Characterization of gem-difluoropropargyl synthons through HF loss from protonated molecules in methane chemical ionization mass spectra

Article abstract of DOI:10.1016/j.jfluchem.2005.10.011

Electron impact and methane chemical ionization mass spectra were obtained following gas chromatography/mass spectrometry for several gem-difluoropropargyl compounds, which had been synthesized as potential intermediates for synthesis of gem-difluoromethy

Full text of DOI:10.1016/j.jfluchem.2005.10.011

Journal of Fluorine Chemistry 127 (2006) 126–132
www.elsevier.com/locate/fluor
Characterization of gem-difluoropropargyl synthons
through HF loss from protonated molecules in
methane chemical ionization mass spectra
a
a
b
Gerald B. Hammond a, , Masayuki Mae , Jiyoung A. Hong , Harrell E. Hurst
*
a Department of Chemistry, University of Louisville, Louisville, KY 40292, USA
b Department of Pharmacology & Toxicology, University of Louisville School of Medicine, Louisville, KY 40292, USA
Received 7 October 2005; received in revised form 25 October 2005; accepted 25 October 2005
Abstract
Electron impact and methane chemical ionization mass spectra were obtained following gas chromatography/mass spectrometry for several
gem-difluoropropargyl compounds, which had been synthesized as potential intermediates for synthesis of gem-difluoromethylene-containing C-3
acetylenes. EI spectra were variable with respect to the presentation of molecular ions, depending on substituent functional groups present.
Methane-CI spectra were characterized by loss of 19 mass units from molecular weight with all compounds examined. These [M À 19]+ ions often
presented as base peaks of the CI spectra, and were more reliably present and abundant than [M + 1]+ ions for these compounds. These ions could
have been formed by elimination of HF from the protonated molecules under conditions of methane chemical ionization.
# 2005 Elsevier B.V. All rights reserved.
Keywords: CF bond activation; gem-Difluoropropargyl; Neutral loss of HF; Fast atom bombardment; Fluoroorganic
1. Introduction
During our investigations on the synthesis of silylated or
stannylated gem-difluoropropargyl synthons, RCBBC–
CF2Si(Sn) [8], we noticed that the analysis of products 1–4
using mass spectrometry did not yield the expected M + 1 peak
under conditions of methane chemical ionization (methane-CI).
We now report the phenomenon of C–F bond cleavage under
methane-CI conditions, which leads to characteristic ions
definitive of several gem-difluoropropargyl synthetic inter-
mediates. Additionally, electron-impact ionization (EI) spectra
of these fluorinated compounds are presented for comparison.
Mass spectrometry has been increasingly used in the
analysis of fluorinated compounds. It has been used in the
determination of stability of fluorinated surfactants in advanced
oxidation processes [1], and their metabolites in sewage sludge
[2]; in the characterization of fluorinated copolymers [3]; or in
the separation and detection of oxidation products of
fluorodeoxyglucose [4]. Some of the techniques used in these
analyses include flow injection-mass spectrometry, liquid
chromatography with mass spectrometry and tandem mass
spectrometry, and liquid chromatography–electrospray ioniza-
tion mass spectrometry. In the cases of small, partially
fluorinated organic compounds, the mass spectrometric
behavior, under electron impact and fast atom bombardment,
has been studied for fluorinated steroids [5], deoxofluorinated
sugars [6], and epoxyethers [7]. In none of these cases,
however, [M À 19]+ ions have been presented as base peaks of
the chemical ionization mass spectra.
2. Results and discussion
We recently reported [8] a practical synthesis of gem-
difluoropropargylsilanes 1, 3, 4 and gem-difluoropropargyl
stannane 2 using Mg(0)-promoted reductive debromometala-
tion of 3-bromo-3,3-difluoropropyne (Eq. (1)). The g-alkyl, -
silyl or -aryl-substituted difluoropropargyl bromide precursor
was prepared in high yield by monitoring concentration and
temperature in the reaction between CF2Br2 and g-substituted
lithium acetylide [9]. These compounds are fitted with
synthetically useful acetylene functionality and therefore could
* Corresponding author. Tel.: +1 502 852 5998.
