Organometallic chemistry of bromodifluoromethyl substituted phosphines. The development of a novel nickel catalysed P-C bond forming reaction

Article abstract of DOI:10.1039/b311799e

A new nickel catalysed coupling reaction between phosphines of type Ph₂PCF₂Br and silyl-substituted phosphines is reported, along with the first example of a microwave assisted phosphine synthesis.

Full text of DOI:10.1039/b311799e

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Organometallic chemistry of bromodifluoromethyl substituted
phosphines. The development of a novel nickel catalysed P–C bond
forming reaction†
Matthew L. Clarke,*a A. Guy Orpen,b Paul G. Pringleb and Edward Turleyb
a School of Chemistry, University of St. Andrews, St Andrews, Fife, UK KY16 9ST.
E-mail: mc28@st-andrews.ac.uk; Fax: 01334 463808; Tel: 01334 463850
b School of Chemistry, University of Bristol, Bristol, UK BS8 1TS
Received 24th September 2003, Accepted 23rd October 2003
First published as an Advance Article on the web 29th October 2003
A new nickel catalysed coupling reaction between phos-
phines of type Ph2PCF2Br and silyl-substituted phosphines
is reported, along with the first example of a microwave
assisted phosphine synthesis.
Br readily reacts with [PtCl2(COD)] to yield the expected complex
cis-[PtCl2(κ1-Ph2PCF2Br)2] (3). Ph2PCF2Br reacts with the Rh()
precursor [RhCp*Cl2]2 as a monodentate ligand to give complex,
4. These experiments confirm that the CF2Br group does not
inhibit the co-ordination of this phosphine to transition metals.
This is in contrast to other fluorinated phosphines studied in
these laboratories that do not react or require extended reaction
times to produce the expected Rh() or Pt() complexes.
Phosphite, phosphonite and phosphinite ligands have many
important applications in homogeneous catalysis, including hydro-
cyanation,1 hydroformylation,2 Stille and Suzuki coupling3 and
allylic alkylation.4 Unfortunately, phosphite, phosphonite and
phosphinite ligands are intolerant of moisture, acid and strong
nucleophiles which limits their application in homogeneous cata-
lysis. It is likely that the desirable catalytic properties of phosphite-
derived metal complexes stems from their pronounced π-acceptor
character. Since weak σ-donor/strong π-acceptor phosphine
ligands have shown some promise in transition metal catalysis,5
we are currently interested in developing electron poor phos-
phines that can mimic P–O type ligands.5a,6 Our intention was
therefore drawn to the rarely studied class of phosphines contain-
ing a P–CF2–R linkage. We report here the use of a new nickel
catalysed P–C bond forming reaction to produce diphosphines of
type Ph2PCF2PR2. These phosphines are analogues of the heter-
oatom bridged diphosphines (R2P)2O and (R2P)2NR. The latter
have shown considerable promise in homogeneous catalysis.7
In 1991, Fild and co-workers communicated the synthesis of
Ph2PCF2Br from Ph2PSiMe3 and CF2Br2.8 Our interest lay in
using this phosphine as a precursor to phosphines of type
Scheme 2 Reaction of Ph2PCF2Br with Rh() and Pt() complexes.
Recrystallisation of 4 from CH2Cl2/hexane gave crystals suit-
able for an X-ray crystal structure determination (Fig. 1).11 The
crystal structure shows that 4 has the expected pseudo-octahedral
piano-stool geometry in which ligand 1 adopts monodentate
co-ordination. This structure was of additional interest to us as it
allowed a direct comparison to the related structure, [RhCp*Cl2-
(κ1-Ph PC(᎐O)CH )], that we have reported recently.6 This struc-
᎐
2
3
R2PCF2R1 and R2PCF2PR1 . We have found that the reaction of
2
ture showed that the P–C bond to the acetyl group was amongst
the longest reported for a phosphine metal complex [P–C(O)CH3
= 1.917(2) Å]. This was rationalised as arising in part from
electrostatic interactions between the electropositive acyl carbon
and co-ordinated phosphorus atom. We therefore predicted,
and observed a similarly long P–C bond length within complex 4
[P(1)–C(23) = 1.902 Å].
