Toward metathesis reactions on vinylphosphaalkenes

Article abstract of DOI:10.1080/10426507.2012.743145

Attempts to utilize C-ethylenic phosphaalkenes in metathesis reactions are discussed. Unprecedented reactivity is observed where the vinylphosphaalkene undergoes the first step of the catalytic cycle and cross-metathesis with the phenylmethylene moiety of

Full text of DOI:10.1080/10426507.2012.743145

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Phosphorus, Sulfur, and Silicon and the
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Toward Metathesis Reactions on
Vinylphosphaalkenes
Elisabet Öberg a , Andreas Orthaber a , Marie-Pierre Santoni a
Fredrick Howard a & Sascha Ott a
,
a Department of Chemistry , Ångström Laboratories, Uppsala
University, Box 523 , 75120 , Uppsala , Sweden E-mail:
Accepted author version posted online: 14 Nov 2012.Published
online: 08 May 2013.
To cite this article: Elisabet Öberg , Andreas Orthaber , Marie-Pierre Santoni , Fredrick Howard &
Sascha Ott (2013) Toward Metathesis Reactions on Vinylphosphaalkenes, Phosphorus, Sulfur, and
Silicon and the Related Elements, 188:1-3, 152-158, DOI: 10.1080/10426507.2012.743145
To link to this article: http://dx.doi.org/10.1080/10426507.2012.743145
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Phosphorus, Sulfur, and Silicon, 188:152–158, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.743145
TOWARD METATHESIS REACTIONS ON
VINYLPHOSPHAALKENES
¨
Elisabet Oberg, Andreas Orthaber, Marie-Pierre Santoni,
Fredrick Howard, and Sascha Ott
Department of Chemistry, Ångstro¨m Laboratories, Uppsala University, Box 523,
75120 Uppsala, Sweden, sascha.ott@kemi.uu.se
GRAPHICAL ABSTRACT
Abstract Attempts to utilize C-ethylenic phosphaalkenes in metathesis reactions are discussed.
Unprecedented reactivity is observed where the vinylphosphaalkene undergoes the first step
of the catalytic cycle and cross-metathesis with the phenylmethylene moiety of Grubbs 2nd
generation catalyst. However, homo-metathesis reaction to form 1,6-diphosphahexa-1,3,5-
triene is not observed, presumably due to steric constraints.
Keywords Phosphaalkene; olefin; π-conjugation; metathesis
INTRODUCTION
This paper presents the first attempts to establish synthetic procedures that would
allow olefin-metathesis reactions on C-ethylenic phosphaalkenes. The phosphaalkene motif
has been vastly explored with respect to π-conjugated materials due to many advantageous
properties that arise from the inclusion of a phosphorus heteroatom. Examples of this are
the influence of phosphorus on the optical and electronic properties upon incorporation in
an all-carbon scaffold and its reactivity towards metals, oxidizing agents, etc.1 Therefore,
it is of high interest to find new routes for linking phosphaalkenes with unsaturated carbon
chains. Using olefin metathesis reactions for this purpose would expand the synthetic
arsenal for the construction of phosphorus-containing π-conjugated materials.
Received 10 September 2012; accepted 21 October 2012.
This work was supported by the Go¨ran Gustafsson Foundation, COST action 0802 PhoSciNet and Uppsala
University through the U3MEC molecular electronics priority initiative. A. Orthaber would like to thank the
Austrian Science Fund (FWF) for financial support through an Erwin-Schro¨dinger fellowship (J3193).
Address correspondence to Sascha Ott, Department of Chemistry, Ångstro¨m Laboratories, Uppsala Univer-
sity, La¨gerhyddsva¨gen 1, Box 523, 75120 Uppsala, Sweden. E-mail: sascha.ott@kemi.uu.se
152

