Magnesium iodide promoted defluorinative reactions of 2,2- difluorocyclopropyl aryl ketones with aryl imines: A new, general synthesis of 2-alkylideneazetidines

Article abstract of DOI:10.1016/j.jfluchem.2010.06.021

Upon treatment with anhydrous MgI₂, 2,2-difluorocyclopropyl aryl ketones undergo a ring-opening process leading to complete loss of fluorine. When carried out in the presence of aryl imines, the reaction leads to a novel synthesis of 2-alkylideneazetidenes. A fluorine-free allenyl ketone is proposed as an intermediate in these reactions.

Full text of DOI:10.1016/j.jfluchem.2010.06.021

Journal of Fluorine Chemistry 131 (2010) 958–963
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Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Magnesium iodide promoted defluorinative reactions of 2,2-difluorocyclopropyl
aryl ketones with aryl imines: A new, general synthesis of 2-alkylideneazetidines
Wei Xu a, Ion Ghiviriga a, Qing-Yun Chen b, William R. Dolbier a,
*
a Department of Chemistry, University of Florida, PO Box 117200, Gainesville, FL 32611-7200, United States
b Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Ling-Ling Road, Shanghai 200032, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Upon treatment with anhydrous MgI2, 2,2-difluorocyclopropyl aryl ketones undergo a ring-opening
process leading to complete loss of fluorine. When carried out in the presence of aryl imines, the reaction
leads to a novel synthesis of 2-alkylideneazetidenes. A fluorine-free allenyl ketone is proposed as an
intermediate in these reactions.
Received 27 May 2010
Received in revised form 28 June 2010
Accepted 29 June 2010
Available online 6 July 2010
ß 2010 Published by Elsevier B.V.
Keywords:
2-Alkylideneazetidines
gem-Difluorocyclopropanes
Allenes
Defluorinative reactions
1. Introduction
Our intended strategy was to mimic the Olsson conditions using
2,2-difluorocyclopropyl ketones, with the expectation that either
Recently we reported the acid catalyzed, regioselective ring-
opening of fluorinated cyclopropyl ketones (i.e., 1 in Scheme 1) to
give 3,3-difluoro-4-bromobutan-1-ones (2) [1]. Such reactions as
this exemplify the synthetic exploitation of the inherent zwitterionic
charge affinity of cyclopropyl ketone systems, as shown in Fig. 1.
In following up this work, we wished to determine whether it
might be possible to devise a way to intercept this putative dipole
in a condensation reaction using a carbonyl or imine reactant,
leading essentially to an overall cycloaddition process.
There are a number of reports of such chemistry involving non-
fluorine-substituted cyclopropyl ketones in Lewis acid catalyzed
ring-opening condensation reactions with aldehydes or imines to
prepare five-membered ring heterocycles. One good example is
Carreira’s MgI2-catalyzed condensation of spiro[cyclopropan-1,30-
oxindol] 3 with N-aryl- and alkylsulfonyl aldimines (Scheme 2) [2].
Another example is Oshima’s stepwise enolate formation via
iodide ring-opening of phenyl cyclopropyl ketones, followed by
aldol reaction and subsequent cyclization to form tetrahydrofurans
(Scheme 3) [3]. Lastly there is the report by Olsson of the three
component, magnesium iodide promoted synthesis of substituted
pyrrolidines from cyclopropyl ketones (Scheme 4) [4].
difluoropyrrolidines or fluoro-dihydropyrroles would be formed
(Scheme 5). However, in the event, a completely unexpected and
unprecedented reaction occurred, leading to products that were
completely fluorine-free.
2. Results
When phenyl 2,2-difluoro-3-phenylcyclopropyl ketone (4a)
was allowed to react with imine 5a under Olsson conditions,
the only isolable product was azetidin-2-ylidine compound (6a)
in a modest 32% yield (Scheme 6). The structure of 6a was
consistent with the 1H and 13C NMR spectral data shown in
Fig. 2.
Reactions of 4a with other bis-aromatic imines led to similar
results, as shown in Table 1 (column 7), and X-ray crystal structure
characterization was carried out on product 6i (Fig. 3).
