CO assistance in ligand exchange of a ruthenium racemization catalyst: Identification of an acyl intermediate

Article abstract of DOI:10.1021/ja9038455

(Chemical Equation Presented) An acyl intermediate in the activation of η⁵-(Ph₅Cp)Ru(CO)₂Cl by t-BuOK was identified by means of in situ FT-IR measurements and NMR spectroscopy. This strongly supports the conclusion that t

Full text of DOI:10.1021/ja9038455

Published on Web 06/17/2009
CO Assistance in Ligand Exchange of a Ruthenium Racemization Catalyst:
Identification of an Acyl Intermediate
Jenny B. Åberg, Jonas Nyhle´n, Bele´n Mart´ın-Matute, Timofei Privalov, and Jan-E. Ba¨ckvall*
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm UniVersity, SE-106 91 Stockholm, Sweden
Received May 12, 2009; E-mail: jeb@organ.su.se
Mechanistic studies of hydrogen transfer reactions with ruthe-
nium catalysts have recently attracted considerable interest,1 and
these catalysts have found extensive applications in hydrogenations,
dehydrogenations, and racemizations.1a,2 For example, cyclopen-
tadienylruthenium catalysts 1-4 have successfully been employed
as racemization catalysts in dynamic kinetic resolution (DKR) of
sec-alcohols (Figure 1).3 The racemization catalyst is used in
combination with an enzyme and an acyl donor to afford enan-
tiopure acetates in high yields.
CO bands of 3 at 2049 and 2001 cm-1 (symmetric and asymmetric
stretch, respectively) with the concomitant appearance of a new
peak at 1933 cm-1 (Figure 2). After ∼1 min, the two CO bands of
the tert-butoxide complex 56 began to appear, and the intensity of
the peak at 1933 cm-1 decreased and disappeared in another 1-2
min. The single peak at 1933 cm-1 indicates that there is only one
CO ligand in the intermediate. The new intermediate (1933 cm-1
)
was assigned as acyl intermediate A.7
Figure 1. Ruthenium catalysts employed in dynamic kinetic resolution.
Ruthenium catalyst 3 is very efficient in racemizing sec-alcohols
via fast DKR reactions at room temperature,3a,b and we have
previously studied the racemization mechanism for this catalyst.3b,4
Complex 3 is activated by t-BuOK to give ruthenium tert-butoxide
complex 5 (Scheme 1, step i). In the subsequent alcohol-alkoxide
exchange, sec-alkoxide complex 7 is formed (Scheme 1, step ii).
Racemization via ꢀ-hydride elimination requires a free coordination
site, and this could be formed by either η5 f η3 ring slippage4 or
loss of a carbon monoxide ligand.5 tert-Butoxide complex 5 and
Figure 2. Surface view of in situ FT-IR measurements at room temperature,
showing the peak for acyl intermediate A.
1
sec-alkoxide complex 7 were characterized by H and 13C NMR
spectroscopy.3a,4 In addition, we showed that the ketone formed
from the ꢀ-hydride elimination is coordinated to ruthenium
throughout the racemization cycle and that the hydride
(Ph5Cp)Ru(CO)2H is not an active catalytic intermediate.3b,4
An interesting question is how the catalyst is activated and how
the ligand exchange takes place.4 Recent computational studies in
our group suggested that the ligand exchange takes place via
participation of a CO ligand, leading to an acyl intermediate.5 In
the present communication, we provide experimental evidence for
such an acyl intermediate by in situ FT-IR measurements and NMR
spectroscopy.
Acyl intermediate A has a lifetime of <3 min at room temper-
ature, and in addition to the IR band at 1933 cm-1 (CO), there is
another band at 1596 cm-1 (acyl). The acyl peak could be detected
only under very dry conditions,8 and it correlates well with similar
structures in the literature.9 The shift toward lower wavenumber
of the remaining CO ligand (1933 cm-1 vs 2049 and 2001 cm-1
)
is consistent with its anionic structure.9a The ruthenium center is
more electron-rich, and the stronger back-donation from the metal
makes the CdO bond weaker. At lower temperatures, intermediate
A was still formed very rapidly but was stable for a longer time: at
-70 °C, intermediate A was formed within 3 min and was stable
for more than 3 h, while at -55 °C, it was formed within 2 min
and was stable for at least 3 h (Figure S2 in the Supporting
Information).
Scheme 1. Proposed Mechanism of Racemization of sec-Alcohols
by Ruthenium Catalyst 3
As soon as the temperature was increased to room temperature,
tert-butoxide complex 5 was formed. The transformation into 5
occurred slowly at -20 °C (t1/2 ≈ 100 min) and more rapidly at
0 °C (t1/2 ) 26 min).
