Direct, Sequential, and Stereoselective Alkynylation of C,C-Dibromophosphaalkenes

Article abstract of DOI:10.1002/chem.201601955

The first direct alkynylation of C,C-dibromophosphaalkenes by a reaction with sulfonylacetylenes is reported. Alkynylation proceeds selectively in the trans position relative to the P substituent to afford bromoethynylphosphaalkenes. Owing to the absence

Full text of DOI:10.1002/chem.201601955

DOI: 10.1002/chem.201601955
Full Paper
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Phosphaalkene Synthesis
Direct, Sequential, and Stereoselective Alkynylation of C,C-
Dibromophosphaalkenes
Muhammad A. Shameem, Keyhan Esfandiarfard, Elisabet ꢀberg, Sascha Ott,* and
Andreas Orthaber*[a]
Abstract: The first direct alkynylation of C,C-dibromophos-
phaalkenes by a reaction with sulfonylacetylenes is reported.
Alkynylation proceeds selectively in the trans position rela-
tive to the P substituent to afford bromoethynylphosphaal-
kenes. Owing to the absence of transition metals in the pro-
cedure, the previously observed conversion of dibromophos-
phaalkenes into phosphaalkynes through the phosphorus
analog of the Fritsch–Buttenberg–Wiechell rearrangement is
thus suppressed. The bromoethynylphosphaalkenes can sub-
sequently be converted to C,C-diacetylenic, cross-conjugated
phosphaalkenes by following a Sonogashira coupling proto-
col in good overall yields. By using the newly described
method, full control over the stereochemistry at the P=C
double bond is achieved. The substrate scope of this reac-
tion is demonstrated for different dibromophosphaalkenes
as well as different sulfonylacetylenes.
Introduction
significant stabilization of the LUMO level, while the HOMO is
essentially unaffected.[4]
Organic electronics as well as single-molecule electronics have
become a wide field of interest for theoretical, experimental,
and applied science. Over the last decade, different carbon-rich
organic scaffolds have been utilized in the fabrication of mate-
rials for applications in organic light-emitting diodes (OLEDs),
dye-sensitized solar cells (DSSCs), thin film transistors, etc.[1] Re-
cently, it was demonstrated theoretically and experimentally
that cross-conjugated systems exhibit intriguing conjugative
properties depending on the applied bias.[2] The search for in-
novative motifs with novel optical and electronic properties
has given rise to a completely new area in main-group chemis-
try. Incorporation of heavier main-group elements has long
been known to alter the optical and electronic properties of
conjugated frameworks, but only recently have the first exam-
ples of silicon-, boron-, and phosphorus-containing molecules
for organic electronics applications been presented.[3] Introduc-
ing an acetylenic moiety at the C-terminus of a phosphaalkene
offers the possibility of building larger conjugated or cross-
conjugated frameworks with unsaturated exocyclic P=C units,
linked through an acetylene bridge. Having such a phosphaal-
kene unit in the backbone of a conjugated molecule leads to
Recently, C-monoacetylenic phosphaalkenes—obtained by
a
transition-metal-mediated coupling reaction from their
monobromo precursors[5]—have been used in the dynamic sta-
bilization of gold nanoparticles,[6] and monohalogenated phos-
phaalkenes are valuable precursors in the synthesis of complex
structures featuring P=C units.[7]
C,C-Diacetylenic phosphaalkenes have hitherto not been ac-
cessible directly from halogenated precursors. Unlike dibro-
moalkenes, which can be used to access diethynylethenes in
Pd-mediated Sonogashira couplings,[8] the P counterpart, that
is, C,C-diacetylenic phosphaalkenes, are not accessible through
such a sequence owing to a transition-metal-mediated phos-
phorus version of the Fritsch–Buttenberg–Wiechell (FBW) rear-
rangement,[9] which leads to phosphaalkynes (RÀCꢀP) and
polymeric decomposition products thereof instead of the de-
sired acetylenic phosphaalkenes (Figure 1).[5a] As a matter of
fact, the FBW rearrangement is a common way to access phos-
phaalkynes from dibromophosphaalkenes nowadays (Fig-
ure 1a).[5b,10]
Inspiration for how to circumvent this problem came from
