Characterization of key intermediates in a complex organocatalytic cascade reaction using mass spectrometry

Article abstract of DOI:10.1002/anie.200804353

Chemical scrabble: Mechanistic studies of complex organocatalytic cascade reactions are challenging owing to the presence of many species involved. Electrospray ionization mass spectrometry provides details about a one-pot quadruple organocatalytic cascad

Full text of DOI:10.1002/anie.200804353

Angewandte
Chemie
DOI: 10.1002/anie.200804353
Mass Spectrometry
Characterization of Key Intermediates in a Complex Organocatalytic
Cascade Reaction Using Mass Spectrometry**
Wolfgang Schrader,* Peni Purwa Handayani, Jian Zhou, and Benjamin List*
Dedicated to Professor Jan T. Andersson on the occasion of his 60th birthday
Modular combinations of organocatalytic reactions into
cascades has attracted increasing attention in organic syn-
thesis, as it enables the efficient and stereoselective con-
struction of complex molecules from simple precursors, and
greatly circumvents time, energy, and yield losses associated
with traditional multistep syntheses.[1] Although the under-
lying mechanistic principles of individual steps seem to be
well understood and even used in the design of new cascades,
the mechanistic details of these complex reactions are largely
unexplored. Isolation and characterization of reaction inter-
mediates can be tedious, and widely used analytical techni-
ques, such as NMR and IR spectroscopy, often fail in case of
complex cascade reactions. Electrospray ionization mass
spectrometry (ESI-MS) is an analytical technique that is
rapidly and widely growing as an important tool in molecular
analysis.[2] However, despite its success in elucidating reaction
mechanisms, the implementation of ESI-MS for mechanistic
studies of a complex organocascade reaction involving several
compounds and intermediates has to the best of our knowl-
edge not been reported. Herein we use ESI-MS analysis as a
powerful method for the characterization of key intermedi-
ates in a quadruple organocatalytic cascade reaction, facili-
tating its detailed mechanistic understanding.
ESI-MS is a soft ionization method that allows the
characterization of species that are actually present in
solution. Additionally, ESI-MS/MS techniques enable selec-
tion and fragmentation of one specific ion using collision
induced dissociation (CID) to gain structural information
about important components. ESI-MS has been used in
bioanalysis,[3] chemical reactions in solution[4] or in the gas
phase,[5] and also for analyzing exotic problems, such as the
formation of silicate oligomers.[6] ESI-MS and its tandem
version ESI-MS/MS are particularly useful for the investiga-
tion of catalytic processes, that is, homogenously catalyzed
reactions[7] and in the high-throughput screening of chiral
catalysts.[8] We have previously used mass spectrometry and
ESI-MS/MS techniques for the investigation of organocata-
lytic reaction mechanisms, such as the intramolecular Michael
reaction of aldehydes[9] and a vinylogous Umpolung reac-
tion.[10] We felt that ESI-MS may provide an ideal solution to
the challenges posed by organocatalytic multistep cascade
reactions as it allows for the individual characterization of all
possible intermediates.
Concepts for designing new organocatalytic cascade
reactions that involve enamine catalysis, iminium catalysis,
and Brønsted acid catalysis have been proposed recently.[11]
For example, a highly enantioselective synthesis of 3-substi-
tuted cyclohexylamines 2 from 2,6-diketones, such as 1 using a
combination of catalytic quantities of the chiral Brønsted acid
(R)-TRIP[12] with an achiral amine 3 and Hantzsch ester 4 has
been developed.[13] This cascade reaction is believed to
proceed via intermediates 5, 6, and 7, and to involve an
aldol condensation using enamine catalysis, a conjugate
reduction using iminium catalysis and Brønsted acid catalysis,
and a final reductive amination by Brønsted acid catalysis
(Scheme 1).
We have now carefully investigated the reaction of
diketone 1 to amine 2 using ESI-MS (for additional reactions,
see Supporting Information), which enabled us to identify all
critical intermediates.
[*] Priv.-Doz. Dr. W. Schrader, P. P. Handayani, Dr. J. Zhou,
Prof. Dr. B. List
Max-Planck-Institut fꢀr Kohlenforschung
Kaiser Wilhelm-Platz 1, 45470 Mꢀlheim an der Ruhr (Germany)
E-mail: wschrader@mpi-muelheim.mpg.de
list@mpi-muelheim.mpg.de
[**] We acknowledge generous funding from the Max-Planck-Society
and the DFG SPP 1179 (Organocatalysis). We also thank H. W. Klein
and W. Joppek for technical assistance.
