Catalytic transformation of aldimine to ketimine by Wilkinson's complex through transimination

Article abstract of DOI:10.1021/ol990158s

(equation presented) The C-H bond activation in homogeneous catalysis is important for carbon skeleton construction through C-C bond formation. In this report, a new direct synthesis of ketimine from aldimine bearing no coordination site is demonstrated with high catalytic efficiency. Transimination is the major role for this catalytic reaction.

Full text of DOI:10.1021/ol990158s

ORGANIC
LETTERS
1999
Vol. 1, No. 6
887-889
Catalytic Transformation of Aldimine to
Ketimine by Wilkinson’s Complex
through Transimination
Chul-Ho Jun* and Jun-Bae Hong
Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea
junch@alchemy.yonsei.ac.kr
Received July 9, 1999
ABSTRACT
The C−H bond activation in homogeneous catalysis is important for carbon skeleton construction through C−C bond formation. In this report,
a new direct synthesis of ketimine from aldimine bearing no coordination site is demonstrated with high catalytic efficiency. Transimination
is the major role for this catalytic reaction.
The C-C bond formation by transition metal catalyzed
reaction involving C-H bond activation is one of the most
active and challenging areas in organometallic research.1 Up
to now, the addition of an sp2 C-H bond to an unsaturated
C-C bond has been extensively studied and applied to new
C-C bond formation in organic synthesis.2 The sp2 C-H
bond of aldimine is added to an unsaturated C-C bond to
give ketimine, which is an important synthetic intermediate
since it performs a significant role as a precursor to amine
by the asymmetric catalytic hydrogenation.3 Especially, the
ketimine, produced through alkylation of aldimine, is a
precursor for ketone through a convenient hydrolysis process.
But the C-H bond of aldimine is inert to transition metal
catalysts, except to a model compound bearing a heteroatom
for cyclometalation, which solves the problem of accessibility
between the catalyst and the carbon-hydrogen bond.4 Below,
we report a new type of catalytic system for the direct
conversion of a common aldimine into a ketimine which is
not confined within a specially designed aldimine. This
reaction is a remarkably efficient catalytic system in com-
parison with other catalytic C-H/ olefin coupling reactions.
(1) For recent reviews, see: (a) Ryabov, A. D. Chem. ReV. 1990, 90,
403-424. (b) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879-
2932. For papers, see: (c) Jun, C.-H.; Hwang, D.-C.; Na, S.-J. Chem.
Commun. 1998, 1405-1406. (d) Niu, S.; Hall, M. B. J. Am. Chem. Soc.
1998, 120, 6169-6170.
(2) (a) Ishii, Y.; Chatani, N.; Kakiuchi, F.; Murai, S. Organometallics
1997, 16, 3615-3622. (b) Lim, Y.-G.; Kang, J.-B.; Kim, Y. H. Chem.
Commun. 1996, 585-586. (c) Radu, N. S.; Buchwald, S. L.; Scott, B.;
Burns, C. J. Organometallics 1996, 15, 3913-3915. (d) Cenini, S.;
Raganiani, S.; Tollani, S.; Paone, D. J. Am. Chem. Soc. 1996, 118, 11964-
11965. (e) Chatani, N.; Fukuyama, T.; Murai, S. J. Am. Chem. Soc. 1996,
118, 493-494. (f) Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.;
Sonoda, M.; Chatani, N.; Murai, S. Bull. Chem. Soc. Jpn. 1995, 68, 62-83
and therein references.
In our experiment, N-phenyl-N-(1-phenylmethylidene)-
amine (1a) reacted with 1-hexene (2a) in toluene at 130 °C
for 24 h under a mixture of 0.5 mol % of chlorotris-
(3) (a) Verdaguer, X.; Lange, U. E. W.; Reding, M. T.; Buchwald, S. L.
J. Am. Chem. Soc. 1996, 118, 6784-6785. (b) Buriak, J. M.; Osborn, J. A.
Organometallics 1996, 15, 3161-3169. (c) Brunel, J. M.; Buono, G. Synlett
1996, 177-178. (d) Spindler, F.; Pugin, B.; Blaser, H.-U. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 558-559.
(4) For C-H bond activation, see: (a) Suggs, J. W. J. Am. Chem. Soc.
1979, 101, 489. (b) Jun, C.-H.; Kang, J.-B.; Kim, J.-Y. J. Organomet. Chem.
1993, 458, 193-198. (c) Jun, C.-H.; Han, J.-S.; Kang, J.-B.; Kim, S.-I. J.
Organomet. Chem. 1994, 474, 183-189. For C-C bond activation, see:
(d) Jun, C.-H.; Lee, H. J. Am. Chem. Soc. 1999, 121, 880-881.
10.1021/ol990158s CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/21/1999

