Amino acids as chiral anionic ligands for ruthenium based asymmetric olefin metathesis

Article abstract of DOI:10.1039/c5cc00052a

Several amino acid ligands were introduced into the Hoveyda-Grubbs 2nd generation complex by a facile anionic ligand exchange. The chiral pre-catalysts obtained displayed enantioselectivity in asymmetric ring-closing and ring-opening cross-metathesis reactions. Reduction of the lability of the carboxylate ligands was found to be cardinal for improving the observed enantiomeric product enrichment.

Full text of DOI:10.1039/c5cc00052a

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Several amino acid ligands were introduced to the Hoveyda-
Grubbs 2 generation complex by a facile anionic ligand
exchange. The chiral pre-catalysts obtained displayed
enantioselectivity in asymmetric ring-closing and ring-
opening cross-metathesis reactions. Reduction of the lability
Herein, we report on the synthesis of several ruthenium-
based olefin metathesis complexes bearing amino acids as
chiral anionic ligands and study their efficiency in asymmetric
olefin metathesis reactions. The complexes are produced from
the most ubiquitous chiral entities in a facile and economical
manner, presenting highly tunable options to study this family
nd
of the carboxylate ligands was found to be cardinal for of asymmetric ruthenium pre-catalysts.
improving the observed enantiomeric product enrichment.
Amines are strong σ-donor ligands capable of coordination
to the ruthenium center and thus need to be protected when
Since its universal recognition in 2005, the field of olefin introduced to ruthenium alkylidene solutions. Accordingly, Boc
metathesis in organic synthesis has continued its phenomenal (tert-butyloxycarbonyl) N-substituted amino acids were chosen
1
6
1
2
3
4
6
development in many aspects, such as: Z-selectivity, latency,
as starting materials. The protected amino acids were converted
to their silver carboxylate salts to ensure complete substitution
5
synthesis of sterically hindered alkenes, aqueous based metathesis,
etc. Asymmetric olefin metathesis, as in – ACM (asymmetric cross of both chlorides and 2.1 equivalents of the corresponding
metathesis), ARCM (asymmetric ring closing metathesis) and silver salts were introduced to a THF solution of commercially
7
nd
17
AROCM (asymmetric ring opening cross metathesis) – provides available Hoveyda-Grubbs 2 generation complex (Table 1).
accessible pathways for the production of some elusive chiral Interestingly, the use of methylene chloride as the solvent
8
products. The first examples of asymmetric olefin metathesis were media led to unsatisfactory results and decomposition of the
9
presented by Schrock and Grubbs et al. using chiral molybdenum amino acid ruthenium complexes. Visibly, complexes in
complexes. However, the applicability of these complexes was methylene chloride solution significantly decomposed after 24
reduced by their sensitivity towards air and water and their low hours, affording an aldehyde by-product and exposing the
tolerance for a variety of functional groups. More recent studies have fragility of these complexes compared to their dichloro
produced more stable molybdenum catalysts and significantly counterparts (see Electronic Supplementary Information,
1
0
18
advanced the field of molybdenum asymmetric olefin metathesis.
The first examples of chiral ruthenium complexes date back to 2001 evaporation of the solvent afforded the products as deep purple
ESI). Filtration of the precipitated silver chloride and
1
1
and new asymmetric catalysts are still being produced today.
Notwithstanding the efficiency observed for these chiral Ru-A
solids. Using this straightforward protocol, complexes Ru-G
,
,
Ru-L and Ru-F bearing Boc protected glycine, alanine,
1
2
complexes, they all require intricate and expensive syntheses of leucine and phenylalanine respectively, were prepared. All
chiral NHC ligands. In light of this, the exploration of complexes were obtained in good yields and were duly
straightforward and low cost synthesis protocols of novel functional characterized by NMR spectroscopy, HRMS and specific
group tolerant ruthenium pre-catalysts for asymmetric metathesis has rotation measurements (Table 1 and ESI). As an exception, the
remained highly desirable. Moreover, it has been proven over time use of the Boc-valine and Boc-proline amino acids, produced a
1
3
that there is no "silver bullet" in olefin metathesis reactions and mixture of the starting material along with mono- and di-
different substrates and conditions many times require catalyst substituted ruthenium complexes, probably due to the higher
optimization. Thus, the ability to produce tunable chiral catalysts is steric congestion introduced by the isopropyl and pyrrolidine
also a sought after goal. Even though in most of the commonly used side groups (see ESI). Notably, while complex Ru-F had a
ruthenium alkylidenes the anionic positions are occupied by chloride specific rotation of -37.5°, the specific rotation of Boc-
ligands, the introduction of different halides and pseudo-halides has phenylalanine (Boc-phenylalanine was the only protected
been extensively studied in this area. Fine-tuning of the anionic amino acid used soluble in toluene) was measured to be +27.6°,
ligands can result in different reactivity and selectivity of the pre- strongly suggesting that optical rotation of polarized light is
1
4,15
catalysts.
