Metal free and selective activation of one C-F bond in a bound CF 3 group

Article abstract of DOI:10.1039/c3cc38669d

The first metal free selective C-F bond activation of a CF₃ group was observed with N-heterocyclic silylenes [PhC(NtBu)₂SiCl] (1) and [CH{(CCH₂)(CMe)(2,6-iPr₂C₆H ₃N)₂}Si] (2) with PhNC(CF₃)₂. The reaction proceeds in a 1:1 molar ratio to yield the mono C-F bond activated products 3 and 4 with each containing a CF₂ group. Both the reactions proceed through an unprecedented selective activation of one of the C-F bonds rather than forming the [1+2] cycloaddition product containing the three-membered SiNC rings.

Full text of DOI:10.1039/c3cc38669d

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Metal free and selective activation of one C–F bond in
a bound CF3 group†
Cite this: Chem. Commun., 2013,
49, 1841
Ramachandran Azhakar, Herbert W. Roesky,* Hilke Wolf and Dietmar Stalke*
Received 3rd December 2012,
Accepted 11th January 2013
DOI: 10.1039/c3cc38669d
www.rsc.org/chemcomm
The first metal free selective C–F bond activation of a CF3 group silicon analogues of N-heterocyclic carbenes (NHCs), where
was observed with N-heterocyclic silylenes [PhC(NtBu)2SiCl] (1) the latter find widespread applications in chemistry.11,12 They
Q
Q
and [CH{(C CH2)(CMe)(2,6-iPr2C6H3N)2}Si] (2) with PhN C(CF3)2. have two non-bonding electrons in the HOMO and an empty
The reaction proceeds in a 1 : 1 molar ratio to yield the mono C–F p-orbital as the LUMO which feature both nucleophilic as well
bond activated products 3 and 4 with each containing a CF2 group. as electrophilic reactive sites at the same silicon center and
Both the reactions proceed through an unprecedented selective tend to possess an ambiphilic character and behave as Lewis
activation of one of the C–F bonds rather than forming the [1+2] acids as well as Lewis bases.13 The silylenes are electronically
cycloaddition product containing the three-membered SiNC rings. and coordinatively unsaturated and they are capable of under-
going a variety of reactions.14–16
Bifluorinated organic compounds such as 1,1-difluoro-1-
Recently we reported the C–H as well as C–F bond activation
alkenes find increasing application in chemical, material and of fluoroarenes with [PhC(NtBu)2SiCl] (1) and [CH{(CQCH2)(CMe)-
biological processes because of their unique activity.1 These (2,6-iPr2C6H3N)2}Si] (2).17 With the target of synthesizing difluori-
bifluorinated compounds are generally prepared by the selective nated compounds by utilizing 1 and 2, we selected PhNQC(CF3)2
defluorination of a trifluoromethyl group from a commercially which contains two CF3 groups. The treatment of 1 and 2 with
available trifluoromethyl group containing compounds in the PhNQC(CF3)2 resulted in difluorinated alkene products 3 and 4
presence of a metal center.2 These 1,1-difluoro-1-alkene containing formed by the selective activation of one of the carbon–fluorine
compounds exhibit exceptional chemical reactivities toward addition bonds without any metal catalyst rather than giving the SiNC three-
reactions,3 and they can also be reduced to monofluoroalkenes. membered ring by [1+2]-cycloaddition reaction.
Therefore they are considered as important intermediates for other
classes of fluorinated organic compounds.4 They are also used as PhNQC(CF3)2was treated with 1 and 2 in a molar ratio of 1 : 1
potential inhibitors.5
as shown in Scheme 1. Both reactions involve the selective
Compounds 3 and 4 were obtained in good yields when
In general the activation of the C–F bond is a subject of and facile C–F bond activation of one CF3 group. Compounds
interest because it is the strongest single bond that carbon can 3 and 4 are soluble in common organic solvents. They are
form due to the high electronegativity of fluorine.6 Recently stable, both in the solid state as well as in solution without
Ozerov et al. reported the hydrodefluorination of a C(sp3)–F any decomposition under an inert gas atmosphere. Both
bond using carborane supported, highly electrophilic silylium the compounds were fully characterized by NMR spectroscopy,
compounds as a catalyst.6b,c Moreover, the selective activation EI-MS and elemental analysis. The molecular structures of the
of one of the C–F bonds is difficult to achieve.7
compounds 3 and 4 were unambiguously established by single
Silylenes contain a divalent silicon atom and they are crystal X-ray structural analyses.
