Chiral organomagnesiates as dual reagents for bromine-magnesium exchange of 2-bromopyridine and access to chiral α-substituted 2-pyridylcarbinols

Article abstract of DOI:10.1002/ejoc.201101490

New chiral ligand containing butyl and dibutylmagnesiates have been prepared from a range of ligands and their reactivity studied. The reagents were generally efficient in promoting the clean bromine-magnesium exchange of 2-bromopyridine at room temperature and the subsequent reaction with aldehydes to afford α-substituted 2-pyridylcarbinols in good yields. (R,R)-TADDOLate proved to be the best ligand, leading to acceptable to good enantioselectivities. To the best of our knowledge this is the first example of an organomagnesiate-induced halogen-metal exchange followed by an enantioselective addition.

Full text of DOI:10.1002/ejoc.201101490

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201101490
Chiral Organomagnesiates as Dual Reagents for Bromine–Magnesium
Exchange of 2-Bromopyridine and Access to Chiral α-Substituted
2-Pyridylcarbinols
Delphine Catel,[a] Floris Chevallier,[b] Florence Mongin,[b] and Philippe C. Gros*[a]
Keywords: Magnesium / Exchange reactions / Pyridylcarbinols / Chirality / Enantioselectivity
New chiral ligand containing butyl and dibutylmagnesiates
have been prepared from a range of ligands and their reac-
tivity studied. The reagents were generally efficient in pro-
moting the clean bromine–magnesium exchange of 2-bromo-
pyridine at room temperature and the subsequent reaction
with aldehydes to afford α-substituted 2-pyridylcarbinols in
good yields. (R,R)-TADDOLate proved to be the best ligand,
leading to acceptable to good enantioselectivities. To the best
of our knowledge this is the first example of an organomag-
nesiate-induced halogen–metal exchange followed by an
enantioselective addition.
ically pure form is a critical issue to develop their synthetic
usefulness. The most efficient syntheses of non-racemic pyr-
idyl alcohols involve enantioselective reduction of the corre-
sponding pyridyl ketones.[8] Several diastereoselective ap-
proaches have been also developed, the first being reported
by Chelucci who trapped 2-pyridyllithium intermediates
with naturally occurring optically active ketones.[9] There is,
however, a lack of straightforward methods to access chiral
pyridyl alcohols from a pyridine derivative and a prochiral
carbonyl electrophile.
Some of us have reported the use of the chiral superbase
BuLi–LiPM* [LiPM* = lithium (S)-(–)-N-methyl-2-pyrrol-
idine methoxide][10] promotes a chemo- and regioselective
deprotonation of pyridine derivatives at C-2 and a notable
enantioselection upon reaction with aldehydes. We have
shown recently that BuLi–LiPM* was coordinated to the
pyridine nitrogen atom as a tetrameric aggregate.[11] This
could be a source of configurational instability by possible
side aggregation and, furthermore, could be responsible for
the moderate enantioselectivities obtained.
Iida and Mase have reported the magnesiation of bromo-
pyridines and aromatic halides by using a substoichiometric
amount of nBu3MgLi in apolar solvents.[12] At the same
time, Oshima described the stoichiometric use of nBu-
Me2MgLi[13] to avoid the presence of an excess amount of
reactive butyl ligands and side reactions such as reduction
or alkylation of electrophilic reagents in the trapping step.
Thus, the inclusion of ligands into organomagnesiates[14]
could be a promising alternative for several reasons: (i) only
the expected reactive species should be transferred; (ii) the
ate complex benefits in terms of stability and gentle reac-
tion conditions should be maintained; (iii) tuning of the re-
activity and access to asymmetric synthesis by using appro-
priate ligands should be possible.
