Chiral imidazolylidine ligands for asymmetric hydrogenation of aryl alkenes [14]

Full text of DOI:10.1021/ja016011p

8878
J. Am. Chem. Soc. 2001, 123, 8878-8879
of the process, are less prevalent than for analogous phosphine
complexes D.
Chiral Imidazolylidine Ligands for Asymmetric
Hydrogenation of Aryl Alkenes
Mark T. Powell, Duen-Ren Hou, Marc C. Perry,
Xiuhua Cui, and Kevin Burgess*
Department of Chemistry, Texas A & M UniVersity
PO Box 30012, College Station, Texas 77842-3012
Complexes of the carbenes C were prepared by forming the
iodides 1 from the corresponding tosylates12 and then by using
these to generate the imidazolium salts 2 (Scheme 1). Salts 2
were reacted with an iridium precursor as shown {the anion
exchange was with tetrakis(3,5-bis(trifluoromethyl)phenyl)borate
(BARF-)}.
ReceiVed April 12, 2001
ReVised Manuscript ReceiVed June 14, 2001
Imidazolylidines A, their saturated analogues imidazolinylidines
B, and related heterocyclic electron-rich carbenes, are useful
ligands for transition metals;1 they stabilize low oxidation states
giving robust complexes.2 Such complexes have been used for
several catalytic applications including alkene metathesis,3 hy-
drogenations,4 and C-C5 and C-N6 bond construction. These
reported transformations do not, however, include useful asym-
metric catalysis. Indeed, even though at least 10 different types
of chiral carbene ligands of this kind have been prepared and
complexed,2,7 to the best of our knowledge, the best enantio-
selectivities obtained are less than 76%.8,9
Scheme 1. Synthesis of the Catalysts
Reported here is design and synthesis of the ligand set C that
can give enantioselectivities of up to 98% in asymmetric
hydrogenations of E-aryl alkenes. This is especially notable
because asymmetric hydrogenations of this kind are difficult and
11
there are only a few practical systems for doing this.10,11
Moreover, the results obtained here indicate that competing
mechanistic pathways that could diminish the enantioselectivity
(1) Weskamp, T.; Bo¨hm, V. P. W.; Herrmann, W. A. J. Organomet. Chem.
2000, 600, 12.
(2) Gardiner, M. G.; Herrmann, W. A.; Reisinger, C.-P.; Schwarz, J.;
Spiegler, M. J. Organomet. Chem. 1999, 572, 239; Huang, J.; Schanz, H.-J.;
Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18, 2370.
(3) Lead references: Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001,
34, 18; Weskamp, T.; Kohl, F. J.; Hieringer, W.; Gleich, D.; Herrmann, W.
A. Angew. Chem., Int. Ed. 1999, 38, 2416.
(4) Lee, H. M.; Smith, J., D. C.; Stevens, E. D.; Yi, C. S.; Nolan, S. P.
Organometallics 2001, 20, 794; Lee, H. M.; Jiang, T.; Stevens, E. D.; Nolan,
S. P. Organometallics 2001, 20, 1255.
(5) Illustrative examples: Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher,
C.; Artus, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2371; Herrmann, W.
A.; Reisinger, C.-P.; Spiegler, M. J. Organomet. Chem. 1998, 557, 93;
Weskamp, T.; Bo¨hm, V. P. W.; Herrmann, W. A. J. Organomet. Chem. 1999,
585, 348; Huang, J.; Nolan, S. P. J. Am. Chem. Soc. 1999, 121, 9889;
McGuinness, D. S.; Cavell, K. J.; Skelton, B. W.; White, A. H. Organome-
tallics 1999, 18, 1596; Zhang, C.; Huang, J.; Trudell, M. L.; Nolan, S. P. J.
Org. Chem. 1999, 64, 3804; Bo¨hm, V. P. W.; Gsto¨ttmayr, C. W. K.; Weskamp,
T.; Herrmann, W. A. J. Organomet. Chem. 2000, 595, 186; McGuinness, D.
S.; Cavell, K. J. Organometallics 2000, 19, 741; Zhang, C.; Trudell, M. L.
Tetrahedron Lett. 2000, 41, 595; Peris, E.; Loch, J. A.; Mata, J.; Crabtree, R.
H. Chem. Commun. 2001, 201.
Hydrogenation of E-1,2-diphenylpropene was chosen as a
model reaction to test potential catalysts. Data obtained for the
phenyl-, diphenylmethyl-, and tert-butyl-substituted oxazoline
complexes 3a-c were not encouraging; the maximum enantio-
meric excess obtained using these was 40%. However, against
expectations, the 1-adamantyl oxazoline complex 3d gave almost
complete conversion and a near perfect enantioselectivity. Even
in retrospect, the markedly different enantioselectivities obtained
using tert-butyl- and 1-adamantyl-substituted ligands seem sur-
prising.
(6) Huang, J.; Grasa, G.; Nolan, S. P. Org. Lett. 1999, 1, 1307.
(7) Coleman, A. W.; Hitchcock, P. B.; Lappert, M. F.; Maskell, R. K.;
Mu¨ller, J. H. J. Organomet. Chem. 1983, 250, C9; Herrmann, W. A.; Goossen,
L. J.; Artus, G. R. J.; Kocher, C. Organometallics 1997, 16, 2472; Enders,
D.; Gielen, H.; Raabe, G.; Runsink, J.; Teles, J. H. Chem. Ber. 1997, 130,
1253; Herrmann, W. A.; Goossen, L. J.; Spiegler, M. Organometallics 1998,
17, 2162; Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1,
953; Clyne, D. S.; Jin, J.; Genest, E.; Galluci, J. C.; RajanBabu, T. V. Org.