caused by the complex itself and not by dissociated amino acid
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in solution. Complex Ru-G was characterized also by single of the opposite enantiomeric product to that obtained by the use
2
crystal X-ray spectroscopy, showing the typical  hapticity for of Ru-A (Table 2, entries 10-11) as shown by the different
one of the carboxylate ligands (Figure 2).
chiral HPLC chromatograms (see ESI). For both metathesis
reactions we expected increased ee as the size of the substituent
of the amino acid increased (i.e. highest ee for Ru-F); yet Table
Table 1 Synthesis of ruthenium complexes bearing amino acid
anionic ligands.
2
displays the opposite trend. This seems to imply that a fine
balance between steric hindrance and electronic effects is
required in order to obtain satisfactory asymmetric induction.
While greater steric stress in the asymmetric environment could
contribute to higher enantioselectivity, we hypothesized that
steric hindrance may also lead to enhanced ligand dissociation,
leading to a detrimental outcome. Ligands at the anionic
positions in Hoveyda-Grubbs type complexes are known for
their lability and dynamic nature; and were shown to dissociate
benzylidene
19
R
specific b
in solution as a part of a degenerate ligand exchange. Both
reduced complex stability and ligand dissociation may be the
cause for the low enantioselectivities observed in Table 2.
complex
yield
-
hydrogen
substituent
a
rotation (°)
(ppm)
Hoveyda-
Grubbs II
-
16.72
0.00
Table 2 AROCM and ARCM with chiral Ru pre-catalysts
Ru-G
Ru-A
Ru-L
H
94%
95%
77%
87%
78%
17.99
17.62
17.58
17.73
17.61
0.00
+38.3
-18.2
-37.5
-38.5
a
b
entry substrate
product
catalyst
conversion [%]
ee [%]
CH
CH (CH
CH Ph
CH
3
Hoveyda-
Grubbs II
1
1
3
100
0
2
3 2
)
2
3
4
5
Ru-G
Ru-A
100
100
100
100
0
4
4
4
Ru-F
2
Ru-DA
3
Ru-DA
Ru-F
a
Reaction conditions: THF, 37°C, 3 hours.
spectra see ESI. Specific rotation measured in toluene at [훼]
6 6
C D , 400MHz, for full NMR
b
23
.
퐷
Hoveyda-
Grubbs II
6
2
4
c
100
0
7
8
9
Ru-G
83
93
0
d
Ru-F
0
Ru-L
Ru-A
100
100
100
12
20
20
1
0
1
1
Ru-DA
AROCM conditions: 8.0 mol% catalyst loadings, 13.0 equivalents of styrene,
5mM substrate in THF, 37°C, 2 hours. ARCM conditions: 8.0 mol%
5
a
Fig 2 ORTEP representation (50% probability ellipsoids) of complex Ru-G. catalyst loadings, 55mM substrate in THF, 37°C, 2 hours. Conversion was
b
Hydrogen atoms were omitted for clarity.
monitored by GC-MS. ee was determined by HPLC (Reprosil Chiral-NR,
8
c
d
µm,150x4.5 mm). 2.5 mol% 7.0 mol%
The catalytic efficiency of complexes Ru-G, Ru-A, Ru-L
In order to study the specific lability of the amino acid
ligands used, disproportionation of Ru-A and Ru-F in the
and Ru-F was tested in ARCM and AROCM benchmark
7
b
reactions. The initial results showed excellent conversions for
AROCM of norbornene and ARCM of triene (Table 2), but
presence of Boc-glycine silver salt (Scheme 1) was monitored
1
2
1
by H-NMR. Qualitative inspection of the carbene region
with very poor enantioselectivities. Alanine complex Ru-A
afforded the highest enantioselectivity with just 20% ee.