Ph2PLi with CF2Br2 generates (Ph2P)2CF2 (2) in low to moderate
yield and we experienced even more difficulty in functionalising
isolated Ph2PCF2Br.9 For example, we attempted the reactions of
Ph2PCF2Br with PhLi, PhZnBr, Ph2PK, CF3–SiMe3, Li-pyrollyl
and magnesium. However, all of these reactions resulted in
decomposition (or no reaction), although in the case of Ph2PK, a
mixture that clearly contained (Ph2P)2CF2 as a minor product
was obtained [δP = 5.5, t, 2JP–F = 73 Hz; δF = Ϫ95, t, 2JF–P = 73 Hz].
The chemistry of halodifluoromethylene-substituted compounds
with reactive organometallics is often complicated by the form-
ation of carbene species or surprisingly low electrophilicity. We
elected to study the organometallic chemistry of Ph2PCF2Br with
a view to facilitating new P–C and C–C bond forming reactions.
As a starting point, the ability of Ph2PCF2Br to act as a ligand
to metals in positive oxidation states was confirmed.9,10 Ph2PCF2-
Fig. 1 X-ray structure of complex 4. Important bond lengths (Å):
Rh(1)–P(1) 2.3206(14); P–C(Ph) 1.814(5), 1.821(6).
Scheme 1 Synthesis and reactivity of Ph2PCF2Br.
The reactions of Ph2PCF2Br with low-valent transition metal
complexes such as [Pd(PCy3)2], Ni(COD)2/L and Pt(NBD)3/L
gave mixtures of species that were generally shortlived at room
† Electronic supplementary information (ESI) available: Selected spec-
troscopic data and procedures, and crystal structure information for
trans-[Pd(Cy3P)2Br2]. See http://www.rsc.org/suppdata/dt/b3/b311799e/
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 4 3 9 3 – 4 3 9 4
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Table 1 Nickel catalysed coupling reaction of Ph2PCF2Bra with
of Ph2PCF2Br with NaBH(OMe)3 as nucleophilic reagent
(Table 1, entries 7–9). Other nucleophiles studied include
iPr2PSiCl3 and (NCCH2CH2)2P–SiMe3. A 33% conversion to
Ph2PCF2PiPr2 and 62% conversion to Ph2PCF2P(CH2CH2CN)2
were observed under unoptimised conditions. Thus this new reac-
tion can be used in the synthesis of unsymmetrical diphosphines.
Finally, 30 min of controlled microwave irradiation (80 W,
MeCN, sealed tube, ca. 160 ЊC) allows the coupling of Ph2PCF2-
Br and Ph2PSiMe3 to take place in near quantitative yield with
minimal formation of the side product Ph2PCF2H. This is, to the
best of our knowledge, the first example of a microwave assisted
phosphine synthesis. The microwave assisted reaction was also
applied to the direct Ni catalysed reaction between iPr2PSiCl3 and
CF2Br2 to yield the novel bulky diphosphine (iPr2P)2CF2 in an
unoptimised yield of 85% [15% iPr2PCF2Br].
In conclusion, the reactivity of Ph2PCF2Br with transition
metal complexes and nucleophilic reagents has been studied. This
has allowed us to develop a novel P–C bond forming reaction to
produce phosphines of type Ph2PCF2PR2. This is an unusual
example of the use of transition metal catalysis in the function-
alisation of organofluorine compounds and should allow the
study of new CF2 bridged diphosphines in catalysis.
nucleophiles
(1) (6) Ph2PCF2PR2
Entry Nucleophile t/h T/ЊC
%
%
%, [catalyst]
1
2
Ph2PSiMe3
Ph2PSiMe3
Ph2PSiMe3
Ph2PSiMe3
Ph2PSiMe3
Ph2PSiMe3
24
20
40
18
18
18
110 ∼20 ∼30 ∼40 [no cat.]
