Reactivity of 2-Silyl- and 2-stannyl-substituted phosphirenes

Article abstract of DOI:10.1021/om100083e

Two methodologies have been tested for the functionalization of phosphirenes. In the first one, the C-Si bond of a 2-silylphosphirene is activated by a substoichiometric quantity of fluoride ion (TBAF) in THF at -78 °C. Using this technique, it is possible to perform a protodesilylation or a functionalization by benzaldehyde. However, at room temperature with a stoichiometry of fluoride, a nucleophilic attack takes place at P, leading to a ring-opened fluorophosphine. Stille cross-coupling with a 2-stannylphosphirene in the presence of [PdL₂] as a catalyst leads to an alkynylphosphine by [1,3] migration of tin from C to P.

Full text of DOI:10.1021/om100083e

Organometallics 2010, 29, 1985–1987 1985
DOI: 10.1021/om100083e
Reactivity of 2-Silyl- and 2-Stannyl-Substituted Phosphirenes
Duanghathai Panichakul, Yi Wee Lim, and Franc-ois Mathey*
Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang
Technological University, 21 Nanyang Link, Singapore 637371
Received February 3, 2010
Summary: Two methodologies have been tested for the func-
tionalization of phosphirenes. In the first one, the C-Si bond
of a 2-silylphosphirene is activated by a substoichiometric
quantity of fluoride ion (TBAF) in THF at -78 °C. Using
this technique, it is possible to perform a protodesilylation or a
functionalization by benzaldehyde. However, at room tem-
perature with a stoichiometry of fluoride, a nucleophilic attack
takes place at P, leading to a ring-opened fluorophosphine.
Stille cross-coupling with a 2-stannylphosphirene in the pre-
sence of [PdL2] as a catalyst leads to an alkynylphosphine by
[1,3] migration of tin from C to P.
Introduction
Since their discovery in 1982,1 the chemistry of phosphirenes
has been actively developed, and its foundations are now well
established.2 One problem has not been satisfactorily solved,
however. All of the described C-functional phosphirenes
Figure 1. LUMO of 2-silyl-1,2-dimethylphosphirene pentacar-
bonyltungsten complex as computed by DFT at the B3LYP/
6-31G(d)-Lanl2dz (W) level.
have been made by condensation of functional alkynes with
phosphinidene units, and today, no method allows the instal-
lation of a functional group on a preformed phosphirene. In an
attempt to solve this problem, we have decided to study the
reactivity of 2-silyl- and 2-stannyl-substituted phosphirenes.
We describe our results in this report.
˚
1.843(1) vs 1.791 and 1.763(1) A). This elongation could
have a steric origin since both P and C are substituted by
bulky groups, but we favor an electronic origin because
this elongation is reproduced by the computed structure of
the 2-silyl-1,3-dimethylphosphirene W(CO)5 complex at the
B3LYP/6-31G(d)-Lanl2dz (W) level. The most interesting
result of the computation concerns the shape of the LUMO
(Figure 1). It shows a significant presence at phosphorus
but no presence at silicon. We were planning to activate the
ring-C-Si bond of 1 by fluoride ion, and thus, this finding
casts some doubt on the feasibility of this approach. Our
initial experiments were carried out with nonstoichiometric
amounts of TBAF in the presence of water or benzaldehyde,
and we were delighted to find that the expected protonation
and functionalization do take place (eq 2). Obviously, these
reactions are not under the control of the LUMO. The high
affinity of silicon for F- plays the leading role.
Results and Discussion
As usual for this kind of strained phosphorus-carbon
heterocycle, we decided to work in the coordination sphere of
tungsten for convenience. The products are easier to synthe-
size and more stable. The reaction of [PhP-W(CO)5] as
generated from the appropriate 7-phosphanorbornadiene
precursor3 with PhCtC-SiMe3 affords the expected 2-silyl-
phosphirene (1) in good yield (eq 1).
The X-ray crystal structure of 1 shows a lot of disorder, but
the key parameters of the ring remain reliable. The most
significant finding is that the P-C(Si) bond is elongated by
comparison with the other P-C ring bond (1.812 and
*Corresponding author. E-mail: fmathey@ntu.edu.sg.