Lett. 2000, 2, 1125.
(8) Enders, D.; Gielen, H.; Raabe, G.; Runsink, J.; Teles, J. H. Chem. Ber.
1996, 129, 1483; Herrmann, W. A.; Goossen, L. J.; Kocher, C.; Artus, G. R.
J. Angew. Chem., Int. Ed. Engl. 1996, 35, 2805.
(9) Enders, D.; Gielen, H.; Breuer, K. Tetrahedron: Asymmetry 1997, 8,
3571. Lee, S.; Hartwig, J. F. J. Org. Chem. 2001, 66, 3402.
(10) Broene, R. D.; Buchwald, S. L. J. Am. Chem. Soc. 1993, 115, 12569;
Troutman, M. V.; Appella, D. H.; Buchwald, S. L. J. Am. Chem. Soc. 1999,
121, 4916.
Data for hydrogenations of trisubstituted alkenes using the
adamantyl-derived catalyst 3d are collected in Table 1. The
enantioselectivities for hydrogenations of the E-alkenes ranged
from 84 to 98%, but Z- and 1,1-disubstituted alkenes gave lower
values (entries 6 and 7). Comparisons were performed using
catalyst 3c, but in all cases inferior enantioselectivities were
obtained (see Supporting Information).
Investigation of the analogous phosphine oxazolines D in
asymmetric hydrogenations of aryl alkenes provides data for
comparison with that obtained in this study.14 The results shown
(11) Lightfoot, A.; Schnider, P.; Pfaltz, A. Angew. Chem., Int. Ed. 1998,
37, 2897; Blackmond, D. G.; Lightfoot, A.; Pfaltz, A.; Rosner, T.; Schnider,
P.; Zimmermann, N. Chirality 2000, 12, 442; Hilgraf, R.; Pfaltz, A. Synlett
1999, 1814.
(12) Hou, D.-R.; Burgess, K. Org. Lett. 1999, 1, 1745; Hou, D.-R.;
Reibenspies, J.; Burgess, K. J. Org. Chem. 2000, 66, 206.
(13) Shitangkoon, A.; Vigh, G. J. Chromatogr. A 1996, 738, 31.
10.1021/ja016011p CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/18/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 36, 2001 8879
Table 1. Hydrogenations of Trisubstituted Alkenes
Figure 1. Relative levels of deuteration of alkenes 4 and 5 using catalysts
2
3d and 6, as measured by H NMR.
by deuterating alkenes 4 and 5 using an imidazolylidine oxazoline
complex 3d and a similar phosphine oxazoline complex 6. These
preliminary studies show that the carbene complex gave fewer
products that may be attributed to competing mechanistic
pathways. Indirect addition of hydrogen is a concern when using
either catalyst to hydrogenate Z- and 1,1-disubstitued alkenes,
but this phenomenon is less important for the carbene complex
3d than for the phosphine 6.
This research has demonstrated that chiral imidazolylidine
complexes can induce high enantioselectivities in catalytic
hydrogenations of E-aryl alkenes; this is the first report of high
induction using electron-rich carbene ligands in any catalytic
process. These ligands cannot present edge-face arrays of aryl
groups in the same way that many phosphine complexes do, and
thus control of enantioselectivities must originate from other
topographical features. These might be relatively subtle as
indicated by the higher enantioselectivities obtained for the
adamantyl complex 3d relative to the tert-butyl analogue 3c.
Studies of the Z- and 1,1-disubstituted alkenes 4 and 5, respec-
tively, indicate that the rate ratio for direct hydrogenation relative
to more circumspect pathways is higher for the iridium carbene
complex 3d than it is for the similar phosphine complex 6.
Consequently, subtle mechanistic advantages may be accrued by
using imidazolylidenes in preference to the corresponding phos-
phines. These should be considered along with the more obvious
attributes of the carbenes that relate to ease of ligand preparation
and catalyst stability.
a Determined via GC on a cyclodextrin-based chiral column13 versus
an internal standard. b Reaction temperature 0 °C. c Absolute config-
uration not determined.
in Table 1 are comparable with or superior to those obtained by
Pfaltz and co-workers3 and in our laboratories14 using similar
substrates in hydrogenations mediated by chiral phosphine ox-
azolines.
Our recent studies with analogous iridium complexes of
phosphine oxazolines D have shown that hydrogenations via other
mechanistic pathways can compete with the desired direct
hydrogenation process.14 Z- and 1,1-disubstitued alkenes are
particularly susceptible to this, whereas such complications are
much less prevalent for E-alkenes. Competing mechanistic
pathways such as double bond migrations make enantioselectivi-
ties more difficult to optimize, and they may be a contributing
factor if high induction cannot be obtained. Consequently, it is
relevant to compare the extent of these reactions leading to
“indirect hydrogenations” for analogous carbene C and phosphine
D complexes.
Acknowledgment. Support for this work was provided by Johnson
Matthey and by The Robert A. Welch Foundation.
Supporting Information Available: Data for hydrogenations with
complex 3c, experimental details for the preparation of compounds 1-3,
a general procedure for the hydrogenation (deuteration) experiments,
discussion of determination of absolute configurations and on solvent
effects in these reactions (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Evidence for indirect introduction of hydrogen was obtained
by deuterating, rather than hydrogenating, selected alkenes. Figure
1 shows the relative levels of deuterium incorporation obtained
(14) Hou, D.-R.; Reibenspies, J. H.; Colacot, T. J.; Burgess, K. Chem.s
Eur. J. 2001. In press.
JA016011P