Aerobic and electrochemical oxidative cross-dehydrogenative-coupling (CDC) reaction in an imidazolium-based ionic liquid

Article abstract of DOI:10.1002/chem.201000240

The ionic liquid 1-butyl-3methylimidazolium tetrafluoroborate [BMIm][BF₄] has demonstrated high efficiency when applied as a solvent in the oxidative nitro-Mannich carboncarbon bond formation. The coppercatalyzed cross-dehydrogenative coupling (CDC) between N-phenyltetrahydroisoquinoline and nitromethane in [BMIm][BF₄] occurred with high yield under the described reaction conditions. Both the ionic liquid and copper catalyst were recycled nine times with almost no lost of activity. The electrochemical behavior of the tertiary amine substrate and β-nitroamine product was investigated employing [BMIm][BF₄] as electrolyte solvent. The potentiostatic electrolysis in ionic liquid afforded the desired product with a high yield. This result and the cyclic voltammetric investigation provide a better understanding of the reaction mechanism, which involves radical and iminium cation intermediates.

Full text of DOI:10.1002/chem.201000240

DOI: 10.1002/chem.201000240
Aerobic and Electrochemical Oxidative Cross-Dehydrogenative-Coupling
(
CDC) Reaction in an Imidazolium-Based Ionic Liquid
[
a]
[a]
[a]
[b]
Olivier Baslꢀ, Nadine Borduas, Pauline Dubois, Jean Marc Chapuzet,
[
a]
[b]
[a]
Tak-Hang Chan, Jean Lessard,* and Chao-Jun Li*
Abstract: The ionic liquid 1-butyl-3-
methylimidazolium tetrafluoroborate
under the described reaction condi-
tions. Both the ionic liquid and copper
catalyst were recycled nine times with
almost no lost of activity. The electro-
chemical behavior of the tertiary amine
substrate and b-nitroamine product
was investigated employing [BMIm]-
ACHTUNGTNERUNNG[ BF ] as electrolyte solvent. The poten-
4
tiostatic electrolysis in ionic liquid af-
forded the desired product with a high
yield. This result and the cyclic voltam-
metric investigation provide a better
understanding of the reaction mecha-
nism, which involves radical and imini-
um cation intermediates.
[BMIm]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[BF ] has demonstrated high
4
efficiency when applied as a solvent in
the oxidative nitro-Mannich carbonÀ
carbon bond formation. The copper-
catalyzed cross-dehydrogenative cou-
pling (CDC) between N-phenyltetrahy-
droisoquinoline and nitromethane in
Keywords: cations · CÀC coupling ·
CÀH activation · ionic liquids ·
oxidation
[
BMIm] ACHTUNGTRENNUNG[ BF ] occurred with high yield
4
Introduction
methodology offers formation and ready access to products
3
3
3
2
3
with a variety of new CÀC bonds (Csp ÀCsp , Csp ÀCsp ,
3
The generation of carbocation intermediates followed by
their reaction with carbon nucleophiles is one of the most
powerful strategies for the formation of CÀC bonds. Differ-
Csp ÀCsp bonds).
ent methods have been developed for the generation of
[1]
these reactive intermediates. We recently reported the use
of a cheap copper salt as an efficient catalyst for the oxida-
tive cross-dehydrogenative coupling (CDC) reaction be-
3
Scheme 1. Cross-dehydrogenative-coupling reaction.
tween the Csp ÀH bond adjacent to a nitrogen atom (see 1,
Scheme 1) and different NuÀH nucleophiles (represented as
CÀH). This method resides in the in situ generation of a
highly reactive electrophilic iminium intermediate 2 in the
Electrochemical oxidation has been largely investigated
for the generation of carbocation intermediates; the electro-
chemical cyanation of tertiary amines constitutes a represen-
tative example of CÀC bonds formation employing this
[2]
presence of peroxides or oxygen as oxidant. This new
[3]
method. More recently Yoshida et al. developed a general
two-step method, referred to as the “cation-pool method,”
providing an easy access to a-functionalized tertiary
[
a] Dr. O. Baslꢀ, N. Borduas, P. Dubois, Prof. T.-H. Chan, Prof. C.-J. Li
Department of Chemistry, McGill University
8
01 Sherbrooke Street West, Montreal, QC H3A 2K6 (Canada)
[4]
amines.
