First study on the enantioselective palladium-catalyzed C-P cross-coupling reaction between an alkenyltriflate and a phosphine-borane

Article abstract of DOI:10.1016/j.crci.2010.06.003

In this article, we report the first example of a palladium-catalyzed asymmetric C-P cross-coupling reaction between a racemic secondary phosphine-borane, methylphenylphosphine-borane 1, and an achiral triflate. The influence of various parameters such as

Full text of DOI:10.1016/j.crci.2010.06.003

C. R. Chimie 13 (2010) 1099–1103
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Preliminary communication/Communication
First study on the enantioselective palladium-catalyzed C-P cross-
coupling reaction between an alkenyltriflate and a phosphine-borane
*
Delphine Julienne, Olivier Delacroix, Annie-Claude Gaumont
UMR CNRS 6507, INC3M, FR 3038, ENSICAEN, laboratoire de chimie mole´culaire et thio-organique, universite´ de Caen Basse-Normandie, 6, boulevard Mare´chal-
Juin, 14050 Caen, France
A R T I C L E I N F O
A B S T R A C T
Article history:
In this article, we report the first example of a palladium-catalyzed asymmetric C-P cross-
coupling reaction between a racemic secondary phosphine-borane, methylphenylpho-
sphine-borane 1, and an achiral triflate. The influence of various parameters such as the
structure of the chiral ligand, the temperature and the nature of the solvent on the activity
and the selectivity of the reaction is reported. Enantiomeric ratios up to 78:22 were
obtained using (S,S)-Me-DUPHOSPdCl2 as catalyst. A kinetic resolution process is proposed
to account for this selectivity.
Received 8 February 2010
Accepted after revision 3 June 2010
Available online 8 July 2010
Keywords:
P-alkenylphosphines
Phosphine-boranes
Phosphine ligands
C-P cross-coupling
Enantioselectivity
´
ß 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
´
´
R E S U M E
Mots cle´s :
Dans cet article, nous de´crivons le premier exemple de couplage C-P asyme´trique catalyse´
Phosphines P-vinyliques
Phosphines borane
´
´
´
au palladium entre une phosphine-borane secondaire racemique, la methylphenylpho-
´
`
sphine-borane 1 et un triflate achiral. L’influence de differents parametres tels que la
´
Ligands phosphores
structure du ligand chiral, la tempe´rature, la nature du solvant et de la base, sur l’activite´ et
Couplage C-P
´
´
´
´ ´ ´
´
´
´
´ ´
la selectivite de la reaction a ete etudiee. Un rapport enantiomerique de 78:22 a ete obtenu
´
´
´
Enantioselectivite
´
´
lorsque le catalyseur (S,S)-Me-DUPHOSPdCl2 est utilise. Un processus de dedoublement
cine´tique est propose´ pour justifier cette se´lectivite´.
ß 2010 Acade´mie des sciences. Publie´ par Elsevier Masson SAS. Tous droits re´serve´s.
1. Introduction
chemistry. Recently, they were successfully used as ligands
in homogeneous catalysis. Various synthetic methods for
Alkenylphosphines are
a
class of compounds that
accessing alkenylphosphine derivatives have been devel-
oped. The main methodologies are related to the prepara-
tion of racemic or achiral alkenylphosphines. They involve
reaction between halophosphines and vinylic organometal-
lic species, hydrophosphination of alkynes or C-P cross-
coupling reactions [2]. Contrarily, enantiopure or enan-
tioenriched derivatives are rare. If a few C-stereogenic
derivatives have been prepared by C-P cross-coupling of
enantiopure alkenyl-triflates [6–8] or halides [9], P-stereo-
genic derivatives are neglected and their synthesis
requires resolution processes [5,10]. A more appealing
route to P-chirogenic phosphines relies on the use of
asymmetric catalysis. To the best of our knowledge, the only
example dealing with the synthesis of an enantioenriched
undergo numerous useful synthetic transformations due
to the carbon-carbon double bond, which is directly linked
tothe phosphorusatom[1–3]. Forexample, alargevarietyof
nucleophiles can add to the double bond to provide
bifunctional or polyfunctional adducts (polyphosphines,
dendrimers, polymers). Alkenylphosphines can also be used
as dienophiles in cycloaddition reactions [4,5]. Due to the
presence of both phosphorus and carbon-carbon double
bond, they are also of great interest in coordination
* Corresponding author.