a study by Smorada and Truce who demonstrated a coupling
of in situ generated organolithium species at low temperatures
with arylsulfonylacetylenes to afford the anti-Michael acetylenic
coupling products with CArylÀCsp bond formation.[11] Recently,
Garcꢁa Ruano et al. have extended this strategy and demon-
strated its high stereo- and regioselectivity.[12] The wide scope
[a] M. A. Shameem, K. Esfandiarfard, Dr. E. ꢀberg, Prof. Dr. S. Ott,
Dr. A. Orthaber
Department of Chemistry - ꢁngstrçm Laboratories
Molecular Inorganic Chemistry Uppsala University
Box 523, 75120 Uppsala (Sweden)
of this reaction was demonstrated to enable Csp2ÀCsp and Csp3
À
Csp bond formations as well as having a large tolerance for the
various electronic natures of the employed reactants.[13] Con-
sidering the limitations of transition-metal-mediated coupling
reactions of C,C-dibromophosphaalkenes owing to the FBW re-
arrangement, we opted to investigate the scope and limita-
E-mail: sascha.ott@kemi.uu.se
andreas.orthaber@kemi.uu.se
Supporting information and the ORCID identification numbers for the au-
thors of this article can be found under http://dx.doi.org/10.1002/
chem.201601955.
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identified products above À508C.[17] The selectivity of the
metal halogen exchange is lost at À788C, giving mixtures of
the cis- and trans-lithium phosphacarbenoids (Scheme 1). For
our initial experiments, we chose dibromophosphaalkene
Mes*-P=CBr2 (1) as the starting material as 1 benefits from the
kinetic stabilization provided by the Mes* group (Mes*=2,4,6-
tBu3Ph). The reaction of intermediate 2 at low temperatures
with sulfonylacetylenes 3a–c (for R1 =a: Ph, b: 4-Br-C6H4, c: Fc)
results in the formation of acetylenic phosphaalkenes 4, which
are characterized by their downfield shifted 31P NMR resonan-
ces (291.0 ppm (4a), 294.2 ppm (4b), and 280.3 ppm (4c)) rela-
tive to that of 1 (271.0 ppm).[18] After chromatographic purifica-
tion, the targeted acetylenic phosphaalkenes were obtained as
1
colorless solids in moderate to good yields. The H NMR data
also underline the trans relationship of the introduced acety-
lene relative to Mes* through retention of the configuration at
the Li–carbenoid intermediate. For derivatives 4a and 4b, the
absence of distinct aromatic protons at higher fields
(ꢁ6.9 ppm) preclude positioning of the phenyl substituent in
the ring current of the Mes* substituent, as previously ob-
served for other phenylacetylene substituted phosphaalk-
enes.[14b,15] Unambiguous confirmation of the molecular struc-
ture was obtained by X-ray diffraction of single crystals (see
below). It is important to note that the Li/Br exchange reaction
must be carried out at temperatures below À908C; otherwise,
the selectivity in the Li/X exchange reaction is lost and a mix-
ture of cis- and trans-isomers is obtained. Reaction of 1 with
nBuLi at around À808C followed by the addition of sulfonyl-
acetylene 3a gives cis-4a as well as the opposite isomer trans-
4a. In contrast, controlled temperatures (below À1008C)
during the Li/halogen exchange step give cis-4a exclusively.
The only exception in this series of sulfonylacetylenes was ob-
served in the attempted coupling of trimethylsilyl (TMS) termi-
nated acetylene 3d (R1 =TMS). This reagent does not show
a clean alkynylation. Among several phosphorus containing
species, we could identify a TMS coupling reaction that result-
ed from a b-attack to afford the C-silylated phosphaalkene 5,
which was identified by X-ray crystallography and on the basis
of literature NMR data.[17] On the other hand, the successful in-
troduction of a para-bromophenylacetylene substituent as in
4b shows the functional group tolerance of this method and
indicates future possibilities for the preparation of larger oligo-
meric and cyclic structures. Surprisingly, the ferrocenylacety-
lene derivative 4c was unstable towards chromatographic
Figure 1. a) Commonly used strategy to convert dibromoolefins to acetylen-
ic alkenes by using transition-metal (TM)-catalyzed reactions. b) TM-mediat-
ed phospha-FBW rearrangement of C,C-dibromophosphaalkene to phos-
phaalkynes, which precludes the formation of acetylenic phosphaalkenes.