Fax : (+1)-514-398-3797
E-mail: cj.li@mcgill.ca
Room temperature ionic liquids (IL) are composed en-
tirely of ions and exist in the liquid state at and around
room temperature. They have several archetypal proper-
[
b] Dr. J. M. Chapuzet, Prof. J. Lessard
Department of Chemistry, Sherbrooke University, 2500
boul. de l’Universitꢀ Sherbrooke
[5]
ties such as intrinsic conductivity, high thermal stability,
Sherbrooke QC J1K 2R1 (Canada)
Fax : (+1)-819-821-8017
E-mail: Jean.Lessard@USherbrooke.ca
low-volatility, high polarity, and potentially wide electro-
[6]
chemical windows. As a result, they are being increasingly
[7]
used in many electrochemical experiments. Combining all
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000240.
the above principles, we envisaged an oxidative carbonÀ
8162
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8162 – 8166

FULL PAPER
carbon bond formation in an IL via the anodic oxidation of
an amine 1, N-phenyltetrahydroisoquinoline (4) (see
Table 1), to generate the corresponding iminium intermedi-
ate 2, which would subsequently react with a carbon nucleo-
phile (Scheme 1).
methane is also efficient in an IL. The next step was to
verify if such an oxidative coupling could be carried out
electrochemically. ILs as electrolytes are known to have a
[6]
potentially wide electrochemical window. Indeed, the elec-
trochemical window of [BMIm][BF ] covers about 4 V
AHCTUNGTRENNUNG
4
(
(
from À2.5 to 2.2 V vs Ag/AgNO 0.01m at a glassy carbon
3
GC) electrode and between À1.8 and 1.8 V at a Pt elec-
Results and Discussion
trode (voltammograms not shown)). In both cases, the oxi-
dation limit occurs at a potential significantly more positive
than those of the oxidation of tertiary N-phenylamine 4 and
of b-nitroamine 5 (see Figures 1 and 2). As seen in Figure 1,
The chemical CDC reaction thus far has been performed in
water or in an organic solvent. For our investigation in ionic
liquids, it was necessary to determine the feasibility of the
chemical CDC reaction in this medium. We therefore select-
ed a well established coupling reaction between N-phenylte-
trahydroisoquinoline (4) and nitromethane, to yield b-nitro-
amine 5 (see Table 1 for the structures), which had been car-
ried out in water under an atmosphere of oxygen using
the cyclic voltammogram (CV) of amine 4 in [BMIm] ACHTUNGTRENNUNG[ BF ]
at a GC electrode reveals a non-reversible oxidation peak at
4
a potential of 0.55 V vs Ag/AgNO , which corresponds to
3
the oxidation of a tertiary N-phenylamine. The CV of the b-
nitroamine 5 (Figure 2) shows a similar oxidation peak but
at a more positive potential of 0.75 V. The tertiary N-phe-
nylnitroamine 5 is more difficult to oxidize due to the induc-
tive effect of the nitro group. In the CVs of Figure 1 and
Figure 2, on the first negative scan after reversal of the po-
tential, a reduction peak appears around 0.2 V (Figure 1)
and 0.3 V (Figure 2) to which corresponds an oxidation
peak at 0.2 and 0.4 V respectively in the second positive
scan. The increase of current of these redox couples upon
cycling indicates that the species giving rise to these reversi-
ble systems do accumulate on the electrode. This is most
probably due to the formation of conducting polymers or
oligomers on the electrode surface. Such polymerization/oli-
gomerization would be responsible for the partial passiva-
tion of the electrode seen in these CVs by the decrease of
the oxidation peak current, upon repetitive scanning, for the
oxidation of amines 4 and 5 (at 0.55 V in Figure 1 and
[2e]
CuBr as catalyst. b-Nitroamines such as 5 can be readily
[8]
reduced to 1,2-diamines, which are important molecules in
[9]
medicinal chemistry and are useful ligands for catalysis.
[
a]
Table 1. Recycling of both catalyst and ionic liquid, NMR yields based
on the tertiary amine 4, using an internal standard.
Run
1
2
3
4
5
6
7
8
9
[
a]
Yield
80
90
>99
>99
>99
95
90
85
80
0
.75 V in Figure 2) and the decrease of the reduction current
of the nitro group of 5 at À1.7–1.8 V in Figure 2.