very important synthons in organosilicon chemistry.8 The first
The 29Si NMR spectrum of 3 shows a double resonance
isolable N-heterocyclic silylene (NHSi) was reported by West centered at d ꢀ106.9 ppm (JSi–F = 251 Hz), which is upfield
et al.,9 after which many stable N-heterocyclic silylenes (NHSis) shifted when compared with that of 1 (d 14.6 ppm).10a Compound 3
have been documented.8,10 They are considered to be the exhibits four 19F NMR resonances at d ꢀ60.81 to ꢀ60.95 (m, 3F,
CF3), ꢀ72.89 (b, 1F, SiF), ꢀ83.60 to ꢀ83.78 (m, 1F, CF2), and
¨
¨
¨
Institut fur Anorganische Chemie, Universitat Gottingen, Tammannstrasse 4,
ꢀ84.52 to ꢀ84.73 (m, 1F, CF2) ppm. The assignment of the Si–F
¨
resonance was confirmed by the HSQC (19F–29Si) technique. The
37077 Gottingen, Germany. E-mail: hroesky@gwdg.de,
dstalke@chemie.uni-goettingen.de
1
tBu protons of compound 3 in the H NMR spectrum exhibit a
† Electronic supplementary information (ESI) available: Experimental section.
CCDC 912595 (3) and 912594 (4). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3cc38669d
double resonance of equal intensity for the tBu groups that
reside on nitrogen atoms (d 0.79 and 1.15 ppm). In addition,
c
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(Si–Cl in 1 is 2.156(1) Å).10a The bond length of C22–C23 is
1.492(3) Å and C22–C24 is 1.311(3) Å which indicates both
single and double bond character.
Like 3, compound 4 also shows a double resonance centered
at d ꢀ65.1 ppm (JSi–F = 286 Hz), in its 29Si NMR spectrum, which
is upfield shifted when compared with that of 2 (d 88.4 ppm).10b
Compound 4 also exhibits four 19F NMR resonances d ꢀ62.08 to
ꢀ62.51 (m, 3F, CF3), ꢀ77.19 to ꢀ77.34 (m, 1F, CF2), ꢀ80.51 to
ꢀ80.93 (m, 1F, CF2), and ꢀ137.35 (s, 1F, SiF) (by the HSQC
(19F–29Si) technique) ppm. The g-CH proton for compound 3 in
the 1H NMR spectrum is observed at d 5.40 ppm and is upfield
shifted when compared with that of 2 (d 5.44 ppm).10b The
NCCH2 protons in 4 exhibit sharp singlets at 3.37 and 4.01 ppm
(d for 2 3.32 and 3.91 ppm). In addition 3 shows its molecular
ion in the mass spectrum at m/z 685.
Compound 4 crystallizes in the orthorhombic space group
Pnma with half a molecule in the asymmetric unit. The struc-
ture is depicted in Fig. 2. In 4 the silicon atom is four
coordinate and displays a distorted tetrahedral geometry com-
prising three nitrogen atoms (two from the amidinato ligand
and one from the amino moiety) and one fluorine atom. The
crystallographic mirror plane through the molecule leads to
disorder between the CF2 and the CF3 group and between a
CCH3 and a CQCH2 group. There is shortening of bonds
observed between the Si atom and the nitrogen atoms of the
supporting ligand. The Si–N bond length from the backbone
ligand in 4 is 1.708(2) Å, whereas in 2 it is 1.7345(10) Å.20 The
bite angle (N–Si–N) at the silicon atom with the backbone
ligand is 106.39(16)1 compared to 99.317(54)1 in 2.10b
Herein, we report the first metal free selective and facile C–F
bond activation of a bound CF3 group by the N-heterocyclic silylenes
[PhC(NtBu)2SiCl] (1) and [CH{(CQCH2)(CMe)(2,6-iPr2C6H3N)2}Si] (2).