This work: novel approach for the direct alkynylation of a lithium phospha-
carbenoid intermediate by using sulfonylacetylenes, followed by a TM-medi-
ated second alkynylation.
tions of an arylsulfonyl-mediated alkynylation of a phosphaal-
kene through an in situ generated phosphavinylidene carbe-
noid species (Scheme 1, 2). It is important to note that prior to
2008, the introduction of two acetylenic units at the C-termi-
nus of a phosphaalkene was inconceivable.[14] However, stereo-
specific control of the acetylenic substituents is precluded by
using the “propargylic” approach and the stereochemistry is
predetermined by the acetylenic substituents.[4b,15]
Results and Discussion
The current work demonstrates a sequential coupling of two
acetylenes that avoids the transition-metal-mediated coupling
of the first acetylenic fragment. With this strategy, we have full
control over the stereochemistry at the P=C double bond,
which enables a reliable retro-synthetic approach towards
complex motifs.
It is known that dihalophosphaalkenes can be selectively
lithiated below À908C to give the kinetically favored cis-lithi-
um phosphacarbenoids, which are stable up to approximately
À608C,[16] but decompose to phosphaalkynes and other un-
Scheme 1. Reaction of dibromophosphaalkene Mes*-P=CBr2 (1) with arylsulfonylacetylenes R1-CꢀC-(SO2)-Tol (3a: R1 =Ph, 3b: R=4-Br-C6H4, 3c: R=Fc, 3d:
R=TMS), leading to desired bromoethynylphosphaalkenes 4a–c and undesired 5. Conditions: i) nBuLi, À1008C, Et2O; ii) R-CꢀC-(SO2)-Tol (3a–d), À1008C.
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workup and could only be characterized as a crude product by
dibromophosphaalkene 9 into the monoacetylenic derivative
10. Isolated yields (30%) are compromised by the slow decom-
position of the product during column chromatography. Simi-
larly, by using this reagent (12), we could demonstrate good
conversion of 1 into 4a, slightly improving the NMR yields to
81% compared with the original tosyl alkynylating reagent 3a
(Scheme 2b). Variation of the sulfonyl substituent presumably
has an impact on the solubility and consequently the availabili-
ty of the sulfonyl reagent especially at low temperatures.
Consequently, this transition-metal-free coupling reaction cir-
cumvents all problems associated with conventional coupling
reactions of dihalophosphaalkenes using transition-metal catal-
ysis. Having a second reactive CÀBr moiety available opens
new possibilities for the construction of larger and complex
frameworks. We and others have demonstrated that Pd-medi-
ated Sonogashira-type couplings of monobromophosphaal-
kenes gives an inversion of the stereochemistry through for-
mation of a three-membered cationic palladacycle.[5] To con-
firm this reactivity and selectively synthesize asymmetrically
substituted phosphaalkenes, we attempted to convert 4a into
its diacetylenic congener through a CÀC cross-coupling with
ferrocenylacetylene (Scheme 3). As expected, we obtain the
corresponding trans-isomer 7. Characteristic upfield shifted ar-
omatic protons and a unique 31P NMR resonance at 324.4 ppm
unambiguously support the formation of this product.[22] Con-
versely, the other stereoisomer was obtained by inverting the
sequence of the coupling steps, that is, by using in situ gener-
ated 4c in the coupling reaction with phenyl acetylene, which
afforded the diacetylenic phosphaalkene 8 with a 31P NMR res-
onance at 322.5 ppm. The two steps, that is, sulfonyl coupling
followed by the Sonogashira coupling, give the desired diacet-
ylenic phosphaalkene 8 in 32% combined yield.
