Exploitation of an unprecedented silica-promoted acetylene-allene rearrangement for the preparation of C,C-diacetylenic phosphaalkenes

Article abstract of DOI:10.1039/b916640h

C,C-Diacetylenic phosphaalkenes have been obtained from a 1-chloro-3-ethynyl-1,2-allene which becomes accessible from a silica-promoted rearrangement of a 3-chloropenta-1,4-diyne; oxidative acetylene homo-coupling of the phosphadiethynyl-ethene is describ

Full text of DOI:10.1039/b916640h

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Exploitation of an unprecedented silica-promoted acetylene–allene
rearrangement for the preparation of C,C-diacetylenic phosphaalkenesw
Xue-Li Geng and Sascha Ott*
Received (in Cambridge, UK) 13th August 2009, Accepted 28th September 2009
First published as an Advance Article on the web 19th October 2009
DOI: 10.1039/b916640h
C,C-Diacetylenic phosphaalkenes have been obtained from a
1-chloro-3-ethynyl-1,2-allene which becomes accessible from a
silica-promoted rearrangement of a 3-chloropenta-1,4-diyne;
oxidative acetylene homo-coupling of the phosphadiethynyl-
ethene is described for the first time and affords a dimeric,
cross-conjugated product which features a largely decreased
HOMOÀLUMO gap compared to all-carbon-based reference
compounds.
most prominently one at d = 211.7 ppm.20 In addition, EI-MS
of 2a and 2b showed identical peaks for the molecular ions,
supporting the notion that a rearrangement has occurred.
Based on this analysis and in analogy to reported analytical
data of a bromoallene,21 we identified 2b as the chloroallene
shown in Scheme 1.
1H-NMR experiments unambiguously identified silica as the
responsible reagent for this transformation as its addition to a
solution of 2a in CDCl3 resulted in complete conversion within
1 day (Fig. 1). Similar experiments were conducted with a
range of different acids to probe whether the effect can be
attributed to the acidity of silica. Whereas no reaction was
observed with toluenesulfonic acid (5À10 mM in CD3CN,
5 mM of 2a) or acetic acid (20 v/v in CDCl3) after 36 and
60 hours, respectively, addition of CF3COOH catalyzed the
transformation within 2 hours. The high acidity that is
required to drive the transformation and that has been
observed to be necessary for a somewhat related acid-catalyzed
acetylene–allene rearrangement22 is however not provided by
silica. The observed activity thus has to be attributed mostly to
silica itself.
Well-defined, monodisperse oligoacetylenes have attracted
chemists’ attention for many decades as their large degree of
unsaturation gives rise to a plethora of interesting (opto)-
electronic properties with many conceivable applications.1–5
Improved synthetic protocols for cross-conjugated structures
such as dendralenes,6 expanded radialenes7 and radiaannulenes8
have recently refueled the interest in these compounds.9
Despite the vast literature on oligoacetylenes, it is surprising
that heavy analogues of these structures where one or more
carbon centers are substituted by a heavy main group element
are essentially unknown. Considering the analogy between
phosphorus and carbon,10 we are currently aiming at introducing
phosphorus heteroatoms in the form of phosphaalkenes into
oligoacetylene frameworks.11 The inclusion of phosphorus
into related aromatic p-conjugated assemblies has brought
forward a number of compounds and materials with intriguing
properties.12–14 For example, it has been shown that the
pyramidalization of the phosphorus center in phospholes
causes
a diminished participation of the lone-pair in
p-delocalization and ultimately leads to decreased HOMO–
LUMO gaps.15–17 It could furthermore be shown that
phosphaalkenes and diphosphenes can be incorporated in
poly(phenylphosphaalkene)18 and diphosphenes,19 respectively.
We have recently reported that treatment of alcohol 1 with
thionyl chloride gives access to diacetylenic chloride 2a, the
1H-NMR spectrum of which features the chloromethyl proton
at d = 5.50 ppm with the 13C-NMR spectrum showing four
signals in the customary acetylene region.11 We were surprised
to find that attempts to remove small amounts of impurities by
silica gel column chromatography (hexane) resulted in the
disappearance of 2a and the formation of a new product 2b
Fig. 1 Transformation of 2a (5 mM, CDCl3, 20 1C) to 2b in the
presence of silica gel (15 mg). Shown is the characteristic region of the
1
chloromethyl (2a) and allenic proton (2b) of the H-NMR spectra.
Our interest in heavy ethynylethene analogues prompted us
to investigate the reaction of 2b with Mes*PCl2 (Mes* =
2,4,6-(tBu)3Ph) in the presence of LDA (Scheme 1). In contrast
to the reactivity of 2a which forms 1-phosphahexa-1-ene-3,5-
diyne 3a,11 allene 2b gives rise to diacetylenic phosphaalkenes
E/Z-3b, in a ratio of 3 : 1, as evidenced by two signals at
d = 338.3 ppm and 339.9 ppm, respectively, in the 31P-NMR
spectrum of the crude product mixture. The isomers were
successfully separated by preparative HPLC on a C18 chromato-
graphic column using a mixture of THF and methanol as
which features
d = 5.75 ppm and very different 13C-NMR chemical shifts,
a
singlet in its 1H-NMR spectrum at
˚
Department of Photochemistry and Molecular Science, Angstrom
¨
Laboratories, Uppsala University, Box 523, 75120 Uppsala, Sweden.
E-mail: sascha.ott@fotomol.uu.se; Fax: +46-18-471-6844
w Electronic supplementary information (ESI) available: Experimental
details, crystal data of 4. CCDC 743457. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b916640h