E-mail address: annie-claude.gaumont@ensicaen.fr (A.-C. Gaumont).
1631-0748/$ – see front matter ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.06.003

1100
D. Julienne et al. / C. R. Chimie 13 (2010) 1099–1103
alkenylphosphine derivative by asymmetric catalysis was
reported recently by some of us [11] in the course of our
studies on the hydrophosphination reaction [12–16]. We
report here the preliminary results dealing with the
enantioselective coupling between an achiral alkenyltriflate
and a racemic secondary phosphine-borane. Indeed, al-
thoughexamplesofenantioselectivePd-catalyzed C-Pcross-
coupling reaction between an arylhalide and a secondary
phosphine have recently appeared in the literature [17,18],
no enantioselective variant of the coupling reaction involv-
ing alkenyl derivatives has ever been reported.
–
–
Conditions A: Daicel Chiralpack OJ-H (250 Â 4.6 mm,
L Â ID) column (n-heptane/isopropanol 90:10, 1 mL/min,
tR1 = 9.34 min, tR2 = 10.95 min) at 20 8C;
Conditions B: Daicel Chiralpack AD-H (250 Â 4.6 mm,
L Â ID) column (n-heptane/(MeOH/EtOH 50:50) 98:2,
0.2 mL/min, tR1 = 36.41 min, tR2 = 40.13 min) at 12 8C.
Given enantiomeric excesses of compound
2 are
average of at least two experiments.
2.2. General procedures
2. Experimental
Enantiopure catalytic systems, (R)-Tol-BINAPPdCl2
[20], (R,R)-BDPPPdCl2 [21], (R)-i-Pr-PHOXPdCl2 [22] and
(S,S)-Me-DUPHOSPdCl2 [23] were synthesized according
to modified literature procedures [24].
2.1. General comments
All reactions were carried out under nitrogen atmo-
sphere. All glassware was flamed before use. Cyclohex-1-
en-1-yltrifluoromethanesulfonate, commercially avail-
able, was distilled under reduced pressure (Kugelro¨hr)
before use. Methylphenylphosphine-borane was prepared
according to literature procedure [19]. All commercially
available ligands were used as purchased.
Dimethylsulfoxide (DMSO) was distilled from calcium
hydride and degassed before use. DMF was stored on
molecular sieves, freshly distilled and degassed before use.
Toluene and THF were purified by an innovative technolo-
gy Pure Solv. device (activated alumina column containing
a copper catalyst and molecular sieves). Pentane was
distilled from calcium hydride before use.
2.2.1. General procedure for the preparation of L2PdCl2
In a two-necked round bottomed flask, containing a
solution of (CH3CN)2PdCl2 (0.077 M, 1 equiv.) in toluene, a
solution of ligand L2 (0.077 M, 1 equiv.) in toluene is added.
The mixture is stirred overnight at room temperature. The
obtained precipitate is filtered, washed with freshly
distilled pentane and dried under vacuum to give
quantitatively the desired complex.
2.2.2. Typical procedure for palladium-catalyzed asymmetric
phosphination
In a schlenk tube, flushed under N2, L2PdCl2 (0.01 mmol,
4.8 mol%) is dissolved into the solvent (0.3 mL). Cyclohex-
Column chromatography was performed on Merck
silica gel Si 60 (40–63 mm). Solvents were used as
purchased. Thin layer chromatography (TLC) was per-
formed on silica gel 60 F-254 plates (0.1 mm) with iodine
or UV detection.
1-en-1-yltrifluoromethanesulfonate (38
equiv.), the base (0.26 mmol, 1.2 equiv.) and methylphe-
nylphosphine-borane 1 (30 L, 0.22 mmol, 1 equiv.) are
mL, 0.22 mmol, 1
m
then introduced. The mixture is stirred during a time t at
the desired temperature T. The reaction is monitored by
31P NMR spectroscopy. When the expected conversion is
reached, the medium is hydrolyzed with degassed
water (0.5 mL). The aqueous layer is extracted with
diethyl ether (3 Â 2 mL). The combined organic layers are
dried over MgSO4 and concentrated under reduced
pressure. The crude product is then purified by silica-
gel column chromatography with pentane/AcOEt (95/5,
Rf = 0.30) as eluent affording pure coupling product 2 as
a yellow oil. All attempts to crystallize the oily product
failed. Er values are measured by HPLC using conditions
A or B.