TOWARD METATHESIS REACTIONS ON VINYLPHOSPHAALKENES
153
RESULTS AND DISCUSSION
Olefin metathesis reactions on alkenes that are in conjugation to phosphaalkenes
have, to the best of our knowledge, not been described in the literature before. There are
however numerous examples where olefins connected to phosphines, protected phosphines,
phosphonates or phosphonamides have been used in ring-closing diene metathesis reac-
tions, homometathesis and cross-metathesis reactions.2 The aim of this study was thus to
investigate whether C-ethylenic phosphaalkenes, available from C-bromophosphaalkenes,
could be converted to the corresponding 1,6-diphosphahexa-1,3,5-triene using an olefin
metathesis protocol (Scheme 1).
t-Bu
t-Bu
t-Bu
t-Bu
Br
t-Bu
t-Bu
P
P
t-Bu
t-Bu
P
P
t-Bu
t-Bu
t-Bu
t-Bu
MgBr
Scheme 1 Retrosynthetic approach to 1,6-diphosphahexa-1,3,5-triene.
t-Bu
t-Bu
Br
P
t-Bu
t-Bu
(i)
P
t-Bu
t-Bu
2
1
Scheme 2 Synthesis of C-ethylenic phosphaalkene 2. (i) vinylmagnesium bromide, Pd(dba)2, PPh3, THF, reflux
5 h, 2 93%.
The starting material, C-bromomethylphosphaalkene 1 (Scheme 2) was obtained
from Mes∗P CBr2 (Mes∗ = 2,4,6-(tert-Bu)3Ph) using known procedures.3 Compound 1
was treated with vinylmagnesium bromide in THF and catalytic amounts of Pd(dba)2 and
PPh3, which afforded 2 in excellent yield. As proven by X-ray analysis (Figure 1) and in
accordance with the finding of van der Sluis et al.,3(a) an isomerization occurs during the
Pd-mediated coupling reaction that converts the cis-bromophosphaalkene 1 to the trans-
ethylenic product 2.4
In the following step, 2 was subjected to different olefin metathesis reaction condi-
tions. Exposing 2 to the Hoveyda-Grubbs 2nd generation catalyst (Scheme 3) in refluxing
CH2Cl2 for 7 days, shows no reactivity and 2 can be recovered quantitatively. Even an
increase in temperature to refluxing toluene does not promote any reaction. Changing the
catalyst to Grubbs 2nd generation catalyst, initiates the formation of a small amount of a
new product that is characterized by a 31P-NMR chemical shift of δ = 267 ppm in contrast
to the signal of 2 at δ = 264 ppm. Addition of larger amounts of catalyst increases the
amount of product to an extent that it can be isolated and purified by column chromatog-
raphy. Initial NMR spectroscopic investigations of the compound were inconclusive, but

¨
E. OBERG ET AL.
154
Figure 1 ORTEP plot (POV-Ray representation) of phosphaalkenes 2, left and 3, right (ellipsoids are drawn at
30% probability level). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): 2; P1-C4 1.860(4),
P1 C3 1.687(5), C2-C3 1.458(7), C1 C2 1.323(8) C1-C2-C3-P1 -180.0(4)◦. 3; P1-C1, 1.8493(18), P1 C19
1.6908(18), C19-C20 1.455(2), C20 C21 1.349(2), C21-C22 1.459(3). (Color figure available online).
ultimate proof of the product identity could be obtained from single crystal X-ray crys-
tallography (Figure 1). Much to our surprise, the isolated product 3 is not the expected
1,6-diphosphahexa-1,3,5-triene, but instead contains a phenyl group at the alkene portion.
Compound 3 thus stems from a cross-metathesis reaction that occurs with the phenylmethy-
lene moiety of Grubbs 2nd generation catalyst. After this initial step, the reaction comes to
a halt and no further activity is observed. At present, we believe that this lack of reactivity
is due to the steric hindrance from the Mes∗ group that disallows the formation of the
desired 1,6-diphosphahexa-1,3,5-triene at some stage of the catalytic cycle. The catalyst
is still active after the formation of 3, as the addition of styrene leads to the formation of
diphenylethylene, while 2 remains untouched.
When exposing compound 2 to equimolar amounts of the “catalyst,” compound 3
can be obtained in 23% isolated yield, along with recovered starting material 2. Figure 1
shows the X-ray crystallographic structures of 2 and 3. The X-ray analysis shows that the
P C and C C bond distances lie within the expected range. The phosphaalkene and the
ethylene moieties are in one plane, providing efficient pathways for π-conjugation. In case
of 3, the phenyl group is also a part of this plane.
The UV/Vis spectra of solutions of 2 and 3 in CH2Cl2 are shown in Figure 2. C-
vinylphosphaalkene 2 exhibits a longest wavelength absorption maximum at 295 nm. The
t-Bu
t-Bu
P
t-Bu
Hoveyda-Grubbs
2nd Generation
N
N
P
t-Bu
t-Bu
(i)
N
N
t-Bu
t-Bu
t-Bu
t-Bu
Cl
Ru
Cl
Ph
P
Cl
Ru
Cl
t-Bu
P
O
2
Grubbs
2nd Generation
t-Bu
(ii)
Grubbs
2nd Generation
Hoveyda-Grubbs
2nd Generation
P
t-Bu
3
Scheme 3 Attempts for metathesis reaction of 2. (i) CH2Cl2 or toluene, reflux, 7 days. 2 left intact. (ii) CH2Cl2,
35◦C, 2 days, 1 eq. of Grubbs 2nd Generation catalyst, 3 23%.

