Synthesis of a Bolm's 2,2′-Bipyridine Ligand Analogue and Its Applications

Article abstract of DOI:10.1002/adsc.201800452

A new method of synthesis of an analogue of Bolm's 2,2′-bipyridine ligand based on the catalytic [2+2+2] cyclotrimerization of 1-halodiynes with nitriles was developed. Crucial step of the whole synthesis turned out to be homodimerization of a substituted 2-bromopyridine to the corresponding bipyridine, that was studied and optimized. The newly prepared bipyridine (S,S)-2 was then tested as a chiral ligand in metal-catalyzed enantioselective reactions. Out of the studied reactions the most promising results were obtained in epoxide ring opening (82% yield, 98% ee) and Mukaiyama aldol reaction (>96% yield, 99/1 dr, 92% ee). In the case of Mukaiyama-aldol reaction as well as in the Michael addition, novel ligand 2 proved its robustness compared to Bolm's ligand as it was less sensitive to the purity of used reagents. (Figure presented.).

Full text of DOI:10.1002/adsc.201800452

Title: Synthesis of a Bolm’s 2,2'-Bipyridine Ligand Analogue and Its
Applications
Authors: Eva Bednářová, Martin Dračínský, Štefan Malatinec, Ivana
Císařová, Frederic Lamaty, and Martin Kotora
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800452
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10.1002/adsc.201800452
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Synthesis of a Bolm’s 2,2'-Bipyridine Ligand Analogue and Its
Applications
Eva Bednářová,a,d Martin Dračínský,b Štefan Malatinec,a Ivana Císařová,c Frédéric
Lamaty,d* and Martin Kotoraa*
a
Department of Organic Chemistry, Faculty of Science, Charles University, Hlavova 8, 128 43 Praha 2, Czech Republic
+420 221 951 058, +420 221 951 326, martin.kotora@natur.cuni.cz
Institute of Organic Chemistry and Biochemistry, Academy of Sciences, Flemingovo náměstí 2, 166 10, Praha, Czech
b
Republic
c
Department of Inorganic Chemistry, Faculty of Science, Charles University, Hlavova 8, 128 43 Praha 2, Czech
Republic
d
Institut des Biomolécules Max Mousseron IBMM), UMR 5247, CNRS, Université de Montpellier, ENSCM, Campus
Triolet, Place Eugène Bataillon, 34095 Montpellier cedex 5, France
+33 (0) 467 143 847, : +33 (0) 467 144 866, frederic.lamaty@umontpellier.fr
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A new method of synthesis of an analogue of Mukaiyama aldol reaction (>96% yield, 99/1 dr, 92% ee).
Bolm’s 2,2'-bipyridine ligand based on the catalytic [2+2+2] In the case of Mukaiyama-aldol reaction as well as in the
cyclotrimerization of 1-halodiynes with nitriles was Michael addition, novel ligand 2 proved its robustness
developed. Crucial step of the whole synthesis turned out to compared to Bolm’s ligand as it was less sensitive to the
be homodimerization of a substituted 2-bromopyridine to the purity of used reagents.
corresponing bipyridine, that was studied and optimized. The
newly prepared bipyridine (S,S)-2 was then tested as a chiral
ligand in metal-catalyzed enantioselective reactions. Out of
the studied reactions the most promising results were
obtained in epoxide ring opening (82% yield, 98% ee) and
Keywords: catalysis; enantioselective synthesis;
cyclotrimerization; bipyridines; chiral ligands
caerulomycins,[3] orellanine,[4] lavendamycin,[5] (±)-
streptonigrin,[6] and (+)-complanadine[7] (Figure 1).
Introduction
2,2'-Bipyridines and their derivatives form an
important class of heteroaromatic compounds with
application in various fields of chemistry such as
supramolecular and macromolecular chemistry,
photochemistry and electrochemistry but more
significantly, thanks to their unique coordination
abilities, as ligands able to complex almost all
imaginable metals.[1] On the contrary to regularly
used phosphine or cyclopentadienyl ligands,
bipyridine ligands are generally more stable both to
moisture and oxidation (air), which is simplifying
their synthesis, handling, and storage. In this manner
bipyridines have found numerous applications in
transition
metal-catalyzed
racemic
and
enantioselective reactions. For example, chiral 2,2'-
bipyridine ligands have found applications in metal-
catalyzed
asymmetric
reactions
such
as
cyclopropanation of olefins, alkylation and allylation
of aldehydes, hydrosilylation of ketones, allylic
oxidation of cycloalkenes.[2] It is also worth
mentioning that the bipyridine scaffold can be found
Figure 1. Natural compounds possessing the 2,2'-
bipyridine scaffold.
in
several
natural
compounds
such
as
1
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10.1002/adsc.201800452
Advanced Synthesis & Catalysis
One of the most comprehensively studied member
bipyridine ligand had a positive effect on the
performance of Cr-bipyridine catalytic system.[27]
Our intended approach to the modified Bolm’s
ligand was based on the following retrosynthetic
analysis (Scheme 1): a) catalytic homocoupling of a
bicyclic 2-halopyridine that in turn could be prepared
by b) catalytic cyclotrimerization of 1-halohepta-1,6-
diyne with electron-poor nitriles. We have recently
developed a new type of Ru-complex catalyzed
[2+2+2] cyclotrimerization of halodiynes with
electron-poor nitriles,[28] which would support this
approach.
