Cyanide initiated perfluoroorganylations with perfluoroorgano silicon compounds

Article abstract of DOI:10.1016/S0022-1139(01)00513-9

Cyanophenylphosphanes, Ph₂PCN or PhP(CN)₂, do not react with Me₃SiCF₃ or Me₃SiC₆F₅ in the absence of cyanide ions. Catalytic amounts of ionic cyanides such as [NEt₄]CN

Full text of DOI:10.1016/S0022-1139(01)00513-9

Journal of Fluorine Chemistry 112 ,2001) 283±286
Cyanide initiated per¯uoroorganylations with per¯uoroorgano
silicon compounds
Patricia Panne, Dieter Naumann, Berthold Hoge*
È È È È
Institut fur Anorganische Chemie, Universitat zu Koln, Greinstr. 6, D-50939 Koln, Germany
Received 27 June 2001; accepted 31 August 2001
Dedicated to Professor Karl Otto Christe on the occasion of his 65th birthday
Abstract
Cyanophenylphosphanes, Ph2PCN or PhP,CN)2, do not react with Me3SiCF3 or Me3SiC6F5 in the absence of cyanide ions.
Catalytic amounts of ionic cyanides such as [NEt4]CN, [18-crown-6-K]CN or NaCN initiate per¯uoroorganylation reactions. The
tri¯uoromethylphosphines, Ph2PCF3 and PhP,CF3)2, as well as the penta¯uorophenylphosphines, Ph2PC6F5 and PhP,C6F5)2, are isolated in
60±75% yield. In dimethylformamide or acetone solutions, side reactions are observed while reactions in CH2Cl2 and acetonitrile proceed
selectively. Me3SiCF3 addition to the carbonyl groups of dimethylformamide and acetone occurs on treatment of Me3SiCF3 solutions of
these solvents with catalytic amounts of cyanide, cyanate or thiocyanate salts even at low temperature. The formation of the reactive silicate
[Me3Si,CN)CF3]À in reactions with an excess of [18-crown-6-K]CN is proved by low temperature NMR investigations. # 2001 Elsevier
Science B.V. All rights reserved.
Keywords: Silicon; Phosphorus; Fluorine; Tri¯uoromethylation; Penta¯uorophenylation
1. Introduction
intermediates of this type may also occur in the tri¯uoro-
methylation of hexa¯uorotricyclophosphazene [13] as well
as in the tri¯uoromethylation of thiocyanates or selenocya-
nates with Me3SiCF3 in the presence of catalytic amounts of
tetrabutylammonium ¯uoride [14]:
Since our research interests lie in the synthesis of novel
bis,per¯uoroorgano)phosphane ligands [1], we are looking
for a convenient access to per¯uoroorgano phosphanes.
While the direct synthesis of tri¯uoromethyl phos-
phanesÐreacting white or red phosphorus with CF3IÐ
proceeds under high pressure giving product mixtures [2],
Ruppert's procedure using ,P,NEt2)3/CF3Br [3]) suffers
from the dif®cult access of CF3Br as a starting material.
The use of Ruppert's reagent ,tri¯uoromethyltrimethylsi-
lane, Me3SiCF3) as a convenient tri¯uoromethylating reagent
especially in organic chemistry has become of considerable
interest during the last decade [4,5]. The ¯uoride initiated
tri¯uoromethylation capacity of Me3SiCF3 is not limited to
organic compounds as has been demonstrated in the synthesis
of several tri¯uoromethyl compounds, e.g. of silver [6,7],
hypervalent bismuth [8], tellurium [9], iodine [9], phosphorus
[10] and silicon [11,12]. The hypervalent silicon compounds,
[Me3Si,F)CF3]À and [Me3Si,CF3)2]À, are of special interest
because they have been proved to be the reactive intermedi-
ates in ¯uoride initiated tri¯uoromethylations [11]. Reactive
‰NBu4ŠF
RÀEÀCN ‡ Me3SiCF3 ! RÀEÀCF3 ‡ Me3SiCN;
ꢀE ˆ S; Se†
The authors of the last mentioned reaction additionally
assume that in this nucleophilic substitution reaction, cyanide
ions activate Me3SiCF3. To verify this assumption, we started
to investigate the cyanide-induced nucleophilic perfluoroor-
ganylation with perfluoroorgano silicon compounds.