In considering the possible mechanism for the reaction, which
probably required multiple iodide involvement in the reaction,
optimization experiments were carried out utilizing excess MgI2,
with the reaction proceeding to give considerably enhanced yields
when using four equivalents of MgI2 (final column, Table 1). It was
also found that best results were obtained using freshly prepared
anhydrous MgI2 via the reaction of Mg powder with I2 in refluxing
anhydrous diethyl ether.
* Corresponding author. Tel.: +1 352 392 0591; fax: +1 352 846 1962.
E-mail addresses: chenqy@mail.sioc.ac.cn (Q.-Y. Chen), wrd@chem.ufl.edu
(W.R. Dolbier).
Only diaryl imines were utilized in this study, largely because of
ease of preparation and stability. The one ketone substrate not
0022-1139/$ – see front matter ß 2010 Published by Elsevier B.V.
doi:10.1016/j.jfluchem.2010.06.021

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W. Xu et al. / Journal of Fluorine Ch
T
emistry 131 (2010) 958–963
959
Scheme 1.
G
[(ig._2)TD$FIG]
Scheme 6.
Fig. 1. Inherent zwitterionic charge affinity of cyclopropyl ketones.
[(Schme_2)TD$FIG]
Fig. 2. 1H and 13C NMR data for azetidinylidene product 6a.
Scheme 2.
bearing a phenyl substituent on the cyclopropane ring (4c)
underwent the reaction similarly, but with a reduced yield
(Scheme 7).
Lastly, in an experiment designed to provide mechanistic
insight, the reaction was carried out under otherwise identical
conditions, but in the absence of an imine, with the result given
3. Discussion
A reasonable mechanism for the observed results requires
iodide ion to act both as a nucleophile and as a reducing agent
during the course of the reaction. Also, an allenyl ketone (10) is a
likely intermediate in the reaction. The action of iodide ion as a
nucleophile in the initial Mg2+-catalyzed ring-opening process to
form enolate intermediate 8, which then undergoes elimination of
one of the fluorines to form iodide intermediate 9 is consistent
with the similar ring-opening results depicted in Schemes 2–4.
Unique to our system, and distinctly different from the ultimate
regiochemistry of imine attack that is observed in these earlier
in Scheme 8. The structure of iodofuran product
7 was
confirmed by comparison of its 1H and 13C NMR spectral data
(Fig. 4) with those reported for the compound [5]. Iodofuran 7
could also be detected as a very minor product in many of the
other reactions of 4a described in Table 1, although it was never
isolated.
[(Schme_3)TD$FIG]
Scheme 3.
[(Schme_4)TD$FIG]
Scheme 4.
[(Schme_5)TD$FIG]
Scheme 5.

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W. Xu et al. / Journal of Fluorine Chemistry 131 (2010) 958–963
Table 1
MgI2-induced reactions of 2,2-difluorocyclopropyl ketones 4 with imines 5.
[
.
Entry
Ketone
Imine
R3
R4
Product
Yield (%)a 1 equiv. MgI2
Yield (%)a 4 equiv. MgI2
1
2
3
4
5
6
7
8
9
4a
4a
4a
4a
4a
4a
4a
4a
4b
5a
5b
5c
5d
5e
5f
C6H5
C6H5
6a
6b
6c
6d
6e
6f
32
13
33
27
21
23
30
33
35
57
59
60
60
64
61
60
66
49
4-MeC6H4
4-MeOC6H4
4-ClC6H4
4-BrC6H4
4-FC6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
5g
5h
5a
4-MeC6H4
4-ClC6H4
C6H5
6g
6h
6i
C6H5
C6H5
a
Isolated yields.
[(Schme_7)TD$FIG]
mixture, which color is removed by washing with NaHSO3 during
work-up of the reaction.
In the absence of imine, the allenyl ketone apparently under-
goes electrophilic attack at the central allenic carbon of 10 by I2,
either synchronous with cyclization or via intermediate 11 with
subsequent cyclization to form intermediate 12, which deproto-
nates to form iodofuran product 7.