of this class of compounds is diol 1 possessing the
2,2'-bipyridine scaffold (Bolm’s ligand) with (R,R)
configuration (Figure 2) that was designed and
synthesized by Bolm et al. in 1990.[8] The synthetic
approach was slightly modified by Kobayashi et al. in
2005, who prepared (S,S)-1.[9] Bolm’s ligand 1
proved to be a useful chiral ligand in various types of
metal-catalyzed asymmetric reactions as e.g.
allylation
of
aldehydes,[10]
opening
of
epoxides,[11,12,13] Michael addition,[14] Mukaiyama-
aldol reaction,[15] hydroxymethylation,[16] C-H
activation of indols,[17] and most recently Diels-Alder
reaction.[18] In addition, its complexes with non-toxic
metal salts (e.g. Fe, Bi, Sc) also showed interesting
behavior in the aqueous media making it an important
player in the field of green chemistry.[16,19,20]
Figure 3. Bolm’s ligand analog 2.
Figure 2. Bolm’s 2,2'-bipyridine ligand (R,R).
Despite the abovementioned interesting results,
studies of the dependence of the activity of the ligand
on changes in its structure have been almost
exclusively limited to the variations of the t-Bu part
of the alcohol moiety in 1 to some other groups (Me,
i-Pr, Ph, Napth etc.)[9,11] Only one example of a
derivative with substituents in 3,3' position (Me), was
efficiently applied in alcoholysis and aminolysis of
meso-epoxides.[11,21]
Scheme 1. Retrosynthetic analysis of an approach to a
modified Bolm’s ligand.
Synthesis of ligand 2. Starting hepta-1,6-diyne 3
was deprotonated with LiHMDS followed by reaction
with trimethylsilyl chloride to furnish the
monosilylated diyne 4. Its subsequent deprotonation
with n-BuLi and reaction with NBS provided diyne 5
in 52% isolated yield after two steps (Scheme 2). It
should be noted that the standard conditions for the
synthesis of bromoalkynes by using AgNO3, NBS
could not be applied, because of the unwanted
removal of the TMS-group during the course of the
reaction. Ensuing cyclotrimerization of diyne 5 with
pivaloyl cyanide under the standard conditions[28]
(Cp*RuCl(cod) (10 mol%), dichloroethane, 20 °C)
provided a mixture of two regioisomers 6a and 6b in
3.5:1 ratio furnishing the former in 56% and the latter
in 15% isolated yields. As expected, the desired
regioisomer 6a was formed preferentially.
Unfortunately, the use of a simpler starting material
such as 1-bromo-1,6-heptadiyne was not feasible,
because of undesirable homocyclotrimerization. The
TMS-group was then removed by treatment with
tetrabutylammonium fluoride yielding ketone 7
(92%). It was necessary to remove the TMS-group
prior to the reduction of the carbonyl, because its
steric hindrance hampered the reaction. Ketone 7 was
Results and Discussion
Ligand design considerations. From the point of
view ligand design, further elaboration of the basic
skeleton of Bolm’s ligand is desirable, because it
could lead to development of new ligands capable of
producing even higher asymmetric induction. One
option is to add additional rings onto the pyridine
scaffold that would restrict rotation along the C-C
bond connecting the pyridine rings as in the modified
Bolm’s ligand 2 (Figure 3). This could potentially
induce a conformationally dynamic chirality.[22] If the
pyridine rings cannot lie in one plane, resulting
conformational rigidity of the transition state will be
higher. This has been already demonstrated in the
case of Co[23] or Re[24] complexes, and may serve as
typical examples. In addition, it is reasonable to
expect that the presence of an alkyl group in the 3 and
3' as well as 4 and 4' positions would increase,
through its inductive effect, electron density on the
nitrogen atom[25,26] and thus enhance the coordination
to a transition metal. In this respect it has been
noticed that the presence of alkyl group in the
positions 3 and 3' and as well as 4 and 4' of the
then
reduced
by
enantioselective
transfer
hydrogenation catalyzed by RuCl[(S,S)-Tsdpen](p-
cymene)[9] to the chiral alcohol 8 (98%, 93% ee). Its
absolute
S
configuration was unequivocally
2
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10.1002/adsc.201800452
Advanced Synthesis & Catalysis
confirmed by a single crystal X-ray analysis[29]
(Figure 4).
(Entry 3);[32] and finally, d) Ni(0)-catalyzed coupling
by using a mixture of NiCl2/Zn/PPh3 in DMF (Entry
4), the original method used by Bolm et al. in the first
synthesis of bipyridine 1.[8]
In all cases, the main side product was
dehalogenated pyridine 9 (Table 1), the formation of
this type of product was also observed in most of the
published procedures. It should be noted that
formation of the side product could be reduced by
using carefully degassed solvents. The unexpected
products were compounds 10 (formed when method b
was used), and 11 (observed in method c) arising
after coupling with butyl and phenyl group
respectively. The highest HPLC yield (50%) of
compound 2 was obtained by Ni(0)-catalyzed
homodimerization (Entry 4), but because of the
complicated separation, the product was isolated in
just 35% yield.
