Unexpected formation of a cyclopentadienylruthenium alkoxycarbonyl complex with a coordinated C=C bond

Article abstract of DOI:10.1021/ja905741w

(Chemical Equation Presented) Reaction of (η⁵-Ph ₅C₅)Ru(CO)₂(Ot-Bu) with 5-hexen-2-ol produces two diastereomers of an alkoxycarbonyl complex having the double bond coordinated to ruthenium. The complexes were c

Full text of DOI:10.1021/ja905741w

Published on Web 09/04/2009
Unexpected Formation of a Cyclopentadienylruthenium Alkoxycarbonyl
Complex with a Coordinated CdC Bond
Jenny B. Åberg, Madeleine C. Warner, and Jan-E. Ba¨ckvall*
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm UniVersity, SE-106 91 Stockholm, Sweden
Received July 11, 2009; E-mail: jeb@organ.su.se
Ruthenium catalyst 1 has successfully been used in combination
with lipases to convert racemic sec-alcohols 2 into enantiopure
acetates 3 in high yields and excellent ee’s via dynamic kinetic
resolution (DKR) (Scheme 1).1 In connection with our work on
chiral diols,2 one strategy involved DKR of 5-hexen-2-ol (4)
followed by transformation of the double bond into a chlorohydrin
and subsequent DKR of the chlorohydrin alcohol.1c However, DKR
of olefinic alcohol 4 has not been successful to date because of
very slow racemization, which suggests that the double bond may
coordinate to ruthenium and inhibit racemization. In the present
work, we have studied the reaction of Ru catalyst 1 with olefinic
alcohol 4 by NMR spectroscopy and in situ FT-IR measurements
and obtained evidence for the formation a novel (alkoxycarbonyl)Ru
olefin complex.
in toluene-d8 in an NMR tube. 1H NMR analysis showed the
disappearance of 6 and the formation of a 1:1 mixture of two
diastereomers8 that have the CdC bond coordinated to ruthenium.
Initially, three possible structures, 8a-b (only one CO), 9a-b
(η3-Ph5C5), and 10a-b (acyl compound) were considered (Figure
1). We argued that the olefin may coordinate to the open site on
Ru required for ꢀ-hydride elimination, which would dramatically
slow the racemization. For simplicity, the two diastereomers that
are formed are herein designated as a and b.
Scheme 1. DKR of sec-Alcohols (CALB ) Candida antarctica
Lipase B)
Figure 1. Possible structures of the two diastereomers having a coordinated
double bond.
1
From the H NMR spectrum, it is clear that the double bond is
coordinated to ruthenium. The proton at the internal carbon of the
double bond (H5) appears at 4.52 and 4.28 ppm for a and b,
respectively. This corresponds to an upfield shift of 1.2-1.5 ppm
relative to the corresponding proton of 4 (5.74 ppm) and can be
compared to the 0.6 ppm upfield shift of the double-bond protons
of trans-3-hexene upon coordination to ruthenium.9 The terminal
double bond protons (H6 and H6′) of both complexes appear as a
multiplet at 2.22-2.37 ppm, which corresponds to an upfield shift
of ∼2.6 ppm relative to the analogous protons of 4 (5.01 and 4.93
ppm). An upfield shift of similar magnitude upon coordination to
ruthenium has previously been reported in the literature.10 Also,
the 13C NMR (toluene-d8) signals of the double-bond carbons are
shifted upfield. The internal olefin carbon (C5) is shifted from 139.0
ppm (free alcohol) to 85.6 and 84.5 ppm for b and a, respectively.
This is in agreement with the literature values for similar com-
plexes.9 The terminal carbon (C6) is shifted even further upfield,
from 114.5 ppm (4) to 1.9 (a) and 19.4 (b) ppm.
