The selective activation of a C-F bond with an auxiliary strong Lewis acid: a method to change the activation preference of C-F and C-H bonds

Article abstract of DOI:10.1039/C6DT03235D

The selective activation of the C-F bonds in substituted (2,6-difluorophenyl)phenylimines (2,6-F₂H₃C₆-(C=NH)-n′-R-C₆H₄ (n′ = 2, R = H (1); n′ = 2, R = Me (2); n′ = 4, R = tBu (3))) by Fe(PMe₃)₄ with an auxiliary strong Lewis acid (LiBr, LiI, or ZnCl₂) was explored. As a result, iron(ii) halides ((H₅C₆-(C=NH)-2-FH₃C₆)FeX(PMe₃)₃ (X = Br (8); Cl (9)) and (n-RH₄C₆-(C=NH)-2′-FH₃C₆)FeX(PMe₃)₃ (n = 2, R = Me, X = Br (11); n = 4, R = tBu, X = I (12))) were obtained. Under similar reaction conditions, using LiBF₄ instead of LiBr or ZnCl₂, the reaction of (2,6-difluorophenyl)phenylimine with Fe(PMe₃)₄ afforded an ionic complex [(2,6-F₂H₃C₆-(C=NH)-H₄C₆)Fe(PMe₃)₄](BF₄) (10) via the activation of a C-H bond. The method of C-F bond activation with an auxiliary strong Lewis acid is appropriate for monofluoroarylmethanimines. Without the Lewis acid, iron(ii) hydrides ((2-RH₄C₆-(C=NH)-2′-FH₃C₆)FeH(PMe₃)₃ (R = H (13); Me (14))) were generated from the reactions of Fe(PMe₃)₄ with the monofluoroarylmethanimines (2-FH₄C₆-(C=NH)-2′-RC₆H₄ (R = H (4); Me (5))); however, in the presence of ZnCl₂ or LiBr, iron(ii) halides ((2-RH₄C₆-(C=NH)-H₄C₆)FeX(PMe₃)₃ (R = H, X = Cl (15); R = Me, X = Br (16))) could be obtained through the activation of a C-F bond. Furthermore, a C-F bond activation with good regioselectivity in (pentafluorophenyl)arylmethanimines (F₅C₆-(C=NH)-2,6-Y₂C₆H₃ (Y = F (6); H (7))) could be realized in the presence of ZnCl₂ to produce iron(ii) chlorides ((2,6-Y₂H₃C₆-(C=NH)-F₄C₆)FeCl(PMe₃)₃ (Y = F (17); H (18))). This series of iron(ii) halides could be used to catalyze the hydrosilylation reaction of aldehydes. Due to the stability of iron(ii) halides to high temperature, the reaction mixture was allowed to be heated to 100 °C and the reaction could finish within 0.5 h.

Full text of DOI:10.1039/C6DT03235D

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The selective activation of a C–F bond with an
auxiliary strong Lewis acid: a method to change
the activation preference of C–F and C–H bonds†
Cite this: DOI: 10.1039/c6dt03235d
Lin Wang,a Hongjian Sun,a Xiaoyan Li,*a Olaf Fuhrb and Dieter Fenskeb
The selective activation of the C–F bonds in substituted (2,6-difluorophenyl)phenylimines (2,6-F2H3C6-
(CvNH)-n’-R-C6H4 (n’ = 2, R = H (1); n’ = 2, R = Me (2); n’ = 4, R = tBu (3))) by Fe(PMe3)4 with an auxiliary
strong Lewis acid (LiBr, LiI, or ZnCl2) was explored. As a result, iron(II) halides ((H5C6-(CvNH)-2-FH3C6)
FeX(PMe3)3 (X = Br (8); Cl (9)) and (n-RH4C6-(CvNH)-2’-FH3C6)FeX(PMe3)3 (n = 2, R = Me, X = Br (11); n =
4, R = tBu, X = I (12))) were obtained. Under similar reaction conditions, using LiBF4 instead of LiBr or
ZnCl2, the reaction of (2,6-difluorophenyl)phenylimine with Fe(PMe3)4 afforded an ionic complex [(2,6-
F2H3C6-(CvNH)-H4C6)Fe(PMe3)4](BF4) (10) via the activation of a C–H bond. The method of C–F bond
activation with an auxiliary strong Lewis acid is appropriate for monofluoroarylmethanimines. Without the
Lewis acid, iron(II) hydrides ((2-RH4C6-(CvNH)-2’-FH3C6)FeH(PMe3)3 (R = H (13); Me (14))) were gener-
ated from the reactions of Fe(PMe3)4 with the monofluoroarylmethanimines (2-FH4C6-(CvNH)-2’-RC6H4
(R = H (4); Me (5))); however, in the presence of ZnCl2 or LiBr, iron(II) halides ((2-RH4C6-(CvNH)-H4C6)
FeX(PMe3)3 (R = H, X = Cl (15); R = Me, X = Br (16))) could be obtained through the activation of a C–F
bond. Furthermore, a C–F bond activation with good regioselectivity in (pentafluorophenyl)arylmethan-
imines (F5C6-(CvNH)-2,6-Y2C6H3 (Y = F (6); H (7))) could be realized in the presence of ZnCl2 to produce
iron(II) chlorides ((2,6-Y2H3C6-(CvNH)-F4C6)FeCl(PMe3)3 (Y = F (17); H (18))). This series of iron(II) halides
could be used to catalyze the hydrosilylation reaction of aldehydes. Due to the stability of iron(II) halides
to high temperature, the reaction mixture was allowed to be heated to 100 °C and the reaction could
finish within 0.5 h.
Received 16th August 2016,
Accepted 13th October 2016
DOI: 10.1039/c6dt03235d
www.rsc.org/dalton
regioselectivity, the following approaches have been reported.