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Scheme 1 Synthesis of diacetylenic phosphaalkenes E-3b and Z-3b from allene 2b. (1) SOCl2, DMF (cat.), Et2O, 3 h, rt (20 1C). (2a/2b = 10/1,
estimated from Fig. 1). (2) Column chromatography on silica gel using hexane as eluent. (2b, 72%). (3) Mes*PCl2, LDA, THF, 1 h, À98 1C to
À40 1C. (55%, E-3b/Z-3b = 3 : 1, identified by 31P-NMR).
eluent. Attempts to grow single crystals of 3b for X-ray analysis
failed and the structures of the two isomers could only be
established by the experiments described below. 13C-NMR
spectra of both isomers show the characteristic chemical shift
for the PQC carbon of C,C-diacetylenic phosphaalkenes
around d = 141, which are thus also remarkably shifted upfield
compared to that of 3a (above d = 160 ppm).11
The mechanistic proposal for the formation of diacetylenic
phosphaalkene 3b starts with the abstraction of the acidic
proton in 2b by LDA. Nucleophilic substitution on Mes*PCl2
establishes a single bond between phosphorus and the central
carbon of 2b. Lithium–chloride exchange at this intermediate,
followed by elimination of LiCl, gives rise to E/Z-3b, the ratio
of which is determined in the final elimination step.
an E-configuration across the PQC bond. Secondly, P1 and
C1ÀC5 all lie within the same plane within 0.02 A. The
p-system extends even over the peripheral phenyl groups
which are twisted by only 6.41 out of the PC5 plane. Thirdly,
the phenyl ring on the acetylene terminus and the aromatic
portion of the supermesityl group are nearly perpendicular to
each other as described by an inter-planar angle of 961. H1 of
the phenyl ring thus points towards the center of the Mes* ring
with a vertical distance of 3.02 A. This distance is within the
range of a relatively strong intramolecular hydrogen bond
which makes E-3b thermodynamically more stable and may
be the reason for its preferred formation over Z-3b. This
interaction can also be observed in solution by 1H-NMR
spectroscopy. Whereas H1 in E-3b resonates at d = 6.86 ppm
owing to the additional shielding above the aromatic Mes*
group, the signal of the corresponding proton in Z-3b features
at d = 7.53 ppm.
The UV/Vis absorption spectra of compound 3b and 4 are
shown in Fig. 3. As expected, the isomeric E-3b and Z-3b
possess almost identical absorption profiles with longest
wavelength absorption maxima around 367 nm. Due to the
larger p-system, dimeric 4 exhibits a longest wavelength
absorption maximum at 446 nm that is thus red-shifted by
79 nm. In comparison with similarly substituted all-carbon-
based diethynylethenes,23 the acetylenic phosphaalkenes 3b
In attempts to further develop the chemistry of acetylenic
phosphaalkene, it was investigated whether E-3b was a suitable
substrate for Eglinton homo-coupling reactions (Scheme 2).
Employing a one-pot deprotection–coupling protocol,11 the
reaction proceeds smoothly to afford dimeric 4 in 68% yield. It
is important that the reaction time is kept short due to the
limited stability of 4 under the basic Eglinton conditions.
31P-NMR of the crude reaction mixture showed only one
signal in the PQC region at d = 348.9 ppm, and that the
stereochemistry across the PQC bond is conserved during the
reaction.
Single crystals of 4 were successfully grown from a mixture
of CH2Cl2 and methanol and their analysis by X-ray diffraction
revealed the trans relationship between the butadiyne
(C2–C1–C1A–C2A) and Mes* (Fig. 2). Since no isomer
scrambling was observed during the homo-coupling, it is safe
to assume that also the employed 3b starting material features
Fig. 2 Molecular structure of dimer 4 (25% probability ellipsoids).
All hydrogen atoms except H1 and H1A are omitted for clarity.
Selected bond lengths [A] and angle [1]: C1A–C1 1.373, C1–C2
1.201, C2–C3 1.429, C3–C4 1.426, C4–C5 1.193, C5–C6 1.441,
C3–P1 1.705, P1–C3–C2 115.2, P1–C3–C4 124.7, C2–C3–C4 120.1.
Dihedral angle between phenyl and Mes* ring planes [1]: 96. Distance
from H1 to Mes* ring plane [A]: 3.02.
Scheme 2 Homo-coupling reaction of E-3b to form 4. (1) K2CO3,
Cu(OAc)2, MeOH, pyridine, 1 h, 20 1C, 68%. TMS = trimethylsilyl.
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The discovery of an unprecedented silica-promoted acetylene–
allene rearrangement has paved the way to C,C-diacetylenic
phosphaalkenes. Dimeric oligoacetylene 4 can be regarded as
an expanded dendralene where the peripheral methylene
groups are exchanged by phosphorus centers. This modification
leads to greatly decreased HOMOÀLUMO gaps which
make our phosphorus-containing oligoacetylenes appealing
compounds for future molecular electronics applications.25
The general applicability of the rearrangement reaction, efforts
to further extend the unsaturated framework in 4 and
to prepare expanded phospharadialenes are subjects of
ongoing work.
Fig. 3 UV/Vis absorption spectra of compounds E-3b (---), Z-3b (Á Á Á)
and 4 (—) in CH2Cl2 at 25 1C.
This work was supported by the Swedish Research Council,
the Goran Gustafssons Foundation, the Carl Trygger
¨
Foundation and Uppsala University through the U3MEC
molecular electronics priority initiative. We thank Mr Jia
Wang and Prof. Jin Qu (Nankai University, China) for the
X-ray structure determination of 4.
Table 1 Electrochemical data for 3b and 4a
Reduction Ep,c/V
PQC–C
Oxidation Ep,a/V
PQC–C
Compound
E-3b
Z-3b
4
À2.04
À1.67b, À2.2c
1.03
1.02
0.95
Notes and references
À2.04
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7208 | Chem. Commun., 2009, 7206–7208