TOWARD METATHESIS REACTIONS ON VINYLPHOSPHAALKENES
155
Figure 2 UV/Vis spectra of 2 and 3 in CH2Cl2 at room temperature. λmax [nm](ε)[M−1cm−1]; 2 295 (34700);
3 346 (62700).
introduction of a phenyl group leads to a bathochromic shift of ∼50 nm of the lowest energy
absorption maximum to 346 nm. Compared to our previous work in which we have syn-
thesized both bis-phenyl- and bis-TMS substituted C,C-diacetylenic phosphaalkenes and
1-phosphahex-1-en-3,5-diynes, the absorption profiles of the C-ethylenic phosphaalkenes
are considerably blue-shifted.1(k,m)
The cyclic voltammograms of the ethylenic phosphaalkenes 2 and 3 are displayed
in Figure 3. While the reduction of 2 is outside the solvent window, the introduction of
the phenyl group in 3 renders the compound easier to reduce, and an irreversible reduction
wave is observed at −2.45 V.
A similar trend can be observed on the oxidation side. While the oxidation of
2 occurs at 1.05 V, the phenyl group in 3 significantly lowers the oxidation potential
to 0.80 V. It thus seems that both the HOMO and the LUMO of both compounds are
mainly localized on the π-system of the compounds, also extending over the phenyl group
in 3.
Figure 3 Cyclic voltammogram of 1 mM solutions of 2 and 3 in CH2Cl2 (0.1 M NBu4PF6), ν = 100 mV/s. All
potentials are given vs Fc+/0
.