(1) Marinetti, A.; Mathey, F.; Fischer, J.; Mitschler, A. J. Am. Chem.
Soc. 1982, 104, 4484.
(2) Reviews on phosphirenes: Mathey, F. Chem. Rev. 1990, 90, 997.
Heydt, H. Sci. Synth. 2001, 9, 85.
(3) Marinetti, A.; Mathey, F.; Fischer, J.; Mitschler, A. J. Chem. Soc.,
Phosphirene 2 is characterized by the expected high-field
shift of its 31P resonance (δ31P -156.6 ppm in CD2Cl2) and
its highly deshielded vinylic proton at 8.48 ppm with a 2JH-P
coupling of 21 Hz. Phosphirene 3 exists as a 1:1 mixture
of two nonseparable diastereomers with their phosphorus
Chem. Commun. 1982, 667.
r
2010 American Chemical Society
Published on Web 03/22/2010
pubs.acs.org/Organometallics

1986 Organometallics, Vol. 29, No. 8, 2010
Panichakul et al.
resonances at high fields (δ31P -149.5 and -149.3 ppm in
CD2Cl2). The 13C spectrum shows the two sp3 C-O reso-
nances at 72.45 and 72.77 ppm.
(2JC-P = 13.4 Hz) and 78.52 ppm (1JC-P = 82.4 Hz). It
appears that [PdL2] readily catalyzes the [1,3] shift of tin from
carbon to phosphorus. This is not a real surprise since it is
known that Pd(0) readily inserts into the P-C ring bonds of
phosphirenes.5
From these series of experiments, it clearly appears that
the functionalization of preformed phosphirenes is a very
tricky problem. Even mild nucleophilic functionalization
methodologies are limited by the competing nucleophilic
ring-opening promoted by the significant localization of
the LUMO at P. On the other hand, electrophilic methodo-
logies have not produced any useful results until now, and
catalytic methodologies are limited by the easy activation of
the P-C ring bonds. More work is obviously needed to solve
this problem.
The result was entirely different when operating at room
temperature with a stoichiometric equivalent of TBAF. We
also observed a desilylation, but it was followed by an attack
of the fluoride ion at phosphorus with formation of the ring-
opened vinylic phosphinous fluoride 4 (eq 3). The resulting
phosphinous fluoride 4 was characterized by its 31P reso-
nance at very low fields (δ31P 197.2 ppm in CDCl3), its
1
huge JF-P coupling of 850 Hz, and its 19F resonance at
-132.8 ppm (PhCF3). It was accompanied by a minor isomer
(δ31P 201.8 ppm) whose formula was established by metha-
nolysis (see later). The major product 4 results from the
preferential cleavage of the P-C(Ph) ring bond of 2 favored
by the stabilizing effect of the phenyl substituent on the
resulting carbanion, whereas the minor product comes from
the cleavage of the P-CH bond. The formula of 4 was
further ascertained by hydrolysis and methanolysis.
Experimental Section
General Procedures. NMR spectra were obtained on a JEOL
ECA400 or JEOL ECA400SL spectrometer. All spectra were
recorded at 298 K unless otherwise specified. Elemental analyses
were performed in the Division of Chemistry and Biological
Chemistry, Nanyang Technological University. HRMS spectra
were obtained in ESI mode on a Finnigan MAT95XP HRMS
system (Thermo Electron Corp.). X-ray crystallographic ana-
lyses were performed on a Bruker X8 APEX diffractometer. All
reactions were performed under argon. Silica gel (230-400
mesh) was used for the chromatographic separations. All com-
mercially available reagents were used as received from the
suppliers.