1
31P and H NMR spectroscopy after extraction with pentane. In-
terestingly, reacting dibromophosphaalkene 1 with 2.1 equiva-
lents of nBuLi (Et2O, À788C) followed by addition of sulfonyl-
acetylene 2a gives rise to two major products, which are attri-
buted to the cis-monoacetylenic phosphaalkene cis-4a and the
symmetrically disubstituted C,C-diacetylenic phosphaalkene.[4b]
Although formation of a dilithiated intermediate is unlikely,
a four-step sequence of a consecutive Li/halogen exchange
then an alkynylation step followed by a second Li/halogen ex-
change and alkynylation can be excluded owing to the imme-
diate reaction of nBuLi with tosyl acetylenes. The exact mecha-
nism underlying the disubstituted derivative remains elusive at
this stage.
To assess the further scope of this reaction, we have also
tested a different dibromophosphaalkene. The Dmp substitu-
ent (Dmp=2,6-bis-(2,4,6-trimethylphenyl)phenyl) is a common-
ly used meta-terphenyl-based moiety for kinetic stabilization of
phosphaalkenes[19] and other unsaturated main-group com-
pounds.[20] Thus, our newly developed coupling protocol was
also tested on Dmp-P=CBr2 (9; Scheme 2a).[21]
Scheme 2. a) Coupling of 2,6-dimesitylphenyl substituted dibromophos-
phaalkene 9 with sulfonylacetylene 3a and 12. Mes=2,4,6-trimethylphenyl.
b) Coupling of dibromophosphaalkene 1 with phenylsulfonylacetylene 12.
Structural and electrochemical characterization
The monobromophosphaalkene cis-4a crystallizes in the
monoclinic space group P21/c as colorless blocks, whereas
trans-4a crystallizes in the space group P21/n as colorless
plates. Crystal structure analysis confirms the NMR assignment
of the two isomers. Both structures show very similar parame-
ters for the P=C double bond (1.682(4) and 1.684(4) ꢃ) and the
Utilizing tolyl-sulfonyl acetylene 3a, we obtained the corre-
sponding acetylenic phosphaalkene 10 only as a minor prod-
uct of the reaction whereas the major product stems from
simple hydrolysis of the lithium phosphacarbenoid. Alternative-
ly, phenyl-sulfonyl acetylene 12 gave almost full conversion of
Scheme 3. Stereoselective synthesis of diacetylenic phosphaalkene 7 and its isomer 8 through the sequential ethynylation with sulfonylacetylene, followed by
Sonogashira coupling. Conditions: [PdCl2(PPh3)2], CuI, THF, pyridine, NEt3.
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angle between the Mes* and the P=C unit (101.9(2) and
101.6(2)8; Figure 2). Interestingly, the angle between the acety-
lene and Br substituents is significantly affected by the relative
stereochemistry. Generally, the angle of the substituent cis to
Mes* is wider (>1258) than for the trans-substituent (<1238).
In compound 5, the P=CÀSi angle (trans-substituent) is the
largest in this series as a result of the steric demand of the
SiMe3 moiety. The P=C bond in 5 shows similar parameters
compared to the acetylenic derivatives cis-/trans-4a, indicating
similar delocalization involving the silyl substituent. In contrast,
the dibromophosphaalkene 9 exhibits a shorter P=C bond
(1.673(2) ꢃ), which is, however, longer than the P=C bond in
the closely related Mes*-P=CBr2 (1: 1.65(2) ꢃ),[23] The P=CÀBr
angles are nevertheless remarkably similar in dibromophos-
phaalkenes 1 and 9.
a conscious choice of substituents and their positions relative
to the P substituent. We have previously reported that phos-
phaalkene-based reductions are highly sensitive to minuscule
changes in the extended p-system.[4b–d] We hypothesized that
steric interactions of the ferrocenyl substituent might be re-
sponsible for substantial geometric changes and alterations in
the extended conjugation, thus influencing the phosphaal-
kene-based reduction. To tap the full potential of this strategy,
it is, however, crucial to have full control over the stereochem-
istry at the P=C double bond through methodologies such as
the one reported herein.