¨
E. OBERG ET AL.
156
CONCLUSIONS
In conclusion, we made a first exploratory study to utilize metathesis reactions of
a C-ethylenic phosphaalkene in order to expand the possible synthetic pathways towards
phosphorus-containing π-conjugated materials. An interesting reactivity is observed, where
the C-ethylenic phosphaalkene does not undergo homo-metathesis but reacts with the
phenylmethylene group of Grubbs 2nd generation catalyst. Future work will include a
screening of catalysts and further in-depth studies to identify the impeded step of the
catalytic cycle.
EXPERIMENTAL
NMR Spectroscopy: 1H-NMR spectra were recorded on a JEOL Eclipse + 400 MHz
spectrometer (operating at a proton frequency of 399.8 MHz). Chemical shifts are given in
ppm and referenced internally to the residual solvent signal (CHCl3, δH = 7.26 ppm). 13C-
NMR spectra were recorded on the same instrument (operating at 100.5 MHz) and were also
referenced internally to the residual solvent signal (CHCl3, δc = 77 ppm, central signal).
1
31P-{ H}-NMR spectra were also measured on JEOL Eclipse + 400 MHz spectrometer
(operating at 161.8 MHz).
UV/Vis Spectroscopy: UV/Vis was performed using a Varian Cary 50 instrument.
The measurements were performed in 1 × 1 cm2 optical quartz cells as solutions in CH2Cl2.
Electrochemistry: The electrochemical measurements were conducted in dry
CH2Cl2 (distilled from calcium hydride). An Autolab potentiostat with a GPES electro-
chemical interface (Eco Chemie) were used for the cyclic voltammetry measurements
with a glassy carbon disc (diameter 3 mm, freshly polished) for voltammetry as the
working electrode. The counter electrode was a glassy carbon rod and the reference
electrode a non-aqueous Ag+/Ag electrode (a silver wire immersed into 10 mM AgNO3
in acetonitrile) with a potential of –0.08 V vs. the ferrocenium/ferrocene (Fc+/Fc) couple
in CH2Cl2 as an external standard. The cyclic voltammograms were conducted at a
scan rate of ν = 100 mV/s. All solutions were conducted on 1 mM solutions of the
phosphaalkenes in dry CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate
(NBu4PF6) as supporting electrolyte. Before all measurements, oxygen was removed
by bubbling with argon and during the measurement all samples were kept under
argon.
Mass Spectrometry: Low-resolution mass spectrometry was performed on a Thermo
LCQ Deca XP Max with direct injection electrospray ionization (ESI). A small amount of
the compounds was dissolved in a solution of ∼2.5 mM AgBF4 in MeOH. High-resolution
mass spectrometry (HRMS) were performed on a high-resolution and FTMS+pNSI mass
spectrometer (orbitrapXL) at University of Muenster.
Crystallographic Studies: All measurements were performed using graphite
monochromatized Mo Kα radiation at 100(1) K on a Bruker Smart APEXII. Data
integration and absorption correction was carried out with SAINT and SADABS, respec-
tively. Structures were solved by direct methods (Shelxs-97)5 and refined by full-matrix
least-squares techniques against F2 (Shelxl-97) using WinGX.6 Graphical representations
are prepared with ORTEP for Windows6 and POV-Ray. The non-hydrogen atoms were
refined with anisotropic displacement parameters. The H atoms of the phenyl rings were
put at the external bisector of the C–C–C angle at a C–H distance of 0.95 Å. The H
atoms of the methyl groups were refined with common isotropic displacement parameters