Preparation of Phosphirene 1. The 7-phosphanorbornadiene
complex (0.30 g, 0.458 mmol) and trimethyl(phenylethynyl)-
silane (0.241 g, 1.37 mmol) in 5 mL of toluene was heated at
110 °C for 15 h. After evaporation, the residue was chromato-
graphed on silica gel with 100% hexane as eluent to give crystals
of 1 (0.242 g, 87% yield). 31P NMR (162 MHz, CDCl3):
δ -177.5 (d, JPW = 273.6 Hz, 1P). 1H NMR (400 MHz, CDCl3):
δ 7.75 (d, J = 6.9 Hz, 2H, ArH), 7.55-7.48 (m, 3H, ArH),
7.40-7.32 (m, 5H, ArH), 0.38 (s, 9H, 3 ꢀ CH3). 13C NMR
(100 MHz, CDCl3): δ 198.0 (d, J = 31.6 Hz, trans CO), 196.3
(d, J = 8.6 Hz, cis CO), 145.7 (d, J = 16.3 Hz, dC(Ph)), 139.1 (d,
J = 11.5 Hz, ipso C PhP), 131.7 (d, J = 35.5 Hz, dC-SiMe3)),
131.2 (CH (PhP)), 131.2 (CH (PhP)), 131.1 (CH (PhP)), 130.6
(CH (PhP)), 130.6 (CH), 130.1 (CH), 129.4 (2 ꢀ CH (PhP)),
128.5 (CH), 128.4 (CH), 127.6 (d, J = 7.7 Hz, ipso C (PhC)),
-0.70 (3 ꢀ CH3). The 13C assignments were made by compari-
son with previous data.6 Anal. Calcd for C22H19O5PSiW: C,
43.58; H, 3.16. Found: C, 43.69; H, 3.61.
General Procedure for the Preparation of Compounds 2 and
3. 2-Phenylmethanol-1,3-diphenylphosphirene (3). 2-Trimethyl-
silyl-1,3-diphenylphosphirene tungsten complex (1) (0.235 g,
0.387 mmol) was dissolved in 5 mL of THF, and benzaldehyde
(0.041 g, 0.387 mmol) was then added. Tetra-n-butylammonium
fluoride(TBAF) (0.08mL, 0.08 mmol) was added dropwisetothe
mixture at -78 °C and stirred overnight at room temperature.
After evaporation, the residue was chromatographed on silica gel
with 100% hexane as eluent and later 90:10 hexane/ethyl acetate
to give 3 as a pale yellow solid (0.126 g, 51% yield) and 2 as a
brownsolid (71mg, 34%yield). 3: 31P NMR (162 MHz, CD2Cl2):
δ -149.5 and -149.3 (d, JPW = 269 and 282 Hz). 1H NMR (400
MHz, CD2Cl2): δ 7.78-7.76 (m, 1H, ArH), 7.73-7.70 (m, 1H,
ArH), 7.52-7.47 (m, 9H, ArH), 7.42-7.35 (m, 4H, ArH),
7.33-7.23 (m, 5H, ArH), 6.25 (d, J = 2.8 Hz, 0.5 H, CH-Ph),
6.18(d, J = 5.0Hz, 0.5H, CH-Ph), 2.68 (br s, 1H, OH). 13C NMR
The hydrolysis product 5 shows the two expected vinylic
2
carbons at 142.90 (dCHPh, JC-P = 9.6 Hz) and 129.00
ppm (dCHP, 1JC-P = 34.5 Hz). The CHPh proton shows a
3JH-P coupling of 18 Hz, in agreement with an E geometry.4
The structural assignment of 6 is straightforward. More
interesting is the structure of the minor product 7 that we
were able to get in the pure state. The key finding is that
it contains a vinylic CH2 carbon at 129.54 ppm (2JC-P
=
14.4 Hz). The interpretation of these results is as follows. At
low temperature, the fluoride ion induces the desilylation
of phosphirene 1 to give phosphirene 2. Then, at room
temperature, F- attacks the phosphorus of 2 with preferen-
tial cleavage of the P-C(Ph) ring bond. This means that this
technique can be used for the functionalization of phosphir-
enes only at low temperature and with highly reactive co-
reagents such as water and aldehydes.
Looking for other potential methods of functionalization
of phosphirenes, we prepared the 2-stannyl-substituted
phosphirene 8 as shown in eq 4.
We expected to use 8 as a partner in a Stille cross-coupling
reaction. In fact we observed that 8 readily rearranges to the
secondary alkynylphosphine 9 in the presence of the [PdL2]
catalyst. Secondary phosphine 9 displays a 31P resonance at
1
-59.9 ppm (CDCl3) with a JH-P coupling of 372.5 Hz.
Its 13C spectrum shows the two sp carbons at 109.20
(5) Carmichael, D.; Hitchcock, P. B.; Nixon, J. F.; Mathey, F.;
Pidcock, A. J. Chem. Soc., Chem. Commun. 1986, 762.