Scheme 7.
In the presence of imine, allenyl ketone intermediate 10
undergoes Michael attack by the nucleophilic nitrogen of the imine
to form enolate, zwitterionic intermediate 13, which can undergo
cyclization to form the much more thermodynamically-stable of
the two potential azetidinylidene products, 6a (Scheme 9).
Alternatively, intermediate 9 might itself undergo Michael
addition of imine, followed by fluoride loss to form intermediate
14, which upon reductive elimination of iodine would lead to
intermediate 13 and subsequent cyclization to product 6a (Scheme
10).
[(Schme_8)TD$FIG]
Scheme 8.
The results were unexpected mainly because fluoride is
generally observed to be a poor leaving group, and in this reaction
two fluorides have been lost during the course of the reaction.
Perhaps that is why these reactions proceed at best with only
modest yields. Thus, it would be interesting to see whether the
dichloro analogues of 4 might well produce better yields in this
reaction. In fact, 2,2-dichlorocyclopropyl ketones are even less well
known than the difluoro ones, and although they have been
observed to undergo ring-opening reactions in the presence of
nucleophiles, apparently by elimination-addition mechanisms
(Scheme 11) [6,7], no reactions under Lewis acid catalysis have
been reported.
results is the proposed reductive
b-elimination of intermediate 9
by iodide ion to eliminate the second fluorine and form allene
intermediate 10. Molecular iodine would be formed in this step,
and this is reflected by the increasingly red color of the reaction
[(ig._3)TD$FIG]
Compounds with the structural features present in our
observed products (6a–j) could well be of pharmaceutical/
medicinal interest, but they have thus far only infrequently been
mentioned in the literature. There are a few papers related to the
synthesis of 2-methyleneazetidines, where they have been
[i(g._4)TD$FIG]
Fig. 4. 1H and 13C NMR data for iodofuran 7.
Fig. 3. Ortep drawing of azetidinylidine product 6i.

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W. Xu et al. / Journal of Fluorine Chemistry 131 (2010) 958–963
961
Scheme 9. Proposed mechanism.
[(Schme_10)TD$FIG]
Scheme 10. Alternative, allene-free mechanism for formation of azetidene products.
[(Schme_1)TD$FIG]
Scheme 11.
[(Schme_12)TD$FIG]
Scheme 12.
[(Schme_13)TD$FIG]
Scheme 13.

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W. Xu et al. / Journal of Fluorine Chemistry 131 (2010) 958–963
prepared either by methylenation of
b
-lactams using dimethylti-
7.24–7.45 (m, 15H), 7.83–7.86 (m, 2H); 13C NMR,
d 57.8, 75.3, 91.8,
tanocene [8,9], or by cyclization reactions [10–12]. 2-Alkylide-
neazetidine compounds with a conjugated enone structure like
those in 6 are even less frequently encountered. They have been
117.7, 123.7, 126.4, 127.2, 127.4, 127.8, 128.4, 128.9, 129.0, 129.4,
129.7, 131.5, 137.8, 138.4, 139.8, 140.5, 164.7, 189.0; HRMS (EI)
calcd for C29H23NO, 401.1780, found 401.1781. Anal. Calcd for
prepared from
b
-lactams through reactions with stabilized Wittig
C29H23NO: C, 86.75; H, 5.77; N, 3.49. Found: C, 86.59; H, 5.46; N,
reagents, as in making the penicillin derivative shown in Scheme
12 [13,14], and they have also been prepared by the [2 + 2]
cycloaddition of a conjugated aldimine with an allenyl ketone
(Scheme 13) [15], a reaction that resembles closely our own
proposed mechanistic process.
3.30.