Various ruthenium catalysts have been successfully employed
in (transfer) hydrogenation and racemization.3,4 We have previously
studied the racemization mechanism of sec-alcohols by Ru catalyst
1.5 Complex 1 is activated by t-BuOK to give tert-butoxide complex
6. We recently demonstrated that this ligand exchange takes place
via assistance of coordinated CO with the formation of an acyl
intermediate A (Scheme 2),6 which undergoes rapid alkoxide
migration from carbon to ruthenium at room temperature to give
complete conversion to tert-butoxide complex 6 within a few
minutes. Complex 6 subsequently undergoes an alcohol-alkoxide
exchange to give a sec-alkoxide complex 7. Racemization via
ꢀ-hydride elimination requires a free coordination site, which could
be formed by either η5 f η3 ring slippage5 or loss of CO.7
Scheme 2. Mechanism of the Racemization of sec-Alcohols by 1
The R-CH protons (H2) of the diastereomers a and b appear at
4.21 and 3.90 ppm, respectively, i.e., they are shifted ∼0.5 ppm
downfield relative to the free alcohol (3.49 ppm).11 This is similar
to the downfield shift of the R-CH proton of the 1-phenylethoxide
ligand of 7 (Scheme 2) and would seem to support structures 8
and 9 (Figure 1). However, a downfield shift for the R-CH protons
is also expected for structures 10. In addition, the 13C NMR peaks
of the R-carbon (C2) are shifted downfield from 67.1 ppm to 75.1
and 74.1 ppm for b and a, respectively.
Since η5 f η3 ring slippage1b,5 or loss of CO7 is suggested in
the racemization, we were eager to find out whether a and b had
one or two CO groups remaining on Ru. The 13C NMR spectrum
showed three peaks in a 2:1:1 ratio between 203 and 204 ppm
(Figure 2).
The reaction between alcohol 4 and complex 6 was first studied
at room temperature. Complex 6 was mixed with 4 (1.1 equiv)
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C O M M U N I C A T I O N S
is that the acyl carbon is expected to shift further upfield than the
peaks observed for a and b.14 Since only one signal was detected
in the 13C NMR spectrum for the cyclopentadienyl carbons (at 108.0
ppm for both complexes), the ligand can be η3-coordinated only if
there is a fast equilibrium between the five carbons of the
cyclopentadienyl ring. This would make these five carbons equiva-
lent at room temperature. Therefore, we studied the diasteromeric
complexes a and b at low temperature. At 0 °C, the peak at 108.0
ppm splits into two peaks in a 1:1 ratio. At lower temperatures,
these peaks are further separated: e.g., at -55 °C they appear at
107.71 and 107.74 ppm, respectively, and at -78 °C they appear
at 107.42 and 107.46 ppm, respectively. There is no significant
change in the line width in the low-temperature 13C NMR spectra,
which rules out exchange at low temperature and makes the η3-
coordinated structures 9 unlikely. The observed data are therefore
best explained by the two diastereomers a and b having η5-
coordinated cyclopentadienyl ligands, coincidently appearing at the
same chemical shift at room temperature and separating at lower
temperatures. Therefore, structures 10 seem to be the only pos-
sibility for alkene complexes a and b. The 13C NMR shifts of the
acyl carbon (203.48 and 203.55 ppm, respectively) are further
downfield than those for similar compounds in the literature.14
However, a similar unexpected downfield shift in the signal of the
acyl carbon of complex A was recently observed.6
Figure 2. 13C NMR spectrum of complexes a and b from the reaction
between tert-butoxide complex 6 and olefin alcohol 4.
The reaction between olefinic alcohol 4 and tert-butoxide
complex 6 was also studied by in situ FT-IR measurements. The
reaction of 4 with complex 6 was slow and could not be followed
to completion, since decomposition started after ∼1 h. One CO
peak (1982 cm-1) in the 1900-2100 cm-1 region was formed
at the same rate at which the CO peaks of complex 6 (2021 and
1964 cm-1) disappeared. In addition, it was possible to detect a
peak in the region where acyl peaks commonly appear (1644
cm-1) (Figure 5).6,14a,b,15
Figure 3. Carbonyl signals of 13CO-labeled complexes a and b (55% 13CO).
These results could be interpreted in different ways, but we were
able to understand the appearance of the 13C NMR spectrum when
13CO-enriched tert-butoxide complex 6 (55% 13C) was used in the
reaction with olefinic alcohol 4. The 13C NMR spectrum shows a
coupling (JCC ≈ 8.2 Hz) between the 13CO groups. In addition to
these doublets, singlets from the 13CO groups having a neighboring
12CO group were observed. Overall this made the signals look like
triplets (Figure 3). The coupling between two CO groups indicates
that the alkene complexes have two CO groups each. The signal at
lower field (204.0 ppm) corresponds to two overlapping CO groups.