2. Results and discussion
Cyanodiphenylphosphane ,Ph2PCN), slowly reacts with
Me3SiCF3 at room temperature to Ph2PCF3 if catalytic
amounts of CsF or tetramethylammonium ¯uoride ,0.2 eq.)
have been added. As pointed out by Langlois and co-workers
[14], the use of catalytic amounts of ¯uoride ions indicates
a cyanide ion generation during nucleophilic displace-
ment reactions ,Scheme 1). Following this assumption that
* Corresponding author. Fax: ‡49-221-470-5196.
E-mail address: b.hoge@uni-koeln.de ,B. Hoge).
0022-1139/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 , 0 1 ) 0 0 5 1 3 - 9

284
P. Panne et al. / Journal of Fluorine Chemistry 112 -2001) 283±286
equilibrium reduces the concentration of ``free'' cyanide
ions and thus leads to the prolongation of the reaction time.
Further addition of cyanide salts shortens, as expected, the
reaction time but due to a shift of the equilibrium to the
right side, decomposition of the [PhP,CN)3]À anion into
[PhPCN]À and ,CN)2 [15] occurs. As a consequence, the
yield of PhP,CF3)2 is reduced. Cyanide initiated reactions of
P,CN)3 and Me3SiCF3 did not yield a uniform product so far.
In a similar manner, per¯uoroarylation is achieved by
cyanide initiated reactions of Me3SiC6F5 and Ph2PCN and
PhP,CN)2, respectively
Scheme 1. Fluoride initiated reaction of Me3SiCF3 with cyanophosphanes.
Ph3ÀnPꢀCN† ‡ nMe3SiC6F5
n
CNÀ
! Ph3ÀnPꢀC6F5† ‡ nMe3SiCN; n ˆ 1; 2
n
cyanide ions are involved in the activation of Me3SiCF3, we
decided to replace the very hygroscopic and scarcely soluble
¯uoride salts with cyanide salts for initiation.
The products Ph2PC6F5 and PhP,C6F5)2 are isolated after
aqueous work-up in up to 70% yield.
Indeed, cyanodiphenylphosphane reacts with Me3SiCF3
and catalytic amounts of NaCN at room temperature in
CH3CN solution slowly but selectively to Ph2PCF3:
To optimize the reaction conditions, several solvents, such
as CH2Cl2, CH3CN, acetone and DMF, were tested. Ethers
such as THF or glyme, exhibit a very low solubility for
[NEt4]CN and [18-crown-6-K]CN. While the reactions in
CH2Cl2 and CH3CN proceed selectively to the pure pro-
ducts, in acetone and DMF formal addition of Me3SiCF3 to
the carbonyl groups occurs as a side reaction.
To characterize the by products, Me3SiCF3 has been
treated in separate reactions in acetone and DMF solutions
with catalytic amounts of cyanides. While the formal addi-
tion product of acetone, Me3SiOCMe2CF3, has been isolated
in 75% yield, the Me3SiCF3 addition to DMF is less selective
and leads after an aqueous work-up to a 20% yield of
Me3SiOC,H),NMe2)CF3:
NaCN
Ph2PCN ‡ Me3SiCF3 ! Ph2PCF3 ‡ Me3SiCN
Substituting the moderately soluble ionic cyanide by well
soluble cyanide sources such as [18-crown-6-K]CN and
tetraethylammonium cyanide the reaction is significantly
accelerated. Thus, after mixing Ph2PCN and Me3SiCF3 with
0.2 eq. of [18-crown-6-K]CN in CH3CN at À40 8C and
gently warming to ambient temperature, complete conver-
sion of Ph2PCN to Ph2PCF3 occurs within 30 min. A very
low cyanide concentration prolongs the reaction time, while
an excess of cyanide leads to the formation of great amounts
of HCF3 in a side reaction. If an excess of Me3SiCF3 is used,
a quantitative trifluoromethylation of the phosphane occurs
in each case.