5.3.2. (E)-1-Phenyl-2-(1,3-diphenyl-4-(4-methylphenyl)azetidin-2-
ylidene)ethanone, 6b
59%; yellow solid, mp 153 8C, 1H NMR,
d
2.35 (s, 3H), 4.69 (t,
J = 1.5 Hz, 1H), 5.24 (d, J = 2.4 Hz, 1H), 6,79 (d, J = 1.5 Hz, 1H), 7.04–
7.45 (m, 17H), 7.84–7.87 (m, 2H); 13C NMR,
57.9, 75.3, 91.7, 117.8,
4. Conclusions
d
123.6, 126.4, 127.2, 127.3, 127.8, 128.4, 128.9, 129.7, 130.1, 131.5,
134.8, 138.5, 138.8, 139.8, 140.6, 164.8, 189.0; HRMS (EI) calcd for
Thus, an attempt to synthesize fluorine substituted five-
membered-ring heterocycles via Lewis acid catalyzed ring-
opening reactions of aldimines with 2,2-difluorocyclopropyl
ketones led, instead, to complete loss of fluorine from these
compounds with the formation of novel 2-alkylideneazetidinyl
derivatives. The observed reactions likely proceed via [2 + 2]
cycloaddition of the imines to an allenyl ketone intermediate, and
they should constitute a good, general method for synthesis of this
potentially interesting molecular system.
C30H25NO, 415.1936, found 415.1942. Anal. Calcd for C30H25NO: C,
86.71; H, 6.06; N, 3.37. Found: C, 86.81; H, 6.25; N, 3.17.
5.3.3. (E)-1-Phenyl-2-(1,3-diphenyl-4-(4-methoxyphenyl)azetidin-
2-ylidene)ethanone, 6c
60%; yellow solid, mp 153 8C, 1H NMR,
d 3.81 (s, 3H), 4.70 (t,
J = 1.5 Hz, 1H), 5.23 (d, J = 2.4 Hz, 1H), 6.78 (d, J = 1.5 Hz, 1H), 6.88–
6.91(m, 2H), 7.07–7.13 (m, 3H), 7.22–7.43 (m, 12H), 7.84–7.87 (m,
2H); 13C NMR,
d 55.5, 57.9, 75.1, 91.6, 114.8, 117.8, 123.6, 127.2,
5. Experimental
127.3, 127.8, 127.9, 128.4, 128.9, 129.6, 129.8, 131.5, 138.4, 139.7,
140.6, 160.1, 164.8, 189.0; HRMS (EI) calcd for C30H25NO2,
431.1885, found 431.1849. Anal. Calcd for C30H25NO2: C, 83.50;
H, 5.84; N, 3.25. Found: C, 83.18; H, 5.72; N, 3.18.
5.1. Materials and general information
Unless otherwise specified, proton and carbon NMR spectra
were obtained in CDCl3 at 300 and 75.5 and 282 MHz, respectively,
and chemical shifts are reported upfield relative to TMS. All 2,2-
difluorocyclopropyl ketones have been previously reported and
were prepared by the reaction of difluorocarbene reagent,
trimethylsilyl 2-fluorosulfonyl-2,2-difluoroacetate (TFDA), with
5.3.4. (E)-1-Phenyl-2-(1,3-diphenyl-4-(4-chlorophenyl)azetidin-2-
ylidene)ethanone, 6d
60%; solid, mp 168 8C, 1H NMR,
d
4.66 (t, J = 1.8 Hz, 1H), 5.25 (d,
J = 2.1 Hz, 1H), 6.79 (d, J = 1.5 Hz, 1H), 7.06–7.11 (m, 3H), 7.24–7.45
(m, 14H), 7.83–7.86 (m, 2H); 13C NMR,
57.7, 74.5, 92.0, 117.6,
d
the respective
a
,
b
-unsaturated ketones [1,16]. All imines were
123.8, 127.2, 127.5, 127.8, 128.5, 129.0, 129.7, 129.8, 131.6, 134.8,
136.3, 138.1, 139.5, 140.3, 164.4, 189.1; HRMS (EI) calcd for
prepared in the usual manner from corresponding aldehydes and
anilines in ethanol under reflux.
C29H22ClNO, 435.1390, found 435.1407. Anal. Calcd for
29H22ClNO: C, 79.90; H, 5.09; N, 3.21. Found: C, 79.97; H, 5.12;
C
5.2. Magnesium (II) iodide
N, 3.03.