Low-valent organophosphorus conjugated backbones are
known to have a great impact on the electrochemistry of ferro-
cene substituents, giving rise to interesting opto-electronic
materials.[24] Having the isomeric compounds 7 and 8 in hand,
we investigated potential effects that the opposite substitution
patterns may have on the redox chemistry of these cross-con-
jugated C,C-diacteylenic phosphaalkenes by cyclic voltammetry
(Figure 3). Both isomers show reversible oxidation, which is as-
cribed to the Fe3+/2+ redox couple. The oxidation of derivative
7 is observed at 0.01 V versus Fc/Fc+, whereas isomeric com-
pound 8 is oxidized at 110 mV higher potentials. Interestingly,
only phosphaalkene 7 shows a quasi-reversible reduction
around À2.02 V, whereas compound 8 shows no reduction up
to À2.3 V (vs. Fc/Fc+). These findings clearly demonstrate that
the electronic properties of the compounds can be tuned by
Figure 3. Cyclic voltammograms of diacetylenic phosphaalkenes 7 (5 mm)
and 8 (2 mm). Scan-rate 100 mV, supporting electrolyte NBu4PF6, Ag/AgNO3,
CH2Cl2, calibrated versus Fc/Fc+.
Figure 2. ORTEP representation of monobromophosphaalkenes cis-4a and trans-4a and silyl derivative 5 as well as dibromophosphaalkene 9. Hydrogen
atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [ꢃ] and angles [8]: cis-4a: C8ÀP1 1.836(4), P1À
C7 1.682(4), C7ÀC6 1.409(6), C6ÀC5 1.206(6); C7-P1-C8 101.92(18), C6-C7-Br1 113.2(3), P1-C7-Br1 126.0(2), P1-C7-C6 120.8(3). trans-4a: P1ÀC6 1.851(3), P1ÀC7
1.684(4), C8ÀC9 1.203(5); C7-P1-C6 101.57(16), P1-C7-Br1 115.1(2), P1-C7-C8 129.0(3). 5: P1ÀC1 1.834(4), P1ÀC19 1.686(5), Br1ÀC19 1.924(4); C19-P1-C1
103.9(2), P1-C19-Br1 125.1(3), P1-C19-Si1 122.8(3). 9: P1ÀC15 1.673(2), P1ÀC7 1.826(2), Br1ÀC15 1.878(2); Br2-C15-1.892(2), C15-P1-C7 106.36(10), P1-C15-Br1
129.76(12), P1-C15-Br2 116.36(11).
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Synthesis of monoacetylenic phosphaalkene 4a
Conclusions
Dibromophosphaalkene 1 was dissolved in THF and cooled to
À1008C and treated with nBuLi (1.1 equiv) while maintaining the
temperature between À908C and À110 8C for 30–45 min. A solu-
tion of sulfonylacetylene 3a was added and the mixture was al-
lowed to slowly reach room temperature. The solvent was evapo-
rated and the crude mixture was extracted with dichloromethane
and purified by column chromatography using hexane as the
eluent. The fractions containing cis-4a were combined to give
78 mg (37%) of the analytically pure product. 1H NMR (CDCl3,
400 MHz): d=7.50–7.52 (m, 2H, Ar), 7.43 (broad s, 2H Ar), 7.32–
7.34 (m, 3H, Ar), 1.52 (s, 18H, tBu), 1.34 ppm (s, 9H, tBu); 13C NMR
(CDCl3, 100 MHz): d=153.4, 151.6, 137.1 (d, 1JPC =55 Hz, P=C),
We have demonstrated for the first time that full stereochemi-
cal control of acetylene substituents introduced at phosphaal-
kenes can be obtained by choosing the correct methodology.
We have developed a straightforward approach towards
diacetylenic phosphaalkenes, which are interesting cross-conju-
gated building blocks for opto-electronic applications. The ste-
reospecific synthesis is achieved by a sequential reaction with
sulfonylacetylenes, followed by conventional Sonogashira-type
reactions. By applying this novel methodology, the transition-
metal-mediated Fritsch–Buttenberg–Wiechell rearrangement of
dibromophosphaalkenes to phosphaalkynes can be avoided.