(6) Tran Huy, N. H.; Perrier, E.; Ricard, L.; Mathey, F. Organo-
metallics 2006, 25, 5176.
(4) Nelson, J. H. Nuclear Magnetic Resonance Spectroscopy; Pearson
Education: Upper Saddle River, NJ, 2003; p 161.

Note
Organometallics, Vol. 29, No. 8, 2010 1987
(100 MHz, CD2Cl2): δ 198.64 and 198.52 (2d, J = 29.6 and 30.5,
trans CO), 196.66 and 196.58 (2d, J = 18.1 Hz, cis CO), 140.50 (d,
J = 8.6 Hz, dC(Ph)), 138.85 and 138.35 (2d, J = 5.7 and 8.6 Hz,
dC(CH)), 133.85 and 133.45 (2d, J = 13.4 Hz, ipso C (PhP)),
131.8 (CH), 131.7 (CH), 131.5 (CH), 131.3 (CH), 131.1 (CH),
130.9 (CH), 129.7 (CH), 129.5 (CH), 129.4 (CH), 129.2 (CH),
129.1 (CH), 129.0 (CH), 128.9 (CH), 128.8 (CH), 127.1 (d, J =
28.6 Hz, CH (Ph)), 126.3 (d, J = 5.7 Hz, ipso C (Ph-CdC)), 72.7
(br, CH-OH), 72.4 (br, CH-OH). Exact mass: calcd for
C26H17O6PW 640.0272, found: 640.0264
J = 17.4 Hz, 1H, CH), 3.55 (d, J = 12.8 Hz, 3H, OCH3). 13
NMR (100 MHz, CDCl3): δ 199.9 (d, J = 25.9 Hz, trans-CO),
196.6 (d, J = 8.6 Hz, cis-CO), 146.9 (d, J = 13.4 Hz, CH), 138.5
(d, J = 46.0 Hz, C), 135.2 (d, J = 16.3 Hz, C), 130.9 (CH), 130.3
(CH), 129.7 (CH), 129.6 (CH), 129.2 (2 ꢀ CH), 129.0 (CH),
128.9 (CH), 127.9 (2 ꢀ CH), 125.7 (d, J = 38.3 Hz, CH), 54.3 (d,
J = 6.7 Hz, OCH3). Exact mass: calcd for C20H15O6PW
566.0116, found 566.0106.
