Metal-free frustrated lewis pair catalyzed 1,4-hydrogenation of conjugated metallocene dienamines

Article abstract of DOI:10.1021/om900963p

Frustrated Lewis pairs, highly active in heterolytic dihydrogen splitting, were used as active and very selective hydrogenation catalysts for organometallic substrates. Conjugated dienamines fixed at the ferrocene framework and at the zirconocene nucleus

Full text of DOI:10.1021/om900963p

Organometallics 2010, 29, 1067–1069 1067
DOI: 10.1021/om900963p
Metal-Free Frustrated Lewis Pair Catalyzed 1,4-Hydrogenation
of Conjugated Metallocene Dienamines
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Sina Schwendemann, Tulay Aslı Tumay, Kirill V. Axenov, Ilona Peuser, Gerald Kehr,
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Roland Frohlich, and Gerhard Erker*
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Organisch-Chemisches Institut der Universitat Munster, Corrensstrasse 40, 48149 Munster, Germany
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Received November 4, 2009
Summary: Frustrated Lewis pairs, highly active in heterolytic
dihydrogen splitting, were used as active and very selective
hydrogenation catalysts for organometallic substrates. Conju-
gated dienamines fixed at the ferrocene framework and at
the zirconocene nucleus were selectively 1,4-hydrogenated
under catalytic conditions at ambient temperature.
The [3]ferrocenophane systems 1 are readily obtained by
Mannich coupling of 1,10-diacetylferrocene.7 We had used
these compounds extensively as starting materials for the
preparation of ferrocenophane-based ligands:8 e.g. in asym-
metric catalysis.9 These syntheses were usually initiated by
catalytic hydrogenation of both CdC double bonds of the
1,3-connected dienamine bridge by conventional means (i.e.
Pd/C, H2) to yield the corresponding fully saturated [3]ferro-
cenophanes 2. It would be desirable to have a method at hand
that allows for a stepwise, selective catalytic hydrogenation
of the conformationally restricted bridging dienamine to
yield derivatives (such as 3, for example) of these useful
frameworks of a higher functionality.
Heterolytic cleavage and activation of dihydrogen by
frustrated Lewis pairs is receiving quite some attention
lately.1 An increasing number of examples is emerging from
the literature, where such systems are used for metal-free
catalytic hydrogenation.2-6 It seems that catalysts derived
from frustrated Lewis pairs are especially suited for carry-
ing out selective catalytic hydrogenation reactions of sensi-
tive organometallic substrates.6 We here wish to describe
examples of such an application of these novel catalyst
systems in metallocene chemistry.
We recently showed that the intramolecular frustrated
Lewis pair Mes2P-CH2CH2-B(C6F5)2 (4) readily activates
dihydrogen heterolytically to yield the zwitterion 5.3,4 The
4/5 system turned out to be an active catalyst for enamine
hydrogenation under mild conditions.4 We have now used
the catalyst system 4/5 for a rather selective 1,4-hydrogena-
tion of the dienamine unit of the [3]ferrocenophane systems
1 and even of the related ansa-zirconocene derivative 6.
Treatment of 1a (-NR2=-NMe2) with dihydrogen (2.5 bar)
in toluene solution in the presence of 5 mol % of the prehy-
drogenated metal-free catalyst system 5 at ambient temperature
resulted in the rapid formation of the hydrogenation products 3a
and 2a in a 77:23 ratio (Scheme 1).10 We isolated the mixture
2a/3a in a combined yield of about 86%. Compound 3a was
characterized by X-ray diffraction (see Figure 1). It features the
newly formed methyl substituent at the sp2-carbon atom of the
*To whom correspondence should be addressed. E-mail: erker@
uni-muenster.de.
(1) (a) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W.
Science 2006, 314, 1124–1126. (b) McCahill, J. S. J.; Welch, G. C.; Stephan,
D. W. Angew. Chem. 2007, 119, 5056-5059; Angew. Chem., Int. Ed.
2007, 46, 4968-4971. (c) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc.