Introduction
The asymmetric addition of achiral alkylmetals to car-
bonyl derivatives is a powerful methodology to access chiral
alcohols, an important class of compounds. It has been in-
vestigated extensively during the two past decades.[1] The
principle is to coordinate the metal to a chiral ligand and
thus create an intermediate able to next induce facial selec-
tion during approach to the aldehyde or ketone. Several alk-
ylmetals have been used for this purpose, the most popular
being diorganozinc,[2] alkyllithium,[3] lithium amides,[4] and
Grignard reagents.[5]
The use of dilithium dialkylmagnesiates, obtained by re-
action of Grignard reagents with lithium BINOLate, has
been reported by Noyori to give high enantioselectivities in
the alkylation of prochiral electrophiles.[6] The interest in
such an approach is to have a well-arranged bimetallic rea-
gent with saturated metal coordination sites avoiding side
aggregation and subsequent loss of chirality. An additional
benefit effect is a potential cooperative effect of lithium and
magnesium in the activation of both the nucleophile and
electrophile.
2-Pyridylcarbinols are important compounds as such or
as precursors of chiral ligands for asymmetric synthesis and
kinetic resolution.[7] Thus, their preparation in enantiomer-
[a] SOR, SRSMC, Nancy Université, CNRS
Boulevard des Aiguillettes, 54506 Vandoeuvre-Lès-Nancy,
France
Fax: +33-3-83684785
E-mail: philippe.gros@srsmc.uhp-nancy.fr
[b] Chimie et Photonique Moléculaires, UMR 6510 CNRS,
Université de Rennes 1, Bâtiment 10A, Case 1003, Campus
Scientifique de Beaulieu
35042 Rennes Cedex, France
Fax: +33-2-23236955
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101490.
Eur. J. Org. Chem. 2012, 53–57
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
53

D. Catel, F. Chevallier, F. Mongin, P. C. Gros
SHORT COMMUNICATION
Examination of the literature revealed that the ability of
Having in hand the appropriate experimental conditions
chiral ligand containing organomagnesiates to achieve bro- for performing the bromine–magnesium exchange using by
mine–magnesium exchange of bromopyridines as well as homoleptic butyl magnesiate, nBu3MgLi, we then examined
the reactivity of the formed organometallics toward car- the reactivity of heteroleptic magnesiates in this reaction.
bonyl electrophiles still remain unexplored.
From the works of Noyori and co-workers on the asymmet-
Herein we report the preparation of organomagnesiates ric alkylation of carbonyl derivatives,[6] BINOL was first
containing chiral ligands, their reactivity in bromine–mag- chosen as a ligand, and the less-expensive racemic form was
nesium exchange, and the subsequent asymmetric pyridyl used for reactivity experiments (Table 2). rac-BINOL was
transfer from the formed pyridylorganometallics to alde- first deprotonated with nBuLi (2 equiv.) in THF and the
hydes (Scheme 1).
formed BINOLate was treated with nBuMgCl (1 equiv.) to
afford putative (BINOLate)BuMgLi (Scheme 2). 2-Bromo-
pyridine (1) was then treated with a stoichiometric amount
of generated (BINOLate)BuMgLi magnesiate, and the mix-
ture quenched with p-anisaldehyde (Table 2).
Table 2. Optimization of the exchange of 2-bromopyridine by using
(BINOLate)BuMgLi followed by trapping with p-anisaldehyde.
Scheme 1. The exchange–trapping sequence for the synthesis of chi-
ral pyridylcarbinols from chiral ligand containing magnesiates.
Results and Discussion
Ida and Mase[12] have shown that the reaction between
2,6-dibromopyridine and nBu3MgLi and subsequent for-
mylation had to be performed at –5 °C in toluene as the
main solvent (THF is present in low amounts from the
preparation of the magnesiate). Consequently, we initially
selected these experimental conditions to examine the reac-
tivity of nBu3MgLi towards 2-bromopyridine and the sub-
sequent trapping by p-anisaldehyde (Table 1). nBu3MgLi
was prepared by mixing nBuLi (2 equiv.) with nBuMgCl
(1 equiv.). The halogen–metal exchange of 2-bromopyridine
by using the formed nBu3MgLi (1/3 equiv.) was found to be
quantitative after 1 h. We observed that the trapping step
had to be performed at –60 °C (then r.t.) instead of –5 °C
to prevent the formation of reduction and alkylation prod-
ucts 3 and 4 and to afford pyridylcarbinol 2a with complete
conversion (Table 1, Entry 3).
Entry
Solvent 1, conc.