The design of the substrates is the most common way to
Introduction
One of current research hotspots in organic chemistry con- achieve the aim, and in this regard carbonyl, nitryl, imine,
cerning organometallics is the activation of C–F bonds by tran- pyridyl, furyl, phthalocyanin, etc. are able to play the role of
sition-metal complexes.1 On the one hand, the activation and the directing group to realize cleavage of the C–F bond selec-
functionalization of a C–F bond could help deal with the tively.4 The other effective method to improve the selectivity is
environment problems caused by fluorinated waste,2 while on to draw support from additives.5 For instance, Thomas Braun’s
the other hand, it is also an efficient method to prepare build- group revealed that silane as well as a germyl compound could
ing blocks containing fluorinated entities.3 However, there is benefit the selective activation of C–F bonds.6 Indeed, the for-
no denying that the selective activation of C–F bonds is still a mation of strong Si–F and Ge–F bond promotes the cleavage of
challenging task. To improve the chemical selectivity as well as C–F bonds.7 Similarly, with the aid of borane8 and phosphine,9
the activation of the C–F bond by transition-metal complexes
have become feasible. In recent years, various interesting
aSchool of Chemistry and Chemical Engineering, Key Laboratory of Special
methods, such as photocatalysis,10 were utilized in the acti-
Functional Aggregated Materials, Ministry of Education, Shandong University,
vation of C–F bonds. Nevertheless, two aspects still need to be
improved in this field. First, the methods to change the acti-
vation preference of inert bonds in the same substrate are
limited and changing the metal center is almost the only way
to achieve the goal.11 Secondly, according to the literature,
besides noble metal complexes, such as rhodium, ruthenium,
palladium, and platinum complexes,12 more eco-friendly and
Shanda Nanlu 27, 250199 Jinan, People’s Republic of China.
E-mail: xli63@sdu.edu.cn
bInstitut für Nanotechnologie (INT) und Karlsruher Nano-Micro-Facility (KNMF),
Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1,
76344 Eggenstein-Leopoldshafen, Germany
†Electronic supplementary information (ESI) available. CCDC 1487183, 1497971
and 1046640. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c6dt03235d
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inexpensive metal complexes are barely involved in the acti- nitrile and arylmagnesiumbromide, and a subsequent quench-
vation of C–F bonds.13 To solve these two problems, adding ing with methanol in diethyl ether solutions.19 Infrared
additives to reduce the difficulty of C–F bond activation must spectra (4000–400 cm−1), as obtained from Nujol mulls
be considered an appropriate approach. According to the lit- between KBr disks, were recorded on a Bruker ALPHA FT-IR
1
erature, Lewis acids, such as AlCl3, could be the catalyst for the instrument. H, 19F, 31P and 13C {H} NMR spectra (300, 282.4,
halodefluorination of C–F bonds.14 Furthermore, in recent 121.5, and 75 MHz, respectively) were recorded on a Bruker
years, Ogoshi’s group proposed a method for the activation of Avance 300 spectrometer with C6D6 or CDCl3 as the solvent
C–F bonds in fluorinated olefins using an auxiliary Lewis without an internal reference at room temperature. 13C, 19F,
acid.15 By means of a similar method, our group also suc- and 31P {H} NMR resonances were obtained with broad band
ceeded in activating the C–F bond in perfluoropropylene by proton decoupling. Elemental analyses were carried out on an
using nickel complexes.16 Therefore, we chose a Lewis acid as Elementar Vario ELIII. Melting points were measured in capil-
the auxiliary reagent for study. In our previous work, the reac- laries sealed under nitrogen and are reported herein
tion of Fe(PMe3)4 with (2,6-difluorophenyl)phenylmethani- uncorrected.
mine tended to afford iron(II) hydride via the activation of the
C–H bonds.17 Herein, we report the selective activation of C–F
bonds in the same substrate using Fe(PMe3)4 as an activation
X-ray crystal structure determinations
reagent in the presence of a Lewis acid, such as LiBr, LiI, and Single crystal X-ray diffraction data of the complexes were col-
ZnCl2 (Scheme 1). Not only could the C–F bond in difluoroaryl- lected on a Bruker SMART Apex II CCD diffractometer
methanimine be activated selectively, but also the C–F bond in equipped with graphite-monochromated Mo Kα radiation (λ =
monofluoroarylmethanimine could also be cleaved through 0.71073 Å). During collection of the intensity data, no signifi-
this method. In contrast, the iron(II) hydride via activation of cant decay was observed. The intensities were corrected for
C–H bond was produced from the direct reaction of mono- Lorentz-polarization effects and empirical absorption with the
fluoroarylmethanimine with Fe(PMe3)4 in the absence of a SADABS program. The structures were resolved by direct or
Lewis acid. With the support of a Lewis acid, a series of iron(II) Patterson methods with the SHELXS-97 program and were
halides were synthesized. This type of iron(II) halide showed refined on F2 with SHELXTL. All the non-hydrogen atoms were
good catalytic activity not only for the hydrosilylation reaction refined anisotropically. The hydrogen atoms were included in
of aldehydes but also for the selective reduction reaction of the calculated positions and were refined using a riding
carbonyl groups in α,β-unsaturated aldehydes as well.
model.
A summary of the crystal data, the data collection para-
meters, and the structure refinement details is given in the
ESI.† CCDC 1487183 (10), 1497971 (11), and 1046640 (12)
contain supplementary crystallographic data for this paper.
Synthesis of 8. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in
20 mL of diethyl ether was added into 30 mL of a diethyl ether
solution of (2,6-difluorophenyl)phenylmethanimine 1 (0.26 g,
1.18 mmol) and LiBr (0.12 g, 1.39 mmol) at 20 °C. Then, the
mixture was kept stirring at the same temperature for 4 h.
During this period, the color of the mixture changed from tan
to red. After careful filtration of the mixture, the mother liquor
was placed at 0 °C. Red crystals of 8 were obtained from the
diethyl ether solution at 0 °C. Yield: 0.50 g, 76%; m.p. 156 °C.