Scheme 1. Quadruple organocatalytic cascade reaction for the syn-
thesis of 3-substituted cyclohexylamines from 2,6-diketones via pro-
posed intermediates. PEP=p-ethoxyphenyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804353.
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One of our questions concerned the mechanism of the
fragmentation pattern of the ion m/z 360 obtained from an
ESI-MS/MS (positive mode) experiment, this intermediate
corresponds mostly to enamine 5, although additional minor
signals point to the presence of 10. Peaks at m/z 479, which
would support a Mannich pathway via intermediates 11 and
12, are entirely absent. This fact, along with our observation
of aldolization intermediates[14] let us conclude that the initial
aldol condensation does in fact proceed via an aldolization to
b-hydroxy ketone intermediate 8 rather than a Mannich
condensation (Scheme 2). For a better understanding we have
collected the detailed fragmentation experiments from the
reactions in the Supporting Information together with GC/
MS data that support the findings.
initial aldolization: does this step proceed via an enamine
aldol or via a Mannich-condensation? The aldol mechanism
would involve intermediates 5 and 10, both at m/z 360,
whereas a Mannich mechanism would require two equiva-
lents of the amine and would proceed via intermediates 11
and 12 (m/z 479). Under our standard conditions, the
aldolization is rapid and full conversion of diketone 1 into
imine 6 is observed within minutes.
Under these conditions the intermediates were hard to
intercept, but by slightly modifying the reaction conditions,
that is, not using molecular sieves (see Supporting Informa-
tion), we were able to slow down this rapid conversion step
such that intermediates could actually be observed. A typical
spectrum obtained after five minutes is shown in Figures 1
and 2. The spectrum shows the signals for the starting
materials, the reagents, the catalyst, and the cyclodehydration
product 9 (m/z 223), and its imine 6 (m/z 342). Moreover, a
characteristic signal at m/z 360 is observed. This signal is
consistent with enamine intermediate 5, its imine tautomer
(not shown), or aldolization product 10. Judging from the
Scheme 2. Plausible and detected intermediates of the aldolization
step; m/z ratios refer to protonated species. PEP=p-ethoxyphenyl,
Naph=naphthyl.
The following reduction steps[15] in the cascade are
proposed to involve ion-pair intermediates, such as 6·TRIP
and 7·TRIP (Scheme 3). Rather to our delight and surprise,
we not only detected all critical cationic iminium ions and
ammonium ions (protonated 6 [m/z 342], 7 [m/z 344], 2 [m/z
346], 10 [m/z 360]), but also the correlating ion pairs. For
example, in addition to free [TRIP + H]+ (m/z 753), peaks at
m/z 1113 and m/z 1095, which are consistent with iminium
salts [10·TRIP + H]+ and [6·TRIP + H]+ are already seen
Figure 1. ESI-MS spectrum (positive mode) of the cascade reaction for
the synthesis of 3-substituted cyclohexylamines, 5 min after the start of
the reaction.
Figure 2. ESI-MS (positive mode) spectrum of the cascade reaction for
the synthesis of 3-substituted cyclohexylamines, 5 min after the start of
the reaction. Detection of protonated ion-pair intermediates are
presented in the spectrum.
Scheme 3. Formation of ion pair intermediates; m/z ratios refer to
protonated species. PEP=p-ethoxyphenyl, Naph=naphthyl.
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Chemie
after five minutes (Figure 2). Moreover, after 4 h the first
hydrogenation product at m/z 344 [7 + H]+ appeared and
replaced its precursor [6 + H]+ at m/z 342. The corresponding
TRIP salt of this intermediate [6·TRIP + H]+ was also
detected at m/z 1097. After further conversion, the final
product was also detected as a protonated ion pair [2·TRIP +
H]+ at m/z 1099. For a detailed evaluation of these presumed
ion pairs,[16] a number of different experiments were con-
ducted, with one example of [7·TRIP + H]+ displayed in
Figure 3. CID studies in both positive and negative mode
were performed using different collision energies from 0 up to
30 eV (see Supporting Information).
342) was clearly shown to correspond to an intermediate,
which is converted into product 2, by following the course of
the reaction over time. The data are shown in Figure 4 with
data points taken every five minutes during the first hour of
Figure 4. Formation of ions and ion pairs during 80 h of reaction time.
the reaction and every two hours for the rest of the reaction
time. Similar to the first intermediate 6, which is formed and
then disappears relatively fast, the second intermediate 7
ultimately disappears, during the formation of product 2.
