Bromodifluoromethylation of aromatic Grignard reagents with CF2Br2

Article abstract of DOI:10.1016/j.tetlet.2014.10.082

The bromodifluoromethylation of aromatic Grignard reagents having electron-withdrawing groups with dibromodifluoromethane (CF₂Br₂) was developed. The reaction proceeded to give the corresponding (bromodifluoromethyl)benzene derivatives via a difluorocarbene-mediated mechanism.

Full text of DOI:10.1016/j.tetlet.2014.10.082

Tetrahedron Letters 55 (2014) 6839–6843
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Bromodifluoromethylation of aromatic Grignard reagents with
CF2Br2
⇑
Masahiro Shiosaki, Munenori Inoue
Sagami Chemical Research Institute (SCRI), 2743-1 Hayakawa, Ayase, Kanagawa 252-1193, Japan
Department of Electronic Chemistry, Tokyo Institute of Technology, Nagatsuta, Midori-Ku, Yokohama 226-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The bromodifluoromethylation of aromatic Grignard reagents having electron-withdrawing groups with
dibromodifluoromethane (CF2Br2) was developed. The reaction proceeded to give the corresponding (bro-
modifluoromethyl)benzene derivatives via a difluorocarbene-mediated mechanism.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 6 August 2014
Revised 4 October 2014
Accepted 14 October 2014
Available online 19 October 2014
Keywords:
Bromodifluoromethylation
Dibromodifluoromethane
Aromatic Grignard reagents
Difluorocarbene
Organofluorine compounds
Nu−
Organofluorine compounds have been widely utilized in the
areas of medicinal chemistry, agricultural chemistry, and material
science.1 To enable facile access to these fluorinated compounds,
recent researches focus on the development of introducing
fluorine-containing functional groups with accompanying
CF2Br2
F
M
CF2Br
F
Nu
FG
M = Li, MgX
FG
FG
Nu = F, 18F, H,
OAr, etc.
carbonAcarbon bond formation. Among fluorinated functional
groups, the bromodifluoromethyl (CF2Br) group is a versatile one
because various bromodifluoromethylated aromatics have served
as precursors of [18F]trifluoromethylated compounds for positron
emission tomography (PET) imaging,2 liquid crystal (LC) com-
pounds containing a –CF2OAr moiety,3 fluoroparacyclophanes,4
and fluorinated polymers (Scheme 1).5 In addition, the CF2Br group
can be regarded as an equivalent of trifluoromethyl (CF3)2 and
difluoromethyl (CHF2)6 groups. To introduce the CF2Br group via
carbonAcarbon bond formation, dibromodifluoromethane (CF2Br2)
has been used as a source of the CF2Br group and several bromod-
ifluoromethylations of carbon nucleophiles, such as acetylides7 and
enolates,6c,8 were reported. Furthermore, recently, several electro-
philic bromodifluoromethylating reagents (+CF2Br) have been
developed by Xiao,9 Magnier,10 and Shibata,11 and the reactions
of acetylides and malonates with these S-(bromodifluorometh-
yl)diarylsulfonium salts and S-(bromodifluoromethyl)sulfoximines
were demonstrated.
Scheme 1. Bromodifluoromethylation and derivatization.
On the other hand, in terms of aromatic bromodifluoromethyla-
tions, only a few reactions using arylmetal reagents were reported.
South et al. disclosed that treatment of the dianion of 2-trifluoro-
acetoamido-4-(trifluoromethyl)thiazole with CF2Br2 at À98 °C
produced the corresponding 5-(bromodifluoromethyl)thiazole
derivative.12 They commented that this reaction is unusual
because the reaction of 2-(trifluoromethyl)phenyllithium with
CF2Br2 gave a quantitative yield of 1-bromo-2-(trifluoromethyl)
benzene. In addition, as shown in Scheme 2, Matsui and co-work-
ers reported that treatment of 2,6-difluorophenyllithium 1 with
CF2Br2 below À70 °C produced 1-(bromodifluoromethyl)-2,6-
difluorobiphenyl derivative 2 and aryl bromide 3 with a ratio of
8:2.3 Although this method has been used in the LC compound syn-
thesis, generally two fluorine atoms substituted at the C-2 and C-6
positions of 1 are required to proceed the reaction. To our best
knowledge, these were the only two examples of the bromodifluo-
romethylation of arylmetal species with a +CF2Br equivalent. The
substrates were limited and the generality of the reaction is
⇑
Corresponding author. Tel.: +81 467 77 4208; fax: +81 467 77 4113.