1
We also studied intermediate A by H and 13C NMR spectros-
copy. In our previous NMR studies of alkoxides 5 and 7,
intermediate A was not observed.3a,4 This is not surprising, since
catalyst 3 and t-BuOK were mixed at room temperature before they
were put into the NMR spectrometer, and the lifetime of 3 at this
temperature is <3 min. One problem with studying this reaction
Reaction of ruthenium chloride 3 with t-BuOK at room temper-
ature resulted in a color change from yellow to orange-red. In situ
FT-IR monitoring showed the immediate disappearance of the two
9
9500 J. AM. CHEM. SOC. 2009, 131, 9500–9501
10.1021/ja9038455 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
by NMR spectroscopy is the poor solubility of ruthenium chloride
3 in toluene and the high viscosity of toluene at low temperature.
Sample preparation was performed by quick transfer of a -78 °C
preformed solution of the intermediate to an NMR tube via a
cannula. The sample was then frozen (-196 °C) and quickly
transferred to the precooled probe of the NMR spectrometer. The
low solubility and therefore low concentration made it difficult to
run 13C NMR experiments, since it was not possible to get a
reasonable signal-to-noise ratio for the carbonyl peaks. This problem
was solved by employing 13CO-labeled catalyst 3 (for its prepara-
tion, see the Supporting Information). With the 13CO-enriched
ruthenium chloride 3, it was possible to monitor the reaction by
13C NMR at low temperature. Acyl intermediate A was studied
and characterized at -32 °C (Scheme 2).
previous NMR studies of the alkoxide exchange,4 and therefore,
the reaction was slower here than in the previous experiments. The
failure to detect an intermediate in the alcohol-alkoxide exchange
is in accordance with the low computed barrier of activation for
this reaction via CO participation (12 kcal/mol).5,14
In conclusion, we have provided experimental evidence for CO
ligand participation in the exchange of chloride for tert-butoxide
in η5-(Ph5Cp)Ru(CO)2Cl (3). An acyl intermediate, A, was observed
by in situ FT-IR measurements and low-temperature NMR spec-
troscopy prior to formation of tert-butoxide complex 5. This shows
how high-energy intermediates (produced via ring slip or dissocia-
tion) can be avoided in ligand exchange15 and that this may be
common in complexes bearing CO ligands.
Acknowledgment. The Swedish Research Council, the Berze-
lius Center EXSELENT, and the Knut and Alice Wallenberg
Foundation are gratefully acknowledged for financial support.
Scheme 2. Detection of Acyl Intermediate A by 13C NMR in
Toluene-d8 at -32 °C
Supporting Information Available: Experimental procedures and
characterization data for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Samec, J. S. M.; Ba¨ckvall, J.-E.; Andersson, P. G.; Brandt, P. Chem.
Soc. ReV. 2006, 35, 237. (b) Casey, C. P.; Singer, S. W.; Powell, D. R.;
Hayashi, R. K.; Kavana, M. J. Am. Chem. Soc. 2001, 123, 1090. (c) Casey,
C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998. (d) Guan, H.; Iimura,
M.; Magee, M. P.; Norton, J. R.; Zhu, G. J. Am. Chem. Soc. 2005, 127,
7805. (e) Casey, C. P.; Clark, T. B.; Guzei, I. A. J. Am. Chem. Soc. 2007,
129, 11821. (f) Åberg, J.; Ba¨ckvall, J.-E. Chem.sEur. J. 2008, 14, 9169.
(g) Hamilton, R. J.; Bergens, S. H. J. Am. Chem. Soc. 2008, 130, 11979.
(2) (a) Gladiali, S.; Alberico, E. Chem. Soc. ReV. 2006, 35, 226. (b) Ikariya,
T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300.
(3) (a) Mart´ın-Matute, B.; Edin, M.; Boga´r, K.; Ba¨ckvall, J.-E. Angew. Chem.,
Int. Ed. 2004, 43, 6535. (b) Mart´ın-Matute, B.; Edin, M.; Boga´r, K.; Kaynak,
F. B.; Ba¨ckvall, J.-E. J. Am. Chem. Soc. 2005, 127, 8817. (c) Ko, S. B.;
Baburaj, B.; Kim, M. J.; Park, J. J. Org. Chem. 2007, 72, 6860. (d) Choi,
J. H.; Choi, Y. K.; Kim, Y. H.; Park, E. S.; Kim, E. J.; Kim, M.-J.; Park,
J. J. Org. Chem. 2004, 69, 1972. (e) Choi, J. H.; Kim, Y. H.; Nam, S. H.;
Shin, S. T.; Kim, M.-J.; Park, J. Angew. Chem., Int. Ed. 2002, 41, 2373.
(f) Mart´ın-Matute, B.; Ba¨ckvall, J.-E. Curr. Opin. Chem. Biol. 2007, 11,
226.