Remarkably, similar results are obtained with the correspond-
ing ion pairs [6·TRIP + H]+ (m/z 1095), [7·TRIP + H]+ (m/z
1097), and [2·TRIP + H]+ (m/z 1099) (inset in Figure 4).
In conclusion, we have successfully intercepted and
structurally characterized important intermediates of an
organocatalytic cascade reaction. Fragmentation experiments
as a function of time, and accurate mass data from FT-ICR
MS experiments, allow insights into the mechanistic details of
this complex reaction and confirm the previously proposed
catalytic cycle. With our methods, chiral ion pair structures of
similar reactions can be easily detected.[17] Further studies
along these lines are under investigation in our laboratories.
Figure 3. ESI-MS/MS (positive mode) spectrum of ion pair
[7·TRIP+H]+.
In the negative ESI mode, only the dissociated acid was
found, as expected, whereas the ion pair complex could not be
detected. In the positive ESI mode, even the relatively low
collision energy of 10 eV leads to fragmentation into both
[TRIP + H]+ (m/z 753) and iminium ion [7 + H]+ (m/z 344).
The rather weak stability of these ions and similar complexes,
and the detection of the ion pair of product 2, led us to
conclude that the higher mass peaks observed correspond to
ion pairs rather than possible covalent adducts. These results
are further confirmed using accurate mass data that have been
obtained for all cationic intermediates, product, and ion pairs
using Fourier-transform ion cyclotron resonance mass spec-
trometry (FT-ICR MS, Table 1).
Received: September 3, 2008
Published online: January 15, 2009
The peak at m/z 344 is consistent with either 1,4-reduction
product 7 or with the corresponding 1,2-reduction product 13
(Scheme 3). Interpretation of the corresponding positive-
mode ESI-MS/MS (see Supporting Information) let us
conclude that the reaction proceeds along a 1,4-reduction
pathway. Allyl amine 13 should be unreactive towards further
reduction; however, the peak at m/z 344 (as well as that at m/z
Keywords: cascade reactions · complexe reactions ·
electrospray mass spectrometry · iminium catalysis ·
organocatalysis
.
[1] a) K. C. Nicolaou, T. Montagnon, S. A. Snyder, Chem. Commun.
2003, 551; b) J. C. Wasilke, S. J. Obrey, R. T. Baker, G. C. Bazan,
Chem. Rev. 2005, 105, 1001.
[2] a) C. M. Whitehouse, R. N. Dreyer, M. Yamashita, J. B. Fenn,
Anal. Chem. 1985, 57, 675; b) J. B. Fenn, M. Mann, C. K. Meng,
S. F. Wong, C. M. Whitehouse, Science 1989, 246, 64.
Table 1: High resolution mass data of detected intermediates.
Species
Mass
Mass
Formula
Error [ppm]
(measured) (theoretical)
[3] a) W. Schrader, S. Dꢀhring, W. Joppek, Angew. Chem. 2004, 116,
6826; Angew. Chem. Int. Ed. 2004, 43, 6657; b) W. Schrader,
H. W. Klein, Anal. Bioanal. Chem. 2004, 379, 1013.
[4] a) J. Griep-Raming, S. Meyer, T. Bruhn, J. O. Metzger, Angew.
Chem. 2002, 114, 2863; Angew. Chem. Int. Ed. 2002, 41, 2738;
b) S. Meyer, R. Koch, J. O. Metzger, Angew. Chem. 2003, 115,
4848; Angew. Chem. Int. Ed. 2003, 42, 4700; c) S. Meyer, J. O.
Metzger, Anal. Bioanal. Chem. 2003, 377, 1108; d) L. S. Santos,
[5/10+H]+
[6+H]+
360.19637
342.18478
344.20083
346.21654
360.19581
342.18524
344.20089
346.21654
C24H26NO2
C24H24NO
C24H26NO
C24H28NO
C74H81NO5P 1.0
C74H83NO5P 1.0
C74H85NO5P 0.5
1.6
1.3
0.2
0.0
[7+H]+
[2+H]+
[6·TRIP+H]+ 1094.58363 1094.58469
[7·TRIP+H]+ 1096.60141 1096.60034
[2·TRIP+H]+ 1098.61543 1098.61599
Angew. Chem. Int. Ed. 2009, 48, 1463 –1466
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1465

Communications
L. Knaack, J. O. Metzger, Int. J. Mass Spectrom. 2005, 246, 155;
e) L. S. Santos, Eur. J. Org. Chem. 2008, 235.
