The synthesis of tris(perfluoroalkyl)phosphines

Article abstract of DOI:10.1039/b507752d

Tris(perfluoroalkyl)phosphines, of interest as turtable alternatives to the carbon monoxide ligand, can be synthesised by the nucleophile mediated reaction of perfluoroalkyltrimethylsilanes with triphenylphosphite; the method can be extended to diphosphines. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b507752d

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The synthesis of tris(perfluoroalkyl)phosphines{
Makeba B. Murphy-Jolly, Lesley C. Lewis and Andrew J. M. Caffyn*
Received (in Cambridge, UK) 1st June 2005, Accepted 5th July 2005
First published as an Advance Article on the web 8th August 2005
DOI: 10.1039/b507752d
linkages. To date, however, these strategies have not been
Tris(perfluoroalkyl)phosphines, of interest as tunable alterna-
tives to the carbon monoxide ligand, can be synthesised by the
nucleophile mediated reaction of perfluoroalkyltrimethylsilanes
with triphenylphosphite; the method can be extended to
diphosphines.
successfully applied to the synthesis of tris(perfluoroalkyl)-
phosphines, P(Rf)3 (Rf 5 CnF2n+1). We now report that not only
CF3SiMe3, but other RfSiMe3 compounds can be used to
synthesise P(Rf)3 from triphenylphosphite [eqn (1)]. In addition,
we show that this method can easily be extended to previously
difficult to synthesise (fluoroalkyl)diphosphines.
The ability to systematically modify ligand properties is a desirable
goal in ligand design. It is through the modification of the steric
and electronic properties of attached ligands that the characte-
ristics of metal complexes can be finely tuned in a predictable
manner. This capacity for property modulation is a key attribute
for organometallic and coordination complexes and is particularly
important when considering their potential applications such as
homogeneous catalysts. There is already a wide array of available
electron-rich ligands amenable to modification. With moderately
electron-withdrawing ligands, the options are much more limited.1
The situation is worse, however, for strongly electron-withdrawing
ligands. With ligands of equivalent p-acceptor strength to carbon
monoxide, no tunable alternatives have yet emerged.2 Thus,
although carbon monoxide is one of the most important and
versatile ligands in transition metal chemistry, and one of the few
ligands capable of stabilising metals in positive, zero, or negative
oxidation states, it cannot be modified. Other prototypical strong
p-acceptor ligands, for example NO3 and PF3,4 also lack a suitable
‘‘handle’’ for modification. Perfluoroalkylphosphines, however,
possess both these attributes, since as p-acceptors they are as
strong as CO5 and yet they do have the ‘‘handle’’ intrinsic to
phosphine ligands.
CsF
P(OPh)3 z 3Rf SiMe3 DCCA P(Rf )3 z 3PhOSiMe3
(1)
Rf ~CF3, C2F5, C3F7, C4F9
The results of our study are summarised in Table 1. Reaction of
P(OPh)3 with 3 equivalents of CF3SiMe3 in the presence of
3 equivalents of CsF at 25 uC produced P(CF3)3 in 98% yield.
Under the same conditions, C2F5SiMe3 gave P(C2F5)3 also in high
yield (Table 1, entry 3). The reactions with n-C3F7SiMe3, and
n-C4F9SiMe3 were considerably slower and gave the corresponding
extended chain tris(perfluoroalkyl)phosphines in reduced yields
(Table 1, entries 7, 8). The extended chain compounds, P(C3F7)3,
P(C4F9)3 and P(C6F13)3 could be made in high yield, however,
by replacing P(OPh)3 with the more reactive phosphite, P(O-
p-C6H4CN)3 (Table 1, entries 9–11). The same procedure could
also be applied to the synthesis of arylbis(perfluoralkyl)phosphines
and diarylperfluoroalkylphosphines14,15 (Table 1, entries 12–15).
Of particular interest was the extension of the procedure
to perfluoroalkyl substituted diphosphines [eqn (2)].
(C2F5)2PCH2CH2P(C2F5)2 (1), a ligand used extensively by the
Roddick group, has previously been synthesised by the potentially
hazardous reaction of C2F5Li with Cl2PCH2CH2PCl2.16 Reaction
Tris(trifluoromethyl)phosphine was first prepared over 50 years