Anal. Calcd for C22H36BrFFeNP3 (562.20 g mol−1): C, 47.00; H,
6.45; N, 2.49. Found: C, 47.35; H, 6.30; N, 2.59. IR (Nujol,
cm−1): 3298 ν(N–H), 1623 ν(CvN), 1586, 1548, 1520 ν(CvC),
Experimental section
General procedures
All the operations were conducted utilizing standard Schlenk
techniques under a nitrogen atmosphere. Diethyl ether, THF,
and toluene were dried via distillation from Na-benzophenone
under nitrogen before use. The CDCl3, C6D6, and C3D6O for
NMR were degassed and dehydrated. Fe(PMe3)4 was prepared
according to the literature method.18 Fluoroarylmethanimines
were obtained by the addition reaction of 2,6-difluorobenzo-
1
937 ρ(PCH3). H NMR (300 MHz, C6D6, 298 K, ppm): δ 1.00 (s,
18H, P(CH3)3), 1.44 (s, 9H, P(CH3)3), 6.53–7.56 (m, 8H, Ar–H),
9.68 (s, 1H, CvN–H). 31P {H} NMR (121.5 MHz, C6D6, 298 K,
2
2
ppm): δ 20.7 (t, JP, P = 58.3 Hz, 1 P, P(CH3)3), 14.7 (d, JP, P
=
58.3 Hz, 2 P, P(CH3)3). 19F {H} NMR (282.4 MHz, C6D6, 298 K,
ppm): δ −105.2 (s, F, Ar–F). 13C {H} NMR (75 MHz, C6D6,
298 K, ppm): δ 16.8 (dd, JP, = 18.7 Hz, JP, = 10.5 Hz,
(CH3)3P), 21.6 (d, JP, C = 18.7 Hz, (CH3)3P)), 104.8 (d, JF, C
52.5 Hz, CAr), 116.3 (d, JF, C = 52.5 Hz, CAr), 124.5 (CAr), 125.8
(CAr), 126.5 (CAr), 129.1 (CAr), 130.4 (d, JF, = 7.5 Hz, CAr),
1
3
C
C
1
2
=
2
3
C
3
140.0 (CAr), 144.6 (d, JF, = 7.5 Hz, CAr), 150.2 (CAr), 160.8
C
Scheme 1 Selective activation of C–H bond versus C–F bond in
fluoroarylimine.
(CAr), 176.2 (d, 1JF, C = 235.5 Hz, F–CAr), 205.8 (CvNH).
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Synthesis of 9. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in ppm): δ 1.08, 1.45 (m, 27H, P(CH3)3), 2.31 (s, 3H, Ar–CH3),
20 mL of diethyl ether was added into 30 mL of a diethyl ether 6.46–7.66 (m, 7H, Ar–H), 9.54 (s, 1H, CvN–H). 31P {H} NMR
2
solution of (2,6-difluorophenyl)phenylmethanimine 1 (0.26 g, (121.5 MHz, C6D6, 298 K, ppm): δ 22.1 (t, JP, P = 60.5 Hz, 1 P,
1.18 mmol) and ZnCl2 (0.19 g, 1.39 mmol) at 20 °C. Then, the P(CH3)3), 14.4 (dd, 2JP, P = 60.5 Hz, 2JP, P = 7.3 Hz, 2 P, P(CH3)3).
mixture was kept stirring at the same temperature for 4 h. 19F {H} NMR (282.4 MHz, C6D6, 298 K, ppm): δ −110.8 (s, F,
During this period, the color of the mixture changed from tan Ar–F). 13C {H} NMR (75 MHz, C6D6, 298 K, ppm): δ 16.2 (t′,
1
to red. After careful filtration of the mixture, the mother liquor |1JP, C
+
3JP, C| = 21.0 Hz, (CH3)3P), 20.6 (d, JP, C = 18.0 Hz,
2
was placed at 0 °C. Red crystals of 9 were obtained from the (CH3)3P)), 30.0 (CH3–Ar), 111.7 (d, JF, C = 25.5 Hz, CAr), 118.1
diethyl ether solution at 0 °C. Yield: 0.44 g, 73%; m.p. 168 °C. (CAr), 125.2 (CAr), 129.2 (CAr), 130.4 (CAr), 144.4 (CAr), 151.5
Anal. Calcd for C22H36ClFFeNP3 (517.75 g mol−1): C, 51.04; H, (CAr), 159.9 (d, 1JF, C = 246.8 Hz, F–CAr), 167.9 (CvNH).
7.01; N, 2.70. Found: C, 50.75; H, 7.19; N, 2.57. IR (Nujol,
Synthesis of 12. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in
cm−1): 3305 ν(N–H), 1624 ν(CvN), 1585, 1572, 1550, 1519 20 mL of diethyl ether was added into 30 mL of a diethyl ether
ν(CvC), 939 ρ(PCH3). 1H NMR (300 MHz, C6D6, 298 K, ppm): δ solution of (2,6-difluorophenyl)4-tert-butylphenylmethanimine
2
1.10 (s, 18H, P(CH3)3), 1.52 (d, JP,
= 6 Hz, 9H, P(CH3)3), 3 (0.32 g, 1.18 mmol) and LiI (0.19 g, 1.39 mmol) at 20 °C.
H
6.57–7.72 (m, 8H, Ar–H), 9.79 (s, 1H, CvN–H). 31P NMR {H} Then, the mixture was kept stirring at the same temperature
2
(121.5 MHz, C6D6, 298 K, ppm): δ 26.0 (t, JP, P = 60.8 Hz, 1 P, for 4 h. During this period, the color of the mixture changed
P(CH3)3), 16.2 (d, JP, P = 60.8 Hz, 2 P, P(CH3)3). 19F {H} NMR from tan to red. After careful filtration of the mixture, the
2
(282.4 MHz, C6D6, 298 K, ppm): δ −109.0 (s, F, Ar–F).