110
85
80
0
0
0
25 65 [cat = 5]
24 76 [cat = 5]
3c
4b
5b
6b
7
5
95 [cat = 5]
80 25 30 45 [Ni(dppe)Cl2]
80 10 90 [Ni(dcypx)COD]
0
NaBH(OMe)3 16
NaBH(OMe)3 16
NaBH(OMe)3 20
90 30 50 — [5]
90 50 30 — [5]
8
9d
10b
125
5
79 — [5]
iPr2PSiCl3
40
40
0.5 ∼165
80 25 41 34 (R = IPr) [5]
11b, e RЈ2PSiMe3
75
0
0
38 62 [5] R = NCCH2CH2
92 [5]
e
12b
Ph2PSiMe3
8
a The reactions were carried out in toluene solvent using 1.2 equiv.
nucleophile and 3 mol% [Ni(dippf )Cl2] unless stated. Product yields
based on Ph2PCF2Br and determined by 19F NMR. Product identity
was further confirmed by 31P NMR and FAB mass spectra. Unidenti-
fied side products account for any remaining mass balance. b Reactions
carried out in the dark, acetonitrile used as solvent. c 1.8 Equiv. of
Ph2PSiMe3 used. d 2 Equiv. of NaBH(OMe)3 used. e 2 Equiv. of
(NCCH2CH2)2PSiMe3 used as nucleophile.
Acknowledgements
temperature and require further study. For example, adding
[Pd(PCy3)2] to a solution of Ph2PCF2Br generates, after one hour,
[trans-PdBr2(PCy3)2] as the major product (δP = 26 ppm, also
characterised by X-ray diffraction).9 These experiments suggest
that the C–Br functionality is cleaved in the presence of
low-valent transition metal complexes.
The authors wish to thank Mr Damien Zaheer who prepared
[Ni(dippf )Cl2] in connection with another project, and Mr Chris
Mason of CEM Ltd. for the trial loan of a CEM discovery
microwave. PGP wishes to thank the Leverhulme Trust for a
Senior Research Fellowship.
The use of a transition metal catalyst to facilitate C–Br cleav-
age presented itself as an intriguing possibility. We elected to
develop a nickel catalysed P–C bond forming reaction between
R2PCF2Br and R2PSiMe3. Nickel complexes undergo facile oxid-
ative addition and reductive elimination and therefore should
make ideal catalysts if the reaction were to proceed through a
cross-coupling reaction. The novel catalyst [NiCl2(dippf )] (5)
(dippf = bis-diisopropylphosphinoferrocene) was therefore pre-
pared as we felt that it should be particularly reactive due to its
high basicity, bulkiness and wide bite angle.
Notes and references
1 U. S. Patent 3 766 237 (1973) to Dupont.
2 (a) P. W. N. M van Leeuwen and C. F. Rooboeek, J. Organomet.
Chem., 1983, 258, 343; (b) E. Billig, A. Abatjoglou and D. R. Bryant,
(Union Carbide) U. S. Patent 4748261, 1988; 4769498, 1988.
3 R. B. Bedford, S. L. Hazelwood and D. A. Abbison, Organo-
metallics, 2002, 21, 2599; D. A. Abbison, R. B. Bedford and P. N.
Scully, J. Chem. Soc., Chem. Commun., 1998, 2095; R. B. Bedford
and S. L. Welch, J. Chem. Soc., Chem. Commun., 2001, 129.
4 R. Takeuchi and M. Kashio, Angew. Chem., Int. Ed. Engl., 1997, 36,
263; R. Pretot and A. Pfaltz, Angew. Chem., Int. Ed. Engl, 1998, 37,
323.
5 (a) K. L. Mason, PhD Thesis, University of Bristol, 1997; (b) M. L.
Clarke, unpublished results; (c) H. Klein, R. Jackstell, K-D. Wiese,
C. Borgmann and M. Beller, Angew. Chem., Int. Ed. Engl., 2001,
40, 3408 and references therein.; (d ) D. F. Foster, D. J. Adams,
D. Gudmundsen, A. M. Stuart, E. G. Hope and D. J.
Cole-Hamilton, J. Chem. Soc., Chem. Commun., 2002, 722.