Although in most of the publications studying the
dimerization of similar compounds the tolerance of
free hydroxyl groups is described, in our hands the
reactions proceeded only in low to moderate yields
(Entries 1-4 Table 1). We decided to study
homocoupling of ketone 7 to diketone 12 with intent
to reduce it to the diol afterwards. Homocouplings
were carried out under the same reaction conditions
as in the above mentioned cases (Table 2). The yields
of diketone 12 obtained under the used conditions
were improved in all cases and the best yield of 12
(70% HPLC yield, 66% isolated yield) was obtained
when the catalytic system composed of a mixture of
NiCl2/Zn/PPh3 was used (Entry 4). Surprisingly, the
next step, proved to be more challenging.
Scheme 2. Synthesis of (S)-8.
Although the racemic reduction of diketone 12 by
using NaBH4 provided a mixture of rac and meso
product in 90% combined isolated yield (See SI),
enantioselective reduction turned out to be more
challenging (Table 3). Reduction with (–)-Ipc2BCl
brought about no conversion of the starting material
(Entry 1). On the other hand, transfer hydrogenation
by using 5 mol% of RuCl[(S,S)-Tsdpen](p-cymene)[9]
and HCOOH/Et3N mixture at 20 °C gave rise to
product 16, in which only one of the ketone moieties
was reduced (Entry 2). Both the higher amount of
catalyst (10 mol%, Entry 3) and heating the reaction
mixture to 40 °C (Entries 3 and 4) provided
compound 16 in higher yields (35% and 50%,
respectively), but only with traces of the desired diol
2. Increasing the temperature to 60 °C along with 10
mol% of the Ru-catalyst finally furnished the diol 2
in excellent enantiomeric excess (97% ee), although
only in a moderate yield of 48% (Entry 5).
The next strategy, for the synthesis of diol 2 in
high yields, was based on homocoupling of the
acetyl-protected alcohol 17. Chiral alcohol 8 was
protected to give 17 in 93% isolated yield under the
standard conditions (Ac2O, pyridine), then was
homocoupled under the same four conditions as in
the previous cases (Table 4). The best results were
obtained again by Ni-catalyzed dimerization (Entry
4) furnishing the diacetate 18 in 68% isolated yield.
Figure 4. View on the molecule of 8. Displacement
ellipsoids are drawn on 30% probability level.
The crucial step of the whole pathway to
bipyridine 2 was the reductive dimerization of (S)-8.
Although, a plethora of various conditions for
homocoupling of pyridines has been reported, they
mainly deal with simple 2-halopyridines.[30]
Moreover, it was observed that yields were highly
dependent on the structure of the starting
halopyridine. Out of a number of available reaction
conditions four best (in terms of yields) were selected
to screen the homocoupling of (S)-8 (Table 1).
Among
homocoupling
Pd(II)/tetrakis(dimethylamino)ethylene
these
belong:
a)
Pd(0)-catalyzed
by
using
(TDAE)
(Entry 1) the original method used by Kobayashi et al.
in the synthesis of Bolm’s ligand 1;[9] b) Pd-catalyzed
cross-coupling of the in situ generated tributylstannyl
pyridine (Entry 2);[31] c) Ni(0)-catalyzed dimerization
by using a mixture of NiBr2(PPh3)2/Zn/Bu4NI in THF
3
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10.1002/adsc.201800452
Advanced Synthesis & Catalysis
Table 1. Reductive dimerization of alcohol (S)-8 to diol (S)-2.
Yield (%)a)
Entry Catalyst
mol%
Additives 1
Additives 2
TDAE (2 eq.) DMF
- toluene MW 150+180 2+2 10
Solvent T (°C)
t (h)
2
9
10
60 ND ND
35 35 ND
60 ND 20
50 (35) 50 ND ND
11
1
2
3
4
PdCl2(PhCN)2 20
80
120 30
Pd(PPh3)4
NiBr2(PPh3)2
NiCl2
20 + 10 (n-Bu)6Sn2 (1.15 eq.)
100
120
Zn (5 eq.)
Zn (1.3 eq.)
Bu4NI (5eq.) THF
PPh3 (4.8 eq.) DMF
60
70
48
15
20
a) HPLC-yield (isolated yield). ND – not detected.
Table 2. Reductive dimerization of the ketone 7 to diketone 12.
Yield (%)a)
13 14
50 ND ND
30 20 20
60 ND 10
70 (66) 30 ND ND
Entry Catalyst
mol%
Additives 1
Additives 2
Solvent T (°C)
t (h) 12
15
1
2
3
4
PdCl2(PhCN)2 20
TDAE (2 eq.) DMF
-
Bu4NI (5eq.) THF
PPh3 (4.8 eq.) DMF
80
120 50
Pd(PPh3)4
NiBr2(PPh3)2
NiCl2
20 + 10 (n-Bu)6Sn2 (1.15 eq.)
100
120
toluene MW 150+180 4+2 20
Zn (5 eq.)
Zn (1.3 eq.)
60
70
120 30
15
a) HPLC-yield (isolated yield). ND – not detected.
4
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10.1002/adsc.201800452
Advanced Synthesis & Catalysis
Table 3. Transfer hydrogenation of the diketone 12 to diol 2.
Yield (%)a)
Entry Catalyst
mol% Reductant 1
Solvent T (°C) t (h) 16
(S,S)-2
1
2
3
4
5
-
20
5
(–)-Ipc2BCl
HCOOH, Et3N
HCOOH, Et3N DCM
HCOOH, Et3N DCM
HCOOH, Et3N DCM
THF
-
20
20
20
40
60
120 NR NR
RuCl[(S,S)-Tsdpen](p-cymene)
48
18
ND
RuCl[(S,S)-Tsdpen](p-cymene) 10
RuCl[(S,S)-Tsdpen](p-cymene) 10
RuCl[(S,S)-Tsdpen](p-cymene) 10
216 35b) traces
216 50b) traces
96
37
48 (97% ee)
a) Isolated yield unless otherwise noted. NR – no reaction; ND – not detected. b) 1H-NMR yield.