1H, 13C, 19F and 31P NMR spectra were obtained on
BRUKER DPX 250 or AC 400 spectrometers, 11B NMR
spectra were recorded on a BRUKER AC 400 spectrometer.
1H and 13C NMR chemical shifts are reported relative to
Me4Si used as an internal standard. 31P, 19F and 11B NMR
chemical shifts are reported relative to respectively H3PO4
(85%), CFCl3 and BF3.Et2O used as external references.
Coupling constants are reported in Hertz (Hz).
Abbreviations are used as follows: s = singlet, d = dou-
blet, t = triplet, q = quartet, quint = quintuplet, sept = sep-
tuplet, oct = octuplet, m = multiplet and br = broad.
Mass spectroscopy was performed on a QTOF Micro
WATERS.
2.3. Experimental data
High performance liquid chromatography (HPLC)
separations were achieved using the following compo-
nents:
Coupling-product 2 [934338-23-5] [8]
Anal. calcd for C13H20BP (218.1): C 71.60, H 9.24. Found:
C 71.08, H 9.79. 31P NMR (161.9 MHz, CDCl3):
d
11.4 (q,
1JPB = 64.6 Hz). 1H NMR (400.1 MHz, CDCl3):
d 7.70-7.62 (m,
–
For flow up to 0.5 mL/min, the components are: a Waters
600 pump, a Waters 996 photodiode array detector
(190–250 nm) and Millenium Software;
3
2H), 7.52-7.43 (m, 3H), 6.57 (dm, 1H, JHP = 18.8 Hz), 2.28-
2.18 (m, 2H), 2.11-1.93 (m, 2H), 1.67-1.63 (m, 4H), 1.63 (d,
2
1
3H, JHP = 10.0 Hz), 0.79 (q, 3H, JHB = 92.0 Hz). 13C NMR
–
For flow below 0.5 mL/min, the components are: a
Waters Alliance 2695 pump, a Waters 2996 photodiode
array detector (190–250 nm) and Empower Software.
2
(100.6 MHz, CDCl3):
2JCP = 9.1 Hz), 130.8(d,4JCP = 2.4 Hz), 129.9(d, 1JCP = 54.3 Hz),
128.7 (d, JCP = 9.9 Hz), 128.5 (d, JCP = 52.5 Hz), 26.6
(d, 3JCP = 12.8 Hz), 24.7 (d, 2JCP = 6.9 Hz), 22.3 (d,
3JCP = 6.4 Hz), 21.4 (d, JCP = 1.3 Hz), 9.3 (d, JCP = 40.5 Hz).
d
141.0 (d, JCP = 10.2 Hz), 131.5 (d,
3
1
Enantiomeric excess of compound 2 was measured by
HPLC using two possible different conditions:
4
1

D. Julienne et al. / C. R. Chimie [S(mhc1_e)TDIG]F$13 (2010) 1099–1103
1101
1
11B NMR (128.4 MHz, CDCl3):
1JBH = 92.0 Hz).
d -35.6 (dq, JBP = 64.6 Hz,
Side-product 3 was isolated as a pale yellow oil (eluent:
pentane/AcOEt: 95/5, Rf = 0.41). 31P NMR (161.9 MHz,
1
CDCl3):
CDCl3):
d
d
109.4 (q, JPB = 63.6 Hz). 1H NMR (400.1 MHz,
7.90-7.83 (m, 2H), 7.61-7.48 (m, 3H), 5.16-5.11
(m, 1H), 2.14-2.04 (m, 2H), 2.02-1.94 (m, 2H), 1.77 (d, 3H,
Scheme 1. Asymmetric C-P coupling of cyclohexenyl triflate and
phosphine-borane 1.