The reaction of 1 and 2 with PhNQC(CF3)2 yielded the five
coordinate silicon containing compound 3 and the four coordinate
silicon containing product 4, respectively. These reactions proceed
Scheme 1 Synthesis of 3 and 4.
compound 3 shows its fragment ion [M+ ꢀ F] in the mass
spectrum at m/z 516.
Compound 3 crystallizes in the monoclinic space group
P21/n, the molecular structure of which is shown in Fig. 1.
The silicon atom is five coordinate and the coordination environ-
ment is made up of three nitrogen atoms (two from the amidinato
ligand and one from the amino moiety), one chlorine atom and a
fluorine atom.
The structural index t which defines the extent of deviation
from trigonal bipyramidal to square pyramidal geometry (t = 1
for perfect trigonal bipyramidal; t = 0 for perfect square based
pyramid)18 is 0.69 indicating strong deviation from the square
pyramidal geometry and closer to regular trigonal bipyramidal
geometry. The Si1ꢀF6 bond length is 1.6476(13) Å, which is
quite comparable to that in PhC(NtBu)2SiF3.19 The N1–Si1–N2
bite angle between the silicon atom with the backbone ligand is
69.51(8)1. The bond length between the Si and the amino
nitrogen atom is 1.7470(18) Å. The Si–Cl bond length in 3
is 2.0935(8) Å, which is shorter when compared with that in 1
Fig. 1 Molecular structure of 3. Anisotropic displacement parameters are
depicted at the 50% probability level. Hydrogen atoms have been omitted for
clarity. Selected bond lengths (Å) and bond angles (1): N1–Si1 1.7959(18),
N2–Si1 1.9507(18), N3–Si1 1.7470(18), F6–Si1 1.6476(13), Cl1–Si1 2.0935(8),
C22–N3 1.423(3), C22–C23 1.492(3), C22–C24 1.311(3), C24–F1 1.318(3), C23–
F3 1.322(3), C22–N3 1.423(3); N1–Si1–N2 69.51(8), N2–Si1–N3 96.62(8), Cl1–Si1–
F6 91.67(5), F6–Si1–N2 165.53(8), F6–Si1–Cl1 118.08(6), N3–Si1–N1 124.17(8).
Fig. 2 Molecular structure of 4. Anisotropic displacement parameters are
depicted at the 50% probability level. Hydrogen atoms have been omitted for
clarity. Selected bond lengths (Å) and bond angles (1): N1–Si1 1.708(2),
N2–Si1 1.727(3), F1–Si1 1.584(3), N2–C22 1.429(5), C22–C23 1.397(4), C(1)–C(2)
1.398(3), C(2)–C(3) 1.415(4), N1–Si1–N10 106.39(16), F1–Si1–N2 98.61(14).
c
1842 Chem. Commun., 2013, 49, 1841--1843
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J. H. Reibenspies and M. Y. Darensbourg, Inorg. Chem., 2011, 50,
8541–8552.
12 (a) S. Chuprakov, J. A. Malik, M. Zibinsky and V. V. Fokin, J. Am.
Chem. Soc., 2011, 133, 10352–10355; (b) W. I. Dzik, P. Zhang and
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without any catalyst with selective activation of one of the
carbon–fluorine bonds rather than complete defluorination of
the CF3 group. Furthermore no [1+2]-cycloaddition products
were observed by oxidative addition of PhNQC(CF3)2 at the low
valent silicon atoms of 1 and 2, respectively.
We thank the Deutsche Forschungsgemeinschaft for
supporting this work (RO224/60–1). R.A. is thankful to the
Alexander von Humboldt Stiftung for a research fellowship.
D.S. and H.W. are grateful to the DNRF funded Center for
Materials Crystallography (CMC) for support and the Land
Niedersachsen for providing a fellowship in the Catalysis of
Sustainable Synthesis (CaSuS) PhD program.
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