Undernitrogen, ina dry 100 mLof three-neckedflaskwereadded
magnesium powder (2.43 g, 100 mmol) and anhydrous ethyl ether
(50 mL). Iodine (5 g, 20 mmol) was added portwise to keep a gentle
reflux. After addition, the refluxing was continued until the color of
iodine disappeared. The hot mixture was filtered under nitrogen and
the solution was cooled to ꢀ20 8C. The precipitated white solid was
collected and dried in a vacuum container.
5.3.5. (E)-1-Phenyl-2-(1,3-diphenyl-4-(4-bromophenyl)azetidin-2-
ylidene)ethanone, 6e
64%; solid; 1H NMR,
d
4.66 (t, J = 1.5 Hz, 1H), 5.23 (d, J = 2.4 Hz,
1H), 6,79 (d, J = 1.5 Hz, 1H), 7.06–7.11 (m, 3H), 7.24–7.45 (m, 14H),
7.83–7.86 (m, 2H); 13C NMR,
57.7, 74.6, 92.0, 117.6, 122.9, 123.9,
d
127.2, 127.5, 127.8, 128.1, 129.0, 129.8, 131.6, 132.6, 136.8, 138.1,
139.5, 140.0, 164.3, 189.1; HRMS (EI) calcd for C29H22BrNO,
479.0885, found 479.0909. Anal. Calcd for C29H22BrNO: C, 72.50; H,
4.62; N, 2.92. Found: C, 72.43; H, 4.67; N, 2.85.
5.3. Typical procedure for reaction of 2,2-difluorocyclopropyl ketones
with imines
5.3.6. (E)-1-Phenyl-2-(1,3-diphenyl-4-(4-fluorophenyl)azetidin-2-
Under nitrogen, into a 50 mL three-necked flask were added
phenyl cyclopropyl ketone (0.258 g, 1 mmol), N-benzylideneani-
line (181 mg, 1 mmol) and anhydrous tetrahydrofuran (10 mL).
Magnesium iodide (1.016 g, 4 mmol) was added to the solution in
one portion with stirring. The mixture was heated to reflux and
stirred for 12 h, during which time the reaction became increasing
red in color. After cooling to room temperature, 5 mL of saturated
sodium thiosulfate solution was added, and the reaction mixture
was extracted with diethyl ether (3ꢁ 20 mL). The combined
organic layers were dried over sodium sulfate, filtered, and the
solvent removed with a rotary evaporator. The residue was
purified by silica flash column chromatography.
ylidene)ethanone, 6f
61%; solid; 1H NMR,
d
4.66 (t, J = 1.5 Hz, 1H), 5.23 (d, J = 2.4 Hz,
1H), 6,79 (d, J = 1.5 Hz, 1H), 7.06–7.11 (m, 3H), 7.22–7.52 (m, 14H),
7.83–7.85 (m, 2H); 13C NMR,
57.8, 74.5, 91.9, 116.3, 116.6, 117.7,
123.8, 127.1, 127.5, 127.8, 128.1, 128.2, 128.4, 129.0, 129.7, 131.6,
133.6, 138.1, 139.5, 140.4, 164.4, 189.0; HRMS (EI) calcd for
d
C29H22FNO, 419.1685, found 419.1667. Anal. Calcd for C29H22FNO:
C, 83.03; H, 5.29; N, 3.34. Found: C, 82.88; H, 5.42; N, 3.23.