2007, 129, 1880–1881. (d) Welch, G. C.; Cabrera, L.; Chase, P. A.; Hollink,
E.; Masuda, J. D.; Wei, P.; Stephan, D. W. Dalton Trans. 2007, 3407–3414.
€
(e) Sumerin, V.; Schulz, F.; Nieger, M.; Leskela, M.; Repo, T.; Rieger, B.
Angew. Chem. 2008, 120, 6090–6092; Angew. Chem., Int. Ed. 2008, 47,
6001-6003. (f) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.;
€
€
Leskela, M.; Repo, T.; Pyykko, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130,
14117–14119. (g) Chase, P. A.; Stephan, D. W. Angew. Chem. 2008, 120,
7543-7547; Angew. Chem., Int. Ed. 2008, 47, 7433-7437. (h) Holschumacher,
D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.; Tamm, M. Angew. Chem.
2008, 120, 7538-7542; Angew. Chem., Int. Ed. 2008, 47, 7428-7432.
˚
C3 bridge (values from molecule A: C6-C8=1.338(3) A, C1-
C6-C7 = 114.7(2)°, C1-C6-C8 = 124.0(2)°, C6-C8-C9 =
128.8(2)°). The saturated carbon atom C9 now bears a hydrogen
in addition to the amino substituent (C10-C9-C8=116.6(2)°,
€
(i) Huber, D. P.; Kehr, G.; Bergander, K.; Frohlich, R.; Erker, G.; Tanino, S.;
Ohki, Y.; Tatsumi, K. Organometallics 2008, 27, 5279–5284. (j) Geier, S. J.;
Gilbert, T. M.; Stephan, D. W. J. Am. Chem. Soc. 2008, 130, 12632–12633.
(k) Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131,
52–53. (l) Ramos, A.; Lough, A. J.; Stephan, D. W. Chem. Commun. 2009,
1118–1120. Reviews: (m) Stephan, D. W. Org. Biomol. Chem. 2008, 6,
1535–1539. (n) Kenward, A. L.; Piers, W. E. Angew. Chem. 2008, 120,
38-42; Angew. Chem., Int. Ed. 2008, 47, 38-41. (o) Stephan, D. W.; Erker,
G. Angew. Chem. 2010, 122, 50-81; Angew. Chem., Int. Ed. 2010, 49,
46-76.
˚
C8-C9-N1=108.9(2)°, C8-C9=1.514(3) A). Compound 3a
features a typical set of eight 1H NMR signals of the pair of Cp
€
€
(7) (a) Knuppel, S.; Frohlich, R.; Erker, G. J. Organomet. Chem.
€
1999, 586, 218–222. (b) Kn€uppel, S.; Frohlich, R.; Erker, G. J. Organomet.
(2) (a) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008,
1701–1703. (b) Chen, D.; Klankermayer, J. Chem. Commun. 2008, 2130–
2131.
Chem. 2000, 595, 308–312. (c) Liptau, P.; Kn€uppel, S.; Kehr, G.; Kataeva,
€
O.; Frohlich, R.; Erker, G. J. Organomet. Chem. 2001, 637-639, 621–630.
€
(8) (a) Liptau, P.; Tebben, L.; Kehr, G.; Wibbeling, B.; Frohlich, R.;
€
(3) (a) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Frohlich, R.;
Erker, G. Eur. J. Inorg. Chem. 2003, 3590–3600. (b) Liptau, P.; Carmona,
D.; Oro, L. A.; Lahoz, F. J.; Kehr, G.; Erker, G. Eur. J. Inorg. Chem. 2004,
4586–4590.
Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072–5074. (b) Spies,
€
P.; Kehr, G.; Bergander, K.; Wibbeling, B.; Frohlich, R.; Erker, G. Dalton
€
Trans. 2009, 1534–1541.
