Alternative synthesis of A C,C-diacetylenic phosphaalkene

Article abstract of DOI:10.1080/10426507.2012.743146

Bis(trimethylsilyl)-terminated C,C-diacetylenic phosphaalkene was prepared from Mes PCl₂ and a propargylic Grignard reagent that in turn was formed from 3-bromo-1,5-bis(trimethylsilyl)penta-1,4-diyne and Rieke-Mg.

Full text of DOI:10.1080/10426507.2012.743146

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Phosphorus, Sulfur, and Silicon and the
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Alternative Synthesis of A C,C-
Diacetylenic Phosphaalkene
Elisabet Öberg a & Sascha Ott a
a Department of Chemistry, Ångström Laboratories, Uppsala
University, Box 523 , 75120 , Uppsala , Sweden E-mail:
Accepted author version posted online: 15 Nov 2012.Published
online: 08 May 2013.
To cite this article: Elisabet Öberg & Sascha Ott (2013) Alternative Synthesis of A C,C-Diacetylenic
Phosphaalkene, Phosphorus, Sulfur, and Silicon and the Related Elements, 188:1-3, 164-167, DOI:
10.1080/10426507.2012.743146
To link to this article: http://dx.doi.org/10.1080/10426507.2012.743146
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Phosphorus, Sulfur, and Silicon, 188:164–167, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.743146
ALTERNATIVE SYNTHESIS OF A C,C-DIACETYLENIC
PHOSPHAALKENE
¨
Elisabet Oberg and Sascha Ott
Department of Chemistry, Ångstro¨m Laboratories, Uppsala University, Box 523,
75120 Uppsala, Sweden, sascha.ott@kemi.uu.se
GRAPHICAL ABSTRACT
Abstract Bis(trimethylsilyl)-terminated C,C-diacetylenic phosphaalkene was prepared from
Mes∗PCl2 and a propargylic Grignard reagent that in turn was formed from 3-bromo-1,5-
bis(trimethylsilyl)penta-1,4-diyne and Rieke-Mg.
Keywords Phosphaalkene; alkyne; π-conjugation; Rieke magnesium
INTRODUCTION
Phosphaalkenes are one of many interesting classes of organophosphorus com-
pounds.1 By incorporating phosphaalkenes into π-conjugated systems, both we2 and others3
have shown that the phosphorus centers has a profound impact on the frontier molecular
orbitals of the systems, thereby altering their optical and electronic properties. In order
to expand the synthetic procedures to C,C-diacetylenic phosphaalkenes (C,C-A2PAs), we
have developed an alternative route to the bis(trimethylsilyl)-substituted C,C-A2PA which
constitutes an important building block for more elaborate oligomers.2(e,f) The new pro-
cedure omits an acetylenic chloroallene intermediate that has been used previously, but is
sometimes available only in lower yields.
RESULTS AND DISCUSSION
We have recently shown that C,C-A2PA 2 can be synthesized from an allenic chloride
1, whereas propargylic chloride 3 gives rise to butadiyne-substituted phosphaalkene 4
(Scheme 1).2(c) Both chlorides were reacted with Mes∗PCl2 (Mes∗ = 2,4,6-(tert-Bu)3Ph)
Received 12 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.
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
164