Acyl intermediate A has CO peaks at 209.5 (CO) and 208.7 ppm
(acyl).10 The peak of the CO ligand is shifted downfield. This is
comparable to the chemical shifts reported for other anionic
complexes (e.g. 208.1 ppm for [PPN]+[Ru3(CO)11(CO2CH3)]- (8)
in THF-d8).11 The acyl peak of A appears at an unusually high
chemical shift compared with those of similar structures in the
literature.9b,d,11 For example, the acyl peak of 8 appears at 189
ppm, and the neutral cyclopentadienylruthenium complexes
[Cp*Ru(CO)2(COOMe)] and [Cp∧Ru(COOMe)(CO)2]12 also have
acyl peaks in this region [191.3 ppm (C6D6)9d and 193.7 ppm
(CD2Cl2),9b respectively]. However, none of these structures has a
chloride bound to ruthenium. We estimated the 13C NMR shifts of
intermediate A using density functional theory (DFT) calculations
and found that they are in agreement with the expected values for
this structure.13 When the temperature was raised to 0 °C, the peaks
at 209.5 and 208.7 ppm for intermediate A slowly disappeared,
and the peak at 202.8 ppm for the CO groups of tert-butoxide
complex 5 appeared. At room temperature, this transformation was
rapid.
In addition, we studied the alcohol-alkoxide exchange (Scheme
1, step ii), which also should occur via CO assistance.5 However,
no acyl intermediate similar to A has been detected to date by in
situ FT-IR measurements at room temperature and below (Figure
S1). At room temperature, the alcohol-alkoxide exchange was very
rapid (<1 min), but at -78 °C the tert-butoxide ligand of complex
5 was not exchanged by phenylethanol for at least 1 h. However,
at -50 °C, the reaction slowly took place (t1/2 ) 15 min) and the
CO peaks of sec-alkoxide complex 7 at 2026 and 1971 cm-1
(symmetric and asymmetric stretch, respectively) appeared. The rate
of disappearance of tert-butoxide complex 5 was the same as the
rate of formation of sec-alkoxide complex 7, and the reaction was
complete within 2.5 h. The ruthenium concentration in this
experiment was 12 mM, which is ∼5 times lower than in our
(4) Mart´ın-Matute, B.; Åberg, J. B.; Edin, M.; Ba¨ckvall, J.-E. Chem.sEur. J.
2007, 13, 6063.
(5) Nyhle´n, J.; Privalov, T.; Ba¨ckvall, J.-E. Chem.sEur. J. 2009, 15, 5220.
(6) The CO bands of 5 appear at 2021 and 1964 cm-1. For experimental details,
see the Supporting Information.
(7) The reaction mixture containing intermediate A also showed characteristic
peaks in the area of C-O stretching vibrations (t-BuO). However, the
mixture possibly contained t-BuOH (and t-BuOK), and the peaks in this
area were not easy to assign.
(8) H2O vapor in the system absorbed in the region of 1600 cm-1, and a
background spectrum was automatically subtracted from the collected data.
If the amount of H2O vapor was not constant (which it seldom is), a peak
in this region might not be visible. Indeed, in our first experiments, the
system contained so much humidity that a lot of noise in this region made
it difficult to detect the acyl peak.
(9) (a) Gross, D. C.; Ford, P. C. J. Am. Chem. Soc. 1985, 107, 585. (b) Dutta,
B.; Scopelliti, R.; Severin, K. Organometallics 2008, 27, 423. (c) Trautman,
R. J.; Gross, D. C.; Ford, P. C. J. Am. Chem. Soc. 1985, 107, 2355. (d)
Suzuki, H.; Omori, H.; Mor-oka, Y. J. Organomet. Chem. 1987, 327, C47.
(10) The shifts were too close to allow a definite assignment, and they may be
reversed (DFT calculations indicated that δCO > δacyl; see note 13). Even if
the shifts are reversed, the structural conclusions remain the same.
(11) Taube, D. J.; Rokicki, A.; Anstock, M.; Ford, P. C. Inorg. Chem. 1987,
26, 526.
(12) Cp* ) Me5Cp; Cp∧ ) 1-MeO-2,3,4-tris(neopentyl)Cp.
(13) The estimated 13C NMR shifts were 205.3 (acyl) and 211.4 ppm (CO). For
experimental details, see the Supporting Information.
(14) The computed barrier for the corresponding exchange of chloride for tert-
butoxide in 3 is 16.7 kcal/mol (see ref 5).
(15) For a discussion of avoiding high-energy pathways in ligand exchange,
see: Casey, C. P.; Vosejpka, P. C.; Underiner, T. L.; Slough, G. A.; Gavney,
J. A. J. Am. Chem. Soc. 1993, 115, 6680.
JA9038455
9
J. AM. CHEM. SOC. VOL. 131, NO. 27, 2009 9501