E-mail address: inoue@sagami.or.jp (M. Inoue).
http://dx.doi.org/10.1016/j.tetlet.2014.10.082
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

6840
M. Shiosaki, M. Inoue / Tetrahedron Letters 55 (2014) 6839–6843
F
F
F
F
CF2Br2
Pr
Li
Pr
R
THF, −70 °C
(2 : 3 = 80 : 20)
1
2: R=CF2Br
3: R=Br
Scheme 2. Aromatic bromodifluoromethylation by Matsui et al.3
unknown. As a part of our project on the development of new
methods for the introduction of fluorine-containing functional
groups into aromatic rings,13 we began to study the reaction of
arylmetal species with CF2Br2. Herein, we would like to report
the bromodifluoromethylation of aromatic Grignard reagents
possessing electron-withdrawing groups with CF2Br2.
benzyl carbanion.15 Since we thought that the key to success in
the bromodifluoromethylation would be to enhance the stability
of the intermediate E, which could accelerate the carbene insertion
step (A–E) and prevent the decomposition (E–G),16 we then decided
to explore the reaction of the arylmetal species A possessing
electron-withdrawing groups on the aromatic ring with CF2Br2.
At first, the reaction of 4-cyanophenylmagnesium chloride
(10a), prepared from 4-halobenzonitrile 9a and i-PrMgCl by
Knochel’s halogen-magnesium exchange reaction,17 with CF2Br2
was examined as shown in Table 1. Upon addition of a solution
of CF2Br2 in THF to 10a derived from 4-bromobenzonitrile (9aa)
at À78 °C, the desired product 11a was fortunately obtained in
27% GC yield concomitantly with the formation of 9aa (7%) and
benzonitrile 12 (15%) (entry 1). Next, the reverse addition of
10a to CF2Br2 was examined, affording 11a in the improved yield
(37%) (entry 2). Interestingly, we observed the formation of a
small amount of 4-(bromochlorofluoromethyl)benzonitrile (13)
(7%).18–20 When 4-iodobenzonitrile (9ab) was used in place of
9aa, the yield (50%) was improved (entry 3). In the absence of LiCl,
the reaction gave rise to the desired product 11a in 77% GC yield
(45% isolated yield21) with a 3% yield of 13 (entry 4).22 We
regarded that the decrease of the isolated yield was due to the
volatility of 11a. While further investigations to improve the yield
were carried out in several solvents, such as dioxane, DME, and
diglyme (entries 5–7), THF gave the best result. Interestingly,
the yield was diminished in the case of using 4-bromobenzonitrile
Initially, we attempted the reaction of 4-biphenylyllithium (4)
with CF2Br2 as depicted in Scheme 3. Exposure of 4, prepared
from 4-bromobiphenyl and n-BuLi, to CF2Br2 at À78 °C produced
4-bromobiphenyl (6), biphenyl (7), and difluorostilbene 8 in 39%,
16%, and 10% yields, respectively. Unfortunately, formation of the
desired product 5 was not observed. As well as precedents,7,8 we
speculated that aromatic bromodifluoromethylation of 1 proceeded
via a carbene chain mechanism (Scheme 4, M = Li): (i) reaction of
aryllithium A with CF2Br2 produced aryl bromide B and bromodiflu-
oromethyl anion C, (ii) C was immediately decomposed to difluoro-
carbene D14 with releasing LiBr, (iii) difluorocarbene D then
combined with aryllithium A, affording aryldifluoromethyllithium
E, and (iv) bromination of E with CF2Br2 yielded the targeted ArCF2
Br F with the formation of C, which propagated the carbene chain
reaction. In the case of the reaction of 4, the formation of 4-bromo-
biphenyl (6) (=B) as the major product indicated that the carbene
insertion step (A–E) was slow. In addition, we regarded that
difluorostilbene 8 (=H) was produced by dimerization of arylfluoro-
carbene G, which was generated through a-elimination of E accel-
erated by two fluorine atoms that destabilized the sp2-hybridized
CF2Br2
(1.2 eq)
F
Ar
F
Li
R
+
THF
−78 °C
Ar
4
5: R = CF2Br (0%) 8: Ar = 4-biphenylyl (10%)
6: R = Br (39%)
7: R = H (16%)
Scheme 3. Attempt of bromodifluoromethylation of 4-biphenylyllithium (4).