Lowering the temperature to 0°C improved the ee to 27%, yet
significantly reduced the conversion.
An advantage of this methodology is that amino acids are
readily available in their mirror-image forms and thus hold a
valuable key for major product selection. To demonstrate the
ease in which a specific enantiomer may be obtained, readily
available N-Boc protected D-alanine was used to synthesize
Ru-DA under the same conditions described previously. Ru-
DA displayed NMR spectra identical to Ru-A and opposite
specific rotation (Table 1). As expected, asymmetric ring
disclosed a faster exchange of the Boc-phenylalanine ligand by
the Boc-glycine ligand, compared to the Boc-alanine ligand.
This observation falls in line with our assumption that greater
steric stress in the system facilitates ligand dissociation. Since
the chiral induction in our system originates from the anionic
ligand, greater ligand dissociation can severely hamper the
enantioselectivity
obtained.
In
order
to
achieve
enantioselectivity without a chiral NHC ligand, it is reasonable
to assume that both chiral anionic ligands must be associated
with the ruthenium center during the stereochemistry
determining step. In related work, Grela et al. probed the
asymmetric efficiency of a chiral at metal ruthenium pre-
closing of triene
2 catalyzed by Ru-DA readily provided excess
2
0
catalyst. The complex, bearing two, not labile, achiral anionic
2
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ligands (trifluoroacetate and iodide ligands), was isolated in its containing
optically pure form. Lack of enantioselectivity in this system
was explained by a fast racemization mechanism of the
propagating species and underlined the need for a permanent
asymmetric induction derived directly from the ligands for
asymmetric olefin metathesis.
asymmetric
NHC
ligands
that display
enantioselectivity in olefin metathesis reactions. Surprisingly,
the least sterically hindered chiral ligand, Boc-alanine, was the
one to afford the highest ee suggesting that lability of the amino
acid ligands plays an important role in the loss of
enantioselectivity. This hypothesis was supported by the fact
that Boc-phenylalanine was found to exchange more rapidly
than Boc-alanine, and that the addition of excess Boc-alanine
promptly improved the enatioselectivity for ARCM. Moreover,
changing the reaction media to a less polar solvent also had a
beneficial effect on the enantioselectivity, raising the ee over
5
0%. The approach presented herein provides facile synthesis
Scheme 1
protocols using readily available commercial starting materials,
easy access to a specific enantiomeric product by a simple
To further support the hypothesis that ligand lability was
hampering enantioselectivity, an ARCM reaction catalyzed by choice of the appropriate amino acid symmetry and an
Ru-A was carried out in the presence of excess of Boc-alanine extensive chiral pool of readily available amino acid or peptide
silver salt (Table 3, entries 1-2). To our satisfaction, an increase
ligands to study. The introduction of dipeptides and alternative
chiral anionic ligands that may strengthen the binding to the
ruthenium center open a window to combinatorial studies in
this area and are currently being probed to improve these
promising first results.
in ee from 20% to 34% was observed by this protocol
1
1j, 18
(
alongside increased isomerization of the substrate
).
2
0
Continuing with this line of reasoning, the reaction was
repeated in a less polar solvent to inhibit ligand dissociation.
N
Indeed, changing the solvent to benzene (E benzene = 0.111;
T
N
22
ET THF = 0.207) increased the ee to 44% (Table 3, entry 3),
while the addition of excess of Boc-alanine silver salt in this
solvent afforded an enantiomeric excess value of 56% (Table 3,
entry 4). Thus, just by changing the solvent polarity and adding
excess anionic ligand, the enantioselectivity could be steadily
increased from 20% to 56%. Unfortunately, further reduction of
Notes and references
Chemistry Department, Ben-Gurion University of the Negev, Beer-
a
Sheva 84105, Israel.
b
School of Chemistry, Tel-Aviv University, Tel-Aviv 69978, Israel..
†
Electronic Supplementary Information (ESI) available: [Synthesis,
solvent polarity by using benzene/hexane 1:1 and 2:1 mixtures
N
led to extremely low conversions (ET hexane = 0.009). The characterization and activity of all new complexes. CIF file for Ru-G.].
above experiments clearly confirm the profound impact the See DOI: 10.1039/c000000x/
dynamic nature of the anionic ligand has on the course of this
reaction and dictates the path for future optimization of this
promising protocol in asymmetric olefin metathesis.
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4
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