Table 4. Reductive dimerization of acetate 17 to diacetate 18.
Yield (%)a)
Entry Catalyst
mol%
Additives 1
Additives 2
TDAE (2 eq.) DMF
toluene MW 150+180 2+2 10
Solvent T (°C)
t (h) 18
19 20
21
1
2
3
4
PdCl2(PhCN)2 20
80
48
10
60 ND ND
Pd(PPh3)4
NiBr2(PPh3)2
NiCl2
20 + 10 (n-Bu)6Sn2 (1.15 eq.)
100
120
-
40 20
30 ND 20
70 (68) 30 ND ND
30
Zn (5 eq.)
Zn (1.3 eq.)
Bu4NI (5eq.) THF
PPh3 (4.8 eq.) DMF
60
70
48
14
50
a) HPLC-yield (isolated yield). ND – not detected.
5
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10.1002/adsc.201800452
Advanced Synthesis & Catalysis
Deprotection of the hydroxyl groups gave the
around the biphenyl/bipyridine main axis revealed a
striking difference between compound 2 and 2,2'-
dimethylbiphenyl (Figure SI-20). While 2,2'-
dimethylbiphenyl has a clear maximum, which has to
be passed to achieve racemization, no such maximum
is apparent in the scan of compound 2. On the other
hand, the conformation with planar (antiperiplanar)
arrangement of the two pyridine rings has minimal
energy. This can be explained by stabilization due to
extended conjugation in the planar conformation and
at the same time, a decrease of steric hindrance due to
missing hydrogen atoms in positions 1 and 1' in
bipyridine. These data are in agreement with
previously reported calculations regarding rotational
barrier of 2,2'-bipyridine.[36] Optimization of the
transition-state structure of the racemization in 2,2'-
dimethylbiphenyl provided the rotational barrier of
16.7 kcal/mol, which is reasonably close to the
experimental value of 18.1 kcal/mol.[34]
desired bipyridine diol 2 as an optically pure
compound after isolation (Scheme 3). The S,S
stereochemistry of the furnished diol 2 was confirmed
by a single crystal X-ray analysis[29,33] (Figure 5).
Scheme 3. Synthesis of the target compound 2 by
deprotection of the diacetate 18.
In order to aquire some information on chiral
transition metal pre-catalyst, we tried to crystallize
various transition metal salts (FeBr3, Fe(ClO4)2,
Bi(OTf)3, CoCl2, [RuCl2(p-cymene)]2, Cu(OTf)2,
Cu(BF4)2, CuCl2, [CuCl(cod)]2, Cu(MeCN)4PF6,)
with 2. Out of these attempts only crystallizations
with various Cu salts provided partially useful results.
However, unlike the previous reports that described
Cu complexes as monomeric species with square
pyramidal,[37] or trigonal bipyramidal[38] geometry for
Cu(II) or Y-shaped planar[38] geometry for Cu(I), we
obtained different types of species. Thus the
crystallization of [CuCl(cod)]2 with 2 provided light
green crystals suitable for X-ray analysis.
Interestingly, unlike in other cases of Bolm’s ligand
Cu-complexes, a cluster composed of four Cu atoms
and two ligands 2-CuCl was obtained[29] (Figure 6).
In addition, green crystals 2-CuX1 were obtained
from other crystallization attempts with CuCl2. It was
also a cluster, but with tetragonal geometry in which
four Cu atoms were coordinated to the chloride anion
(Figure SI-24). In a similar manner blue crystals were
obtained by crystallization of 2 with Cu(MeCN)4PF6.
The crystals showed presence of dimeric complexes
2-CuPF6, each Cu atom had square pyramidal
geometry coordinating one ligand 2 (Figure SI-25).
Unfortunately, the last two examples provided highly
disordered crystals that did not allow exact
determination of the structure (the obtained
parameters did not meet the requirements for
structure determination).
Figure 5. View on the molecule of 2. Displacement
ellipsoids are drawn on 30% probability level.
We intended to determine the rotational barrier in
compounds 2 and 12 by 1H-NMR spectroscopy,
because of speculation that the barriers might be
close to that of 2,2'-dimethylbiphenyl (ΔG# = 18.1
kcal/mol at 25 °C, see Figure SI-19).[34,35]
Although non-equivalency of the hydrogen atoms
of the CH2 groups in 2 due to diastereotopicity was
observed in the spectra of compound 2 (the result of
the presence of centers of chirality in the molecule),
no signal splitting due to the restricted rotation
around the axis of chirality in 1H-NMR spectra of this
compound as well as of compounds 2 or 12 was
observed even at -80°C (Figure SI-19). This
observation may be explained either by very low
rotational barrier or by very small difference of
resonance frequencies of the pairs of diastereotopic
hydrogen atoms.