CF2
D
M-CF2Br
- M-Br
C
CF2Br2
CF2−M+
M
CF2Br
CF2
D
FG
A
FG
E
FG
F
CF2Br2
− M-Br
M-CF2Br
C
F
F
FG
Br
FG
F
FG
FG
B
G
H
(M = Li or MgCl)
Scheme 4. Plausible reaction mechanism of the bromodifluoromethylation.

M. Shiosaki, M. Inoue / Tetrahedron Letters 55 (2014) 6839–6843
6841
Table 1
Bromodifluoromethylation of aryl Grignard reagent 10a
i-PrMgCl (1.06 eq)
NC
9aa: X=Br
X
NC
+
MgCl
LiCl, solvent
temp.
10a
9ab: X=I
CF2Br2 (1.2 eq)
NC
CF2Br
NC
R
THF
−78 °C to rt, 8 h
11a
9aa: R = Br
12: R = H
13: R = CFClBr
Entry
X
LiCl (equiv)
Solvent
Method
Temp (°C)
Yielda (%)
11a
9aa
12
13
1
2
3
4
5
6
7
8
9e
Br
Br
I
I
I
I
I
Br
I
2.4
2.4
2.4
0
0
0
0
0
0
THF
THF
THF
THF
dioxane
DME
diglyme
THF
Ab
Bc
B
B
B
B
B
B
B
À40 to rt
À40 to rt
À40
27
37
50
7
9
13
5
9
13
5
15
3
3
5
3
3
5
13
7
0
7
7
3
7
7
3
2
3
À40
77 (45)d
37
rt
À40
50
50
30
50
À40
À40 to rt
À40
8
10
THF
a
b
c
Determined by GC analysis using undecane as an internal standard.
Method A: A solution of CF2Br2 in THF was added dropwise into the Grignard reagent.
Method B: The Grignard reagent was added dropwise into a solution of CF2Br2 in THF.
Isolated yield.
d
e
2.4 equiv of CF2Br2 were used.
9aa (entry 8)23 and increasing amount of CF2Br2 (2.4 equiv) did
not improve the yield (entry 9). Unfortunately, the reactions using
the corresponding arylzinc and arylcopper reagents did not
proceed.
parentheses indicate Hammett rp and rR values, respectively,24
and we regarded the resonance effect of the substituent may
facilitate the reaction. We regarded that this bromodifluoromethy-
lation also proceeded via a carbene-mediated mechanism as
described in Scheme 4 (M = MgCl), in which the intermediate aryl-
difluoromethyl anion E was stabilized by an electron-withdrawing
group (=FG) through inductive and resonance effects, eventually
affording the desired product F. Generally, (bromodifluorometh-
yl)benzene derivatives were prepared by radical bromination of
With the optimized reaction condition in hand, we next evalu-
ated the substrate scope of this reaction by using several aryl Grig-
nard reagents 10 having electron-withdrawing groups, prepared
from the corresponding aryl halides 9, to obtain the (bromodifluo-
romethyl)benzene derivatives 11 as shown in Table 2.21,23 The
reactions of m- and o-cyanophenyl Grignard reagents gave 11b
and 11c in 41% and 71% isolated yields, respectively. In the case
of the cyanophenyl Grignard reagents with halogen atoms, the
desired products 11d–11f were obtained in 54–71% yields. In addi-
tion, p-cyanophenyl Grignard reagents with electron-donating
groups, such as allyloxy and methoxy groups, also produced the
corresponding bromodifluoromethyl products 11g–11h in 54%
yields. Next, we examined aryl Grignard reagents 10 with other
electron-withdrawing groups, such as methoxycarbonyl, penta-
fluorosulfanyl, and arylsulfonyl groups. These electron-withdraw-
ing groups also worked to stabilize the intermediate
aryldifluoromethyl anion, giving rise to 11i–11l in moderate yields
(33–41%). While the yields were decreased to around 20% yields,
the reactions of some trifluoromethylated aryl Grignard reagents
with CF2Br2 also proceeded to give 11m–11o. In addition, poly-
haloaryl compound 11p was obtained in moderate yield (39%)
and 2-(trifluoromethyl)-5-pyridyl Grignard reagent produced a
29% yield of bromodifluoromethylated pyridine 11q. Unfortu-
nately, the reactions of phenyl and 4-methoxyphenyl Grignard
reagents with CF2Br2 gave no desired products (11r and 11s) and
the corresponding bromobenzene derivatives were mainly
produced. At the moment, we conclude that it is essential to use
aromatic Grignard reagents with electron-withdrawing groups in
this reaction. The cyano group is the most preferred one with the
following order: –CN (0.66, 0.18) > –SO2Ar (0.69, 0.12) > –CO2Et
(0.45, 0.16) ꢀ –SF5 (0.68, 0.07) > –CF3 (0.54, 0.09). The values in
a,a a,a,a-tri-
-difluorotoluenes2a,b,4a,25 and defluorobromination of
fluorotoluenes by BBr3,26 however, these reactions required harsh
reaction conditions and/or hazardous reagents. Compared with
these reactions, our developed method is a simple way to access
(bromodifluoromethyl)benzene derivatives.