An important feature of the CVs of amines 4 and 5 is the
reduction peak at À1.2 V in the reverse scans which, as will
be seen below, corresponds to the reduction of the iminium
The above reaction was performed in [BMIm]
vent. Such 1,3-dialkyl imidazolium based ILs have consider-
ACHTUNGTNERNUNG[ BF ] as sol-
4
[7]
able interest in both catalysis and electrochemistry.
BMIm][BF ] was synthesized according to the reported
method and was obtained as a colorless liquid.
BMIm][BF ] appeared as an effective solvent for the CDC
[
ACHTUNGTRENNUNG
4
[10]
The IL
[
ACHTUNGTRENNUNG
4
reaction between tertiary amine 4 and nitromethane and the
desired product 5 was obtained in a 80% NMR yield
[2e]
(
Table 1, run 1) compared to a 90% NMR yield in water.
Interestingly, the IL and the catalyst could be recycled nine
times with almost no lost of activity (Table 1). It is also im-
portant to note that an increase of yield was observed after
the first recycling. In fact, a better yield of the desired cou-
pling product 5 was obtained in runs 2 to 6, suggesting a
possible activation of the copper catalyst by the IL solvent.
Imidazolium-based ILs can dramatically accelerate the elec-
[11]
tron transfer from metal complexes to oxygen molecule.
Moreover, the in situ generation of a carbene intermediate
in an ionic liquid, which can play the role of ligand for
[12]
metal centers, has also been clearly demonstrated.
Figure 1. Cyclic voltammogram of N-phenyltetrahydroisoquinoline 4,
The results summarized in Table 1 clearly show that the
chemical CDC reaction between a tertiary amine and nitro-
À2
(
[4]=5.10 m) in [BMIm]
A
T
N
T
E
N
N
[BF
4
] at a GC electrode and a scan rate of
À1
0.1 Vs
.
Chem. Eur. J. 2010, 16, 8162 – 8166
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8163

C.-J. Li, J. Lessard et al.
Electrolysis of amine 4 was then performed under con-
trolled potential (potentiostatic electrolysis) in a standard
two-compartment electrochemical H-cell with a glass frit as
separator. A large area Pt electrode was used as working
electrode instead of a GC electrode for practical reasons
(
see Experimental Section). The CV behavior on a Pt mi-
croelectrode was identical to that on a GC electrode. The
anodic potential was fixed at 0.7 V, 150 mV more positive
than the oxidation peak potential of amine 4 (see the Exper-
imental Section and Supporting Information for details).
The electrochemical reduction of the 1,3-dialkyl imidazoli-
um cation (C2-hydrogen) was responsible for the massive
hydrogen evolution observed at the cathode during the elec-
[13]
trolysis. The number of Coulombs consumed correspond-
ed to the exchange of one electron per molecule of 4. After
the electrolysis, two equivalents of nitromethane were
added to the cathodic compartment and the resulting mix-
ture was stirred for 2 h. No organic compound (other than
nitromethane) was detected after extraction with diethyl
ether, suggesting that only charged species insoluble in di-
ethyl ether and stabilized in the ionic liquid were present in
the final electrolytic solution. On the other hand, when two
equivalents of both triethylamine and nitromethane were
added to the solution after electrolysis, the CV of Figure 3
was obtained showing the presence of an equimolar ratio of
the desired b-nitroamine 5 and starting amine 4 (oxidation
peaks of amine 4 at 0.55 V and b-nitroamine 5 at 0.75 V of
equal intensity). Other features of the CV of Figure 3 are:
the cathodic peak at À1.2 V corresponding to the reduction
À2
Figure 2. Cyclic voltammogram of the b-nitroamine 5, ([5]=5.10 m) in
À1
[
BMIm]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[BF
4
] at a GC electrode and a scan rate of 0.1 Vs
.
cation generated by oxidation of the amine: iminium cation
from the oxidation of 4 (see Scheme 2) and iminium
6
+
cation 5 generated by oxidation of b-nitroamine 5.
of both the iminium cation 6, generated by the oxidation of
+
4
5
and the iminium cation 5 , generated by the oxidation of
; the peak corresponding to the reduction of the nitro
group of 5; and the nitromethane reduction peak. Note that
there is no iminium cation present in the solution since the
first scan of Figure 3 recorded towards the negative poten-
tials does not show any reduction peak at À1.2 V. The imini-
um reduction peak is generated only after the first cycle in
oxidation. The formation of an equimolar amount of b-ni-
Scheme 2. Postulated mechanism for the formation of b-nitroamine 5 by
oxidation of amine 4.
