Tmsch<sub>2</sub>li and tmsch<sub>2</sub>lilidmae: efficient reagents for noncryogenic halogenlithium exchange in bromopyridines
TMSCH2Li and TMSCH2Li-LiDMAE: Efficient SCHEME 1. Metal-Halogen Exchanges Reported in the Reagents for Noncryogenic Halogen-Lithium Literature Exchange in Bromopyridines
Abdelatif Doudouh,† Christopher Woltermann,‡ and
Synthe`se Organome´tallique et Re´actiVite´, UMR CNRS 7565,Nancy UniVersite´, UniVersite´ Henri Poincare´, BouleVard desAiguillettes, 54506 VandoeuVre-le`s-Nancy, France, and FMCCorporation, Lithium DiVision, Highway 161, Box 795,Bessemer City, North Carolina 28016SCHEME 2. Metalation of Electrophilic Halogenopyridines with TMSCH2Li-LiDMAE
or -78 °C), solvent (toluene), as well as dilution was neededto avoid C-2 to C-5 isomerization and degradation (Scheme 1,eq 2).
An alternative to this sensitive lithiation process has been
reported recently by Song,6 who realized the magnesium halogenexchange at C-2 under noncryogenic conditions (0 °C) using
TMSCH2Li and TMSCH2Li-LiDMAE have been used
i-PrMgCl in THF. The reaction proceeded smoothly allowing
efficiently for bromine-lithium exchange in 2-bromo-,
the preparation of a range of C-2-substituted derivatives.
3-bromo-, and 2,5-dibromopyridines under noncryogenic
However, since the magnesation of 2,5-dibromopyridine was
conditions, while low temperatures (-78 to -100 °C) are
known to occur mainly at C-5,7 the authors had to exchange
always needed with n-BuLi. The aminoalkoxide LiDMAE
bromine at C-2 for iodine to direct the reaction toward the
induced a remarkable C-2 selectivity with 2,5-dibromopyr-
desired position, thus implying an additional step and added
idines in toluene at 0 °C, which was unprecedented at such
expense to the transformation (Scheme 1, eq 3).
a temperature. The lithiopyridines were successfully reacted
Thus, the search for new reagents able to promote the clean
with electrophiles also under noncryogenic conditions giving
bromine-lithium exchange in pyridines under easily applicable
the expected adducts in good yields.
Recently, we have reported a new lithiating agent TMSCH2-
Li-LiDMAE (with LiDMAE ) Me2N(CH2)2OLi)8-10 which
Metal-halogen exchange in 2,5-dibromopyridine 3 has been
promoted the clean C-6 deprotonation of chloropyridines and
the subject of much attention motivated by the usefulness of
even of the highly sensitive fluoropyridines at 0 °C when used
this doubly reactive intermediate for the synthesis of ligands1,2
in hexane (Scheme 2).8 This unprecedented reactivity contrasted
and biologically active compounds.3 First studies by Parham4
with those of our previous reagent BuLi-LiDMAE for which
and further developments by other groups1,2 clearly established
low temperatures (-78 to -100 °C) were needed to prevent
that the C-5 position could be lithiated selectively with n-BuLi
in THF at -78 or -100 °C (Scheme 1, eq 1).
This high level of functional tolerance at 0 °C led us to
In contrast, Wang5 reported the control of the C-2 lithiation
consider TMSCH2Li for the selective bromine-lithium ex-
to be more problematic. Due to the instability of 2-lithio-5-
change in 2,5-dibromopyridine under noncryogenic condi-
bromopyridine, careful attention to reaction temperature (-50
(6) Song, J. J.; Yee, N. K.; Tan, Z.; Xu, J.; Kapadia, S. R.; Senanayake,
C. H. Org. Lett. 2004, 6, 4905.
(1) Bolm, C.; Ewald, M.; Felder, M.; Schlingloff, G. Chem. Ber. 1992,
(7) Trecourt, F.; Breton, G.; Bonnet, V.; Mongin, F.; Marsais, F.;
Queguiner, G. Tetrahedron Lett. 1999, 40, 4339.
(2) Romero-Salguero, F. J.; Lehn, J.-M. Tetrahedron Lett. 1999, 40, 859.
(8) Doudouh, A.; Gros, P. C.; Fort, Y.; Woltermann, C. Tetrahedron
(3) Nicolaou, K. C.; Sasmal, P. K.; Rassias, G.; Reddy, M. V.; Altmann,
2006, 62, 6166.
K.-H.; Wartmann, M.; O’Brate, A.; Giannakakou, P. Angew. Chem., Int.
(9) Gros, P. C.; Doudouh, A.; Woltermann, C. Chem. Commun. 2006, Ed. 2003, 42, 3515.
(4) Parham, W. E.; Piccirilli, R. M. J. Org. Chem. 1977, 42, 257.
(10) Gros, P. C.; Doudouh, A.; Woltermann, C. Org. Biomol. Chem.
(5) Wang, X.; Rabbat, P.; O’Shea, P.; Tillyer, R.; Grabowski, E. J. J.;
2006, 4, 4331.
Reider, P. J. Tetrahedron Lett. 2000, 41, 4335.
(11) Choppin, S.; Gros, P. C.; Fort, Y. Org. Lett. 2000, 2, 803.