The procedure opens new tools to synthesize and study com-
plex heteroatomic cross-conjugated systems with a full control
over their stereochemistry in the future. Electrochemical analy-
sis of the isomeric compounds 7 and 8 clearly highlights the
electronic impact of the stereochemistry at the P=C double
bond that underlies the high demand for controlled synthetic
methodologies.
1
3
133.0 (d, JPC =45 Hz, PÀC), 131.8, 129.0, 128.4, 122.4, 97.6 (d, JPC
=
2
22 Hz, CꢀC), 92.5 (d, JPC =20 Hz, CꢀC), 38.0 (tBu), 35.2 (tBu), 33.0
(tBu), 31.4 ppm (tBu); 31P-{1H}-NMR (CDCl3, 162 MHz): d=
291.0 ppm.
Synthesis of diacetylenic phosphaalkene 7
Phosphaalkene cis-4a (114 mg, 0.213 mmol) was dissolved in de-
aerated THF (10 mL). [PdCl2(PPh3)2] (8.6 mg, 0.012 mmol), CuI
(5.6 mg, 0.024 mmol), Et3N (5 mL), and ethynylferrocene (56.4 mg,
0.268 mmol) were added to this solution. The reaction mixture was
deaerated again for 5 min. The reaction progress was followed by
TLC and continued until the starting material was fully consumed.
The organic phase was washed with sat. NH4Cl(aq.) followed by
brine. The crude product was purified by column chromatography
Experimental Section
Unless otherwise stated, all reactions were carried out under an
inert atmosphere of Ar or N2 by using modified Schlenk tech-
niques. All solvents were freshly distilled over benzophenone and
metallic sodium unless otherwise specified. Dichloromethane
(CH2Cl2) was distilled from calcium hydride and acetonitrile from
1
using toluene/hexane to give 7 (50 mg, 34% yield). H NMR (CDCl3,
400 MHz): d=7.49 (broad s, 2H, Ar), 7.16 (m, 3H, Ar), 6.89 (d, J=
8 Hz, 2H, Ar), 4.55 (m, 2H, Fc), 4.28 (m, 2H, Fc), 4.27 (s, 5H, Fc),
1.55 (s, 18H, tBu), 1.32 ppm (s, 9H, tBu); 13C NMR (CDCl3, 100 MHz):
1
1
1
d=154.3, 150.6, 142.6 (d, JPC =36 Hz, PÀC), 135.8 (d, JPC =57 Hz,
phosphorus pentoxide prior to use. The H NMR spectra were mea-
3
P=C), 131.5, 129.2, 128.0, 122.3, 102.8 (d, JPC =9 Hz, CꢀC), 96.5 (d,
sured in CDCl3 as solvent by using a JEOL Eclipse+ 400 MHz spec-
trometer. 13C-{1H} and 31P-{1H}-NMR spectra were measured on the
same instrument operating at 100 MHz and 162 MHz, respectively.
Electrochemical analysis was performed with a Pgstat Metrohm au-
tolab in a three-electrode setup using a glassy carbon working
electrode and a Ag/AgNO3 reference electrode. Reported potentials
are referenced externally to Fc/Fc+.
3JPC =17 Hz, CꢀC), 89.5 (d, JPC =21 Hz, CꢀC), 86.8 (d, JPC =26 Hz,
CꢀC), 71.7 (Fc), 70.2 (Fc), 69.3 (Fc), 65.4 (ipso-Fc), 38.3 (tBu), 35.2
(tBu), 33.2 (tBu), 31.4 ppm (tBu); 31P-{1H}-NMR (CDCl3, 162 MHz): d=
324.4 ppm.
2
2
X-ray crystallography
CCDC 145800, 1458001, 1458002, 1458003, 1458004, 1458005, and
1458006 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre.