Next, the cyclic voltammogram of a solution of amine 4
plus two equivalents of nitromethane in [BMIm] ACHTUGNTERNN[UNG BF ] was
4
recorded in order to see if the b-nitroamine 5 could be
formed in situ upon oxidation of amine 4. This CV did not
show any peak corresponding to the b-nitroamine 5 (voltam-
mogram not shown). The latter was not formed under these
conditions. These voltammetric results will be discussed
below together with the potentiostatic electrolysis results.
Figure 3. Cyclic voltammogram of the electrolysis solution after addition
of 2 equiv of triethylamine and 2 equiv of nitromethane at a GC elec-
À1
trode and a scan rate of 0.1 Vs
.
8164
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Chem. Eur. J. 2010, 16, 8162 – 8166

Oxidative Cross-Dehydrogenative-Coupling
FULL PAPER
troamine 5 and starting amine 4 was confirmed by ether ex-
traction of the final solution and NMR analysis of the crude
product.
clearly that the cation 6 has been preformed in [BMIm]-
AHCTUNGTNERNU[G BF ] (cation pool) by complete oxidation of 4. Then, the
4
addition of two equivalents of nitromethane and triethyl-
amine to the ionic liquid cation pool produced the b-nitro-
amine 5 in an 80% NMR yield with high current efficiency
(Scheme 3).
According to the electrolysis and voltammetric results,
[3]
and literature precedent, a reaction mechanism can be
postulated as shown in Scheme 2. It involves the generation
+
of the radical cation 4 C by a one-electron oxidation of
amine 4 which would be deprotonated to the radical inter-
mediate (4C) by the amine 4. Thus, one equivalent each of
+
radical 4C and protonated amine 4H would be formed. The
neutral radical 4C would be further oxidized to the iminium
cation 6. In the absence of Et N, there is no nucleophile
3
present in the solution to react with the iminium cation 6
+
and the charged species 4H and 6 are much more soluble
in the ionic liquid than in diethyl ether. The protonated
+
amine 4H is not oxidized in the potential region studied as
evidenced by the absence of an oxidation peak in the CV of
+
À
4
H
BF4 in [BMIm] AHCTUNGERTNNUGN[ BF ] (voltammogram not shown). In
4
+
the presence of two equivalents of Et N, 4H and nitrome-
3
thane would be deprotonated to regenerate the starting
amine 4 and form the nucleophilic nitronate anion. The
latter then would react with the iminium cation 6 to gener-
ate the desired b-nitroamine 5. Hence a total of one elec-
tron per molecule of amine 4 is consumed to produce an
equimolar amount of b-nitroamine 5 and starting amine 4.
Scheme 3. Results of the cation pool method in an ionic liquid.
+
A similar mechanism involving the radical cation 4 C and
To evaluate further the potential of this new cation-pool
the radical 4C might also take place in the copper-catalyzed
method, we investigated the formation of a CÀP bond using
aerobic chemical cross-dehydrogenative-coupling reaction.
In fact, the addition of radical scavengers such as 2,2,6,6-tet-
ramethyl-1-piperidinyloxy (TEMPO) and 4-methyl-2,6-di-
tert-butylphenol (BHT) to the standard conditions of
copper-catalyzed nitro-Mannich reaction decreased signifi-
cantly the yield of the coupling product in both ionic liquid
and water. There would be a competition between oxidation
of the radical 4C to the iminium cation 6 and its reaction
with the scavengers (coupling with TEMPO and H-abstrac-
tion from BHT).