10.1021/jo070620j CCC: $37.00 2007 American Chemical Society
J. Org. Chem. 2007, 72, 4978-4980 TABLE 1. Bromine-Lithium Exchange in 1 and 2 with TABLE 2. Metalation of 3 with TMSCH2Li-Based Reagentsa TMSCH2Lia 1b 2b 1a, 94c 1a, 70d 1a, 47d 2a, 88c
Reaction performed on 1.84 mmol of 3. b GC yields. S.M.: starting 1b, 86c 2b, 88c a Reaction performed on 1.84 mmol of 1 or 2. b Isolated yield. c The SCHEME 4. Proposed Intermediate for Stabilization of
GC analysis revealed conversions >98%. d The remainder was unreacted
5-Bromo-2-lithiopyridine SCHEME 3. Proposed Pathway for TMSCH2Li Consumption
Since no data was available about the reactivity of TMSCH
effects. The metalation mixtures were quenched with MeOH
Li in such halogen-metal exchange reaction, we first investi-
gated its behavior at 0 °C toward 2- and 3-bromopyridine in
As shown, in toluene, which was the best solvent reported
hexane (Table 1). Bromine-lithium exchange in such substrates
for selective C-2 lithiation with n-BuLi at -78 °C,5 TMSCH2-
was known to need very low temperature (-78 to -100 °C)
Li led to a mixture of 1 and 2 at 0 °C. Extended reaction time
with n-BuLi in THF to avoid dimerization, side deprotonation
led to C-2 to C-5 isomerization and degradation of 3 (entries 1
and subsequent aryne formation with the latter.12
exchange at 0 °C very cleanly with the two substrates. The
lation with a small amount of C-5 metalation (entry 3). The
exchange product was obtained exclusively in high yield. In
effect of TMSCH2Li-LiDMAE on the reaction outcome was
particular, no product resulting from side deprotonation of 2
also examined. In hexane, the aminoalkoxide induced a complete
was detected. The yields decreased proportionally to the amount
conversion but a loss in selectivity was noted. The effect of
incorporating LiDMAE was remarkable in toluene leading to
2Li, and 2 equiv of TMSCH2Li were necessary for
completion of the exchange. An explanation is the formation
the C-2 metalation in 95% yield with only 4% of the other
isomer after 30 min at 0 °C. Extended reaction time did not
produce notable isomerization of the lithiated species (entries
present in NMR spectra and GC analysis of the crude mixtures
The effect of coordinating solvents was also examined (entries
The metalation and the electrophilic condensation step were
8 and 9) to attempt the metalation of the C-5 position. In THF,
realized in the same solvent and at similar temperatures. Large
only degradation was observed while diethyl ether led to the
excesses of electrophiles were not necessary despite the use of
expected C-5 lithiation in 65% yield.
2Li-LiDMAE was, to our knowledge, the first
reagent to regioselectively lithiate 2,5-dibromopyridine at 0 °C.
The selectivity could be explained by chelation of 2-lithio
intermediate by LiDMAE ensuring stabilization and preventing
2Li was found to be practical and selective for the
bromine-lithium exchange in monobromopyridines at 0 °C.
the C-2 to C-5 isomerization (Scheme 4). The reason for a lower
This result was strongly encouraging for trying it in the selective
selectivity in hexane remains unclear but could be due to a
C-2 lithiation of 2,5-dibromopyridine 3. The reaction was
slower formation of the 2-lithiopyridine in this less polar solvent.
investigated under various conditions with focus on solvent
The synthetic usefulness of this new lithiation process was
then finally illustrated by reaction with a set of electrophilicreagents (Table 3). All of the electrophiles reacted efficiently
(12) Cai, D.; Larsen, R. D.; Reider, P. J. Tetrahedron Lett. 2002, 43,
providing the expected products in good yields comparable with
J. Org. Chem, Vol. 72, No. 13, 2007 4979 TABLE 3. Reaction with Electrophilic Reagentsa
reasonable amounts of electrophiles (20-50% excess compared to 3) despite the use of 2 equiv of TMSCH2Li, and the condensation step could be realized efficiently at 0 or -20 °C in toluene.
In summary, we have discovered a new reactivity of
TMSCH2Li and TMSCH2Li-LiDMAE reagents. These reagentsare suitable for selective bromine-lithium exchange and subse-quent functionalization of several bromopyridines in apolarsolvents. The effect of LiDMAE on the selectivity in bromine-lithium exchange of 2,5-dibromopyridine is remarkable. Thetransformation proceeds under noncryogenic conditions con-suming only small excesses of electrophiles opening access toa potentially scalable process. Experimental Section Procedure for Bromine-Lithium Exchange in 2,5-Dibro- mopyridine. To a solution of 2-dimethylaminoethanol (164 mg, 1.84 mmoles) in toluene (6 mL) cooled at 0 °C was added dropwise (trimethylsilyl)methyllithium (5.52 mmol, 6 mL of a 0.92 M solution in hexane) under a nitrogen atmosphere. After being stirred for 30 min at the same temperature, a solution of 2,5-dibromopyr- idine (436 mg, 1.84 mmoles) in toluene (2 mL) was added dropwise. The obtained red solution was then stirred for 30 min at 0 °C and treated dropwise with a solution of the appropriate electrophile (2.2 or 2.76 mmol) in toluene (2 mL) at 0 or -20 °C. After 1 h of stirring, the mixture was hydrolyzed with water (10 mL). The organic layer was then extracted with diethyl ether (10 mL) and dried over MgSO4, and the solvents were evaporated. The crude product was subjected to GC analysis and finally purified by column chromatography using hexane-AcOEt mixtures as eluent. Acknowledgment. We thank the FMC Corporation (Lithium a Reaction performed on 1.84 mmol of 3. b Isolated yield after column
chromatography. The conversions were >97% in each case. Supporting Information Available: Experimental details and
characterization data for all compounds. This material is availablefree of charge via the Internet at http://pubs.acs.org.
those obtained using BuLi at low temperatures (-78 °C)5 orby the magnesation process.6 The quenching step consumed
4980 J. Org. Chem., Vol. 72, No. 13, 2007
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