Afeworki, J. Am. Chem. Soc., 2002, 124,
12932; H. Hagiwara, Y. Sugawara, K. Isobe, T. Hoshi and T. Suzuki,
Org. Lett., 2004, 6, 2325.
3 J. Bodis, T. E. Mu¨ller and J. A. Lercher, Green Chem., 2003, 5, 227.
4 See, e.g., H. M. Senn, P. E. Blo¨chl and A. Togni, J. Am. Chem. Soc.,
2000, 122, 4098.
Notes and references
{ EMIm 5 1-ethyl-3-methylimidazolium trifluoromethanesulfonate,
BMIm 5 1-butyl-3-methylimidazolium trifluoromethanesulfonate,
HM2Im 5 1-hexyl-2,3-dimethylimidazolium trifluoromethanesulfonate.
§ The amount of palladium leached into the reaction solution was below
the detection limit of AAS. No further reaction was observed when the
filtered reaction mixture was maintained at 125 uC.
1 See, e.g., M. Kawatsura and J. F. Hartwig, J. Am. Chem. Soc., 2000, 122,
9546; I. Kadota, A. Shibuya, L. M. Lutete and Y. Yamamoto, J. Org.
Chem., 1999, 64, 4570; R. Q. Su and T. E. Mu¨ller, Tetrahedron, 2001, 57,
6027; J. Penzien, C. Haeßner, A. Jentys, K. Ko¨hler, T. E. Mu¨ller and
J. A. Lercher, J. Catal., 2004, 221, 302.
5 See, e.g., U. Nettekoven and J. F. Hartwig, J. Am. Chem. Soc., 2002, 124,
1166.
6 L. Crowhurst, N. L. Lancaster, J. M. P. Arlandis and T. Welton, J. Am.
Chem. Soc., 2004, 126, 11549.
2976 | Chem. Commun., 2006, 2974–2976
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