Since the bromodifluoromethyl (–CF2Br) group is a potential
precursor of trifluoromethyl (–CF3), difluoromethyl (–CHF2), and
(aryloxy)difluoromethyl (–CF2OAr) groups, we then examined to
derivatize the bromodifluoromethyl compound 11k as shown in
Scheme 5. Thus, fluorination of 11k by exposure to tetra-n-butyl-
ammonium fluoride under heating in toluene provided (trifluoro-
methyl)benzene derivative 14 in 80%. Reduction of 11k with
tri-n-butyltin hydride offered difluoromethyl compound 15 in
90%. Additionally, heating of 11k in the presence of p-nitrophenol
and K2CO3 in DMF at 80 °C afforded a 65% yield of difluoromethyl
ether 16.
In summary, we have developed the novel aromatic bromodi-
fluoromethylation of aryl Grignard reagents containing electron-
withdrawing groups, such as –CN, –CO2Me, –SF5, –SO2Ar, –CF3,
and halides, with CF2Br2. We assumed the reaction proceeded via
a carbene chain mechanism. The resulting bromodifluoromethyl
compounds were successfully converted to the corresponding tri-
fluoromethyl, difluoromethyl, and (aryloxy)difluoromethyl deriva-
tives. Further studies on the mechanism and scope of this aromatic
bromodifluoromethylation are currently under investigation in our
laboratory.

6842
M. Shiosaki, M. Inoue / Tetrahedron Letters 55 (2014) 6839–6843
Table 2
Scope of the bromodifluoromethylation of several Grignard reagentsa
CF2Br2
(1.2 eq)
i-PrMgCl
(1.06 eq)
X
MgCl
CF2Br
THF
−40 °C, 3 h
THF
−78 °C to rt, 8 h
FG
9 (X = Br, I)
FG
10
FG
11
CN
NC
CN
CF2Br
11c (71%)b
Cl
NC
CF2Br
CF2Br
F
11b (41%)b
11d (71%)c
F
AllylO
NC
CF2Br
11g (54%)c
NC
CF2Br
CF2Br
11f (54%)c
11e (54%)c
MeO
MeO2C
CF2Br
F5S
CF2Br
11j (33%)b
CF2Br
NC
CF2Br
11i (35%)b
11h (54%)c
O
S
R
CF2Br
F3C
O
11k (R = Me, 41%)b,d
11l (R = F, 40%)b,d
F
11m (22%)b
Cl
F
F
F
Br
CF2Br
F3C
CF2Br
F3C
CF2Br
Cl
11p (39%)b
F
11o (21%)c,e
11n (19%)c
(22%)c,e
CF2Br
CF2Br
MeO
CF2Br
11s (0%)c,e
F3C
11q (29%)c
N
11r (0%)c,e
a
Isolated yield.
b Aryl iodide 9 was used for the reaction.
c Aryl bromide 9 was used for the reaction.
d 1.5 equiv of CF2Br2 were used.
e Mg (1.2 equiv) was used instead of i-PrMgCl.
O
S
O
Supplementary data
TBAF (1.1 eq)
Me
CF3
toluene, 80 °C
overnight
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.10.082.