(4) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Frohlich, R.;
(9) (a) Liptau, P.; Tebben, L.; Kehr, G.; Frohlich, R.; Erker, G.;
€
Hollmann, F.; Rieger, B. Eur. J. Org. Chem. 2005, 1909–1918. (b) Liptau,
€
Erker, G. Angew. Chem. 2008, 120, 7654-7657; Angew. Chem., Int. Ed.
2008, 47, 7543-7546.
P.; Seki, T.; Kehr, G.; Abele, A.; Frohlich, R.; Erker, G.; Grimme, S.
Organometallics 2003, 22, 2226–2232.
€
(5) Wang, H.; Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun.
(10) It is likely that the small amount of the fully saturated hydro-
genation product 2 is a slowly formed follow-up product of 3 and is
potentially formed from 3 via an isomerization pathway. For details see
the Supporting Information.
2008, 5966–5968.
(6) Axenov, K. V.; Kehr, G.; Frohlich, R.; Erker, G. J. Am. Chem.
€
Soc. 2009, 131, 3454–3455.
r
2010 American Chemical Society
Published on Web 02/08/2010
pubs.acs.org/Organometallics

1068 Organometallics, Vol. 29, No. 5, 2010
Schwendemann et al.
Figure 1. Molecular structure of compound 3a (atoms are shown
at the 50% probability level).
Scheme 1
Figure 2. Molecular structure of the salt 8-[HB(C6F5)3] (atoms
are shown at the 50% probability level).
Scheme 2
ligands, the 6-CH3 resonance at δ 1.99, the signal of the single
olefinic 8-H proton at δ 5.82, and the 9-H resonance at δ 4.09
(1H). The NMe2 signal was found at δ 2.24 (s, 6H). We carried
out the catalytic hydrogenation of the derivatives 1b (-NR2 =
-NC5H10) and 1c (-NR2=-NC4H8O) analogously using the
metal-free frustrated Lewis pair derived catalyst 5. The hydro-
genation reactions of the more bulky dienamines (1b,c) with the
frustrated Lewis pair catalyst 5 are more selective. Under the
applied reaction conditions we observed practically only the
formation of the 1,4-hydrogenation products 3b,c. Both were
isolated in ca. 90% yield.
It may be assumed that these catalytic 1,4-hydrogenation
reactions proceed by proton transfer from 5 followed by
hydride attack on the resulting in situ generated iminium ion.
We have generated the iminium ion 8 by an alternative
protonation route. Treatment of the dienamine 1a with a
stoichiometric amount of HCl in ether gave the conjugated
iminium ion system 8-[Cl] with a Cl- counteranion (Scheme 3).
The anion could be exchanged by treatment with AgBF4 to give
8-[BF4]. Treatment of 1a with the organometallic ammonium/
hydridoborate system {[(CpCH2NH2Ar)2ZrCl2]2þ/2[HB-
(C6F5)3]-} (9) gave the salt 8-[HB(C6F5)3], which was char-
acterized by X-ray diffraction (see Figure 2 and the Supporting
Information).
The metal-free Mes2PHþ(CH2CH2)BH-(C6F5)2 catalyst 5
can even be used for the selective hydrogenation of the
conjugated dienamine moiety at the ansa-zirconocene com-
plex 6, which was prepared by a related Mannich condensa-
tion route as previously described.11 Treatment of
6 with H2 under our typical nearly ambient conditions in
the presence of 20 mol % of the catalyst 5 resulted in selective
1,4-hydrogenation of the bridging unit to yield the product
The salt 8-[HB(C6F5)3] is stable at room temperature.12
Upon heating (80 °C, C6D6) it is converted to a ca. 1:1
mixture of the products 1013 and 11,14 which are potentially
follow-up products derived from 2a and 1a, respectively. We
then treated the related 8-[BF4] salt with a stoichiometric
1
7 (Scheme 2) (isolated in 27% yield by crystallization; H
NMR δ 1.58 (s, 3H, 6-CH3), δ 5.75 (dCH-), δ 4.26 (9-H),
δ 1.78 (s, 6H, 9-NMe2); for further details see the Supporting
Information).