1
Because the H-NMR determination of rotational
barriers in compounds 2 and 12 was not conclusive,
we performed density functional theory (DFT)
calculations of the barrier in compound 2. For a
validation of the computational method, we included
calculations of the rotational barrier in 2,2'-
dimethylbiphenyl. An energy scan upon the rotation
6
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10.1002/adsc.201800452
Advanced Synthesis & Catalysis
5). It should be emphasized, that during the course of
this study we found that the results of aldol reaction
carried out with Bolm’s ligand 1 were highly
sensitive to the purity of all reactants. In order to
achieve reproducible results all reactants had to be
meticulously purified prior to the reactions. This can
be nicely exemplified by comparison of reactions
carried out with the reactants purified (distilled) prior
to the reactions (see Entries 1-6) with those used as
purchased (see Entries 7-10). Interestingly, the
decrease in enantioselectivity was more pronounced
in the cases when ligand 1 was used, than in cases
when ligand 2 was used.
By using conditions C the published level of
selectivity (dr 91/9, 92% ee) with Bolm’s ligand 1
was not achieved and 23 was obtained in dr 88/12
and 84% ee (Entries 11 and 12).[19] This could be
attributed to difficulties encountered during the
preparation of Fe(DS)2. Gratifyingly, the use of
ligand 2 gave aldol 23 with better diastereoselectivity
(93/7) and enantioselectivity (88% ee) (Entry 13).
Figure 6. View on the molecule of 2-CuCl. The
displacement ellipsoids are drawn on 30% probability level.
The solvating methanol molecules as well as second
position of disordered C14 atom are omitted for clarity.
The complex exhibits the symmetry of two fold axis of the
space group C2, therefore position of half of the atoms are
generated via operation with symmetry code (i) 1-x, y, 1-z.
Table 5. Fe(II)-catalyzed Mukaiyama-aldol reaction of
silicon enolate 22 and benzaldehyde in the presence of
Bolm’s ligand 1 and ligand 2.
Application of 2 in enantioselective catalysis.
Application of ligand
2
in several catalytic
enantioselective reactions was examined. For this
purpose five typical reactions in which Bolm’s ligand
1 proved its activity were chosen: a) Fe(II)-catalyzed
Mukaiyama-aldol reaction,[15,19] b) Bi(III)-catalyzed
hydroxymethylation,[20] c) Sc(III)-catalyzed Michael
addition,[14a] d) Pd(II)-catalyzed C-H activation of
indols,[17] and e) Sc(III)-catalyzed opening of the
epoxides.[13] The conditions from the literature were
used without any optimization. In order to ensure
comparability of the results obtained with (S,S)-
Bolm’s ligand 1 and our new analogue (S,S)-2, both
of the reactions were done in parallel.
Entry Cond.a) Ligand Yield
(%)b)
dr
ee (%)d,e)
(syn/anti)c)
(syn))
1f)
2
A
A
A
B
B
B
A
A
B
B
C
C
C
(S,S)-1 90
(S,S)-1 >96
(S,S)-2 >96
(S,S)-1 85
(S,S)-1 95g)
(S,S)-2 >96
(S,S)-1 >96c)
(S,S)-2 96
(S,S)-1 >96c)
(S,S)-2 90
(S,S)-1 80
(S,S)-1 88c)
(S,S)-2 87
97/3
97/3
99/1
97/3
97/3
98/2
95/5
97/3
96/4
97/3
91/9
88/12
93/7
91
93
92
88
91
90
86
92
76
83
92
84
88
3
4f)
5
6
Ligand
Mukaiyama-aldol reaction of propiophenone-derived
silicon enolate ((Z)-trimethyl(1-phenylprop-1-
2 was applied to iron(II)-catalyzed
7g)
8g)
9g)
10g)
11h)
12
enyloxy)silane) 22 and benzaldehyde, for which were
selected three conditions reported by Kobayashi et
al.[15] and Ollevier et al.[19] (Table 5). Conditions A
used Fe(OTf)2, pyridine and a ligand, all in catalytic
13
amounts
in
dimethoxyethane-water
mixture
a)
Conditions A: Fe(OTf)2 (3 mol%), pyridine (7.2 mol%),
ligand (3.6 mol%), DME/H2O 9/1, 0 °C.[15] Conditions B:
Fe(ClO4)2 (3 mol%), PhCOOH (3.6 mol%), ligand (3.6
mol%), DME/H2O 7/3, 0 °C.[15] Conditions C: Fe(DS)2 (5
mol%), PhCOOH (5 mol%), ligand (15 mol%), H2O,
(DME/H2O 9/1) at 0 °C. In conditions B the source of
iron was Fe(ClO4)2 and benzoic acid was used instead
of pyridine in DME/H2O 7/3. In conditions C were
used Fe(II) dodecylsulfate (Fe(DS)2), benzoic acid
and a ligand in pure water at 20 °C. The reaction
under conditions A with ligand 2 proceeded to give
the corresponding aldol product 23 in quantitative
yield with excellent diastereoselectivity (99/1) and
very high enantioselectivity of 92% ee (Entry 3). The
results were comparable with the reported results
obtained by using Bolm’s ligand 1[15] (Entry 1) or that
carried out by us (Entry 2). The reaction was
performed under conditions B by using ligand 2
provided aldol 23 in quantitative yield, good
b)
c)
20 °C.[19]
Isolated yields unless otherwise noted.
1
d)
Determined by H-NMR. Determined by HPLC with a
chiral stationary phase. S,S configuration in all cases.
e)
f)
g)
Data reported in ref. [14]. Reactants were not purified
prior to the reaction. h) Data reported in ref. [19].