Dicyano,phenyl)phosphane,PhP,CN)2) reacts even slower
than Ph2PCN with Me3SiCF3 and 0.2 eq. of [18-crown-6-
K]CN or [NEt4]CN. Again, the reaction time depends on
the cyanide concentration. This behavior indicates that
CNÀ ions are involved in reactions with PhP,CN)2. Deng
and Dillon [15] reported that PhP,CN)2 and cyanides
form the phosphoranide, [PhP,CN)3]À. The 31P NMR
signal of PhP,CN)2 is shifted about 14 ppm to higher
®eld after adding 1 eq. of cyanide. Upon warming to ambi-
ent temperature, the phosphoranide, [PPh,CN)3]À, begins
to decompose ,Scheme 2). The phosphane±phosphoranide
In an attempt to characterize the reactive intermediate in the
system Me3SiCF3/CNÀ, solutions of Me3SiCF3 in CH2Cl2
and an acetonitrile/propionitrile mixture were treated with
1 eq. of [18-crown-6-K]CN at low temperature. While in
CH2Cl2 solution even at À90 8C only CF3H was formed,
NMR spectroscopic investigation of the acetonitrile/propio-
nitrile reaction mixture at À60 8C gave evidence for a new
1
silicon compound. The large JCF coupling constant of
379 Hz, in comparison with 322 Hz for Me3SiCF3, appears
to be characteristic for hypervalently bound trifluoromethyl
groups in 3c±4e units ,Table 1). The 19F NMR resonance
Table 1
19F NMR data of trifluoromethyl silicon compounds [11]
d ,19F) ,ppm)
1JCF ,Hz)
2JSiF ,Hz)
[Me3Si,CF3)2]a
[Me3Si,CF3)F]a
[Me3Si,CF3)CN]À
À62.6
À63.9
À63.7
À65.8
378
375
379
322
6.0
4.5
±
a
Me3SiCF3
36.2
Scheme 2. Cyanide initiated reaction of Me3SiCF3 with dicyanophenyl-
phosphane.
a See [9] and literature cited therein.

P. Panne et al. / Journal of Fluorine Chemistry 112 -2001) 283±286
285
of À63.7 ppm indicates a downfield shift of about 2 ppm with
respect to the neutral compound. A comparable low field
shift of about 2 ppm is also observed for the [Me3Si,CF3)F]À
ion. We interpret these observations in terms of a formation
of a cyanosilicate, [Me3Si,CF3)CN]À. Reaction mixtures
with a 1:1 ratio of Me3SiCF3 and [18-crown-6-K]CN exhibit
a turnover of only 20% at À60 8C. The turnover in a 1:3
mixture increases up to 80%, indicating an equilibrium
reaction of Me3SiCF3 and cyanide ions.
The formation of the [Me3Si,CF3)CN]À anion in most
organic solvents is accompanied by CF3H evolution which
increases with increasing [Me3Si,CF3)CN]À and CNÀ con-
centration, respectively. Evolution of CF3D in the reaction of
Me3SiCF3 with cyanide ions in CD3CN solution clearly
indicates the reaction with the solvent.
MHz; 13C, 50.32 MHz; 1H, 200.13 MHz) with positive shifts
being down®eld from the external standards 85% orthopho-
1
sphoric acid ,31P), CCl3F ,19F) and TMS ,13C and H).
3.2. Preparation of -trifluoromethyl)diphenylphosphane:
general procedure
To a solution containing Ph2PCN ,0.7 g, 3.31 mmol) and
Me3SiCF3 ,0.5, 3.52 mmol) in CH2Cl2 ,10 ml), [18-crown-
6-K]CN ,0.11 g, 0.33 mmol) was added at À78 8C. After
warming to ambient temperature for 2 h, the brown reaction
mixture was treated with Et2O ,200 ml) and ®ltered. The
solvents are removed from the ®ltrate in vacuo. The residue
was treated a second time with Et2O ,100 ml) ®ltered and the
solvent removed in vacuo to give the Ph2PCF3 as a brown oil
,0.6 g, 2.36 mmol, 70%). Spectroscopic data are comparable
Treatment of Me3SiCF3 in CH3CN solution with 0.1 eq. of
[18-crown-6-K]CN results in a conversion into Me3SiCH2-
CN which was isolated after aqueous work-up. Comparable
results were reported for the Me3SiCF3/FÀ system [16]:
1
to literature [19]. H NMR ,CDCl3) 6.8±7.2 ppm ,m); 19F
2
NMR ,CDCl3) À55.3 ppm ,d, JPF ˆ 73 Hz); 31P {1H}
2
NMR ,CDCl3) 2.7 ppm ,q, JPF ˆ 73 Hz). MS m/e 254
‡
‡
,M , 70), 185 ,Ph2P , 100).