(12) For [HB(C6F5)3]- salts see: (a) Parks, D. J.; Blackwell, J. M.;
Piers, W. E. J. Org. Chem. 2000, 65, 3090–3098. (b) Rubin, M.; Schwier, T.;
Gevorgyan, V. J. Org. Chem. 2002, 67, 1936–1940. (c) Schwier, T.;
Gevorgyan, V. Org. Lett. 2005, 7, 5191–5194. (d) Di Saverio, A.; Focante,
F.; Camurati, I.; Resconi, L.; Beringhelli, T.; D'Alfonso, G.; Donghi, D.;
Maggioni, D.; Mercandelli, P.; Sironi, A. Inorg. Chem. 2005, 44, 5030–5041.
€
€
(11) (a) Knuppel, S.; Wang, C.; Kehr, G.; Frohlich, R.; Erker, G.
J. Organomet. Chem. 2005, 690, 14–32 and references cited therein.
€
€
(b) Kn€uppel, S.; Erker, G.; Frohlich, R. Angew. Chem. 1999, 111, 2048–
(13) Liptau, P.; Neumann, M.; Erker, G.; Kehr, G.; Frohlich, R.;
2051; Angew. Chem., Int. Ed. 1999, 38, 1923-1926. (c) Bai, S.-D.; Wei, X.-H.;
Guo, J.-P.; Liu, D.-S.; Zhou, Z.-Y. Angew. Chem. 1999, 111, 2051-2054;
Angew. Chem., Int. Ed. 1999, 38, 1926-1928.
Grimme, S. Organometallics 2004, 23, 21–25.
(14) Compound 11 was independently prepared by treatment of 1a
with B(C6F5)3; for details see the Supporting Information.

Communication
Organometallics, Vol. 29, No. 5, 2010 1069
Scheme 3.a
a Legend: (i) 2M HCl in Et2O; (ii) Ag[BF4] in CH2Cl2; (iii) 0.5 equiv of {(Cp-CH2NH2þAr)2ZrCl2/2[HB(C6F5)3-]} (9) [Ar = 2,6-(iPr)2C6H3] in
CH2Cl2; (iv) 8-[BF4], 5 in C6D6, 70 °C; (v) 8-[HB(C6F5)3] in C6D6, 80 °C.
amount of the compound Mes2PHþ(CH2)2BH-(C6F5)2 (5)
in C6D6 solution. At ambient temperature no reaction was
observed, probably due to low solubility of 8-[BF4] in C6D6.
At 70 °C within 20 min this resulted in clean formation of the
neutral 1,4-hydrogenation product 3a, admixed with 2a,10 in
a similar ratio (3a:2a ≈ 71:29 in a typical experiment), as
observed in the catalytic hydrogenation reaction (see above).
These experiments may indicate that the actual catalytic
hydrogenation reaction of the substrates 1 proceeds by
means of a sequential pathway initiated by proton transfer
followed by hydride reduction of an in situ generated imi-
nium ion intermediate. Our study shows that metal-free
hydrogenation catalysts based on specific frustrated Lewis
pairs can in special cases effect remarkable activities and
selectivities. This indicates an increasing potential of this
novel catalyst type for synthetic applications.
Acknowledgment. Financial support from the Deutsche
Forschungsgemeinschaft, the Fonds der Chemischen Indus-
trie, and the Alexander von Humboldt-Stiftung (stipend
to K.V.A.) is gratefully acknowledged. We are grateful to
Noman Mubarik, Carolin Usener, and Kerstin Unverhau
for a generous donation of the [3]ferrocenophane complex 1a.
Supporting Information Available: Text and figures giving
further experimental and spectroscopic details and CIF files
giving crystallographic data. This material is available free of
charge via the Internet at http://pubs.acs.org.