mother liquor was placed at 0 °C. Red crystals of 12 were
Synthesis of 10. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in obtained from the diethyl ether solution at 0 °C. Yield: 0.44 g,
20 mL of THF was added into 30 mL of a THF solution of (2,6- 56%; m.p. 158 °C. Anal. Calcd for C26H44FFeINP3 (665.31
difluorophenyl)phenylmethanimine 1 (0.26 g, 1.18 mmol) and g mol−1): C, 46.94; H, 6.67; N, 2.11. Found: C, 47.18; H, 6.85; N,
LiBF4·H2O (0.16 g, 1.39 mmol) at 20 °C. Then, the mixture was 2.04. IR (Nujol, cm−1): 3288 ν(N–H), 1585, 1560, 1521 ν(CvC),
kept stirring at the same temperature for 12 h. During this 942 ρ(PCH3). 1H NMR (300 MHz, C6D6, 298 K, ppm): δ 1.16, (t′,
period, the color of the mixture changed from tan to red. After |2JH, P
removal of the solvent at reduced pressure, the solid residue 1.56 (d, JH, P = 9 Hz, 9H, P(CH3)3), 6.56–7.82 (m, 7H, Ar–H),
was extracted with a mixed solvent of THF and diethyl ether 9.84 (s, 1H, CvNH). 31P {H} NMR (121.5 MHz, C6D6, 298 K,
+
4JH, P| = 6 Hz, 18H, P(CH3)3), 1.35, (s, 9H, C(CH3)3),
2
2
2
(v/v = 3 : 1, 30 mL × 2). Red crystals of 10 were obtained from ppm): δ 19.2 (dd, JP, P = 56.9 Hz, 1P, P(CH3)3), 12.9 (d, JP, P
=
the solution at 0 °C. Yield: 0.33 g, 42%; m.p. >260 °C. Anal. 56.9 Hz, 2P, P(CH3)3). 19F {H} NMR (282.4 MHz, C6D6, 298 K,
Calcd for C25H44BF6FeNP4 (663.18 g mol−1): C, 45.28; H, 6.69; ppm): δ −104.9 (s). 13C {H} NMR (75 MHz, C6D6, 298 K, ppm):
N, 2.11. Found: C, 44.71; H, 6.37; N, 2.04. IR (Nujol, cm−1): δ 18.6 (t′, |1JC, P
+
3JC, P| = 22.5 Hz, (CH3)3P), 23.6 ((CH3)3P),
3347 ν(N–H), 1620 ν(CvN), 1577 ν(CvC), 939 ρ(PCH3). 31.3 (C(CH3)3), 34.7 (C(CH3)3) 105.3 (CAr), 116.1 (CAr), 118.5
1H NMR (300 MHz, C3D6O, 298 K, ppm): δ 1.07 (s, 18H, P(CH3)3), (CAr), 126.0 (CAr), 126.6 (CAr), 129.3 (CAr), 135.4 (CAr), 144.9
2
2
1.68 (d, JP, H = 6 Hz, 9H, P(CH3)3), 1.75 (d, JP, H = 6 Hz, 9H, (CAr), 152.1 (CAr), 180.9 (CAr), 207.4 (CvNH).
P(CH3)3), 7.02–8.20 (m, 7H, Ar–H), 9.67 (s, 1H, CvN–H).
Synthesis of 13. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in
31P {H} NMR (121.5 MHz, C3D6O, 298 K, ppm): δ 3.4 (td, 2JP, P = 7.3 20 mL of diethyl ether was added into 30 mL of a diethyl ether
2
2
Hz, JP, P = 36.4 Hz, 1 P, P(CH3)3), 11.6 (dd, JP, P = 36.4 Hz, solution of (2-fluorophenyl)phenylmethanimine 4 (0.23 g,
2JP, P = 51.0 Hz, 2 P, P(CH3)3), 16.1 (td, JP, P = 7.3 Hz, JP, P
=
1.18 mmol) at 20 °C. Then, the mixture was kept stirring at the
2
2
51.0 Hz, 1 P, P(CH3)3). 19F {H} NMR (282.4 MHz, C3D6O, 298 K, same temperature for 24 h. During this period, the color of the
ppm): δ −111.0 (s, 2F, Ar–F), −152.0 (s, 4F, BF4−). 13C {H} NMR mixture changed from tan to violet. After removal of the
1
(100 MHz, C3D6O, 298 K, ppm): δ 25.1 (d, JP, = 20.2 Hz, solvent at reduced pressure, the solid residue was extracted
C
(CH3)3P), 25.8 (d, 1JP, C = 20.2 Hz, (CH3)3P), 27.2 (d, 1JP, C = 20.2 with n-pentane (30 mL × 2). Violet crystals of 13 (a mixture of
1
Hz, (CH3)3P), 28.5 (d, JP, = 20.2 Hz, (CH3)3P), 131.6 (CAr), 13a and 13b) were obtained from the pentane solution at
C
132.7 (CAr), 134.9 (CAr), 135.9 (CAr), 145.5 (CAr), 153.7 (CAr).
−20 °C. Yield: 0.35 g, 62%; m.p. (dec) 118 °C. Anal. Calcd for
Synthesis of 11. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in C22H37FFeNP3 (483.30 g mol−1): C, 54.67; H, 7.72; N, 2.90.
20 mL of diethyl ether was added into 30 mL of a diethyl ether Found: C, 54.53; H, 7.83; N, 2.85. IR (Nujol, cm−1): 3293 ν(N–
solution of (2,6-difluorophenyl)2-methylphenylmethanimine 2 H), 3281 ν(N–H), 1730 ν(Fe–H), 1584 ν(CvC), 934 ρ(PCH3).
(0.27 g, 1.18 mmol) and LiBr (0.12 g, 1.39 mmol) at 20 °C. 1H NMR (300 MHz, C6D6, 298 K, ppm): δ −15.92, −15.26 (td,
Then, the mixture was kept stirring at the same temperature 2JP, H = 21.0 Hz, JP, H = 84.0 Hz, 1 H, Fe–H), 1.05 (m, 27 H,
2
for 4 h. During this period, the color of the mixture changed P(CH3)3), 7.25–8.41 (m, 8H, Ar–H), 9.53, 9.91 (s, 1H, CvN–H).
from tan to red. After careful filtration of the mixture, the 31P {H} NMR (121.5 MHz, C6D6, 298 K, ppm): δ 24.1, 23.8 (t,
mother liquor was placed at 0 °C. Red crystals of 11 were 2JP, P = 36.4 Hz, 1 P, P(CH3)3), 19.9, 19.2 (d, 2JP, P = 36.4 Hz, 2 P,
obtained from the diethyl ether solution at 0 °C. Yield: 0.53 g, P(CH3)3). 19F {H} NMR (282.4 MHz, C6D6, 298 K, ppm):
78%; m.p. 162 °C. Anal. Calcd for C23H38BrFFeNP3 (576.28 δ −108.8, 116.4 (s, F, Ar–F).