Àꢀ0:1 eq:†
Me3SiCF3 ‡ CH3CNCN
!
Me3SiCH2CN ‡ CF3H
3.3. Preparation of bis-trifluoromethyl)phenylphosphane:
general procedure
In contrast to [Me3Si,CF3)F]À generated from the
Me3SiCF3/FÀ system [11], [Me3Si,CF3)CN]À does not react
with excess Me3SiCF3 to the bis,trifluoromethyl)silicate,
[Me3Si,CF3)2]À:
To a solution containing PhP,CN)2 ,0.77 g, 4.81 mmol)
and Me3SiCF3 ,1.36, 9.56 mmol) in CH3CN ,15 ml),
This behavior reflects the lower reactivity of the Me3SiCF3/
CNÀ system compared with the Me3SiCF3/FÀ system. But
this ``lower'' reactivity could be useful for more selective and
moderate reactions. For the reaction of Me3SiCF3 and DMF
several ionic compounds were tested as initiators. The reac-
tion is not only promoted by cyanide or fluoride ions but also
by cyanate or thiocyanate ions. The possibility to use dif-
ferent nucleophiles for the activation step should be useful in
the development of more selective trifluoromethylations.
[NEt4]CN ,0.16 g, 0.1 mmol) dissolved in CH3CN ,5 ml)
was added at À45 8C. After warming to ambient tempera-
ture for 2 h and stirring for 10 h, the brown reaction mixture
was treated with CH2Cl2 ,200 ml) and extracted three times
with water. The organic phase was dried ,MgSO4) and the
product separated from the solvent by fractional condensa-
tion ,À64 8C, CHCl3 slush bath), giving the pure compound
as a colorless liquid ,0.88 g, 3.57 mmol, 75%). NMR spec-
1
troscopic data are comparable to literature [20]. H NMR
,CDCl3) 7.2±7.8 ppm ,m); 19F NMR ,CDCl3) À53.3 ppm
2
,d, JPF ˆ 80 Hz); 31P {1H} NMR ,CDCl3) 0.7 ppm ,sep,
3. Experimental section
‡
‡
2JPF ˆ 80 Hz). MS m/e 246 ,M , 85), 177 ,M À CF3, 177),
‡
127 ,PhCF2 , 100).
3.1. Materials and apparatus
Trimethylsilylacetonitrile was obtained as a colorless
liquid in the reaction of Me3SiCF3 and excess CH3CN
following to the same procedure. NMR spectroscopic data
are comparable to literature [21]. 1H NMR ,CDCl3) 0.1 ppm
,s, Si,CH3)3), 1.7 ppm ,s, CH2). 13C {1H} NMR ,CDCl3)
À2.0 ppm ,s, Si,CH3)3), 4.6 ppm ,CH2), 119.1 ppm ,CN).
Chemicals were obtained from commercial sources and
used without further puri®cation. Literature methods were
used for the synthesis of Ph2PCN and PhP,CN)2 [17].
Solvents were puri®ed by standard methods [18]. Standard
high-vacuum techniques were employed throughout all
preparative procedures; nonvolatile compounds were
handled in a dry N2 atmosphere by using Schlenk techni-
ques. Mass spectra ,20 eV) were measured on a MAT CH5
instrument. The NMR spectra were recorded on a Bruker
Model AC200 spectrometer ,31P, 81.01 MHz; 19F, 188.31
‡
‡
‡
MS m/e 113 ,M , 2), 98 ,M À CH3, 40), 73 ,SiMe3 , 40).
Me3SiOC-Me)2CF3 was obtained as a colorless liquid
in the reaction of Me3SiCF3 and excess acetone following
to the same procedure: 1H NMR ,CDCl3) 0.1 ppm ,s,
Si,CH3)3), 1.4 ppm ,s, C,CH3)2). 13C {1H} NMR ,CDCl3)

286
P. Panne et al. / Journal of Fluorine Chemistry 112 -2001) 283±286
2.0 ppm ,s, Si,CH3)3, 1JSiC ˆ 60 Hz), 24.1 ppm ,s, C,CH3)2),
61.38%; H, 2.86%. Found: C, 61.40%; H, 3.23%. MS m/e
‡
‡
‡
2
74.8 ppm ,q, JCF ˆ 29 Hz, CF3C,CH3)2), 127.1 ppm ,q,
352 ,M , 100), 275 ,PhPC6F5 , 3), 154 ,Ph2 , 40).