The observation of two carbonyl groups in each of compounds a
and b rules out structure 8 as a possible structure for the alkene
complexes. Furthermore, no free CO could be detected at 184.7
ppm.12
This was further confirmed by 2D NMR spectroscopy. Thus,
the HMBC spectrum13 of the 13C-labeled diastereomers a and b
clearly showed cross-peaks between the olefin protons and two CO
groups each (Figure 4).
Of the remaining structures for alkene complexes a and b, acyl
compounds 10 are expected to be more stable than η3-coordinated
compounds 9 (Figure 1). The only argument against complexes 10
Figure 5. FT-IR spectra before (pink, t ) 0 min) and after (blue, t ) 29
min) the addition of 5-hexene-2-ol (4) to tert-butoxide complex 6.
The peak at 1644 cm-1 was formed at the same rate as the CO
peak at 1982 cm-1 (see the peak height analysis in Figure S4 in
the Supporting Information), and this further supports the assign-
ment of the diasteromeric complexes a and b as alkoxycarbonyl
complexes 10a-b (Figure 1).
Possible pathways for the formation of alkoxycarbonyl complexes
10a-b are given in Scheme 3. The first step would be an
alcohol-alkoxide exchange similar to that depicted in Scheme 2,
which would produce alkoxide complex 11. Racemization of sec-
alcohols via alkoxide complexes has been proposed to proceed via
ꢀ-hydride elimination, which requires the formation of a free
coordination site on ruthenium.16 This could be provided by either
η5 f η3 ring slippage5 or dissociation of a CO ligand7 (Scheme 3,
step i). Two competing reactions may fill this free coordination
site: in addition to ꢀ-hydride elimination (step ii), which would
Figure 4. Cross-peaks in the HMBC spectrum additionally confirm that
each of the diastereomers a and b must have two coordinated CO
groups.
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C O M M U N I C A T I O N S
(c) Tra¨ff, A.; Boga´r, K.; Warner, M.; Ba¨ckvall, J.-E. Org. Lett. 2008, 10,
4807–4810.
(2) (a) Leijondahl, K.; Bore´n, L.; Braun, R.; Ba¨ckvall, J.-E. Org. Lett. 2008,
10, 2027–2030. (b) Leijondahl, K.; Bore´n, L.; Braun, R.; Ba¨ckvall, J.-E. J.
Org. Chem. 2009, 74, 1988–1993. (c) Bore´n, L.; Leijondahl, K.; Ba¨ckvall,
J.-E. Tetrahedron Lett. 2009, 50, 3237–3240.
(3) (a) Samec, J. S. M.; Ba¨ckvall, J.-E.; Andersson, P. G.; Brandt, P. Chem.
Soc. ReV. 2006, 35, 237–248. (b) Gladiali, S.; Alberico, E. Chem. Soc.
ReV. 2006, 35, 226–236.
lead to racemization of the alcohol (step iii), the double bond can
coordinate to ruthenium (step iV). Complexes 10a-b would then
be formed via migration of the alkoxy moiety to the CO ligand
(migratory insertion, step V), which would explain the easy
formation of the large ring (10a-b).17 An alternative pathway for
formation of 10a-b would be migration of the alkoxy group to
CO in 11 (step Vi) followed by coordination of the double bond
(step Vii).
(4) (a) Abbel, R.; Abdur-Rashid, K.; Faatz, M.; Hadzovic, A.; Lough, A. J.;
Morris, R. H. J. Am. Chem. Soc. 2005, 127, 1870–1882. (b) Casey, C. P.;
Clark, T. B.; Guzei, I. A. J. Am. Chem. Soc. 2007, 129, 11821–11827. (c)
Hamilton, R. J.; Bergens, S. H. J. Am. Chem. Soc. 2008, 130, 11979–
11987.
Scheme 3. Possible Routes for the Formation of 10a-b
(5) Mart´ın-Matute, B.; Åberg, J. B.; Edin, M.; Ba¨ckvall, J.-E. Chem.sEur. J.
2007, 13, 6063–6072.