Solvent 2, conc.
2a [%][c]
1[d]
PhMe, 0.10 m[a]
PhMe, 0.31 m[b]
Ͼ98 (71)
Ͼ98 (76)
Ͼ98 (75)
80[g]
2[e]
PhMe, 0.21 m[a]
PhMe, 0.63 m[b]
3[e]
–
–
–
–
4[e,f]
[a] Concentration of a solution of 1 in toluene. [b] Concentration
of a solution of p-anisaldehyde in toluene. [c] Conversion deter-
mined by 1H NMR spectroscopy. Isolated yield in brackets.
[d] Magnesiate concentration in THF was 0.07 m. [e] Magnesiate
concentration in THF was 0.14 m. [f] 1 equiv. of p-anisaldehyde was
used. [g] Reduction product 3 was also formed.
Table 1. Optimization of the exchange of 2-bromopyridine by using
nBu3MgLi followed by trapping with p-anisaldehyde.
Scheme 2. Preparation of putative (BINOLate)BuMgLi.
As shown, whatever the conditions, (BINOLate)BuMgLi
promoted the exchange reaction, leading to expected
alcohol 2a as the main product. In a separate experiment,
we checked that, in contrast with iPrMgCl,[15] nBuMgCl
was unable to promote the exchange of 2-bromopyridine,
strongly supporting the formation of the magnesiate by
contact with BINOLate. The (BINOLate)BuMgLi reagent
was found to be efficient in exchanging the bromine of 2-
bromopyridine at room temperature, and the corresponding
pyridine organometallic was readily trapped by p-anisal-
dehyde under experimental conditions similar to those used
with nBu3MgLi (Table 2, Entry 1). This result supports the
hypothesis of the generation of the (BINOLate)BuMgLi
species in the medium under the experimental conditions
used. A clean reaction was consequently obtained, leading
to pyridylcarbinol 2a in 71% isolated yield. A higher con-
Entry
T1
[°C]
T2
[°C]
2a
3 and 4
[%][a]
[%][a]
1[b,c]
2[d]
–5
–5 to r.t.
–5
–5
–5 to r.t.
–60 to r.t. Ͼ98
79
63
16
37
–
3[b]
[a] Determined by 1H NMR integration, bromine–magnesium ex-
change was quantitative. [b] nBu3MgLi magnesiate was prepared at
–5 °C in toluene. [c] 5% (1H NMR integration) of p-anisaldehyde
was recovered. [d] nBu3MgLi was prepared at –5 °C with subse-
quent warming to r.t. in toluene.
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Chiral Organomagnesiates as Dual Reagents
centration of the reaction medium was not deleterious and Table 3. Pyridyl alcohols synthesis with the use of (BINOLate)-
BuMgLi.
also gave 2a in 76% yield (Table 2, Entry 2). Interestingly,
we have shown that the reaction could be conducted with
the same efficiency by using exclusively THF as solvent
(Table 2, Entry 3), whereas toluene was reported to be nec-
essary to ensure the stability of the metalated pyridines af-
ter reaction with Bu3MgLi.[12] Finally, a 50% excess of alde-
hyde was required to avoid a reduction process; indeed, the
use of 1 equiv. instead of 1.5 equiv. of p-anisaldehyde in-
duced a decrease in the conversion into 2a in favor of the
formation of the reduction product (Table 2, Entry 4).
The scope of the exchange–trapping sequence on 2-bro-
mopyridine (1) was investigated by using variously substi-
tuted aldehydes (Table 3). As shown, aromatic and aliphatic
aldehydes reacted efficiently, giving expected alcohols 2b–g
in good to high yields. Electron-donating as well as elec-
tron-withdrawing substituents at the para position of the
benzaldehydes gave similar yields (Table 3, Entries 1 and 2);
alcohol 2c bearing a CF3 group was obtained in 83% yield
(Table 3, Entry 2). The introduction of a methoxy or methyl
group ortho to the carbonyl group did not affect the reactiv-
ity, also giving a high yield of 84 and 81%, respectively
(Table 3, Entries 3 and 4); a similar result was obtained with
[a] The conversion, determined by 1H NMR spectroscopy, was
Ͼ98% except when mentioned. [b] Isolated yield after column
chromatography. [c] 2 h trapping step. [d] Partial sublimation of 2j
occurred upon workup.
bulky 1-naphthaldehyde (Table 3, Entry 5). Trimethylacetal-
dehyde reacted efficiently, and the corresponding alcohol 2g
was isolated in acceptable yield despite a marked trend for
it to undergo sublimation (Table 3, Entry 6).