g mol−1): C, 47.94; H, 6.66; N, 2.43. Found: C, 48.31; H, 6.83; N,
Synthesis of 14. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in
2.24. IR (Nujol, cm−1): 3296 ν(N–H), 1622 ν(CvN), 1586, 1542, 20 mL of diethyl ether was added into 30 mL of a diethyl ether
1517 ν(CvC), 937 ρ(PCH3). 1H NMR (300 MHz, C6D6, 298 K, solution of (2-fluorophenyl)2-methylphenylmethanimine
5
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(0.25 g, 1.18 mmol) at 20 °C. Then, the mixture was kept stir- 118.0 (CAr), 125.4 (CAr), 126.5 (CAr), 128.8 (CAr), 129.4 (CAr),
ring at the same temperature for 24 h. During this period, the 130.8 (CAr), 144.7 (CAr), 150.5 (CAr), 180.7 (CvNH).
color of the mixture changed from tan to violet. After removal
Synthesis of 17. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in
of the solvent at reduced pressure, the solid residue was 20 mL of diethyl ether was added into 30 mL of a diethyl ether
extracted with n-pentane (30 mL × 2). Violet crystals of 14 were solution of (2,6-difluorophenyl)pentafluorophenylmethan-
obtained from the pentane solution at −20 °C. Yield: 0.41 g, imine 6 (0.36 g, 1.18 mmol) and ZnCl2 (0.19 g, 1.39 mmol) at
70%; m.p. (dec) 120 °C. Anal. Calcd for C23H39FFeNP3 20 °C. Then, the mixture was kept stirring at the same temp-
(497.33 g mol−1): C, 55.55; H, 7.90; N, 2.82. Found: C, 55.63; H, erature for 10 min. During this period, the color of the mixture
7.83; N, 2.85. IR (Nujol, cm−1): 3278 ν(N–H), 1755 ν(Fe–H), changed from tan to red rapidly. After careful filtration of the
1588, 1524 ν(CvC), 934 ρ(PCH3). 1H NMR (300 MHz, C6D6, mixture, the mother liquor was placed at 0 °C. Red crystals of
2
298 K, ppm): δ −16.32 (t, JP, H = 84.0 Hz, 1 H, Fe–H), 1.05 (m, 15 were obtained from the diethyl ether solution at 0 °C. Yield:
27 H, P(CH3)3), 2.42 (s, 3 H, CH3), 6.72–7.99 (m, 6H, Ar–H), 0.33 g, 46%; m.p. 178 °C. Anal. Calcd for C22H31ClF6FeNP3
8.88 (s, 1H, CvN–H). 31P {H} NMR (121.5 MHz, C6D6, 298 K, (607.71 g mol−1): C, 43.48; H, 5.14; N, 2.30. Found: C, 43.67;
2
2
ppm): δ 24.1 (t, JP, P = 38.9 Hz, 1 P, P(CH3)3), 19.0 (d, JP, P
38.9 Hz, 2 P, P(CH3)3). 19F {H} NMR (282.4 MHz, C6D6, 298 K, 1585, 1561, 1529 ν(CvC), 942 ρ(PCH3). 1H NMR (300 MHz,
C6D6, 298 K, ppm): δ 1.01 (s, 18H, P(CH3)3), 1.49 (s, 9H,
=
H,5.28; N, 2.26. IR (Nujol, cm−1): 3303 ν(N–H), 1628 ν(CvN),
ppm): δ −113.7 (s, F, Ar–F).
Synthesis of 15. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in P(CH3)3), 6.54–6.74 (m, 3H, Ar–H), 10.48 (s, 1H, CvN–H). 31P
20 mL of diethyl ether was added into 30 mL of a diethyl ether {H} NMR (121.5 MHz, C6D6, 298 K, ppm): δ 13.6 (s, P(CH3)3).
solution of (2-fluorophenyl)phenylmethanimine 4 (0.23 g, 19F {H} NMR (282.4 MHz, C6D6, 298 K, ppm): δ −110.9 (s, 2 F,
1.18 mmol) and ZnCl2 (0.19 g, 1.39 mmol) at 20 °C. Then, the Ar–F), −115.8 (m, 1 F, Ar–F), −140.3 (t′, |3JF, F + 5JF, F| = 39.5 Hz,
3
mixture was kept stirring at the same temperature for 6 h. 1 F, Ar–F), −156.7 (dd, JF, F = 19.8 Hz, 1 F, Ar–F), −166.1 (t′,
During this period, the color of the mixture changed from tan ∣3JF,
+
5JF, F∣ = 39.5 Hz, 1 F, Ar–F). 13C {H} NMR (75 MHz,
F
to red. After careful filtration of the mixture, the mother liquor C6D6, 298 K, ppm): δ 16.1 (t′, |1JP, C + 3JP, C| = 22.5 Hz, (CH3)3P),
1
2
was placed at 0 °C. Red crystals of 15 were obtained from the 22.4 (d, JP, C = 16.5 Hz, (CH3)3P)), 111.8 (d, JF, C = 24.0 Hz,
2
3
diethyl ether solution at 0 °C. Yield: 0.39 g, 67%; m.p. 158 °C. CAr), 111.9 (d, JF, C = 24.0 Hz, CAr), 131.3 (t, JF, C = 10.5 Hz,
Anal. Calcd for C22H37ClFeNP3 (499.76 g mol−1): C, 52.87; H, CAr), 149.0 (d, 3JF, C = 10.5 Hz, CAr), 144.4 (CAr), 159.7 (d, 1JF, C
=
1
7.46; N, 2.80. Found: C, 53.15; H,7.29; N, 2.61. IR (Nujol, 247.5 Hz, F–CAr), 159.8 (d, JF,
cm−1): 3231 ν(N–H), 1589, 1572, 1521 ν(CvC), 945 ρ(PCH3). (CvNH).
= 247.5 Hz, F–CAr), 211.9
C
1H NMR (300 MHz, C6D6, 298 K, ppm): δ 1.02 (m, 18H,
Synthesis of 18. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in
2
P(CH3)3), 1.48 (d, JH, P = 9 Hz, 9H, P(CH3)3), 6.92–7.74 (m, 9H, 20 mL of diethyl ether was added into 30 mL of a diethyl ether
Ar–H), 9.96 (s, 1H, CvN–H). 31P {H} NMR (121.5 MHz, C6D6, solution of phenylpentafluorophenylmethanimine 7 (0.32 g,
2
298 K, ppm): δ 23.5 (t, JP, P = 58.3 Hz, 1 P, P(CH3)3), 17.6 (d, 1.18 mmol) and ZnCl2 (0.19 g, 1.39 mmol) at 20 °C. Then, the
2JP, P = 58.3 Hz, 2 P, P(CH3)3). 13C {H} NMR (75 MHz, C6D6, mixture was kept stirring at the same temperature for 10 min.