14 (80%)
AIBN (0.2 eq)
Bu3Sn-H (1.1 eq)
References and notes
O
S
O
11k
Me
Me
CF2H
1. (a) Hiyama, T.; Kanie, K.; Kusumoto, T.; Morizawa, Y.; Shimizu, M.
Organofluorine Compounds: Chemistry and Applications; Springer: Berlin,
2000; (b) Kirsch, P. Modern Fluoroorganic Chemistry; Wiley-VCH: Weinheim,
2004; (c) Bégué, J.-P.; Bonnet-Delpon, D. Bioorganic and Medicinal Chemistry of
Fluorine; John Wiley & Sons: Hoboken, New Jersey, 2008; (d) Ojima, I. Fluorine
in Medicinal Chemistry and Chemical Biology; Wiley-Blackwell: West Sussex,
2009.
2. (a) Kilbourn, M. R.; Pavia, M. R.; Gregor, V. E. Appl. Radiat. Isot. 1990, 41,
823; (b) Das, M. K.; Mukherjee, J. Appl. Radiat. Isot. 1993, 44, 835; (c)
Johnström, P. S.; Stone-Elander, S. Appl. Radiat. Isot. 1996, 47, 401; (d)
Prabhakaran, J.; Underwood, M. D.; Parsey, R. V.; Arango, V.; Majo, V. J.;
Simpson, N. R.; Heertum, R. V.; Mann, J. J.; Kumar, J. S. D. Bioorg. Med. Chem.
2007, 15, 1802.
toluene, 80 °C
overnight
15 (90%)
p-nitrophenol
(1.1 eq.)
O
S
O
F
F
O
NO2
K2CO3 (3.0 eq)
DMF
80 °C, 24 h
16 (65%)
Scheme 5. Derivatization of 11k.
3. Matsui, S.; Kondo, T.; Sago, K. Mol. Cryst. Liq. Cryst. 2004, 411, 127.
4. (a) Chow, S.-W.; Pilato, L. A.; Wheelwright, W. L. J. Org. Chem. 1970, 35, 20; (b)
Dolbier, W. R., Jr.; Asghar, M. A.; Pan, H.-Q.; Celewicz, L. J. Org. Chem. 1993, 58,
1827; (c) Zhu, S.-Z.; Mao, Y.-Y.; Jin, G.-F.; Qin, C.-Y.; Chu, Q.-L.; Hu, C.-M.
Tetrahedron Lett. 2002, 43, 669.
Acknowledgment
We greatly thank Tosoh F-tech, Inc. for the generous gift of
dibromodifluoromethane.
5. Dolbier, Jr., W. R.; Hedhli, L.; Elsheikh, M.; Rodriguez-Garcia, V.; Wu, K. 2008,
WO2008/045987 A1.

M. Shiosaki, M. Inoue / Tetrahedron Letters 55 (2014) 6839–6843
6843
6. (a) Miethchen, R.; Hein, M.; Reinke, H. Eur. J. Org. Chem. 1998, 919; (b)
Marcotte, S.; Gérard, B.; Pannecoucke, X.; Feasson, C.; Quirion, J.-C. Synthesis
2001, 929; (c) Katagiri, T.; Handa, M.; Matsukawa, Y.; Kumar, J. S. D.; Uneyama,
K. Tetrahedron: Asymmetry 2001, 12, 1303; (d) Wang, H.; Zhu, S.; Xing, C.; Pang,
W.; Deng, Q.; Zhu, S. J. Fluorine Chem. 2006, 127, 1195.
7. (a) Rico, I.; Cantacuzene, D.; Wakselman, C. J. Chem. Soc., Perkin. Trans. 1 1982,
1063; (b) Barlow, M. G.; Tajammal, S.; Tipping, A. E. J. Fluorine Chem. 1993, 64,
61; (c) Xu, B.; Mae, M.; Hong, J. A.; Li, Y.; Hammond, G. B. Synthesis 2006, 803;
(d) Hammond, G. B. J. Fluorine Chem. 2006, 127, 476.
8. (a) Everett, T. S.; Purrington, S. T.; Bumgardner, C. L. J. Org. Chem. 1984, 49,
3702; (b) Iseki, K.; Asada, D.; Takahashi, M.; Nagai, T.; Kobayashi, Y.