C
Alkenylphosphinite 7. 31P NMR (162 MHz, CDCl3): δ 125.9
(d, JPW = 284.8 Hz, 1P). 1H NMR (400 MHz, CDCl3): δ
7.66-7.61 (m, 2H, ArH), 7.50-7.44 (m, 3H, ArH), 7.24-7.17
(m, 3H, ArH), 7.08-7.06 (m, 2H, ArH), 6.18 (d, J = 32 Hz, 1H,
dCH trans P), 6.14 (d, J = 16 Hz, 1H, dCH cis P), 3.50 (d, J =
13.7 Hz, 3H, OCH3). 13C NMR (100 MHz, CDCl3): δ 198.8 (d,
J = 27.8 Hz, trans-CO), 196.9 (d, J = 7.7 Hz, cis-CO), 148.6 (d,
J = 32.6 Hz, C), 137.9 (d, 13.4 Hz, C), 137.1 (d, J = 42.2 Hz, C),
131.1 (CH), 130.8 (CH), 130.7 (CH), 129.5 (d, J = 14.4 Hz,
CH2), 128.8 (CH), 128.6 (CH), 128.3 (2 ꢀ CH), 128.26 (CH),
128.15 (CH), 128.12 (CH), 53.9 (OCH3). Exact mass: calcd for
C20H15O6PW 566.0116, found 566.0109
1,3-Diphenylphosphirene (2). 2-Trimethylsilyl-1,3-phenylphos-
phirene (1) (0.020 g, 0.0330 mmol) was dissolved in 2 mL of
THF, and TBAF (0.02 mL, 0.0165 mmol) was added into the
reaction flask. Three drops of water were added to the mixture
at -78 °C and stirred overnight at room temperature. After
evaporation, the residue was chromatographed on silica gel
with 100% hexane as eluent to give 2 as a brown solid (10 mg,
57% yield). 31P NMR (162 MHz, CDCl3): δ -156.6 (d, JPW
=
273.6 Hz). 1H NMR (400 MHz, CDCl3): δ 8.47 (d, J = 21.0 Hz,
1H, CH), 7.84-7.80 (m, 2H, ArH), 7.58-7.50 (m, 5H, ArH),
7.47-7.40 (m, 3H, ArH). 13C NMR (100 MHz, CDCl3): δ
198.2 (d, J = 31.5 Hz, trans CO), 196.2 (td, J = 62.0, 8.6 Hz, cis
CO), 138.29 and 138.27 (d, J = 9.6 Hz, dC(Ph)þ ipso PhP),
131.6 (CH), 131.4 (CH), 130.8 (CH), 130.5 (d, J = 3.8 Hz, CH),
129.3 (CH), 129.0 (CH), 128.6 (d, J = 10.5 Hz, CH), 127.4
(CH), 126.0 (ipso C), 125.93 (d, J = 6.7 Hz, ipso C), 117.6 (d,
J = 6.7 Hz, CH). Anal. Calcd for C19H11O5PW: C, 42.73; H,
2.08. Found: C, 43.11; H, 1.74.
Preparation of 2-Tributylstannyl-1,3-diphenylphosphirene (8).
The 7-phosphanorbornadiene complex (1.96 g, 3 mmol) and
tributyl(phenylethynyl)stannane (2.35 g, 6 mmol) in 10 mL of
toluene were heated at 110 °C for 15 h. After evaporation, the
residue was chromatographed on silica gel with 100% hexane as
eluent to give 8 as a brown solid (1.26 g, 51% yield). 31P NMR
1
(162 MHz, CDCl3): δ -179.7 (d, JPW = 273.5 Hz). H NMR
(400 MHz, CD2Cl2): δ 7.93 (d, J = 6.9 Hz, 2H, ArH), 7.65-7.54
(m, 5H, ArH), 7.43 (br s, 3H, ArH), 1.76-1.70 (m, 2H, 3 ꢀ
CH2), 1.50-1.41 (m, 12H, 3 ꢀ CH2CH2), 0.98 (t, J = 3.0 Hz,
9H, 3 ꢀ CH3). 13C NMR (100 MHz, CD2Cl2): δ 199.2 (d, J =
30.6 Hz, trans-CO), 197.4 (d, J = 8.6 Hz, cis-CO), 148.6 (d, J =
14.4 Hz, C), 141.0 (d, J = 7.7 Hz, C), 133.1 (C), 132.7 (C), 132.2
(CH), 132,1 (CH), 131.6 (CH), 130.7 (CH), 130.5 (2 ꢀ CH),
130.0 (2 ꢀ CH), 129.0 (CH), 128.9 (CH), 29.9 (t, J = 22.0 Hz,
3 ꢀ CH2), 28.1 (t, J = 59.4 Hz, 3 ꢀ CH2), 14.2 (3 ꢀ CH3), 12.4
(3 ꢀ CH2). Anal. Calcd for C31H37O5PSnW: C, 45.23; H, 4.53.
Found: C, 45.62; H, 4.02.