2JHP = 8.8 Hz), 1.70-1.61 (m, 2H), 1.53-1.44 (m, 2H), 0.90 (q,
1
3H, JHB = 95.5 Hz). 13C NMR (100.6 MHz, CDCl3):
d 149.4
3
1
(d, JCP = 7.0 Hz), 132.8 (d, JCP = 56.0 Hz), 132.1 (d,
4JCP = 2.4 Hz), 130.7 (d, 2JCP = 11.5 Hz), 128.6 (d,
3JCP = 10.4 Hz), 111.2 (d, 2JCP = 6.3 Hz), 28.7 (d, 4JCP = 2.3 Hz),
23.6 (s), 22.7 (s), 21.6 (s), 17.1 (d, 1JCP = 46.6 Hz). 11B NMR
only pre-formed complexes L2PdCl2 in the enantioselective
version in order to avoid a possible catalysis by the
susbtrate. Indeed, chiral bidentate ligands tightly bind to
the metal and no displacement of the chiral ligand from the
metal either by the precursor 1 or the product 2 was
observed, whatever the conditions [28]. It is worthy of note
that no reaction occurred in the absence of the palladium
catalyst. Coupling conditions were based on those devel-
oped for the racemic coupling: 1:1 ratio of precursors
(phosphine 1 and triflate), 1.2 equiv. of K3PO4 as a base, L2
chelating ligand-PdCl2 complex (4.8 mol%) as catalyst in
DMSO at 80 8C (Scheme 1) [8].
The procedure is quite simple: the pre-formed catalyst
L2PdCl2 is dissolved in DMSO. Then cyclohexenyl triflate,
K3PO4 and phosphine-borane 1 are added. The mixture is
then heated to 80 8C and monitored by 31P NMR
spectroscopy. After completion, the reaction is hydrolysed,
the product is extracted and purified on silica gel
chromatography under the air (allowed by the presence
of the borane group) [25]. Conditions for the separation of
the two enantiomers were defined by HPLC on the racemic
compound, which was previously obtained by C-P cross-
coupling between racemic 1 and cyclohexenyltriflate,
catalyzed by dpppPdCl2 [8]. The ability of ligands L1-L4
to promote asymmetry was then evaluated (Fig. 1, Table 1).
After 3 h at 80 8C, excellent conversions were obtained
whatever the ligand. Whereas most of the ligands afforded
racemic 2, (S,S)-Me-DUPHOS L4, an electron-rich and rigid
ligand, induced a promising level of enantioselectivity.
(Table 1, entry 4). Hence, a 64:36 enantiomeric ratio was
determined by HPLC. Consequently, [(S,S)-Me-DUPHOS]
PdCl2 was kept for the rest of the study. A survey of reaction
conditions (temperature, time) was next performed using
L4PdCl2 as catalyst. Following the reaction by 31P NMR, we
observed that with L4, an almost complete conversion (86%)
(128.4 MHz, CDCl3):
d
-36.6 (dq, 1JBP = 63.6 Hz,
ꢀ
1JBH = 95.5 Hz). HMRS calcd for C13H17OP ([M-BH3]+ ):
220.1017, found 220.1016.
Side-product 4 was isolated as a pale yellow oil (eluent:
methanol, Rf = 1). 31P NMR (161.9 MHz, CDCl3):
d
25.1 (q,
1JPB = 66.0 Hz). 1H NMR (400.1 MHz, CDCl3):
d 7.73-7.65 (m,
2H), 7.51-7.36 (m, 3H), 2.55 (s, 1H), 1.78-1.69 (m, 2H),
1.66-1.52 (m, 3H), 1.55 (d, 3H, 2JHP = 10.0 Hz), 1.52-1.48 (m,
1H), 1.48-1.38 (m, 2H), 1.12-0.18 (m, 5H). 13C NMR
2
(100.6 MHz, CDCl3):
d
133.1 (d, JCP = 8.2 Hz), 131.5 (s),
3
1
128.6 (d, JCP = 9.7 Hz), 126.5 (d, JCP = 50.1 Hz), 71.2 (d,
2
2JCP = 39.9 Hz), 41.0 (s), 31.5 (d, JCP = 21.4 Hz), 31.4 (d,
1JCP = 23.1 Hz), 25.0 (s), 20.4 (s), 4.4 (d, JCP = 38.6 Hz). 11B
1
NMR (128.4 MHz, CDCl3): d-35.5 to -40.0 (m). HRMS calcd
for C13H19O11BP [M-3Hꢀ]+: 233.1267. Found: 233.1267.