The second application was Bi(III)-catalyzed
hydroxymethylation of the same silicon enolate 22
with formaldehyde (Table 6).[20] By using ligand 2,
product 24 was furnished in quantitative yield and
diastereoselectivity
(98/2)
and
a
good
enantioselectivity of 90% ee (Entry 6) that were
almost same as by using Bolm’s ligand (Entries 4 and
7
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10.1002/adsc.201800452
Advanced Synthesis & Catalysis
Table 8. Pd(II)-catalyzed C-H activation of indol 27 in the
presence of Bolm’s ligand 1 and ligand 2.
86% ee (Entry 3) a result similar to the one obtained
with Bolm’s ligand (Entry 2).
Table 6. Bi(III)-catalyzed hydroxymethylation of silicon
enolate 22 with formaldehyde in the presence of Bolm’s
ligand 1 and ligand 2.
Entry Ligand Yield (%)a) ee (%)b,c)
1d)
2
3
(S,S)-1 94
(S,S)-1 94e)
(S,S)-2 >96
92
89
72
a)
b)
Entry Ligand Yield (%)a) ee (%)b,c)
Isolated yields unless otherwise noted. Determined by
c)
HPLC with a chiral stationary phase. R configuration in
(S,S)-1 93
(S,S)-1 >96
91
89
86
all cases. d) Data reported in ref. [17]. e) Determined by 1H-
NMR.
(S,S)-2 >96
b)
c)
d)
S configuration in all cases.
Data
Last but not least, the activity of ligand 2 was also
tested in Sc(III)-catalyzed epoxide ring opening of
cis-stilbene oxide 30 by p-methoxybenzyl alcohol in
dichloromethane (Table 9).[13] By using Bolm’s
ligand 1, product 31 was obtained with 90% yield
(1H-NMR) and 95% ee while using ligand 2, product
31 was obtained in 82% isolated yield and with
excellent enantioselectivity of 98%.
Ligand 2 was also used in Sc(III)-catalyzed
Michael addition of β-ketoester 25 to methyl vinyl
ketone (MVK) in dichloromethane (Table 7).[14a] The
Michael adduct 26 was obtained in quantitative yield
and 81% ee (Entry 3), while using Bolm’s ligand 1 its
yield was 94% and 90% ee (Entry 2). However,
without drying of the Sc(III) triflate and distillation of
MVK prior to use ee dropped to 67% (Entry 4). On
the other hand, under the same conditions using
ligand 2 ee remained unchanged at 81% (Entry 5).
Table 9. Sc(III)-catalyzed ring opening of cis-stilbene
oxide 30 in the presence of Bolm’s ligand 1 and ligand 2.
Table 7. Sc(III)-catalyzed Michael addition of β-ketoester
25 and MVK in the presence of Bolm’s ligand 1 and ligand
2.
Entry Ligand Yield (%)a) ee (%)b,c)
1d)
2
3
(R,R)-1 82
(S,S)-1 90f)
(S,S)-2 82
97e)
95
98
a)
b)
Isolated yields unless otherwise noted. Determined by
c)
HPLC with a chiral stationary phase. S,S configuration
Entry Ligand Yield (%)a) ee (%)b,c)
unless otherwise noted. Data reported in ref. [13].e) R,R
d)
configuration. f) Determined by 1H-NMR.
1d)
2
(S,S)-1 94
(S,S)-1 94e)
(S,S)-2 >96
(S,S)-1 >96
(S,S)-2 >96
92
90
81
67
81
3
4f)
Conclusion
5f)
a)
b)
We
have
demonstrated
that
Ru-catalyzed
Isolated yields unless otherwise noted. Determined by
c)
cyclotrimerization of a 1-halodiyne with nitriles is a
suitable reaction allowing preparation of substituted
pyridines. Specifically, cyclotrimerization of (7-
HPLC with a chiral stationary phase. R configuration in
all cases. Data reported in ref. [14a]. e) 1H-NMR yield.
d)
f)
Reactants were not purified prior to the reaction.
bromohepta-1,6-diyn-1-yl)trimethylsilane
with
pivaloyl cyanide provides regioselectively 1-bromo-
6,7-dihydro-5H-cyclopenta[c]pyridine that can be
easily converted into Bolm’s ligand analog 2. One of
the crucial steps along the synthetic route was
homodimerization of the bromopyridine that was
optimized to give a bipyridine intermediate in high
yield.
As far as the use of 2 as ligand in enantioselective
reactions is concerned, the results indicate its
potential applicability. Although the use of 2 in
hydroxymethylation, Michael addition, and C-H
Ligand 2 was also tested in C-H activation of indol
27 followed by conjugated addition to 28 catalyzed
by Pd(II) in the presence of dodecyl sulfate salt in
water (Table 8).[17] In this case were the yield and ee
of product 29 noticeably lower (69%, 72% ee, Entry
3) compared to the use of Bolm’s ligand (94%, 89%
ee, Entry 2).