1JCF ˆ 284 Hz, CF3). 19F NMR ,CDCl3) À83.8 ,s). MS m/e
‡
‡
‡
185 ,M À CH3, 5), 131 ,M À CF3, 10), 89 ,OSiMe3
,
‡
100), 73 ,SiMe3 , 60).
Me3SiOCH-NMe2)CF3 was obtained as a colorless liquid
Acknowledgements
in the reaction of Me3SiCF3 and excess DMF following to
1
We are grateful to the Fonds der Chemischen Industrie for
®nancial support. Thanks to Dr. W. Tyrra for discussion and
providing Me3SiC6F5.
the same procedure: H NMR ,CDCl3) À0.2 ppm ,s, Si-
,CH3)3), 2.0 ppm ,s, N,CH3)2) 4.4 ppm ,q, 3JHF ˆ 5:8 Hz).
13C {1H} NMR ,CDCl3) 0.4 ppm ,s, Si,CH3)3), 39.4 ppm
4
2
,q, JCF ˆ 1:7 Hz, N,CH3)2), 84.3 ,q, JCF ˆ 32:3 Hz,
References
CF3C) 123.6 ppm ,q, JCF ˆ 286 Hz, CF3). 19F NMR
1
12
,CDCl3) À77.2 ppm ,d, JHF ˆ 5:8 Hz), D,dꢀF CÀF13C†
3
[1] B. Hoge, P. Panne, in preparation.
‡
1
0.12 ppm, JCF ˆ 286 Hz). MS m/e 215 ,M , 1), 200
Â
[2] F.W. Bennett, H.J. Emeleus, R.N. Haszeldine, J. Chem. Soc. ,1953)
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‡
‡
,M À CH3, 10), 146 ,M À CH3, 100), 126 ,CF3-
‡
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CH,NMe2), 70), 73 ,SiMe3 , 80).
3.4. Preparation of
bis-pentafluorophenyl)phenylphosphane:
general procedure
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To a solution containing PhP,CN)2 ,0.31 g, 1.94 mmol)
and Me3SiC6F5 ,1.16, 4.83 mmol) in CH3CN ,15 ml), [18-
crown-6-K]CN ,0.16 g, 0.48 mmol) dissolved in CH3CN
,5 ml) was added at À45 8C. After warming to ambient
temperature for 2 h and stirring for 1 h, the brown reaction
mixture was treated with CH2Cl2 ,200 ml) and extracted
with water four times. The organic phase was dried ,MgSO4)
and the solvent removed in vacuo yielding PhP,C6F5)2
,0.56 g, 68%) as a slight brown oil. Spectroscopic data
È
È
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1
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are comparable to literature [22]. H NMR ,CDCl3) 7.1±
7.6 ppm ,m); 19F NMR ,CDCl3) À160.5 ppm ,m, 2F),
À150.1 ppm ,m, 1F), À129.5 to 128.5 ppm ,m, 2F); 31P
3
NMR ,CDCl3) À45.9 ppm ,quin, JPF ˆ 30 Hz).
[17] C.E. Jones, K.J. Coskran, Inorg. Chem. 10 ,1971) 1536.
[18] D.D. Perrin, W.L.F. Armarego, D.R. Perrin, Purification of
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[20] K. Gosling, D.J. Holman, J.D. Smith, B.N. Ghose, J. Chem. Soc. ,A)
,1968) 1909.
-Penta¯uorophenyl)diphenylphosphane was obtained
according to the same procedure as a slight brown solid
,0.66 g, 56%). Spectroscopic data are comparable to litera-
1
ture [22]. H NMR ,CDCl3) 6.9±7.3 ppm ,m); 19F NMR
,CDCl3) À160.5 ppm ,m, 2F), À150.2 ppm ,m, 1F), À126.8
[21] W. Zarges, M. Marsch, K. Harms, G. Boche, Chem. Ber. 122 ,1989)
1307.
to 127.8 ppm ,m, 2F); 31P NMR ,CDCl3) À24.6 ppm
3
[22] D.I. Nichols, J. Chem. Soc. ,A) ,1969) 1471.
,t, JPF ˆ 38 Hz). Anal. Calcd. for C18H10F5P ,352.2): C,