ꢀ
HRMS calcd for C13H19OP [M-BH3]+ : 222.1173. Found:
222.1186.
3. Results and discussion
To investigate the enantioselective version, racemic
methylphenylphosphine-borane 1 and achiral cyclohex-
enyl triflate were selected as model reaction partners.
Phosphine-boranes were selected as phosphine precursors
in order to ensure an easy handling of the phosphine
derivatives and also to activate the P-H bond of the
phosphine [25–27]. Very bulky groups on phosphorus such
as isityl ((2,4,6-(i-Pr)3C6H2)) or mesityl groups were
prohibited since they are not representative substituents.
A preliminary study in the racemic version having shown
that catalysis could take place under certain conditions by
using Pd(OAc)2 and phosphine-borane 1, we decided to use
[(ig._1)TD$FIG]
Fig. 1. Enantiopure ligands used in the C-P coupling reactions.

D. Julienne et al. / C. R. Chimie [(i_.gT)3DF$I]G13 (2010) 1099–1103
1102
Table 1
Screening of ligands.
Entry
Ligand
Conversion of rac-1 (%)a
er of 2 (%)b,c
1
2
3
4
L1
L2
L3
L4
89
94
93
97
50:50
50:50
50:50
64:36
a
Determined by 31P NMR integration.
Fig. 3. By-product 4.
b
Enantiomeric ratio measured by chiral HPLC (average of at least two
experiments).
c
Conditions used: L2PdCl2 (4.8 mol%) as precatalyst, K3PO4 (1.2 equiv.)
We next decided to perform the reaction under kinetic
with a 1:1 ratio of phosphine 1 and triflate, in DMSO (3 h, 80 8C).
resolution conditions. Interestingly, an increase in the
enantioselectivity was observed for a conversion below
50% (Table 2, entries 5–8). Hence at 60 8C, for a conversion
of 32%, the enantiomeric ratio was 69:31. By lowering the
temperature to 40 8C and 20 8C, enantiomeric ratios of
74:26 and 78:22 were obtained, respectively (Table 2,
entries 7 and 8). Although, this enantioselectivity is not as
high as might be desirable, it should be pointed out that an
enantiomeric ratio of 78:22 is the best result obtained for
the synthesis of P-chirogenic alkenylphosphines by
asymmetric catalysis yet.1
Table 2
Asymmetric C-P coupling of cyclohexenyl triflate and phosphine-borane
1a.
Entry
Temperature
Solvent
Time
(h)
Conversion
er of
(8C)
of rac-1 (%)b
2 (%)c
1
80
80
60
20
80
60
40
20
20
20
20
20
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
THF
3
97
86
93
94
23
32
24
23
17
35
23
20
64:36
64:36
64:36
67:33
66:34
69:31
74:26
78:22
75:25
74:26
68:32
75:25
2
0.5
3
3
4
86
5
0.08
0.5
2
With the aim to increase the enantiomeric ratio, various
parameters were modified. First, several inorganic and
organic bases were tested. The change of the base
dramatically affected the rate of the coupling reaction
and led to the formation of several side-products. Hence,
upon replacing K3PO4 by K2CO3, side-product 3 was mainly
obtained. With Cs2CO3 and Me3SiOK, 3 was formed
together with another side-product 4. Characterization
by NMR and HRMS allowed the determination of the
structure of 4 (Fig. 3), which results from the hydropho-
sphination of the carbon-carbon double bond of the
starting vinyltriflate, followed by the hydrolysis of the
6
7
8
14.5
14.5
14.5
14.5
14.5
9d
10e
11
12
DMF
a
Catalyst: [(S,S)-Me-DUPHOS]PdCl2 (4.8 mol%), base: K3PO4 (1.2
equiv.).
b
Determined by 31P NMR integration.
c
Enantiomeric ratio measured by chiral HPLC (average of at least two
experiments); the major enantiomer is e1.
d
One equivalent of KI was added.
e
One equivalent of n-Bu4NI was added.
triflate into the corresponding alcohol (d (
31P) = 24 ppm).
witha similar erof 64:36 was obtained after only30 minutes
of reaction, (Table 2, entry 2) demonstrating the great
efficiencyofL4comparedtotheachiraldppp[8], forwhichat
least6 hwererequiredforacompleteconversionatthesame
temperature(80 8C). Decrease of the temperature, from 80 to
60 then 20 8C (Table 2, entries 2–4) has only little effect on
the enantiomeric ratio (er = 67:33 at 20 8C compared to
64:36 at 80 8C). However, the kinetic of the reaction was
markedly lowered (86 h for a 94% conversion at 20 8C
compared to 3 h for a 97% conversion at 80 8C). Furthermore,
at 20 8C, together with the expected product 2, a small
amount (13%) of a side-product was detected by NMR.