298 K, ppm): δ 16.1 (t′, |1JP, C + JP, C| = 21.0 Hz, (CH3)3P), 20.6 During this period, the color of the mixture changed from tan
3
1
(d, JP, = 18.0 Hz, (CH3)3P)), 117.8 (CAr), 125.7 (CAr), 128.8 to red rapidly. After careful filtration of the mixture, the
C
(CAr), 129.0 (CAr), 137.9 (CAr), 144.9 (CAr), 149.5 (CAr), 179.0 mother liquor was placed at 0 °C. Red crystals of 16 were
(CvNH).
obtained from the diethyl ether solution at 0 °C. Yield: 0.36 g,
Synthesis of 16. Fe(PMe3)4 (0.50 g, 1.39 mmol) dissolved in 53%; m.p. 167 °C. Anal. Calcd for C22H33ClF4FeNP3 (571.7
20 mL of diethyl ether was added into 30 mL of a diethyl ether g mol−1): C, 46.22; H, 5.82; N, 2.45. Found: C, 46.34; H, 5.91; N,
solution of (2-fluorophenyl)2-methylphenylmethanimine
5
2.45. IR (Nujol, cm−1): 3204 ν(N–H), 1621 ν(CvN), 1597, 1576,
1
(0.25 g, 1.18 mmol) and LiBr (0.12 g, 1.39 mmol) at 20 °C. 1524 ν(CvC), 947 ρ1(PCH3). H NMR (300 MHz, C6D6, 298 K,
Then, the mixture was kept stirring at the same temperature ppm): δ 0.71 (s, 18H, P(CH3)3), 1.17 (s, 9H, P(CH3)3), 7.31 (br,
for 6 h. During this period, the color of the mixture changed 5 H, Ar–H), 10.38 (s, 1 H, CvN–H). 31P {H} NMR (121.5 MHz,
from tan to red. After careful filtration of the mixture, the C6D6, 298 K, ppm): δ 15.8 (br, 1 P, P(CH3)3), 15.3 (br, 2 P,
mother liquor was placed at 0 °C. Red crystals of 16 were P(CH3)3). 19F {H} NMR (282.4 MHz, C6D6, 298 K, ppm):
obtained from the diethyl ether solution at 0 °C. Yield: 0.48 g, δ −115.8 (m, 1 F, Ar–F), −132.2 (m, 1 F, Ar–F), −156.3 (m, 1 F,
73%; m.p. 159 °C. Anal. Calcd for C23H39BrFeNP3 (558.23 Ar–F), −165.4 (m, 1 F, Ar–F).
g mol−1): C, 49.49; H, 7.04; N, 2.51. Found: C, 48.97; H,7.22; N,
Representative experimental procedure for the catalytic
2.29. IR (Nujol, cm−1): 3295 ν(N–H), 1570, 1546, 1517 ν(CvC), hydroslilylation of aldehydes. A 25 mL Schlenk tube was
937 ρ(PCH3). 1H NMR (300 MHz, C6D6, 298 K, ppm): δ 1.08 (br, charged with a mixture of para-FC6H4CHO (1.0 mmol), HSi
18H, P(CH3)3), 1.49 (br, 9H, P(CH3)3), 2.15 (s, 3H, CH3–Ar), (OEt)3 (1.2 mmol), and complex 15 (0.02 mmol) in 2 mL of
6.84–7.63 (m, 8H, Ar–H), 9.48 (s, 1H, CvN–H). 31P {H} NMR THF (toluene). The reaction vessel was stirred at 55 °C for 3 h
2
(121.5 MHz, C6D6, 298 K, ppm): δ 23.9 (t, JP, P = 60.8 Hz, 1 P, (or at 100 °C for 0.5 h). The progress of the reaction was moni-
2
P(CH3)3), 15.1 (d, JP, P = 60.8 Hz, 2 P, P(CH3)3). 13C {H} NMR tored by TLC. After cooling to room temperature, CH3OH
(75 MHz, C6D6, 298 K, ppm): δ 17.1 (t′, |1JP, C + 3JP, C| = 21.0 Hz, (3 mL), and 10% NaOH (5 mL) were added to the mixture.
1
(CH3)3P), 20.9 (Ar–CH3), 21.6 (d, JP, = 18.0 Hz, (CH3)3P)), After stirring for 24 h at 60 °C, the product was extracted with
C
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Et2O (30 mL × 2). The combined organic phases were dried was only one signal detected at −105.2 ppm. On the basis of
over anhydrous Na2SO4, filtered, and then the solvent evapor- the characterization data, the molecular structure of complex 8
ated under reduced pressure. The product was purified by was concluded as having an octahedral coordination geometry
column chromatography on silica gel (petroleum ether around the metal center. This structure is similar to that
(60–90 °C)/ethyl acetate 5 : 1, v/v) to afford para-FC6H4CH2OH (Fig. 2) of complex 12, as confirmed through X-ray diffraction
as a colorless liquid (0.12 g, 95%).
analysis.
Encouraged by this result, we tested the effect of various
Lewis acids in promoting the cleavage of the C–F bond. As
expected, the strong Lewis acids, such as ZnCl2 and LiI, could
promote the C–F bond activation. Just like the reaction in
Scheme 2, the three-component reaction of compound 1, Fe
(PMe3)4, and ZnCl2 led to iron(II) chloride 9 (Scheme 3).
Results and discussion
Activation of a C–F bond in (2,6-difluorophenyl)
phenylmethanimine with an auxiliary Lewis acid
However, when weak Lewis acids were used, due to their
slighter influence on the electrophilicity of substrate,16,20 the
activation preference of the inert bonds could not be changed
Based on the work from Ogoshi group15 and our group,16 we
tried to realize the selective activation of a C–F bond by adding
a Lewis acid to the reaction of compound 1 with Fe(PMe3)4. We
chose to add LiBr as the additive (Scheme 2). As a result, the
color of the mixture turned red rapidly rather than violet,
which is the color of iron(II) hydride produced from the direct
reaction of 1 with Fe(PMe3)4. After 4 h, the mixture was filtered
and complex 8 was crystallized as red crystals from the filtrate
at 0 °C. In the IR spectrum of complex 8, the typical ν(CvN–H)
signal was found at 3298 cm−1 as a strong sharp peak.