E-mail address: gb.hammond@louisville.edu (G.B. Hammond).
0022-1139/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2005.10.011

G.B. Hammond et al. / Journal of Fluorine Chemistry 127 (2006) 126–132
127
be key synthetic intermediates in the synthesis of gem-
difluoromethylene-containing C-3 derivatives.
in which the opposing alkyne substituent was aromatic. With
compounds 1d and 1e, which included aliphatic (hexyl) or tri-
(1)
Table 1 shows the structures, formulae, molecular weights,
and GC retention times of gem-difluoropropargyl synthons 1–4.
Their EI and CI mass spectra are presented in Fig. 1. Prominent
ions, their formulae, and assignments are given in Table 2.
When methane-CI ionization was employed, all gem-difluor-
opropargyl synthons produced ions at [M À 19]+. These
presented as base peaks in the CI spectra of all compounds
isopropylsilyl functional groups, intensities of [M À 19]+ ions
were much less, 6 and 13%, respectively. These [M À 19]+ ions
likely result from loss of a neutral species (HF, 20 Da) during
methane-CI. Although C–F bond cleavage has been considered
difficult in general due to the high C–F bond strength, the strong
association of fluorine with boron and aluminum reagents
promotes sometimes the heterolysis of C–F bond. The present
Table 1
Structures, formulae, molecular weights, and GC retention times of gem-difluoropropargyl synthons
Compound Number
Structure
Molecular formula
C12H14F2Si
Molecular weight
224.2
GC retention time (min)
8.57
1a
1b
1c
1d
C13H16F2Si
C13H16F2Si
C12H22F2Si
238.3
238.3
232.4
10.1
10.1
7.80
1e
C15H30F2Si
304.2
10.2
2
3
C12H14F2Sn
314.9
242.3
10.7
12.9
C16H12F2
4
C12H10F2
192.2
7.45

128
G.B. Hammond et al. / Journal of Fluorine Chemistry 127 (2006) 126–132
Fig. 1. EI and CI mass spectra of gem-difluoropropargyl compounds. MM*, mono-isotopic mass.
C–F bond activation may arise through proton-assisted C–F
bond cleavage, which would be favorable from the point of
view of higher H–F bond energy (BDE for C–F, H–F, B–F, and
Al-F, 132, 136, 181, and 159 kcal/mol, respectively). Fig. 2
proposes a mechanism for the formation and stabilization of the
resulting [M À 19]+ ions using 1a as an example. In the first
step, the addition of a proton to the triple bond in 1a produces
species A. This is followed by HF elimination to form the

G.B. Hammond et al. / Journal of Fluorine Chemistry 127 (2006) 126–132
129
Table 2
Mass assignments for selected ions of gem-difluoropropargyl compounds
Compound
m/z; BP
EI
m/z; BP
CI
Formula
Assignment
Formula
Assignment
1a
224
151
132
73
C12H14F2Si
C9H5F2
C9H5F
M+
225
209
205
147
132
73
C12H15F2Si2
C11H11F2Si
C12H14FSi
C6H9F2Si
C9H5F
[M + H]+
[M À Si(CH3)3]+
[M À Si(CH3)3 À F]+
[Si(CH3)3]+
[M À CH3]+
[M À F]+
C3H9Si
[M À C6H5]+
[M À Si(CH3)3 À F]+
C3H9Si
[Si(CH3)3]+
1b
1c
238
223
165
146
115
73
C13H16F2Si
C12H13F2Si
C10H7F2
C6H8F2Si
C9H7
M+
239
223
219
147
146
73
C13H17F2Si2
C12H13F2Si
C13H16FSi
C6H9F2Si
C10H7F
[M + H]+
[M À CH3]+
[M À CH3]+
[M À Si(CH3)3]+