(triphenylphosphine)rhodium(I) (3) and 10 mol % of 2-amino-
3-picoline (4) as a cocatalyst system based upon 1a (Scheme
1). Following the reaction, N-phenyl-N-(1-phenylheptylide-
ne)amine (5a) was isolated in 90% yield by column chro-
matography.5
tion, previously studied: the C-H bond activation of 6 by
3 to give 9, a hydride insertion of 9 into 2a to form 10, and
reductive elimination in 10 to generate 8.4a Then the second
transimination of 8 with 7 produces 5a with regeneration of
4.
To identify the involvement of the transimination of 1a
with 4 in the reaction pathway, 1a was allowed to react with
3 (10 mol %) and 4 (20 mol %) in C6D6 at 130 °C for 2 h
Scheme 1. Alkylation of Aldimine through C-H Bond
1
without olefin 2a, and it was observed by H NMR spectra
Activation
that 50% of 4 was transformed into complex 9, which should
be formed from 6.7
Various imines, 1b-1e, reacted with 2a to give corre-
sponding ketimines, 5b-5e, in fairly high yield except 1c
under identical reaction conditions. The reason for the low
yield of 5c from 1c is that the electron-donating substituent
in aldimine may inhibit facile transimination due to reduced
electrophilicity of the imine-carbon center. Among imines,
hydrazone 1f and oxime 1g did not undergo alkylation with
2a.8 This result can be explained by the fact that 1f and 1g
are too stable to undergo transimination with 4 since the
thermodynamic stability of CdN bond in imine increases
in the order of imine of NH3 < aliphatic amine < aromatic
amine < amine with an adjacent electronegative atom.9
When the concentration of 4, one of the catalysts, was
changed from 0 to 100 mol % based upon 1a, the best results
(94-98% yield of 5a) were obtained with 10-20 mol % of
4 as shown in Figure 1. As the concentration of 4 was
However, when the reaction was carried out without 4,
the starting aldimine 1a was completely recovered, implying
no direct conversion of 1a to 5a. The proposed mechanism
is shown in Scheme 2. The first step must be the formation
Scheme 2. Transimination and Alkylation Process
of aldimine 6 through transimination of 1a with 4, liberating
aniline (7). There are some reports about transimination for
the synthesis of the desired imine from primary imine and
amine.6 The resulting aldimine 6 reacts with olefin to give
ketimine 8 on the rhodium(I) catalyst by hydroiminoacyla-
(5) In a typical experiment, a mixture of imine 1a (117.4 mg, 0.649
mmol) and 1-hexene (2a) (272 mg, 3.24 mmol) in a screw-capped pressure
vial (1 mL) was heated at 130 °C for 24 h with [chlorotris(triphenylphos-
phine)rhodium(I)] (3) (3 mg, 0.00324 mmol) and 2-amino-3-picoline (4)
(7.0 mg, 0.0649 mmol). The reaction mixture was cooled to room
temperature and purified by column chromatography (n-hexane:ethyl acetate
) 5:2) to give 144.5 mg (84%) of N-(1-phenylheptylidene)aniline (5a) and
7.4 mg (6%) of heptanophenone (11a). 5a:1H NMR (250 MHz, CDCl3) δ
(ppm) 7.9 (m, 2H), 7.4-6.8 (m, 8H), 2.6 (t, J ) 7.9 Hz, 2H, R-CH2 to
CdN), 1.4-1.1 (m, 8H), 0.8 (t, J ) 6.8 Hz, 3H, -CH3); 13C NMR (62.9
MHz, CDCl3) δ (ppm) 169.7 (CdN), 150-120 (Cs in phenyl group), 31.4
(R-CH2 to CdN), 30.1 (δ-CH2 to CdN), 29.1 (γ-CH2 to CdN), 27.8 (ꢀ-
CH2 to CdN), 22.3 (â-CH2 to CdN), 13.8 (CH3 in hexyl group); MS m/z
(%) 265 (9) [M+], 208 (42), 193 (30), 173 (34), 129 (35), 117 (100), 115
(70), 93 (31), 77 (42); IR (neat) 3054, 3027, 2952, 2924, 2854, 1685, 1625,
1588, 1485, 1443, 1313, 1206, 1024, 768, 689 cm-1; HRMS calcd for
C19H23N1 (M+) 265.183 050, found 265.183 044.
Figure 1. Effect of 2-amino-3-picoline: 1a (0.65 mmol) reacted
with 2a (3.2 mmol) under 0.5 mol % of 3 (0.0032 mmol) and
0-100 mol % of 4 at 130 °C for 12 h, and the yield of product 5a
is determined by GC.
increased above 30 mol %, the yield of 5a was dramatically
decreased. The phosphine ligand in this catalytic reaction is
very important because it may enforce the facile reductive
elimination in intermediate 10. However, excess use of 4
may inhibit the phosphine ligand-promoted reductive elimi-
888
Org. Lett., Vol. 1, No. 6, 1999