Tetrahedron: Asymmetry 1996, 7, 1205; (c) Amii, H.; Kondo, S.; Uneyama, K.
Chem. Commun. 1998, 1845.
9. Zhang, C.-P.; Cao, H.-P.; Wang, Z.-L.; Zhang, C.-T.; Chen, Q.-Y.; Xiao, J.-C. Synlett
2010, 1089.
Kopp, F.; Korn, T.; Sapountzis, I.; Vu, V. A. Angew. Chem., Int. Ed. 2003, 42, 4302;
(c) Krasovskiy, A.; Knochel, P. Angew. Chem., Int. Ed. 2004, 43, 3333.
18. We speculated that 4-(bromochlorofluoromethyl)benzonitrile (13) was
produced from arylfluorocarbene G. The intermediate G was trapped by ClÀ
from LiCl and/or MgBrCl and the resulting ArCFCl–MgCl was brominated with
CF2Br2 to give 13.
19. For a recent example of the synthesis of bromochlorofluoromethyl compounds,
see: Zhao, Y.; Gao, B.; Hu, J. J. Am. Chem. Soc. 2012, 134, 5790.
20. Further investigation to obtain this compound selectively is in progress in our
laboratory and the results will be disclosed in due course.
21. All reactions gave
a mixture of the desired product (Ar-CF2Br) and the
corresponding bromobenzene (Ar-Br), benzene (Ar-H), and bromochloro-
fluoromethylbenzene (Ar-CFClBr) derivatives. Owing to the difficulty of
separation of this mixture by silica gel column chromatography, the desired
product was purified by recycle GPC to remove the side products.
10. (a) Urban, C.; Macé, Y.; Cadoret, F.; Blazejewski, J.-C.; Magnier, E. Adv. Synth.
Catal. 2010, 352, 2805; (b) Urban, C.; Cadoret, F.; Blazejewski, J.-C.; Magnier, E.
Eur. J. Org. Chem. 2011, 4862.
22. A negligible amount of 4-(2-bromo-1,1,2,2-tetrafluoroethyl)benzonitrile was
detected by GC (<1%). We regarded that this side product was produced from the
intermediate E, which reacted with difluorocarbene
D to afford aryltetra-
11. Liu, G.; Mori, S.; Wang, X.; Noritake, S.; Tokunaga, E.; Shibata, N. New J. Chem.
2012, 36, 1769.
fluoroethyl anion (ArCF2CF2-M) and the resulting anion was brominated with
CF2Br2.
12. South, M. S.; Van Sant, K. A. J. Heterocycl. Chem. 1991, 28, 1017.
13. Araki, K.; Inoue, M. Tetrahedron 2013, 69, 3913.
14. Brahms, D. L. S.; Dailey, W. P. Chem. Rev. 1996, 96, 1585.
23. Although using 4-bromobenzonitrile in place of the iodide diminished the
yield of the reaction (entry 8, Table 1), some of the reactions in Table 2 were
carried out from bromobenzene derivatives due to commercial availability.
Contrary to the result of 4-bromobenzonitrile, they proceeded as well as from
other iodobenzene derivatives.
24. Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
25. (a) Fuqua, S. A.; Parkhurst, R. M.; Silverstein, R. M. Tetrahedron 1964, 20, 1625;
(b) Haas, A.; Spitzer, M.; Lieb, M. Chem. Ber. 1988, 121, 1329.
26. Prakash, G. K. S.; Hu, J.; Simon, J.; Bellew, D. R.; Olah, G. A. J. Fluorine Chem.
2004, 125, 595.
15. The conjugated benzyl carbanion is destabilized by lone pair-p repulsion, see:
(a) Streitwieser, A., Jr.; Mares, F. J. Am. Chem. Soc. 1968, 90, 2444; (b) Lemal, D.
M. J. Org. Chem. 2004, 69, 1.
16. Tverdomed, S. N.; Kolanowski, J.; Lork, E.; Röschenthaler, G.-V. Tetrahedron
2011, 67, 3887.
17. (a) Boymond, L.; Rottländer, M.; Cahiez, G.; Knochel, P. Angew. Chem., Int. Ed.
1998, 37, 1701; (b) Knochel, P.; Dohle, W.; Gommermann, N.; Kneisel, F. F.;