6 A. Baber, M. L. Clarke, A. G. Orpen and D. A. Ratcliffe, J. Organo-
met. Chem., 2003, 667, 112.
The reaction of Ph2PCF2Br with Ph2PSiMe3 in the absence of
catalyst gives a low yield of (Ph2P)2CF2 (20–40%) even after
extended refluxing in xylene. However, in the presence of
[NiCl2(dippf )] (3 mol%), improved yields up to 95% of the
desired product were obtained (Table 1, entries 2–4). In contrast
to the uncatalysed reaction, conversion of Ph2PCF2Br was 100%.
The main side product from these reactions was Ph2PCF2H (6)
(δP = Ϫ10.2, t, 2JP–F = 120 Hz, δF = Ϫ117, 2JF–P = 120 Hz, 2JF–H
=
52 Hz). The analogous compounds (RO)2P(O)CF2H have been
isolated as the major product in the reaction of (RO)2P(O)CF2-
Br with nucleophiles.12 Our results seem to imply that this
side-product is minimised in the absence of sunlight. When
[NiCl2(dppe)] was used as catalyst under identical conditions,
significantly lower yields of 2 were observed (compare Table 1,
entries 4 and 5). This is another example where the use of a bulky,
electron rich phosphine promotes an otherwise difficult transition
metal catalysed coupling reaction.
The reaction seems likely to proceed by oxidative addition of
Ph2PCF2Br to the Ni(0) centre, followed by transmetallation of
the nucleophilic reagent and reductive elimination of the desired
products. Consistent with this mechanism is the finding that the
nickel(0) bis-dicyclohexylphosphino-xylene (dcypx) complex
[Ni(COD)((dcypx))]13 is also a good catalyst for this reaction.
The reduction of [Ni(L2)Cl2] by phosphines is well precedented,
and in this case we believe that the formation of (Ph2P)2 (31P
NMR: δ ∼Ϫ15ppm) from Ph2PSiMe3 delivers the active Ni(0)
species. However, we cannot rule out a radical based mechanism
as halofluorocarbons frequently react by one-electron pathways.
Ph2PCF2H can be prepared as the major product by the reaction
7 A. Carter, S. A. Cohen, N. A. Cooley, A. Murphy, J. Scutt and D. F.
Wass, J. Chem. Soc., Chem. Commun., 2002, 858; S. J. Dossett,
A. Gillon, A. G. Orpen, J. S. Fleming, P. G. Pringle, D. F. Wass and
M. D. Jones, J. Chem. Soc., Chem Commun., 2001, 699.
8 M. Fild, P. G. Jones and K. Ruhman, J. Fluorine Chem., 1991, 54,
388 (Poster 138).
9 See Electronic Supplementary Information (ESI)† for further
details.
10 All compounds reported here were characterised by multinuclear
NMR and FAB mass spectroscopies. Selected analytical data: (3)
Found: C, 33.03; H, 2.50; C27H22F4Br2Pt1P2Cl4 (DCM solvate, 1H
NMR evidence) requires: C, 33.05; H, 2.26. (4) Found: C, 44.74; H,
4.07; C23H20F2Br1Cl2P1Rh1 requires: C, 44.30; H, 4.05. [NiCl2-
(dippf )] Found: C, 47.4; H, 6.60; C22H36Fe1P2 requires: C, 48.0; H,
6.96.
11 Crystal data for 4: C23H25BrCl2F2PRh, M = 624.12, monoclinic,
space group C2/c (No. 15), a = 15.890(3), b = 8.3668(18), c =
36.119(8) Å, β = 92.679(4)Њ, U = 4796.7(18) Å3, Z = 8, T = 173 K,
5500 unique data, R1 = 0.0573. CCDC reference numbers 220351
and 220352. See http://www.rsc.org/suppdata/dt/b3/b311799e/ for
crystallographic data in CIF or other electronic format.
12 D. J. Burton and R. M. Flynn, J. Fluorine Chem., 1980, 15, 263.
13 Complex was generated in situ from [Ni(COD)2] and dcypx.
D a l t o n T r a n s . , 2 0 0 3 , 4 3 9 3 – 4 3 9 4
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