Synthesis of aryl sulfonylacetylene 3a
Cerium ammonium nitrate (CAN, 40.25 g, 73.43 mmol, 2.5 equiv)
was added to a stirred suspension of ethynylbenzene (3.00 g,
29.4 mmol), sodium-p-toluene sulfinate (6.28 g, 35.2 mmol,
1.2 equiv), and sodium iodide (5.26 g, 35.3 mmol, 1.2 equiv) in an-
hydrous acetonitrile (150 mL) at room temperature under an argon
atmosphere. The reaction was monitored by TLC. After completion,
the solvent was evaporated and the solid residue was re-dissolved
in dichloromethane/diethyl ether, then washed with water
(150 mL). The organic phases were combined and washed with
Na2S2O3(aq.) (3ꢄ50 mL) followed by brine. The solvent was removed
and the crude product was purified by chromatography to afford-
ed iodophenylvinyl sulfone (6.69 g, 59% yield).[25] The iodophenyl-
vinyl sulfone (0.34 g, 0.89 mmol) was dissolved in THF and tBuOK
(0.93 g, 0.11 mmol, 1.05 equiv dissolved in THF) was added. The re-
action was stirred for 30 min at 508C. The solvent was evaporated
from the crude mixture and the residue was extract with dichloro-
methane (3ꢄ20 mL). The solution was passed through a plug of
silica and re-crystallization from n-hexane afforded phenylsulfonyl-
acetylene 3a (0.18 g, 80% yield).
All the measurements were performed by using graphite-mono-
chromatized MoKa radiation using a Bruker D8 APEX-II equipped
with a CCD camera. The structures were solved by direct methods
(SHELXS-2014) and refined by full-matrix least-squares techniques
against F2 (SHELXL-2014/7).[26] The non-hydrogen atoms were re-
fined with anisotropic displacement parameters. The H atoms were
refined with common isotropic displacement parameters for the H
atoms of the same group and idealized geometries.
cis-4a: colorless block, 0.42ꢄ0.4ꢄ0.38 mm, monoclinic, P21/c, a=
10.5969(7) ꢃ, b=20.5714(15) ꢃ, c=11.9720(12) ꢃ, b=111.607(4)8,
V=2426.4(3) ꢃ3, 1calc =1.285 MgmÀ3, 2qmax =53.08, MoKa, 0.71073 ꢃ,
w scans, 100(2) K, no. of reflections: 15063 (collected), 4960
(unique), 3559 (>2s), no. of parameters/restraints: 278/0, multi-
scan absorption correction (min/max transmission: 0.5690/0.7454),
Rint =0.0582, R=0.0585 (>2s), wR2=0.1655 (all).
trans-4a: colorless plates, 0.33ꢄ0.20ꢄ0.12 mm, monoclinic, P21/n,
a=9.1058(5) ꢃ, b=29.6319(18) ꢃ, c=9.2001(6) ꢃ, b=100.303(2)8,
V=2442.4(3) ꢃ3,
1calc =1.277 MgmÀ3
,
2qmax =52.928,
MoKa,
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Acknowledgments
The authors would like to thank the Swedish Research Council
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(CM1302), and the Carl-Trygger foundation for financial sup-
port.
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Keywords: alkynylation · cross-conjugation · phosphaalkenes ·
Sonogashira coupling · sulfonyl coupling
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Received: April 26, 2016
Published online on && &&, 0000
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Chem. Eur. J. 2016, 22, 1 – 7
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FULL PAPER
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Phosphaalkene Synthesis
M. A. Shameem, K. Esfandiarfard,
E. ꢀberg, S. Ott,* A. Orthaber*
&& – &&
Step by step: Diacetylenic phosphaal-
kenes were obtained from dibromo-
phosphaalkenes in a sequential step-
wise manner; the first step is a direct re-
action with sulfonylacetylenes with re-
tention of stereochemistry, this is fol-
lowed by a transition-metal-catalyzed
second step that proceeds with stereo-
chemical inversion (see scheme).
Direct, Sequential, and Stereoselective
Alkynylation of C,C-
Dibromophosphaalkenes
Chem. Eur. J. 2016, 22, 1 – 7
www.chemeurj.org
7
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