ago in a seminal piece of work by the Emele´us group.6 Despite
several inherent drawbacks, this route has remained the method of
choice for synthesising tris(trifluoromethyl)phosphine, since sub-
sequent methods have involved the use of more hazardous
reagents.7 Unfortunately, the Emele´us synthesis does not work
for longer chain tris(perfluoroalkyl)phosphines.8 As a result,
perfluoroalkylphosphines have remained a curiosity.9,10 There is,
therefore, a need for a convenient laboratory scale synthesis of
perfluoroalkylphosphines. We sought to develop a simple and
efficient procedure.
17
of (PhO)2PCH2CH2P(OPh)2 with 4 equivalents of RfSiMe3
(Rf 5 CF3, C2F5) in the presence of CsF gave 1 and
(CF3)2PCH2CH2P(CF3)2 in good yields (Table 1, entries 16, 17).18
CsF
(PhO)2PCH2CH2P(OPh)2 z 4Rf SiMe3 DCCA
(Rf )2PCH2CH2P(Rf )2 z 4PhOSiMe3
(2)
(2)
Rf ~CF3, C2F5
No reaction was observed between P(OPh)3 and RfSiMe3
(Rf 5 CF3, C2F5) in the absence of an initiator. With only
0.1 equivalents of CsF however, P(CF3)3 and P(C2F5)3 were
formed in high yield (Table 1, entries 2, 4), demonstrating that the
reaction is catalytic in CsF. Other sources of F2 and, indeed,
alkoxides were found to be effective catalysts. Clearly, the
observation that NaOPh, for example, is an effective catalyst
indicates that P–F intermediates are not implicit in the reaction
mechanism. A catalytic cycle is proposed in Scheme 1. In all
Ruppert’s reagent, CF3SiMe3, has been widely used as a
trifluoromethylating agent to introduce CF3 groups into organic
compounds. It has also been shown that this reagent could be used
13
to create P–CF3 groups from P–F,11 P–CN12 and P–OC6H4NO2
Department of Chemistry, The University of the West Indies,
St. Augustine, Trinidad & Tobago. E-mail: acaffyn@fsa.uwi.tt;
Fax: +1 (868) 645 3771; Tel: +1 (868) 662 2002 (ex3533)
{ Electronic supplementary information (ESI) available: Details of the
preparation and characterisation of the compounds. See http://dx.doi.org/
10.1039/b507752d
examples of the reaction, PhOSiMe3 was observed as
co-product.
a
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Table 1 Preparation of perfluoroalkylphosphines
Entry Phosphite
Silane
Initiator Phosphite:silane:initiator Solvent
Product
Yield (%)a,b
1
2
3
4
5
6
7
8
9
P(OPh)3
P(OPh)3
P(OPh)3
P(OPh)3
P(OPh)3
P(OPh)3
P(OPh)3
P(OPh)3
CF3SiMe3
CF3SiMe3
C2F5SiMe3 CsF
C2F5SiMe3 CsF
CsF
CsF
1:3:3
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
PhCN
Et2O
Et2O–PhCN P(C3F7)3
Et2O–PhCN P(C4F9)3
Et2O–PhCN P(C6F13)3
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
P(CF3)3
P(CF3)3
P(C2F5)3
P(C2F5)3
P(C2F5)3
P(C2F5)3
P(C3F7)3
P(C4F9)3
98 (80)
90
98 (85)
95
75
45
35
20
90
90 (40)
90
90
98 (80)
90
90
1:3:0.1
1:3:3
1:3:0.1
C2F5SiMe3 NaOPh 1:3:0.5
C2F5SiMe3 TBATc
C3F7SiMe3 CsF
C4F9SiMe3 CsF
C3F7SiMe3 CsF
C4F9SiMe3 CsF
C6F13SiMe3 CsF
1:3:0.5
1:3:0.5
1:3:3
1:3:0.5
1:3:3
1:3:3
1:1:0.5
1:1:0.5
1:2:0.1
1:2:0.1
1:4:4
P(O-p-C6H4CN)3
P(O-p-C6H4CN)3
P(O-p-C6H4CN)3
Ph2POPh
10
11
12
13
14
15
16
17
a
CF3SiMe3
C2F5SiMe3 CsF
CF3SiMe3 CsF
C2F5SiMe3 CsF
CsF
(PhO)2PCH2CH2P(OPh)2 C2F5SiMe3 CsF
CsF
Ph2PCF3
Ph2PC2F5
PhP(CF3)2
PhP(C2F5)2
(CF3)2PCH2CH2P(CF3)2
(C2F5)2PCH2CH2P(C2F5)2 90
Ph2POPh
PhP(OPh)2
PhP(OPh)2
(PhO)2PCH2CH2P(OPh)2 CF3SiMe3
95
1:4:4
Yield calculated via integration of one scan 31P and 19F NMR spectra. Isolated yields in parentheses. Tetrabutylammonium
b
c
triphenyldifluorosilicate.
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In conclusion, we have developed a general and high yield
procedure for the conversion of P(OPh)3 into the corresponding
P(Rf)3 by reaction with RfSiMe3. The commercial availability and
relatively non-hazardous nature of the reagents make the
procedure a highly attractive alternative to the previously reported
syntheses of tris(perfluoroalkyl)phosphines and bis(diperfluoroal-
kylphosphino)ethanes.
We gratefully acknowledge BP Chemicals Ltd. (studentship to
M.B.M.-J.) for financial support.
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4480 | Chem. Commun., 2005, 4479–4480
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