TOWARD METATHESIS REACTIONS ON VINYLPHOSPHAALKENES
157
for the H atoms of the same group and idealized geometry with tetrahedral angles,
enabling rotation around the X–C bond, and C–H distances of 0.98 Å. Crystallographic
data (excluding structure factors) for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as supplementary numbers
CCDC-897142 (2) and CCDC-849422 (3). These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data request/cif, or by emailing data request@ccdc.cam.ac.uk, or
by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge
CB21EZ, UK; fax: +44 1223 336033.
The starting material, C-bromomethylphosphaalkene 1 was obtained from the corre-
sponding Mes∗P = CBr2 using known procedures.3
(E)-But-3-en-2-ylidene)(2,4,6-tri-tert-butylphenyl)phosphine (2)
Mes∗P C(CH3)Br (300 mg, 0.78 mmol), Pd(dba)2, (46 mg, 0.080 mmol) and PPh3
(21 mg, 0.080 mmol) were dissolved in 20 ml THF. Vinylmagnesium bromide (2.4 ml,
1 M, 2.4 mmol) was added dropwise and the reaction mixture was refluxed for 5 h. The
mixture was filtered through a plug of silica which was rinsed with CH2Cl2. The solvent
was removed in vacuo. Column chromatography in pentane yielded a white solid, 240 mg,
1
0.73 mmol (93%). Recrystallization from CH2Cl2/acetonitrile yields white crystals. H-
NMR (399.8 MHz, CDCl3): δ 1.31 (d, JPH = 13.0 Hz, CH3) 1.35 (s, 9H, p-tert-butyl), 1.46
(s, 18H, o-tert-butyl), 5.05 (ddd, 1H, 2Jgem = 0.9 Hz, JPH = 4.6Hz, 3Jcis = 10.5 Hz, alkene
CHcis), 5.21 (ddd, 1H,, 2Jgem = 0.9 Hz, JPH = 6.6 Hz, 3Jtrans = 17.0 Hz, alkene CHtrans),
7.03 (ddd, 1H, 3Jcis = 10.5 Hz, JPH = 13.9 Hz, 3Jtrans = 17.0 Hz, alkene-CH), 7.43 (m, 2H,
1
Mes∗). 31P-{ H}-NMR (161.8 MHz, CDCl3): δ 264.3. 13C-NMR(100.5 MHz, CDCl3): 18.3
(d, JPC = 15.7Hz, CH3), 31.4 (s, p-C(CH3)3), 32.4 (s, o-C(CH3)3, 32.5 (s, o-C(CH3)3, 35.0
(s, p-C(CH3)3), 37.9 (s, o-C(CH3)3), 110.2 (d, JPC = 41.2 Hz, CH2), 121.5 (s, meta-Ar),
136.7 (d, P-C, JPC = 56.1 Hz), 141.4 (d, JPC = 43.6 Hz, CH), 150.1 (s, para-Ar), 153.9 (s,
orto-Ar), 179.6 (d, P-C, JPC = 35.9 Hz). ESI MS: m/z [2M+Ag] + 767.6 (100), [M+Ag] +
437.4 (34). Anal. Calcd. for C22H35P; C, 79.95; H, 10.67. Found C, 79.78; H, 10.58.
Single crystals suitable for X-ray diffraction were obtained as colorless needles by
slow evaporation of 2 in CH2Cl2/CH3CN at room temperature. Compound 2 crystallizes
in the triclinic space group P-1(No. 2), C22H35P, M = 330.47g mol−1, crystal dimensions
0.1 × 0.2 × 0.4 mm, a = 6.1572(9) Å, b = 12.7564(18) Å, c = 13.3771(18) Å, α =
91.676(2)◦, β = 95.369(2)◦, γ = 91.575(2)◦, V = 1045.1(3) Å3, Z = 2, 2θmax = 55.3◦, ρ
= 1.050 g∗cm−1, μ(MoKα) = 0.131 mm−1, F000 = 364, –7 ≤ h ≤ 8, –16 ≤ k ≤ 16, –17 ≤ l
≤ 16, 16501 reflections measured, 4793 unique (Rint = 0.0405), R1 = 0.0921 (I>2.0σ(I)),
Rw = 0.2513 (all data), GooF = 1.016, 219 parameters, 0 restraints.
(E)-((E)-4-Phenylbut-3-en-2-ylidene)(2,4,6-tri-tert-butylphenyl)phosphine (3)
Mes∗P = C(CH = CH2)CH3 (114 mg, 0.35 mmol) and Grubbs Catalyst 2nd Gener-
ation (297 mg, 0.35 mmol) were dissolved in 10 ml CH2Cl2. The solution was stirred at
35◦C for 2 days. The solvent was removed in vacuo. Column chromatography in pentane
yielded a white solid, 39 mg, 0.096 mmol (27%) Recrystallization from CDCl3 yields
1
white crystals. H-NMR (399.8 MHz, CDCl3): δ 1.34 (s, 9H, p-tert-butyl), 1.42 (d, JPH
= 12.7 Hz, CH3) 1.46 (s, 18H, o-tert-butyl), 6.57 (m, 1H, CH), 7.21 (m, 1H, CH), 7.32
1
(m, 2H, Ar), 7.44 (s, 2H, Mes∗), 7.51 (m, 3H, Ar). 31P-{ H}-NMR (161.8 MHz, CDCl3):

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E. OBERG ET AL.
158
δ 267.2 ESI MS: m/z [2M+Ag] + 919.5 (100), [M+Ag] + 513.4 (34) HR-MS (ESI): m/z
513.18156 [M+Ag]+; calcd for C28H39PAg 513.18348
Single crystals suitable for X-ray diffraction were obtained as colorless blocks by
slow evaporation of 3 in CDCl3 at 4◦C. Compound 3 crystallizes in the monoclinic space
group P2(1)/c (No. 14), C28H39P, M = 406.56 g mol−1, crystal dimensions 0.13 × 0.21 ×
0.31 mm, a = 21.8508(29) Å, b = 6.2362(8) Å, c = 18.5898(25) Å, β = 100.271(2), V =
2492.56 Å3, Z = 4, 2θmax = 61.0◦, ρ = 1.083 g∗cm−1, μ(MoKα) = 0.12 mm−1, F000
=
888, –31 ≤ h ≤ 29, –8 ≤ k ≤ 8, –26 ≤ l ≤ 26, 46654 reflections measured, 7462 unique
(Rint = 0.090), R1 = 0.0588 (I > 2.0σ(I)), Rw = 0.1655 (all data), GooF = 0.971, 272
parameters, 0 restraints.
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