nation by occupying the coordination site of 10 with 4.
Therefore the reaction scarcely proceed in excess use of 4.
This postulate was confirmed by the fact that when an
identical reaction was carried out under 0.5 mol % of
chlorobis(cyclooctene)rhodium(I) dimer that has no phos-
phine ligand, no alkylation product was obtained. When the
reaction was carried out in increasing order of the additional
triphenylphosphine (30-40 mol %) under 40 mol % of 4,
the catalytic activity of complex 3 was completely restored
as shown in Figure 2.
reductive elimination in 10, probably due to the high
concentration of 6 compared with that of 3.10
The ketimine, produced through alkylation of aldimine,
is a precursor for the ketone since it is easily hydrolyzed to
give ketone. Heptanophenone (11a) was isolated in 88%
yield from 1a through acid hydrolysis of the resulting
ketimine 5a (Scheme 3).
Scheme 3. Ketone Synthesis through Transimination
Figure 2. Effect of PPh3: 1a (0.65 mmol) reacted with 2a (3.2
mmol) under 0.5 mol % of 3 (0.0032 mmol), 40 mol % of 4 (0.26
mmol), and 0-100 mol % of PPh3 at 130 °C for 12 h, and the
yield of product 5a is determined by GC.
This catalytic system is by far the most effective one in
comparison with a previously reported ketone synthesis from
aldehyde and olefin, namely intermolecular hydroacyla-
tion.11,12 Various olefins were used in the alkylation of
aldimine 1a to give the corresponding ketimines at a fairly
high conversion rate, which were isolated in the form of
ketone through acid hydrolysis.
In summary, we have demonstrated that aldimines bearing
no coordination site can be converted into ketimines by a
rhodium(I) catalyst through transimination. This catalytic
system showed excellent efficiency in catalytic turnover
through the use of only 0.5 mol % of Rh complex.
Even with aldimine 6 as a starting material, previously
studied,4a hydroiminoacylation of 1-hexene (2a) barely
proceeds under 0.5 mol % of 3 (<1% yield of 8). The reason
might be that liberated triphenylphosphine cannot induce
(6) (a) Zandbergen, P.; van den Nieuwendijk, A. M. C. H.; Brussee, J.;
van den Gen, A.; Kruse, C. G. Tetrahedron 1992, 48, 3977-3982. (b)
Hulsbos, E.; Marcus, J.; Brussee, J.; van den Gen, A. Tetrahedron:
Asymmetry 1997, 8, 1061-1067. (c) de Vries, E. F. J.; Steenwinkel, P.;
Brussee, J.; Kruse, C. G.; van den Gen, A. J. Org. Chem. 1993, 58, 4315-
4325.
(7) 9: 1H NMR (250 MHz, C6D6) δ (ppm) 2.7 (s, 3H, -CH3), -10.4
(overlapping d of t, JRh-H ) 13.3 Hz, JP-H ) 12.4 Hz, 1H, Rh-H).
(8) The oxime 1g was completely converted to phenylamide by a
rhodium(I)-catalyzed Beckmann rearrangement: Gawley, R. E. In Organic
Synthesis; Kende, A. S., Ed.; John Wiley & Sons: New York, 1988; Vol.
35, pp 14-43.
Acknowledgment. This work was supported by the
Korean Science and Engineering Foundation (Grant 97-05-
01-05-01-3).
(9) Ma¨kela¨, M. J.; Korpela, T. K. Chem. Soc. ReV. 1983, 12, 309-329.
(10) With increasing order of additional triphenylphosphine under
identical reaction conditions, the GC yield of 8 was gradually increased
(7% yield of 8 with 40 mol % of additional PPh3; 21% yield with 100 mol
% of PPh3).
(11) While 5-10 mol % of Rh catalyst has been used in a previously
reported reaction, this system requires only 0.5 mol % of Rh catalyst. The
hydroacylation of 1-hexene (2a) with benzaldehyde in the presence of 4
produced only a 9% yield of 11a under the reaction conditions of 0.5 mol
% of Rh catalyst. The improvement of efficiency for this catalytic reaction
is probably due to the adequate ratio of 6 to 3: (a) Jun, C.-H.; Lee, H.;
Hong, J.-B. J. Org. Chem. 1997, 62, 1200-1201. (b) Jun, C.-H.; Lee, D.-
Y.; Hong, J.-B. Tetrahedron Lett. 1997, 38, 6673-6676. (c) Jun, C.-H.;
Huh, C.-W.; Na, S.-J. Angew. Chem. Int. Ed. 1998, 37, 145-147.
Supporting Information Available: General experimen-
tal procedures for alkylation and the characterization of
compounds 5a-e. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL990158S
(12) (a) Lenges, C. P.; Brookhart, M. J. Am. Chem. Soc. 1997, 119,
3165-3266. (b) Kokubo, K.; Murai, M.; Nomura, M. Organometallics 1995,
14, 4521-4524. (c) Kondo, T.; Akazome, M.; Tsuji, Y.; Watanabe, Y. J.
Org. Chem. 1990, 55, 1286-1291. (d) Kondo, T.; Tsuji, Y.; Watanabe, Y.
Tetrahedron Lett. 1987, 28, 6229-6230.
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