After having demonstrated the efficiency of the ex-
change–trapping sequence by using rac-BINOLate as a the appropriate Grignard reagents.[17,18] Monobutyl
model ligand, we turned to the asymmetric version of the magnesiate LBuMgLi and putative dibutyl magnesiate
reaction (Table 4). Various parameters (solvent, type of LBu2MgLi2 were prepared according to Scheme 3 (L = chi-
magnesiate, and trapping temperature) and several ligands ral ligand). Two routes were investigated to generate the
based on the BINOLate and TADDOLate[16] skeletons LBu2MgLi2 species.
were screened (Figure 1). TADDOL analogues T1 and T2
After deprotonation of the diol with nBuLi (2 equiv.), the
were prepared from trans-dimethyl tartrate acetonide and resulting alkoxide was treated according to two different
Table 4. Screening of chiral diols in the exchange–trapping sequence with magnesiates.[a]
Entry
Ligand
Magnesiate
Solvent, T [°C]
2a [%][b]
er[c]
1
2
3
4
5
6
7
8
(R)-BINOL
(S)-BINOL
LBuMgLi
LBuMgLi
LBuMgLi
LBuMgLi
LBu2MgLi2[e]
LBu2MgLi2[d]
LBu2MgLi2[d]
LBu2MgLi2[d]
LBu2MgLi2[d]
LBu2MgLi2[d]
LBu2MgLi2[d]
LBu2MgLi2
PhMe, 60 to r.t.
PhMe, 60 to r.t.
PhMe, 60 to r.t.
74
88
59
56
64
68
64
56
68
79
54
28
50:50
50:50
61:39
63:37
73:27
70:30
66:34
50:50
54:46
28:72
17:83
88:12
(R,R)-TADDOL
(R,R)-TADDOL
(R,R)-TADDOL
(R,R)-TADDOL
T1
–60
–60
–60
–60
–60
–60
–60
–100
–100
[d]
T2
9
(R)-BINOL
10
11
12
(R)-BIPHEN H2
(R)-BIPHEN H2
(R,R)-TADDOL
[a] The bromine–metal exchange was complete. [b] Yields determined by GC. [c] Determined by chiral GC. [d] Prepared according to
Route A (see Scheme 3). [e] Prepared according to Route B (see Scheme 3).
Eur. J. Org. Chem. 2012, 53–57
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55

D. Catel, F. Chevallier, F. Mongin, P. C. Gros
SHORT COMMUNICATION
while allowing 2a to be prepared in acceptable yield, led to
a racemic product (Table 4, Entry 8).[19] (R)-BINOL was
also compared under the same conditions, leading to poor
enantioselectivity (54:46) but good yield (Table 4, Entry 9).
Another axially chiral diol, BIPHEN H2, gave 2a in 79%
yield and 28:72er (Table 4, Entry 10). Interestingly the op-
posite enantiomer was obtained in this case. Finally, we
were pleased to obtain a substantial improvement by per-
forming the trapping step at –100 °C, leading to a good
17:83er with BIPHEN H2 (Table 4, Entry 11) and 88:12er
with (R,R)-TADDOL (Table 4, Entry 12).
Conclusions
Figure 1. Chiral diols used for screening experiments.
In summary, we have shown that two alkyl chains of tri-
alkylmagnesiates could be replaced by a bidentate ligand
with maintained efficiency in the bromine–magnesium ex-
change of 2-bromopyridine. BINOLate-containing butyl-
magnesiate has been prepared and promoted the bromine–
magnesium exchange of 2-bromopyridine at room tempera-
ture. The formed hetaryl magnesiate was found to be reac-
tive toward various aldehydes, leading to the corresponding
pyridylcarbinols in good to high yields. The same reaction
was performed by using optically pure chiral diols and re-
vealed that the dibutyl magnesiates built from (R,R)-TAD-
DOL or (R)-BIPHEN H2 were the best ligands for the
asymmetric addition of the pyridyl organometallics, leading
to 88:12 and 17:83 enantiomeric ratios, respectively. In con-
trast, (R)- or (S)-BINOL led to racemic pyridylcarbinols.