Meanwhile, the resonance of CvN–H was recorded at
1
9.68 ppm in the H NMR spectrum, while two types of PMe3
signals appeared at 1.44 and 1.00 ppm, with the relative inte-
gral ratio of 1 : 2. The 31P {1H} NMR spectrum is in accord with
the 1H NMR data and showed two types of signals at 20.67 and
14.66 ppm, with the relative integral ratio of 1 : 2. The coupling
constant 2J(P, P) is 58.3 Hz. In the 19F {H} NMR spectrum, there
Scheme 2 Selective activation of C–F bond in compound 1 with auxili-
ary LiBr.
Fig. 2 Molecular structures of complex 11 (top) and complex 12
(bottom); hydrogen atoms omitted for clarity, thermal ellipsoids drawn
at 50% probability. The 2-methylphenyl in complex 11 is in disorder.
Fig. 1 Molecular structure of complex 10; hydrogen atoms omitted for
Scheme 3 Selective activation of C–F bond in compound 1 with auxili-
clarity, thermal ellipsoids drawn at 50% probability.
ary ZnCl2.
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Application scope of the method to activate C–F bonds with an
auxiliary Lewis acid
We synthesized a series of fluoroarylmethanimines with
different substituent groups to test the application scope of
this method. First, we found that with addition of a Lewis
acid, the C–F bond in compounds 2 and 3 could be activated
to afford the corresponding iron(II) halide complexes 11 and
12 (Scheme 5). Crystals of complexes 11 and 12 suitable for
single crystal X-ray diffraction were obtained from their
mother liquors at 0 °C. The structures showed that the halide
atom (Br1 in complex 11 and I1 in complex 12) was located in
the trans-position to the phenyl-C atom rather than the imine-
N atom. The strong trans-influence of the phenyl-C atom
causes this interesting phenomenon.
Scheme 4 Activation of C–H bond in the presence of LiBF4·H2O.
because, for example in the presence of LiCl, iron(II) hydride
was obtained from the reaction of compound 1 with Fe
(PMe3)4. And when LiBF4·H2O was used as the additive, ionic
complex 10 was generated via activation of a C–H bond
(Scheme 4). It should be noted that the reaction must be
carried out in a polar solvent such as THF. Complex 10 was
characterized by NMR spectroscopy (using deuterated acetone
as the solvent) and X-ray diffraction analysis. In the 19F{1H}
Subsequently, monofluoroarylmethanimines were chosen
as the substrates to study the selective activation of C–F and
C–H bonds. At first, the direct reaction of compound 4 with Fe
(PMe3)4 was explored. As a result, iron(II) hydride 13 was
obtained in a yield of 62% (Scheme 6). This indicates the C–H
bond in monofluoroarylimine is easier to be cleaved by Fe
−
NMR spectrum of 10, the resonances of Ar–F and BF4 were
recorded at −111.0 and −152.0 ppm, respectively, with the rela-
tive integral ratio of 1 : 2. This implies that the C–F bond in
compound 1 remained untouched during the reaction. The H
1
1
(PMe3)4. The two sets of signals of the H NMR, 31P NMR, and
19F NMR spectra of 13 in C6D6 indicate that there are two
isomers of 13a and 13b in solution, but 13a and 13b could not
be identified, while the molar ratio of 13a : 13b (or 13b : 13a) is
about 3 : 1 according to the 19F NMR spectrum (Scheme 6). In
the case of compound 5, iron(II) hydride 14 was obtained in a
NMR spectrum of complex 10 showed three types of PMe3
signals, with the relative integral ratio of 1 : 1 : 2. This indicates
that there are four PMe3 ligands coordinating to the iron
center. The molecular structure of 10 confirmed our con-
clusion (Fig. 1). The iron atom was coordinated by four PMe3
ligands, one imine-N atom, and one phenyl-C atom in an octa-
hedral coordination geometry. The iron atom is connected to
the C13 atom belonging to the non-fluorinated phenyl group.
Meanwhile, the BF4− group acted as the counter ion.
1
yield of 70%, while the H NMR and 31P NMR spectra of 14 in
C6D6 indicate that only one isomer of 14 exists in solution.
According to our experience, the C–H bond activation in the
phenyl ring with EWG is easier than that in the phenyl ring
with EDG.13a Therefore, it is considered that 14a should be the
only product in this reaction. However, through the three-com-
ponent reaction of monofluoroarylimine, Fe(PMe3)4, and Lewis
acid, iron(II) halides 15 and 16 via the activation of C–F bond
were obtained (Scheme 7). Both iron(II) halides (15 and 16)
were characterized completely, with the data of complex 15
taken here as an example: in the 1H NMR spectrum of complex
15, the resonance of CvN–H appeared at 9.96 ppm, which is
similar to the data of other iron(II) chlorides. The resonances
of two types of PMe3 were recorded at 1.47 and 1.02 ppm.
Meanwhile, neither a signal of fluorine nor one of the carbon
Scheme 5 Selective activation of C–F bond in (2,6-difluorophenyl)aryl-
methanimine with an auxiliary strong Lewis acid.
Scheme 6 Activation of C–H bond in (2-fluorophenyl)arylmethanimine by Fe(PMe3)4.
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Table 1 Hydrosilylation of p-FC6H4CHO catalyzed by iron(II) halidesa
Entry
Cat.
Solvent
Cat. (%)
T (°C)
t (h)
Conv.b (%)
Scheme 7 Selective activation of C–F bond in (2-fluorophenyl)aryl-
methanimine with an auxiliary strong Lewis acid.
1
2
3
4
5
6
7
8
15
15
15
15
8
9
11
16
THF
THF
THF
Toluene
THF
THF
THF
HSiEt3
3
2
1
2
2
2
2
0.6
55
55
55
100
55
55
3
3
8
0.5
4
4
4
4
99
99
83
99
78
90
67
75
55
70
a Reaction conditions: (1) p-FC6H4CHO (1 mmol); silane (1.2 mmol)
and catalyst in 1 mL THF or 1 mL toluene at T (°C) for t (h). (2) 3 mL
CH3OH and 5 mL 10% NaOH were added into the mixture, 60 °C for
24 h. b Determined by GC. n-Dodecane as the standard.