Isolation of this product by chromatography and full
characterization allowed to identify it as product 3 (Fig. 2)
As for 3, by-product 4 results from a non-catalytic process
and is formed in the absence of the catalyst. With sodium
tert-pentoxide, only 4 was obtained. Moreover, whatever
the base used, no improvement of the enantiomeric ratio
was observed, so the preferred base was K3PO4.
As already reported in the literature, adjunction of a salt
like KI could influence the transmetallation step and thus
affect the efficiency and the selectivity of the reaction [30].
In our case, addition of 1 equiv. of KI or n-Bu4NI showed no
benefit (Table 2, entries 9 and 10). Finally, influence of the
solvent was evaluated. Replacement of DMSO (
(Table 2, entry 8) by a weakly polar solvent (THF,
(Table 2, entry 11) induced a strong decrease in the
enantioselectivity (er = 68:32). Interestingly, DMF ( = 38)
e
= 47)
= 7.5)
e
e
having an oxygen inserted in the newly created C-P bond (
(
d
promoted a clean reaction. Hence, by-product 3, formed at
20 8C in DMSO, was totally suppressed in DMF and a
satisfactory enantiomeric ratio of 75:25 was obtained
(Table 2, entry 12). Conversion versus enantioselectivity
was then measured in DMF at 20 8C. As anticipated, the
enantiomeric ratio decreased when conversion increased
(Table 3) going down from 75:25 for a 20% conversion to
70:30 for a 45% conversion and 64:36 for a full conversion.
31P) = 109 ppm) [29]. This product results from a non-
catalytic process as confirmed by its formation in the
absence of the palladium catalyst.
[(ig._2)TD$FIG]
1
Fig. 2. By-product 3.
Er up to 71:29 was obtained in ref. [11].

D. Julienne et al. / C. R. Chimie 13 (2010) 1099–1103
1103
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Table 3
er versus conversion at 20 8C in DMFa.
Entry
Time (h)
Conversion of rac-1 (%)b
er of 2 (%)c
1
2
3
4
14.5
24
20
26
45
98
75:25
73:27
70:30
64:36
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96
a
Catalyst: L4PdCl2 (4.8 mol%), base: K3PO4 (1.2 equiv.).
b
c
Determined by 31P NMR integration.
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major enantiomer is e1.
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4. Conclusion
In summary, we have demonstrated that an enantioen-
riched P-chirogenic alkenylphosphine could be formed by
asymmetric C-P coupling reaction between an achiral
alkenyltriflate and
a racemic phosphine-borane. An
enantiomeric ratio up to 78:22 was measured by HPLC.
The evolution of the enantiomeric ratio versus the
conversion seems to be in agreement with a kinetic
resolution process. A mechanistic investigation to gain
more information on the enantiodiscrimining step is
currently underway and will be published in due course.
Acknowledgements
This work has been performed within the ‘‘CRUNCH’’
interregional network. We gratefully acknowledge finan-
cial support from the Ministere de la Recherche et des
`
Nouvelles Technologies, Centre national de la recherche
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´
scientifique (CNRS), the region Basse-Normandie and the
European Union (FEDER funding) for financial support.
References
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ˆ
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[1] For a book chapter on the synthesis of alkenylphosphines, see: A.C.
Gaumont, M. Gulea, Science of Synthesis, in: H. Weyl (Ed.), vol. 33,
Georg Thieme Verlag KG, Stuttgart, 2007, p. 711.
[30] T. Ishiyama, H. Kizaki, T. Hayashi, A. Suzuki, N. Miyaura, J. Org. Chem 63
(1998) 4726.
[2] For
a review on the synthesis of alkenylphosphines, see also:
D. Julienne, O. Delacroix, A.C. Gaumont, Curr. Org. Chem. 14 (2010) 457.