8
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10.1002/adsc.201800452
Advanced Synthesis & Catalysis
hexane/EtOAc) = 0.28. [훼]2퐷0= +1.52° (CHCl3, c = 2.62
activation gave products with lower enantioselectivity
than the reported values obtained with Bolm’s ligand
1, other reactions were more promising. The use of 2
in aldol reaction gave products within the same range
of enantioselectivity as Bolm’s ligand 1. However, it
should be pointed out that 2 is more robust, because it
gave exactly the same results with reactants that were
not purified prior to the reaction. In this instance the
use of Bolm’s ligand 1 gave products with decreased
enantioselectivity by 6-7% ee. Its robustness was also
noticed in the case of Michael addition with reactants
that were not purified where the product was obtained
with enantioselectivity of 81% ee, whereas with 1 the
obtained ee was 67%. Gratifyingly, in the ring
opening of epoxides it gave products with slightly
higher enantioselectivity of 98% ee (95% ee with 1 in
our hand, 97% ee the reported value).
g/100 mL)
(S,S)-(6,6',7,7'-Tetrahydro-5H,5'H-[1,1'-
bi(cyclopenta[c]pyridine)]-3,3'-diyl)bis(2,2-
dimethylpropane-1,1-diyl) diacetate (18). To a dry argon
flushed 100 mL flask with NiCl2 (305 mg, 2.3 mmol) dry
degassed DMF (13.5 mL) was added. The mixture was
heated to 70 °C, then PPh3 (2.4 g, 9.24 mmol) and Zn (166
mg, 2.5 mmol) were added. The reaction mixture was
stirred at 70 °C for 1 hour, then 17 (628 mg, 1.9 mmol) in
dry degassed DMF (13.5 mL) was added. After stirring for
14 hours at 70 °C, the reaction mixture was quenched with
water (25 mL) and extracted with Et2O (3×100 mL). The
combined organic layers were washed with brine (50 mL),
dried over MgSO4, filtered and concentrated under reduced
pressure. Column chromatography of the residue on silica
gel (gradient 20/1 → 5/1 hexanes/EtOAc) furnished 321
mg (68%) of the title compound 18 as a colorless solid.
Mp = 173.6 °C. 1H-NMR (400 MHz, CDCl3) δ 7.11 (s, 2H,
2×Ar-H), 5.56 (s, 2H, 2×CH), 3.49–3.38 (m, 2H, 2×CH),
3.14–3.03 (m, 2H, 2×CH), 2.92 (t, J = 7.6 Hz, 4H, 2×CH2),
2.12 (s, 6H, 2×CH3), 2.11–1.94 (m, 4H, 2×CH2), 1.00 (s,
18H, 6×CH3). 13C-NMR (100 MHz, CDCl3) δ 170.43,
154.99, 154.63, 152.27, 138.88, 117.76, 84.08, 35.16,
33.01, 32.99, 26.53, 24.99, 21.32. IR (drift KBr) υmax 2959,
2905, 2870, 1735, 1585, 1558, 1427, 1396, 1370, 1244,
1023, 972 cm-1. HRMS (ESI) m/z calculated for
C30H41N2O4 (M+H) 493.30608, found 493.30600. Rf (10/1
hexane/EtOAc) = 0.23.
Experimental Section
1-(1-Bromo-4-(trimethylsilyl)-6,7-dihydro-5H-
cyclopenta[c]pyridin-3-yl)-2,2-dimethylpropan-1-one
(6a) and 1-(4-bromo-1-(trimethylsilyl)-6,7-dihydro-5H-
cyclopenta[c]pyridin-3-yl)-2,2-dimethylpropan-1-one
(6b). To a dried argon flushed 50 mL flask with
Cp*RuCl(COD) (110 mg, 0.29 mmol), DCE (14 ml)
followed by pivaloyl cyanide (0.7 ml, 5.8 mmol) were
added. Then diyne 5 (700 mg, 2.9 mmol) dissolved in DCE
(18 mL) was added slowly by the syringe pump during the
course of 1 hour and the reaction mixture was stirred at 20
°C After the consumption of 5 (disappearance of the
respective spot on TLC), volatiles were evaporated under
reduced pressure. Column chromatography of the residue
on silica gel (gradient 10/1 → 2/1 hexanes/DCM) provided
566 mg (6a, 56%) and 151 mg (6b, 15%) of the title
compounds as colorless solids.
(S,S)-1,1'-(6,6',7,7'-Tetrahydro-5H,5'H-[1,1'-
bi(cyclopenta[c]pyridine)]-3,3'-diyl)bis(2,2-
dimethylpropan-1-ol) ((S,S)-2). To a solution of 321 mg
of 18 (321 mg, 0.65 mmol) in MeOH (5 mL) in 25 mL
flask, K2CO3 (675 mg, 4.9 mmol) was added. After stirring
of the mixture for 5 hours at 20 °C, the reaction was
quenched with water and extracted with DCM (4×30 mL).
The combined organic layers were dried over MgSO4,
filtered and concentrated under reduced pressure. Column
chromatography of the residue on silica gel (gradient 5/1
→ 3/1 hexanes/EtOAc) provided 254 mg (96%, >99% ee)
of the title compound 2 as colorless crystals.
6a: Mp = 65.7 °C. 1H-NMR (300 MHz, CDCl3) δ 3.09 (t, J
= 7.6 Hz, 2H, CH2), 2.92 (t, J = 7.6 Hz, 2H, CH2), 2.06 (p,
J = 7.6 Hz, 2H, CH2), 1.41 (s, 9H, 3×CH3), 0.28 (s, 9H,
3×CH3). 13C-NMR (75 MHz, CDCl3) δ 210.59, 162.76,
161.12, 141.90, 137.81, 130.33, 44.17, 36.91, 32.79, 28.31,
23.64, 1.69. IR (drift KBr) υmax 2956, 1681, 1520, 1249,
1054, 919, 901, 845 cm-1. HRMS (ESI) m/z calculated for
C14H25NOBrSi (M+H) 354.0889, found 354.0889. Rf (20/1
hexane/EtOAc) = 0.38.