General Procedure for the Preparation of Compounds 4 and
5. Alkenylfluorophosphine (4). 2-Trimethylsilyl-1,3-diphenylpho-
sphirene (1) (15.2 mg, 0.025 mmol) was dissolved in 2 mL of THF,
and TBAF (0.025 mL, 0.025 mmol) was added dropwise at -78 °C
and stirred for 1 h at room temperature. After evaporation,
the crude product was obtained as a dark brown oil. 31P NMR
(162 MHz, CDCl3): δ 197.2 (dd, 1JPF = 850 Hz, 1JPW = 328.7,
319.9 Hz). 19F NMR (376 MHz, CDCl3, PhCF3): δ -132.8.
Alkenylphosphinous acid (5). 2-Trimethylsilyl-1,3-diphenyl-
phosphirene (1) (0.103 g, 0.17 mmol) was dissolved in 5 mL of
THF, and TBAF (0.17 mL, 0.17 mmol) was added dropwise
at -78 °C and stirred for 1 h at room temperature. After
evaporation, the residue was chromatographed on silica gel
with 90:10 hexane/ethyl acetate to give 5 as a white solid
Preparation of Alkynylphosphine 9. The catalyst was prepared
from Pd(dba)2 (16.4 mg, 0.03 mmol) and PPh3 (15.8 mg, 0.06
mmol) in toluene (10 mL). After 10 min of stirring at room
temperature, phosphirene 8 (0.56 g, 0.66 mmol) was added. The
mixture was then heated at 120 °C for 3 h. After evaporation of
the solvent, purification on silica gel with 80:20 hexane/ethyl
acetate gave 9 as a yellow solid (0.114 g, 32% yield). 31P NMR
(26 mg, 28% yield). 31P NMR (162 MHz, CD2Cl2): δ 101.6 (d,
1
J
PW = 280.5 Hz). H NMR (400 MHz, CD2Cl2): δ 7.67-7.62
(m, 2H, ArH), 7.52-7.45 (m, 5H, ArH), 7.38-7.34 (m, 3H,
ArH), 7.14 (dd, J = 18.8 and 17.4 Hz, 1H, dCHPh), 6.84 (dd,
J = 22.9, 17.4 Hz, 1H, dCHP), 4.00 (br s, 1H, OH). 13C NMR
(100 MHz, CD2Cl2): δ 199.7 (d, J = 24.0 Hz, trans CO), 196.5
(d, J = 8.6 Hz, cis CO), 142.9 (d, J = 9.6 Hz, dCHPh), 140.5 (d,
J = 46.9 Hz, ipso C (PhP)), 135.1 (d, J = 15.3 Hz, ipso C (Ph)),
130.7 (CH), 129.9 (CH), 129.1 (d, J = 34.5 Hz, dCHP), 128.9 (2
ꢀ CH), 128.9 (CH), 128.8 (CH), 128.7 (CH), 128.7 (CH), 127.6
(2 ꢀ CH). Anal. Calcd for C19H13O6PW: C, 41.33; H, 2.37.
Found: C, 40.91; H, 2.77.
(162 MHz, CDCl3): δ -59.9 (dd, JPH = 372.5 Hz, JPW
=
1
236.7 Hz). H NMR (400 MHz, CDCl3): δ 7.76-7.71 (m, 3H,
ArH), 7.57-7.49 (m, 5H, ArH), 7.42-7.38 (m, 3H, ArH), 6.48
(d, JPH = 372.0 Hz, 1H, PH). 13C NMR (100 MHz, CDCl3): δ
199.0 (d, J = 23.0 Hz, trans-CO), 195.9 (d, J = 6.7 Hz, cis-CO),
132.6 (2 ꢀ CH), 131.2 (CH), 131.1 (CH), 131.0 (CH), 130.9
(CH), 130.4 (CH), 129.7 (C), 129.3 (CH), 129.2 (CH), 128.7 (2 ꢀ
CH), 128.0 (d, J = 4.8 Hz, C), 109.2 (d, J = 13.4 Hz, tC), 78.5
(d, J = 82.4 Hz, tC). Exact mass: calcd for C19H11O5PW
533.9853, found 533.9865
General Procedure for the Preparation of Compounds 6 and 7.
2-Trimethylsilyl-1,3-diphenylphosphirene (1) (0.121 g, 0.2 mmol)
was dissolved in 5 mL of THF, and 2 mL of MeOH was added to
the reaction. TBAF (0.2 mL, 0.2 mmol) was added dropwise at
-78 °C and stirred for 1 h at room temperature. After evapora-
tion, the residue was chromatographed on silica gel with 80:20
hexane/ethyl acetate to give 6 as a white solid (37 mg, 33% yield)
and 7 (13 mg, 11% yield).
Acknowledgment. The authors thank the Nanyang
Technological University in Singapore for the financial
support of this work and Dr. Li Yong Xin for the X-ray
crystal structure analyses.
Alkenylphosphinite 6. 31P NMR (162 MHz, CDCl3): δ 117.56
Supporting Information Available: X-ray crystal structure
analysis of compound 1. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
(d, JPW = 284.8 Hz). H NMR (400 MHz, CDCl3): δ 7.62-
7.39 (m, 10H, ArH), 7.29 (t, J = 17.9 Hz, 1H, CH), 6.80 (t,