Taking the proposed mechanism in consideration, we
postulated the possibility of generating and accumulating
diethylphosphite as nucleophile in place of the conjugate
base of nitromethane using the same conditions and proce-
[14]
dure described above. The biologically important a-ami-
nophosphonate 7 was synthesized in 85% yield with high
current efficiency (Scheme 3). This result demonstrates the
efficiency of the cation-pool method in ionic liquid. In fact,
when Geniꢃs et al. reported the electrochemical oxidative
phosphonation of ring-substituted N,N-dimethyl anilines in
[3h,i]
acetonitriles,
the desired a-aminophosphonates were ob-
tained only in moderate yields (<30%) with moderate cur-
rent efficiencies. It is important to note that the addition of
nitromethane or diethylphosphite with the chemically pre-
formed iminum salt 6 generated also the desired coupling
products with high efficiency (see Supporting Information).
carbocations electrochemically in [BMIm] ACHTUNGTERNNUN[G BF ] in the ab-
4
[4]
sence of nucleophiles (ꢂꢂcation pool’’). Since the conditions
described above were producing 50% of unreactive proton-
+
ated amine 4H , we envisaged the addition of triethylamine
in the electrolysis solution. The addition of such a base to
Conclusion
the iminium cation 6 would be reversible and the base
+
would then deprotonate 4H to regenerate the starting
To conclude, we have demonstrated that ionic liquids and in
amine 4 which would allow the complete anodic oxidation
of amine 4 to the iminium cation 6. Cyclic voltammetric
studies demonstrated that triethylamine was not oxidized at
the selected potential. Consequently, triethylamine was
particular [BMIm] ACHTUNGERTNNUNG[ BF ] are highly effective solvents for
4
copper-catalyzed cross-dehydrogenative coupling (CDC)
with oxygen as terminal oxidant. By using ionic liquids as
such, both the solvent and the copper catalyst can be recy-
cled nine times without significant lost of activity. Further-
more, we have also demonstrated the possibility of using an
electrochemical method rather than a metal catalyst to carry
out CDC in ionic liquids as solvent and electrolyte. Finally,
a detailed mechanism of the anodic nitro-Mannich carbonÀ
added to the [BMIm]
ACHTUNGTNERUNNG[ BF ] ionic liquid containing the amine
4
4
and the reaction mixture was subjected to potentiostatic
anodic oxidation. The coulometric measurements corre-
sponded to the expected consumption of two electrons per
molecule of 4. The starting amine 4 and the protonated
+
amine 4H were not detected and only the iminium cation
carbon bond formation process was described. Further stud-
ies for the detection and the characterization of proposed
6
reduction peak could be observed in the CV, thus showing
Chem. Eur. J. 2010, 16, 8162 – 8166
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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8165

C.-J. Li, J. Lessard et al.
intermediates along with the extension of the scope of the
CDC reaction in ionic liquids are being pursued.
[
1] a) G. A. Olah, Angew. Chem. 1995, 107, 1519–1532; Angew. Chem.
Int. Ed. Engl. 1995, 34, 1393–1405; b) B. E Maryanoff, H.-C. Zhang,
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1
431–1628; c) J.-i. Yoshida, K. Kataoka, R. Horcajada, A. Nagaki,
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3, 2933–2980; e) A. D. Dilman, S. L. Ioffe, Chem. Rev. 2003, 103,
Experimental Section
9
1
733–772.
General: H NMR spectra were recorded on a Varian 400 MHz spec-
[
2] a) L. Zhao, O. Baslꢀ, C.-J. Li, Proc. Natl. Acad. Sci. USA 2009, 106,
3
trometer in CDCl . Thin-layer chromatography was performed using
4
106–4111; b) C.-J. Li, Acc. Chem. Res. 2009, 42, 335–344; c) W.-J.
Sorbent Silica Gel 60 F254 TLC plates and visualized with ultraviolet
light. All reagents were purchased from Aldrich and Acros, deoxygenat-
ed and back filled with Argon before used. 2-Phenyl-1,2,3,4-tetrahydroi-
Yoo, C. A. Correia, Y. Zhang, C.-J. Li, Synlett 2009, 138–142; d) O.
Baslꢀ, C.-J. Li, Org. Lett. 2008, 10, 3661–3663; e) O. Baslꢀ, C.-J. Li,
Green Chem. 2007, 9, 1047–1050; f) Z. Li, D. S. Bohle, C.-J. Li,
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Eur. J. Org. Chem. 2005, 3173–3176; h) Z. Li, C.-J. Li, J. Am. Chem.
Soc. 2005, 127, 6968–6969; i) Z. Li, C.-J. Li, J. Am. Chem. Soc. 2005,
[
2h]
soquinoline (4) was prepared by the literature method.