(6) Åberg, J. B.; Nyhle´n, J.; Mart´ın-Matute, B.; Privalov, T.; Ba¨ckvall, J.-E.
J. Am. Chem. Soc. 2009, 131, 9500–9501.
(7) Nyhle´n, J.; Privalov, T.; Ba¨ckvall, J.-E. Chem.sEur. J. 2009, 15, 5220–
5229.
(8) Only two of the four possible diastereoisomers were observed [there are
three stereogenic centers: the ruthenium, the R-carbon of the alcohol (C2),
and one of the coordinated carbons of the alkene (C5)].
(9) McWilliams, K. M.; Angelici, R. J. Organometallics 2007, 26, 5111–5118.
(10) Alvarez, P.; Lastra, E.; Gimeno, J.; Brana, P.; Sordo, J. A.; Gomez, J.;
Falvello, L. R.; Bassetti, M. Organometallics 2004, 23, 2956–2966.
(11) The assignments of the protons were made by conventional 2D NMR
experiments (see the Supporting Information for further details).
(12) Selg, P.; Brintzinger, H. H.; Andersen, R. A.; Horva´th, I. T. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 791–793.
(13) A heteronuclear multiple bond correlation (HMBC) spectrum shows cross-
peaks for long-range couplings between protons and carbons.
(14) (a) Dutta, B.; Scopelliti, R.; Severin, K. Organometallics 2008, 27, 423–
429. (b) Suzuki, H.; Omori, H.; Mor-oka, Y. J. Organomet. Chem. 1987,
327, C47–C50. (c) Taube, D. J.; Rokicki, A.; Anstock, M.; Ford, P. C.
Inorg. Chem. 1987, 26, 526–530.
(15) (a) Taube, D. J.; Rokicki, A.; Anstock, M.; Ford, P. C. Inorg. Chem. 1987,
26, 526–530. (b) Trautman, R. J.; Gross, D. C.; Ford, P. C. J. Am. Chem.
Soc. 1985, 107, 2355–2362.
(16) Other mechanisms for racemization are also possible. These include a formyl
intermediate (I), an acyloxy intermediate (II), and a hydroxyl carbene
intermediate (III):
The rate of racemization would be slow because ruthenium is
held as inactive 10a-b. In Scheme 3 we have assumed that the
racemization occurs via ꢀ-hydride elimination. However, other
racemization mechanisms are also possible, such as those involving
CO participation.16 In these alternative mechanisms, the rate of
racemization would also be slowed by the formation of 10a-b.
In conclusion, we have used 1H and 13C NMR spectroscopy and
in situ FT-IR measurements to characterize two diastereomers (10a
and 10b)18 of an alkoxycarbonyl complex having a double bond
coordinated to ruthenium. IR peaks were observed at 1982 cm-1
(CO) and 1644 (acyl) cm-1. These studies have provided further
insight into the mechanism of the highly efficient racemization
catalyst 1.
Acknowledgment. The Swedish Research Council, the Berze-
lius Center EXSELENT, and the K. & A. Wallenberg Foundation
are gratefully acknowledged for financial support.
(17) The easy formation of a seven-membered ring from a six-membered ring
via 1,2-migration is known in a number of rearrangements, e.g., the
Beckman rearrangement and the Baeyer-Villiger oxidation.
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.
(18) Only two of the four possible diatereomers were observed. Density
functional theory calculations indicate that the change in the configuration
at C5 relative to the configuration at Ru leads a significant energy change.
This means that for a given Ru configuration, it is energetically favorable
to coordinate one face of the alkene, whereas coordination of the other
face is very unfavored. The two diastereoisomers observed must therefore
have the same or opposite absolute configuration at C2 and C5 (2R,5R-
2S,5S, or 2R,5S-2S,5R) (Nyhle´n, J., unpublished results).
References
(1) (a) Mart´ın-Matute, B.; Edin, M.; Boga´r, K.; Ba¨ckvall, J.-E. Angew. Chem.,
Int. Ed. 2004, 43, 6535–6539. (b) Mart´ın-Matute, B.; Edin, M.; Boga´r, K.;
Kaynak, F. B.; Ba¨ckvall, J.-E. J. Am. Chem. Soc. 2005, 127, 8817–8825.
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