5.3.7. (E)-1-Phenyl-2-(1-(4-methylphenyl)-1,3-diphenylazetidin-2-
ylidene)ethanone, 6g
59%; yellow solid, mp 153 8C, 1H NMR,
d 2.32 (s, 3H), 4.71 (t,
J = 1.2 Hz, 1H), 5.27 (d, J = 2.4 Hz, 1H), 6.76 (d, J = 1.2 Hz, 1H), 7.02–
5.3.1. (E)-1-Phenyl-2-(1,3,4-triphenylazetidin-2-ylidene)ethanone, 6a
7.04 (m, 2H), 7.14–7.17 (m, 2H), 7.25–7.46 (m, 13H), 7.85–7.88 (m,
57%; yellow solid, mp 162 8C, 1H NMR,
d
4.70 (t, J = 1.5 Hz, 1H),
2H); 13C NMR,
d
57.8, 75.3, 91.3, 117.8, 126.4, 127.2, 127.4, 123.8,
5.27 (d, J = 2.1 Hz, 1H), 6,80 (d, J = 1.5 Hz, 1H), 7.05–7.12 (m, 3H),
128.4, 128.9, 128.9, 129.4, 130.2, 131.4, 133.5, 137.3, 137.9, 138.4,

W. Xu et al. / Journal of Fluorine Chemistry 131 (2010) 958–963
963
140.6, 164.8, 188.9; HRMS (EI) calcd for C30H25NO, 415.1936, found
415.1940. Anal. Calcd for C30H25NO: C, 86.71; H, 6.06; N, 3.37.
Found: C, 86.62; H, 6.15; N, 3.35.
91.8, 117.6, 123.5, 126.4, 127.2, 127.3, 127.9, 128.9, 129.1, 129.4,
129.6, 137.8, 137.9, 138.4, 139.8, 141.9, 164.3, 188.7; HRMS (EI)
calcd for C23H20NO[M+H]+, 326.1539, found 326.1542. Anal. Calcd
for C30H26ClNO: C, 84.89; H, 5.89; N, 4.30. Found: C, 84.91; H, 6.06;
N, 4.23.
5.3.8. (E)-1-Phenyl-2-(1-(4-chlorophenyl)-1,3-diphenylazetidin-2-
ylidene)ethanone, 6h
66%; solid, mp 168 8C; 1H NMR,
d
4.71 (t, J = 1.2 Hz, 1H), 5.23 (d,
J = 2.4 Hz, 1H), 6,73 (d, J = 1.2 Hz, 1H), 7.00–7.03 (m, 2H), 7.24–7.46
(m, 15H), 7.82–7.85 (m, 2H); 13C NMR,
57.8, 75.4, 92.2, 118.8,
Appendix A. Supplementary data
d
126.4, 127.2, 127.5, 127.8, 128.5, 128.8, 129.0, 129.0, 129.5, 129.8,
131.6, 137.4, 138.1, 138.3, 140.3, 164.3, 188.9; HRMS (EI) calcd for
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jfluchem.2010.06.021.
C
C
29H22ClNO, 435.1390, found 435.1398. Anal. Calcd for
29H22ClNO: C, 79.90; H, 5.09; N, 3.21. Found: C, 79.81; H, 5.06;
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N, 3.17.
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49%; solid, mp 146 8C; 1H NMR,
d
2.37 (s,3H), 4.69 (m, 1H), 5.25
(d, J = 2.4 Hz, 1H), 6,78 (d, J = 1.5 Hz, 1H), 7.03–7.40 (m, 17H), 7.74–
7.77 (m, 2H); 13C NMR,
21.8, 57.7, 75.2, 91.8, 117.6, 123.5, 126.4,
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139.8, 141.9, 164.3, 188.7; HRMS (EI) calcd for C30H26NO[M+H]+,
416.2006, found 416.1994. Anal. Calcd for C30H26ClNO: C, 86.71; H,
6.06; N, 3.37. Found: C, 86.31; H, 6.17; N, 3.23.
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5.3.10. (E)-1-Phenyl-2-(1,4-diphenylazetidin-2-ylidene)ethanone, 6j
41%; solid, mp 131 8C, 1H NMR,
d 3.40(dddd, J = 1.5 Hz, 2.7 Hz,
16.5 Hz, 1H), 3.95 (dddd, J = 1.5 Hz, 5.7 Hz, 16.5 Hz, 1H), 5.54 (dd,
J = 2.7 Hz, 5.7 Hz, 1H), 6.68 (t, J = 1.5 Hz, 1H), 7.00–7.06 (m, 3H),
7.28–7.50 (m, 10H), 7.93–7.96 (m, 2H); 13C NMR,
d
21.8, 57.7, 75.2,
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