[M À Si(CH3)3 À F]+
[M À Si(CH3)3 À CF2]+
[Si(CH3)3]+
[M À F]+
[M À CH3C6H5]+
[M À Si(CH3)3 À F]+
[Si(CH3)3]+
C3H9Si
C3H9Si
238
223
165
146
115
73
C13H16F2Si
C12H13F2Si
C10H7F2
C6H8F2Si
C9H7
M+
239
223
219
147
146
73
C13H17F2Si2
C12H13F2Si
C13H16FSi
C6H9F2Si
C10H7F
[M + H]+
[M À CH3]+
[M À CH3]+
[M À Si(CH3)3]+
[M À Si(CH3)3 À F]+
[M À Si(CH3)3 À CF2]+
[Si(CH3)3]+
[M À F]+
[M À CH3C6H5]+
[M À Si(CH3)3 À F]+
[Si(CH3)3]+
C3H9Si
C3H9Si
+
1d
1e
162
109
97
C7H12F2Si
C8H13
[M À C5H11
]
213
93
C12H22FSi
C2H3F2Si
C3H9Si
[M À F]+
[M À Si(CH3)3CF2]+
+
C7H13
[C7H13
]
73
[Si(CH3)3]+
73
C3H9Si
[Si(CH3)3]+
157
73
C9H21Si
C3H9Si
[M À (CH3)3SiCF2]+
285
261
193
137
73
C15H30FSi2
C12H23F2Si2
C12H21Si
C8H13Si
[M À F]+
[Si(CH3)3]+
[M À C3H7]+
C3H9Si
[Si(CH3)3]+
1
C1 H11F2Sn
1
C1 H11F2Sn
2
3
4
301
165
113
[M À CH3]+
301
297
165
[M À CH3]+
[M À F]+
C3H9Sn
C9H5
[(CH3)3Sn]+
C12H14FSn
C3H9Sn
[(CH3)3Sn]+
242
151
91
C16H12F2
C9H5F2
C7H7
M+
243
223
C16H13F2
[M + H]+
[M À F]+
[M À C6H5CH2]+
C16H12F
[C6H5CH2]+
192
151
128
C12H10F2
C9H6F2
C10H8
M+
193
173
151
129
C12H11F2
C12H10
[M + H]+
[M À C3H5]+
F
[M À F]+
C9H5F2
C10H9
[M À C3H5]+
corresponding allenyl cation B, which is stabilized through
resonance with its canonical structure C. Alternatively, if one of
the fluorine atoms present in the difluoromethylene moiety of
1a could accept a proton via a hydrogen bonding-type
mechanism, it would promote dehydrofluorination and forma-
tion of carbocation C [M À 19]+. Loss of neutral molecules
from the molecular ion during ion formation in mass
spectrometry is common. As stated by McLafferty and
abundance with structures 1a–c, 3, and 4, while no protonated
molecular species were observed with compounds 1d, 1e,
and 2 [11].
In summary, these [M À 19]+ ions offer more reliable,
definitive information with respect to the molecular weights of
these gem-difluoropropargyl structures than the usual proto-
nated molecules that often are used for determination of
molecular weight with chemical ionization. Given these
observations, [M À 19]+ ions may be useful for confirmation
of synthesis of similar compounds in the future.
ˇ
Turecek [10], loss of 20 mass units under such conditions
almost always represents loss of HF as a neutral species, and
such loss is of major significance in confirming the molecular
structure. Taken together, these CI spectra of gem-difluor-
opropargyl compounds indicate cleavage of the C–F bond is a
likely event under conditions of methane chemical ionization.
However, such C–F bond cleavage is not common in spectra of
fluorinated carbon structures due to the strength of the C–F
bond. The [M + 1]+ ions were observed at relatively low
3. Experimental
3.1. General
1H, 13C, and 19F NMR spectra were recorded at 500,
126, and 470 MHz, respectively, using CDCl3 as a solvent.

130
G.B. Hammond et al. / Journal of Fluorine Chemistry 127 (2006) 126–132
Fig. 2. Proposed mechanism for the formation and stabilization of [M À 19]+ during methane-CI-MS using 1a as model.