[10] a) C. Burstein, F. Glorius, Angew. Chem. 2004, 116, 6331; Angew.
Chem. Int. Ed. 2004, 43, 6205; b) W. Schrader, P. P. Handayani,
C. Burstein, F. Glorius, Chem. Commun. 2007, 716.
[5] a) C. Hinderling, C. Adlhart, P. Chen, Angew. Chem. 1998, 110,
2831; Angew. Chem. Int. Ed. 1998, 37, 2685; b) D. M. Tomazela,
L. A. B. Moraes, R. A. Pilli, M. N. Eberlin, J. Org. Chem. 2002,
67, 4652; c) D. Schrꢀder, J. Roithova, H. Schwartz, Int. J. Mass
Spectrom. 2006, 254, 197.
[6] a) S. A. Pelster, W. Schrader, F. Schꢁth, J. Am. Chem. Soc. 2006,
128, 4310; b) S. A. Pelster, F. Schꢁth, W. Schrader, Anal. Chem.
2007, 79, 6005.
[7] a) E. C. Meurer, L. S. Santos, R. A. Pilli, M. N. Eberlin, Org.
Lett. 2003, 5, 1391; b) A. Sabino, A. H. L. Machaldo, C. R. D.
Correia, M. N. Eberlin, Angew. Chem. 2004, 116, 2568; Chem.
Int. Ed. 2004, 43, 2514; c) L. S. Santos, C. H. Pavam, W. P.
Almeida, F. Coelho, M. N. Eberlin, Angew. Chem. 2004, 116,
4430; Angew. Chem. Int. Ed. 2004, 43, 4330; d) L. S. Santos, J. O.
Metzger, Angew. Chem. 2006, 118, 991; Angew. Chem. Int. Ed.
2006, 45, 977; e) C. A. Marquez, F. Fabbretti, J. O. Metzger,
Angew. Chem. 2007, 119, 7040; Angew. Chem. Int. Ed. 2007, 46,
6915; f) D. A. Yalalov, S. B. Tsogoeva, T. E. Shubina, I. M.
Martynova, T. Clark, Angew. Chem. 2008, 120, 6726; Angew.
Chem. Int. Ed. 2008, 47, 6624.
[11] a) J. W. Yang, M. T. H. Fonseca, B. List, J. Am. Chem. Soc. 2005,
127, 15036; b) Y. Huang, A. M. Walji, C. H. Larsen, D. W. C.
MacMillan, J. Am. Chem. Soc. 2005, 127, 15051; c) M. Marigo, T.
Schulte, J. Franzen, K. A. Jørgensen, J. Am. Chem. Soc. 2005,
127, 15710; d) S. Brandau, E. Maerten, K. A. Jørgensen, J. Am.
Chem. Soc. 2006, 128, 14986; e) B. List, Chem. Commun. 2006,
819; f) C. Palomo, A. Mielgo, Angew. Chem. 2006, 118, 8042;
Angew. Chem. Int. Ed. 2006, 45, 7876; g) A. Carlone, S. Cabrera,
M. Marigo, K. A. Jørgensen, Angew. Chem. 2007, 119, 1119;
Angew. Chem. Int. Ed. 2007, 46, 1101; h) D. Enders, C. Grondal,
M. R. M. Huettl, Angew. Chem. 2007, 119, 1590; Angew. Chem.
Int. Ed. 2007, 46, 1570.
[12] G. Adair, S. Mukherjee, B. List, Aldrichimica Acta 2008, 41, 31.
[13] J. Zhou, B. List, J. Am. Chem. Soc. 2007, 129, 7498.
[14] The results from GC supported this fact (for details, see the
Supporting Information).
[15] For a review on asymmetric Brønsted acid catalysis, see: T.
Akiyama, Chem. Rev. 2007, 107, 5744, and references therein.
[16] Also see: M. Sickert, C. Schneider, Angew. Chem. 2008, 120,
3687; Angew. Chem. Int. Ed. 2008, 47, 3631.
[8] a) P. Chen, Angew. Chem. 2003, 115, 2938; Angew. Chem. Int.
Ed. 2003, 42, 2832; b) C. Markert, A. Pfalz, Angew. Chem. 2004,
116, 2552; Angew. Chem. Int. Ed. 2004, 43, 2498.
[17] P. P. Handayani, S. Hoffmann, W. Schrader, B. List, unpublished
results.
[9] M. T. Hechavarria Fonseca, B. List, Angew. Chem. 2004, 116,
4048; Angew. Chem. Int. Ed. 2004, 43, 3958.
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