General procedure for the aerobic CDC nitro-Mannich reaction in ionic
liquids: To a mixture of CuBr (1.4 mg, 0.01 mmol), 4 (42 mg, 0.2 mmol),
BMIm] ACHTUNGTRENNUNG[ BF ] (0.4 mL) and nitromethane (21.5 mL, 0.4 mmol) was added.
4
Then the 20 mL test-tube was filled up with molecular oxygen and
sealed. The reaction was stirred using a magnetic stirrer at 608C for 16 h.
The resulting mixture was extracted with diethyl ether. The solvent was
evaporated and the residue was purified by column chromatography on
silica gel (hexane/ethyl acetate 5:1), and the fraction with an R
collected and concentrated to give the desired product 5.
[
1
27, 3672–3673; j) Z. Li, C.-J. Li, Org. Lett. 2004, 6, 4997–4999;
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7
, 351–356; b) N. Shankaraiah, R. A. Pilli, L. S. Santos, Tetrahedron
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2301.
f
=0.5 was
Recycling: Residues of solvents in the ionic liquid were removed under
vaccum and the [BMIm][BF ] containing the copper bromide catalyst
was recharged with 4 and nitromethane. The test tube was sealed and the
reaction was stirred under an atmosphere of oxygen at 608C for 16 h.
The extraction and purification procedures were the same as those de-
scribed above.
A
H
U
G
R
N
U
G
4
General procedure for the anodic nitro-Mannich reaction in ionic liquids:
In a two-compartment electrochemical cell with a glass frit as separator,
the ionic liquid [BMIm] ACHTUNGTRENNUNG[ BF ] (3 mL) was added to the three-neck anodic
4
compartment containing 4 (31 mg, 0.15 mmol). Amine 1 was carefully
dissolved upon heating by stirring using a magnetic stirrer under an
Argon atmosphere. The cathodic compartment was filled with [BMIm]-
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jiwara, J. Am. Chem. Soc. 1999, 121, 9546–9549; b) S. Suga, S.
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2658.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[BF
4
] (3 mL). The platinum mesh wire-netting working electrode (area=
[5] a) P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis Vol. 1, 1st
ed., Wiley-VCH, Weinheim, 2003; b) T. Welton, Chem. Rev. 1999,
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and references therein.
2
1
.3 cm ) was positioned in the anodic compartment and the platinum aux-
iliary electrode in the cathodic compartment (see Figure 1 of the Sup-
porting Information). The oxidation potential was fixed at 0.7 V (vs. Ag/
AgNO ) and the colorless solution was stirred at room temperature until
3
the current reached less than 1% of its initial intensity. To the orange
product mixture, nitromethane (16 mL, 0.3 mmol) and triethylamine
(
44 mL, 0.315 mmol) were added and the solution was stirred for 12 h at
[6] a) F. A. Uribe, R. A. Osteryoung, J. Electrochem. Soc. 1988, 135,
378–381; b) Z. J. Karpinski, S. Song, R. A. Osteryoung, Inorg.
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moto, M. Yanagida, K. Tanimoto, M. Nomura, Chem. Lett. 2000,
room temperature. The resulting mixture was extracted with diethyl
ether and filtered through a short layer of silica gel and eluted with dieth-
yl ether. The solvent was evaporated and the residue was analyzed by
1
thin layer chromatography and H NMR spectroscopy.
9
22–923.
General procedure for the cation pool method in ionic liquids: In The
electrochemical cell, the electrodes and the procedure were the same as
those described above except that triethylamine (28 mL, 0.3 mmol) was
added to the anodic compartment. After completion of the electrolysis,
nitromethane (16 mL, 0.3 mmol) or diethylphosphite (26 mL, 0.3 mmol)
and triethylamine (44 mL, 0.315 mmol) were added and the solution was
stirred for 12 h at room temperature. The resulting mixture was extracted
with diethyl ether and filtered through a short layer of silica gel and
eluted with diethyl ether. The solvent was evaporated and the residue
[
[
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[
1
was analyzed by thin-layer chromatography and H NMR spectroscopy.
[
[
[
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Acknowledgements
[
We are grateful to the Canada Research Chair (Tier 1) foundation (to
C.J.L.), NSERC of Canada, the Canada Foundation for Innovation
(
CFI), FQRNT of Quebec, McGill University, and Sherbrooke University
Received: March 16, 2010
for their financial support.
Published online: June 8, 2010
8166
www.chemeurj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8162 – 8166