The chemical shifts are reported in d (ppm) values relative to
CHCl3 (d 7.26 ppm for 1H NMR and d 77.0 ppm for 13C NMR)
and C6F6 (d 0 ppm for 19F NMR). Coupling constants are
reported in hertz (Hz). All air and/or moisture sensitive
reactions were carried out under argon atmosphere with dry
solvents using PuresolvTM solvent purification systems. All
other reagents and solvents were employed without further
purification.
Compounds eluted according to retention times listed in
Table 1.
3.3. A typical procedure for the synthesis of 3,3-
difluoropropargylsilanes (1)
To the mixture of Mg (194 mg, 8.0 mmol) and chloro-
trimethylsilane (0.51 mL, 4.0 mmol) in dry THF (10 mL), 3-
bromo-3,3-difluoro-1-(4-methylphenyl)propyne (7b) (245 mg,
1.0 mmol) was added dropwise at 0 8C under an argon
atmosphere. The reaction mixture was stirred for 30 min at
0 8C. The residual Mg was removed by decantation, the THF
solution was washed with H2O (5 mL). The aqueous layer was
extracted with hexane (5 mL Â2) and the combined organic
layer was dried over Na2SO4. After evaporation of the solvent,
the crude product was purified by silica gel treated with Et3N/
hexane = 1/9 column chromatography (hexane) to afford 1b
(193 mg, 81%) as a colorless oil.
3.2. Conditions employed in the mass spectral analysis
Methane chemical ionization (CI) and electron-impact
ionization (EI) mass spectra were obtained using a Hewlett-
Packard (HP) 5973 mass spectrometer operated with the
respective CI or EI source. The instrument was tuned to
standard operating parameters using the Autotune software for
EI and methane-CI modes. CI spectra were produced at
10À4 Torr source pressure using an instrumental methane flow
setting of 20 mL/min, with source and quadrupole tempera-
tures set at 250 and 106 8C, respectively. EI spectra relied on
the standard 70 eV electron energy, with source and quadru-
pole temperatures of 230 and 150 8C, respectively. Spectra
were acquired by scanning from m/z 50 to 400 with 4 A/D
samples and a digital threshold of 150. These conditions
enabled acquisition of 4.1 scans/s throughout the GC/MS
analysis.
3.4. 3,3-Difluoro-1-phenyl-3-trimethylsilylpropyne (1a)
1
Colorless oil; 51% yield; IR (neat) 2225 cmÀ1; H NMR
(500 MHz, CDCl3) d 0.29 (s, 9H), 7.34–7.50 (m, 5H); 13C NMR
(126 MHz, CDCl3) d À5.0, 82.2 (t, J = 31.8 Hz), 91.0 (t,
J = 9.6 Hz), 120.5 (t, J = 255 Hz), 120.8 (t, J = 3.5 Hz), 128.4,
129.6 132.0; 19F NMR (470 MHz, CDCl3, C6F6 as an internal
standard) d 57.1 (s, 2F); Anal. Calcd. for C12H14F2Si: C, 64.25;
H, 6.29. Found: C, 64.60; H, 6.23.
The inlet was
a
directly coupled 15 m  0.25 mm
i.d. Â 0.25 mm film thickness ZB-5 ms (5%-polysilarylene
95%-polydimethylsiloxane stationary phase, Phenomenex,
Torrance, CA, USA) capillary column installed in a HP
6890 gas chromatograph. Compounds were diluted to 500 mg/
mL hexane, and 1 mL injections were made via a 250 8C
injector in splitless mode. He carrier gas at 20 kPa pressure
gave an average column velocity of 58 cm/s. The oven
temperature program ramped from 50 to 200 8C at 10 8C/min.