This study reveals that the chiral ligand used in the process
is able to induce asymmetry, indicating its coordination to
the metal center of the putative ate species. To the best of
our knowledge, this is the first example of an organomagne-
siate-induced asymmetric exchange–addition sequence. A
detailed study of this promising strategy for the straightfor-
ward synthesis of enantioenriched chiral α-substituted pyr-
idylcarbinols is in progress and will be published in due
course.
Scheme 3. Preparation of the LBuMgLi and LBu2MgLi2 reagents
from chiral diols.
procedures: either 1 equiv. of nBuMgCl and nBuLi, respec-
tively, were added successively (Route A), or nBu2Mg
(1 equiv.) was added (Route B).
The formed magnesiate (0.5 equiv.) was then treated with
1 and the medium subsequently quenched by the aldehyde.
LBuMgLi reagents prepared from (R)- or (S)-BINOLate
and (R,R)-TADDOLate led to alcohol 2a in 59–88% yield
after complete bromine–metal exchange (Table 4, Entries 1–
3). Whereas BINOLate ligands afforded racemic products,
(R,R)-TADDOLate induced a promising level of enantio-
selectivity. Hence, a 61:39 enantiomeric ratio was deter-
mined by chiral GC (Table 4, Entry 3). When the trapping
step temperature was maintained at –60 °C and toluene was
removed in both exchange and trapping steps (Table 4, En-
try 4), little effect was observed on the reactivity and the
Experimental Section
General Procedure for Br–Mg Exchange Reaction by Using [(rac)-
BINOLate]BuMgLi and Synthesis of Pyridylcarbinols 2a–g: In a
Schlenk tube flushed with argon, rac-BINOL (0.3 g, 1.05 mmol,
1 equiv.) was dissolved in anhydrous THF (7.5 mL). nBuLi (1.6 m
in hexanes, 1.31 mL, 2.1 mmol, 2 equiv.) was slowly added at –5 °C.
enantioselectivity (63:37er). As no benefit was noted in the After stirring at this temperature for 1 h, nBuMgCl (2 m in THF,
0.52 mL, 1.05 mmol, 1 equiv.) was added at –5 °C, and the resulting
solution was stirred for an additional 1 h at the same temperature.
2-Bromopyridine (158 mg, 1 mmol, 1 equiv.) was then added at
–5 °C. The mixture was warmed to room temperature and stirred
for 1 h. The reaction was monitored by TLC (cyclohexane/ethyl
acetate, 8:2.5). The medium was then cooled to –60 °C and the
electrophile (1.5 equiv.) was added. The mixture was warmed to
room temperature and stirred for a given time. The reaction was
quenched with a saturated aqueous solution of NH4Cl. The aque-
ous layer was extracted with ethyl acetate and acidified (pH 3–4)
enantioselectivity, toluene was discarded for the rest of the
study. Much better enantiocontrol was obtained by using
the LBu2MgLi2 magnesiate, as a 73:27er was measured
(Table 4, Entry 5). Pyridylcarbinol 2a was obtained in 64%
yield in this case.
Route A was chosen to investigate the effect of TAD-
DOL analogues T1 and T2. The methyl groups on the
phenyl groups in T1 were found to cause only a small loss
of enantioselectivity compared with (R,R)-TADDOL
(Table 4, Entry 7). In contrast, the naphthyl group in T2, by using 0.4 m HCl. The aqueous solution was then extracted with
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Chiral Organomagnesiates as Dual Reagents
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The authors gratefully acknowledge the financial support of
Agence Nationale de la Recherche (ACTIVATE program) to D. C.
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Received: October 12, 2011
Published Online: November 14, 2011
Eur. J. Org. Chem. 2012, 53–57
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