Mp = 214.0 °C. 1H-NMR (400 MHz, CDCl3) δ 7.09 (s, 2H,
2×Ar-H), 4.71 (brs, 2H, 2×OH), 4.38 (s, 2H, 2×CH), 3.37–
3.27 (m, 2H, 2×CH), 3.25–3.15 (m, 2H, 2×CH), 2.97 (t, J
= 7.6 Hz, 4H, 2×CH2), 2.16–2.05 (m, 4H, 2×CH2), 0.97 (s,
18H, 6×CH3). 13C-NMR (100 MHz, CDCl3) δ 156.47,
155.68, 151.06, 138.76, 118.74, 80.50, 36.66, 33.10, 32.89,
26.29, 25.40. IR (drift KBr) υmax 2969, 2870, 1588, 1558,
1435, 1363, 1226, 1066, 1051, 1014 cm-1. HRMS (ESI)
m/z calculated for C26H37N2O2 (M+H) 409.28495, found
409.28521. Rf (3/1 hexanes/EtOAc) = 0.33. [훼]2퐷0= -20.5°
(CHCl3, c = 0.367 g/100 mL).
(S)-1-(1-Bromo-6,7-dihydro-5H-cyclopenta[c]pyridin-3-
yl)-2,2-dimethylpropan-1-ol (8). To a dried argon flushed
10 mL flask with solution of 7 (332 mg, 1.2 mmol) and
RuCl[(S,S)-Tsdpen](p-cymene) (40 mg, 0.06 mmol) in
DCM (1 mL), 2/1 mixture of Et3N/HCOOH (1.8 mL) was
added. After stirring for 3 days at 20 °C, the reaction was
quenched with water (10 mL) and extracted with EtOAc
(3×30 mL). The combined organic layers were dried over
MgSO4, filtered and concentrated under reduced pressure.
Column chromatography of the residue on silica gel
(gradient 10/1 → 7/1 hexanes/EtOAc) furnished 319 mg
(96%, 94% ee) of the title compound 8 as an amorphous
solid. Optically pure compound could be obtained by
crystallization from CHCl3 by slow evaporation of solvent.
(1S,2S)-2-((4-Methoxybenzyl)oxy)-1,2-diphenylethan-1-
ol (31). To a solution of Sc(OTf)3 (15 mg, 0.03 mmol) and
(S,S)-2 (13 mg, 0.03 mmol) in dry DCM (1.5 mL) in 4 mL
vial under the argon atmospehe were after 5 min of stirring
at 20 °C subsequently added cis-stilbene oxide 30 (59 mg,
0.3 mmol) and p-methoxybenzyl alcohol (86 mg, 0.6
mmol). After 12 hours of stirring at 20 °C, the
corresponding spot of stilbene disappeared, so the volatiles
were removed under reduced pressure. Column
chromatography of the residue on silica gel (5/1
hexanes/Et2O) furnished 83 mg (82%, 98% ee) of the title
compound 31 as colorless crystals.
M.p.= 65.9 °C. 1H-NMR (400 MHz, CDCl3) δ 7.01 (s, 1H,
Ar-H), 4.27 (s, 1H, CH), 3.60 (brs, 1H, OH), 3.00 (t, J =
7.6 Hz, 2H, CH2), 2.92 (t, J = 7.6 Hz, 2H, CH2), 2.13 (p, J
= 7.6 Hz, 2H, CH2), 0.90 (s, 9H, 3×CH3). 13C-NMR (100
MHz, CDCl3) δ 159.75, 155.84, 140.65, 137.61, 118.37,
80.47, 36.36, 33.91, 32.84, 26.01, 23.75. IR (drift KBr)
υmax 2956, 2903, 1598, 1541, 1453, 1415, 1364, 1238,
1108, 1063, 1016 cm-1.HRMS (ESI) m/z calculated for
C13H18BrNO (M) 283.0572, found 283.0580. Rf (10/1
The recorded values were in agreement with the published
data.[13]
9
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10.1002/adsc.201800452
Advanced Synthesis & Catalysis
Acknowledgements
[16] M. Kokubo, C. Ogawa, S. Kobayashi Angew. Chem.
Int. Ed. 2008, 47, 6909–6911.
E. B., Š. M., and M. K. would like to acknowledge financial
support from the Czech Science Foundation (grant No. 17-
07707S), Grant Agency of Charles University (grant No. 243-
250362) and French embassy in Prague and M. D. from Ministry
of Education of the Czech Republic (Project LO1304, Program
“NPU-I”). E. B. and F. L. would like to thank Université de
Montpellier, CNRS and French Ministry of Higher Education and
Research for funding. Last but not least, M. K. would like to
express his gratitude to the Institute for Molecular Science,
Okazaki, Japan for funding of his visiting professorship.
[17] T. Kitanosono, M. Miyo, S. Kobayashi ACS
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Advanced Synthesis & Catalysis
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11
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Advanced Synthesis & Catalysis
FULL PAPER
Synthesis of a Bolm’s 2,2'-Bipyridine Ligand
Analogue and Its Applications
Adv. Synth. Catal. Year, Volume, Page – Page
Eva Bednářová, Martin Dračínský, Štefan
Malatinec, Ivana Císařová, Frédéric Lamaty,* and
Martin Kotora*
12
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