Scheme 8 Regioselective activation of C–F bond in (pentafluoro-
phenyl)arylmethanimine with auxiliary ZnCl2.
Table 2 The hydrosilylation of aldehydes catalyzed by 15 a
connected to the fluorine was detected in the 19F and 13C{1H}
NMR spectra. The above data indicate that the addition of the
Lewis acid changed the activation preference of the C–H and
C–F bonds in monofluoroarylimine. Here is an explanation for
this change. Without the Lewis acid, the electrophilicity of
C–H bond is superior to that of the C–F bond, which makes
Fe(PMe3)4 tend to attract the C–H bond. However, when the
Lewis acid was added, the formation of the byproduct LiF or
ZnCl2 is a thermodynamic driving force to cleave the C–F
bonds in this process.14b,15d
Finally, we studied the effect of adding a Lewis acid on
improving the regioselectivity of C–F bond activation.
(Pentafluorophenyl)arylmethanimines (compounds 6 and 7)
were synthesized. When this type of imine was reacted with Fe
(PMe3)4 alone, complicated products were detected due to the
poor regioselectivity. However, in the presence of 1.2 equivalent
ZnCl2, iron(II) chlorides 17 and 18 were generated in yields of 46%
(17) and 53% (18), respectively, in only 10 min (Scheme 8). The
19F{1H} NMR spectra of these two complexes indicated that C–F
bond activation occurred at the C-2 position of the pentafluorophe-
nyl group. In the spectrum of complex 17, there were five types of
fluorine signals appearing at −110.9, −115.8, −140.3, −156.8 and
−166.2 ppm, with a relative integral ratio of 2 : 1 : 1 : 1 : 1; while for
complex 18, the relative integral ratio of four types of fluorine
signals was 1 : 1 : 1 : 1.
Entry Aldehyde
1
Alcohol
T (°C) t (h) Yieldb (%)
55
3
0.5
94
93
100c
2
3
55
3
0.5
95
95
100c
55
3
0.5
90
88
100c
4
5
55
3
0.5
95
93
100c
55
3
0.5
90
92
100c
6d
7d
8d
55
55
55
7
7
7
89
73
90
9d
55
55
7
7
85
78
Catalytic properties of the iron(II) halides
With the addition of a Lewis acid, various iron(II) halides were
synthesized in good yields under mild conditions. After explor-
ing diverse catalytic reactions, it was found that this type of
iron(II) halide could catalyze the hydrosilylation reaction of
aldehydes. First, p-fluorobenzaldehyde was chosen as the sub-
strate, while 1.2 equivalent HSi(OEt)3 was used as the hydro-
gen source. We found that iron(II) chlorides performed better
than iron(II) bromides (Table 1, entries 5–8). Among the iron(II)
chlorides, complex 15 showed the best efficiency. In the pres-
10d
a Reaction conditions: (1) aldehyde (1 mmol); HSi(OEt)3 (1.2 mmol)
and complex 15 (0.02 mmol) in 1 mL THF or 1 mL toluene at T (°C) for
t (h). (2) 3 mL CH3OH and 5 mL 10% NaOH were added into the
mixture, 60 °C, 24 h. b Isolated yield. c The solvent was toluene. d The
loading of complex 15 was 0.03 mmol.
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ence of 2 mol% catalyst, the reaction could finish in 3 h at
55 °C, with the conversion of 99%. In our previous work, iron
(II) hydride was found to be able to catalyze the hydrosilylation
reaction of aldehydes as well.13a However, the iron(II) hydride
was so sensitive to temperature that they would decompose
under higher temperature. As for complex 15, it can keep
stable and active under pretty high temperature. Furthermore,
when the reaction mixture was heated to 100 °C (with toluene
as the solvent), the hydrosilylation of aldehyde could complete
within 0.5 h (Table 1, entry 4).
Under optimized conditions, a series of aldehydes with
different substituent groups were chosen as the substrates to
test the catalytic application scope (Table 2). The aromatic alde-
hydes could convert to the corresponding alcohols in good
yields in 3 h at 55 °C. When the temperature of the reaction was
raised to 100 °C, the products could be obtained in similar
yields after 0.5 h. In the presence of 3 mol% catalyst, the formyl
group in α,β-unsaturated aldehydes could be selectively hydrosi-
lylated at 55 °C. However, the EWG on the aromatic ring and
the large steric hindrance group at the α-position (Table 2,
entry 9) may affect the coordination of the substrate to the cata-
lyst slightly. This led to relatively lower yields.
References
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Conclusions
In summary, we proposed an effective method to realize the
selective activation of a C–F bond in fluoroarylmethanimines
with the active C–H bond untouched. More specifically, with
the auxiliary strong Lewis acid, iron(II) halides could be
obtained via the activation of the C–F bond from the reaction
of (2,6-difluorophenyl)phenylmethanimine with Fe(PMe3)4.
This reaction would otherwise generate iron(II) hydride via
C–H bond activation in the absence of the Lewis acid. The
acidity of the Lewis acid influenced this process significantly.
From the three-component reaction of (2,6-difluorophenyl)
phenylmethanimine, Fe(PMe3)4, and LiBF4·H2O, an ionic
complex via the activation of the C–H bond was obtained. This
method could also be used in the activation of the C–F bond in
monofluoroarylmethanimine. Furthermore, this method showed
excellent regioselectivity when (pentafluorophenyl)arylmethani-
mines were studied as the substrates. Finally, the catalytic
activity of the iron(II) halides in the hydrosilylation reaction of
aldehydes was explored. Due to the tolerance of iron(II) halides
to high temperature, the hydrosilylation reaction of aromatic
aldehydes was allowed to be occur with heating to 100 °C, which
accelerated the reaction process. Moreover, the hydrosilylation of
the formyl group in α,β-unsaturated aldehydes could be cata-
lyzed by the iron(II) halides selectively, with the CvC double
bond untouched too.
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Acknowledgements
We gratefully acknowledge the financial support by NSF China
no. 21372143/21172132.
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