3.5. 3,3-Difluoro-1-(4-methyl)phenyl-3-
trimethylsilylpropyne (1b)
1
IR (neat) 2225 cmÀ1; H NMR (500 MHz, CDCl3) d 0.29
(s, 9H), 2.37 (s, 3H), 7.16 (d, J = 7.0 Hz, 2H), 7.38
(d, J = 7.0 Hz, 2H); 13C NMR (126 MHz, CDCl3) d À4.9,

G.B. Hammond et al. / Journal of Fluorine Chemistry 127 (2006) 126–132
131
21.6, 81.6 (t, J = 30.6 Hz), 91.4 (t, J = 9.6 Hz), 111.7, 120.6 (t,
3.10. 3,3-Difluoro-1-phenyl-3-trimethylstannylpropyne (2)
J = 254 Hz), 129.2, 131.9, 140.0; 19F NMR (470 MHz, CDCl3,
C6F6 as an internal standard) d 56.8 (s, 2F); Anal. Calcd.
for C13H16F2Si: C, 65.51; H, 6.77. Found: C, 65.24; H,
6.94.
IR (neat) 2223 cmÀ1; 1H NMR (500 MHz, CDCl3) d 0.41 (s,
9H), 7.33–7.49 (m, 5H); 13C NMR (126 MHz, CDCl3) d À9.5,
83.9 (t, J = 27.8 Hz), 92.5 (t, J = 10.6 Hz), 121.0, 124.0 (t,
J = 279 Hz), 128.4, 129.5, 131.8; 19F NMR (470 MHz, CDCl3,
C6F6 as an internal standard) d 68.0 (s, 2F); Anal. Calcd. for
C12H14F2Sn: C, 45.76; H, 4.48. Found: C, 46.24; H, 4.42.
3.6. 3,3-Difluoro-1-(2-methyl)phenyl-3-
trimethylsilylpropyne (1c)
1
Colorless oil; 77% yield; IR (neat) 2222 cmÀ1; H NMR
3.11. Fluoride ion promoted alkylation of 2a with allyl
halide
(500 MHz, CDCl3) d 0.29 (s, 9H), 2.38 (s, 3H), 7.16 (d,
J = 7.5 Hz, 2H), 7.39 (d, J = 8.5 Hz, 2H); 13C NMR (126 MHz,
CDCl3) d À5.0, 21.5, 81.6 (t, J = 31.8 Hz), 91.4 (t, J = 9.7 Hz),
111.7, 120.6 (t, J = 255 Hz), 129.2, 131.9, 140.0; 19F NMR
(470 MHz, CDCl3, C6F6 as an internal standard) d 57.5 (s, 2F);
Anal. Calcd. for C13H16F2Si: C, 65.51; H, 6.77. Found: C,
65.52; H, 6.69.
A solution of 1a (224 mg, 1.0 mmol), allyl bromide
(605 mg, 5.0 mmol), KF (70 mg, 1.2 mmol), and CuI
(286 mg, 1.5 mmol) in DMF (1.5 mL) under an argon
atmosphere was stirred for 5 h at 55 8C. The usual workup
procedure of the mixture provided the crude product. The crude
mixture was chromatographed on silica gel (hexane) to give 3
(125 mg, 65%) as a colorless oil.
3.7. 1,1-Difluoro-1-trimethylsilyl-2-nonyne (1d)
1
Colorless oil; 63% yield; IR (neat) 2234 cmÀ1; H NMR
3.12. 4,4-Difluoro-6-phenyl-1-hexen-5-yne (3)
(500 MHz, CDCl3) d 0.22 (s, 9H), 0.89 (t, J = 7.0 Hz, 3H),
1.26–1.33 (m, 4H), 1.40 (q, J = 7.0 Hz, 2H), 1.55 (q, J = 7.0 Hz,
2H), 2.31 (q, J = 7.0 H, 2H); 13C NMR (126 MHz, CDCl3) d
À5.1, 14.0, 18.7, 22.5, 28.0, 28,4, 31.2, 74.4 (t, J = 31.8 Hz),
93.0 (t, J = 8.7 Hz), 120.4 (t, J = 253 Hz); 19F NMR (470 MHz,
CDCl3, C6F6 as an internal standard) d 58.8 (t, J = 7.0 Hz, 2F);
Anal. Calcd. for C12H22F2Si: C, 62.02; H, 9.54. Found: C,
62.40; H, 9.44.
1
IR (neat) 2243, 1230 cmÀ1; H NMR (500 MHz, CDCl3) d
1
2.87–2.94 (m, 2H), 5.30–5.33 (m, 2H), 5.85–5.94 (m, H),
7.34–7.51 (m, 5H); 13C NMR (126 MHz, CDCl3) d 44.1 (t,
J = 27.8 Hz), 81.4(t, J = 40.6 Hz), 87.3 (t, J = 6.7 Hz),114.0 (t,
J = 234 Hz), 120.2, 121.1, 128.5, 129.9, 132.1; 19F NMR
(470 MHz, CDCl3, C6F6 as an internal standard) d 79.1 (t,
J = 15.5 Hz, 2F); HRMS (EI) m/z Calcd. for C10H10F2,
192.0751. Found, 192.0744.
3.8. 3,3-Difluoro-1-triisopropylsilyl-3-
trimethylsilylpropyne (1e)
3.13. Fluoride ion promoted alkylation of 2a with benzyl
halide
Colorless oil; 82% yield; IR (neat) 1863 cmÀ1; H NMR
1
(500 MHz, CDCl3) d 0.24 (s, 9H), 1.08–1.11 (m, 21H); 13C
NMR (126 MHz, CDCl3) d À5.0, 11.0, 18.5, 94.0 (t,
J = 7.1 Hz) 99.8 (t, J = 28.9 Hz), 119.8 (t, J = 254 Hz); 19F
NMR (470 MHz, CDCl3, C6F6 as an internal standard) d 56.7 (s,
2F); Anal. Calcd. for C15H30F2Si2: C, 59.15; H, 9.93. Found: C,
59.22; H, 10.09.
A solution of 1a (224 mg, 1.0 mmol), benzylbromide
(513 mg, 3.0 mmol), KF (70 mg, 1.2 mmol), and CuI
(286 mg, 1.5 mmol) in DMF (1.5 mL) under an argon
atmosphere was stirred for 8 h at 70 8C. The usual workup
procedure of the mixture provided the crude product. The crude
mixture was chromatographed on silica gel (hexane) to give 4
(87 mg, 36%) as a colorless oil.
3.9. A typical procedure for the synthesis of 3,3-
difluoropropargylstannane (2)
3.14. 3,3-Difluoro-1,4-diphenylbutyne (4)
To the mixture of Mg (194 mg, 8.0 mmol) and chloro-
trimethyltin (4.0 mL of 1.0 M solution) in dry THF (10 mL), 3-
bromo-3,3-difluoro-1-phenylpropyne (231 mg, 1.0 mmol) was
added dropwise at 0 8C under an argon atmosphere. The
reaction mixture was stirred for 1 h at 0 8C. The residual Mg
was removed by decantation. After evaporation of the solvent
and removal of excess amount of chrolotrimethyltin in vacuo
(<0.1 mmHg, r.t.), the hexane solution was washed with H2O
(5 mL), aqueous layer was extracted with hexane (5 mL Â2)
and combined organic layer was dried over Na2SO4. The crude
product was chromatographed on silica gel (hexane) treated
with Et3N/hexane = 1/9 to afford 2 (236 mg, 75%) as a
colorless oil.
IR (neat) 2244 cmÀ1; 1H NMR (500 MHz, CDCl3) d 3.44 (t,
J = 14.0 Hz, 2H), 7.33–7.42 (m, 10H); 13C NMR (126 MHz,
CDCl3) d 45.8 (t, J = 27.0 Hz), 81.4 (t, J = 40.3 Hz) 88.0 (t,
J = 6.7 Hz), 114.2 (t, J = 235 Hz), 120.2, 127.7, 128.3, 128.4,
129.8, 130.7, 132.0, 132.1; 19F NMR (470 MHz, CDCl3, C6F6
as an internal standard) d 79.9 (t, J = 14.0 Hz, 2F); HRMS (EI)
m/z Calcd. for C10H12F2, 242.0907. Found, 242.0905.
Acknowledgements
The authors are grateful to the National Science Foundation
(CHE-0513483) and the University of Louisville Vice-Provost
Office for Research for their generous support.

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[7] D. Favretto, M. Dalpaos, P. Traldi, J.P. Begue, D. Bonnet-Delpon, M.H.
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