SYNTHESIS OF A C,C-DIACETYLENIC PHOSPHAALKENE
165
Scheme 1 Synthesis of C,C-diacetylenic phosphaalkene 2 and butadiyne-substituted phosphaalkene 4.
and elimination of HCl and LiCl was achieved by LDA in order to form the P C double
bond.
In a different approach, we wanted to prepare a Grignard reagent from the propargylic
chloride 3 in order to explore whether it was possible to access a different reactivity as
compared to the LDA procedure, i.e., the reactivity at the central carbon atom and the
formation of C,C-A2PA 2. In the first experiments, it was discovered that the activity of
chloride 3 was not sufficient to allow for the formation of the Grignard reagent, even
when freshly prepared Rieke magnesium was used. Having identified this shortcoming,
propargylic alcohol 5 was reacted with Br2 and PPh3 in CH2Cl2 to afford bromide 6, i.e., a
more reactive starting material for the subsequent Grignard reaction (Scheme 2).4 In analogy
to the procedures reported for the formation of the allenic- and propargylic chlorides,2(c)
the use of CH2Cl2 as solvent gives the propargylic bromide as the major product, which
can be isolated after column chromatography in pentane in 71% yield. The occurrence of
one signal for the TMS groups at δ = 0.20 ppm in the 1H-NMR and δ = –0.5 ppm in the
13C-NMR together with two signals for the acetylenic carbons at δ = 92.2 and 98.9 ppm
confirms the symmetric structure of 6. Subsequently, bromide 6 was dissolved in THF and
added dropwise to a suspension of commerically available Rieke Mg in the same solvent.
The formed Grignard reagent 6’ was transferred to a solution of Mes∗PCl2 in THF at 0◦C.
After stirring at room temperature, DABCO was added to facilitate dehydrochlorination
and the formation of the P=C double bond.
Much to our satisfaction, this procedure yielded A2PA 2 in 23% isolated yield,
with diphosphene, Mes∗P = PMes∗, being the major side product due to the presence of
unreacted Mg.5 As stated above, the use of a Grignard reagent like 6ꢁ favors a fairly different
mechanistic pathway compared to our earlier described method.2c Firstly, the utilization
of a Grignard reagent avoids a proton abstraction which is proposed to be the first step
in the reaction of chlorides 1 and 3 upon treatment with LDA (Scheme 1). Whereas the
lithiated 1 seems to react directly with Mes∗PCl2, the propargyllithium species of 3 is in
an equilibrium with its allenyl lithium species and the reaction occurs at the carbon atom
bonded to the TMS-group, yielding 4.2(c),6 However, this equilibrium does not seem to exist
for the propargylic Grignard reagent 6ꢁ in Scheme 2 or lies on the propargylic side, since no
butadiyne-substituted phosphaalkene 4 that would stem from an allenic magnesiumbromide
species could be detected.
CONCLUSIONS
In conclusion, we have explored an alternative synthetic route to a bis-TMS sub-
stituted C,C-diacetylenic phosphaalkene. The reaction of a propargylic Grignard reagent
with Mes∗PCl2 and subsequent treatment with base gives the bis-TMS substituted C,C-
diacetylenic phosphaalkene. The observed reactivity of 6ꢁ is thus fundamentally different

¨
E. OBERG AND S. OTT
166
to that of the previously used propargylic lithium reagents, which give rise to butadiyne-
substituted phosphaalkenes.
EXPERIMENTAL
NMR Spectroscopy: 1H-NMR spectra were recorded on a JEOL Eclipse + 400 MHz
spectrometer (operating at 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
1
internally to the residual solvent signal (CHCl3, δc = 77 ppm, central signal). 31P-{ H}-
NMR spectra were also measured on JEOL Eclipse + 400 MHz spectrometer (operating at
161.8 MHz).
Compound 5 was prepared using known procedures.2(c)
1,5-Bis(trimethylsilyl)-3-bromopenta-1,4-diyne (6): To a solution of PPh3 (2.07 g,
7.90 mmol) in 50 mL CH2Cl2, Br2 (0.40 mL, 7.90 mmol) was added dropwise at 0◦C.
The solution was stirred for 30 min and 1,5-bis(trimethylsilyl)penta-1,4-diyne-3-ol (1.76 g,
7.85 mmol) was added in portions. The reaction mixture was stirred for 2 h. After addition
of 10% Na2S2O3 (aq) to neutralize unreacted Br2, the phases were separated. The aqueous
phase was extracted with pentane, and the combined organics dried with Na2SO4 and
concentrated in vacuo. Purification by column chromatography (silica, pentane). Yield:
1
1.62 g (5.64 mmol, 71%). H-NMR (400 MHz, CDCl3): δ = 5.17 (s, 1H, CH), 0.20 (s,
18H, CH3). 13C-NMR (75 MHz, CDCl3): δ = 98.9, 92.2, 19.0, -0.5
(Bis(trimethylsilylethynyl))(2,4,6-tri-tert-butylphenyl)phosphaethene (2): To a
suspension of Rieke Mg (1.40 mL of a 25 mg/L Rieke Mg in THF, appr. 1.44 mmol) in THF
(15 mL) at room temperature, 1,5-bis(trimethylsilyl)-3-bromopenta-1,4-diyne (0.139 M in
THF, 1.39 mmol) was added drop wise during 45 minutes. The greenish brown suspension
was stirred for 30 minutes and transferred to a solution of Mes∗PCl2 (0.500 g, 1.44 mmol) at
0◦C. After warming to room temperature and stirring for 3 h, 1,4-diazabicyclo[2.2.2]octane
(0.314 g, 2.80 mmol) was added slowly. The reaction mixture was stirred for 14 h, filtered
through celite, dried with Na2SO4 and concentrated in vacuo. Purification by column chro-
matography (1% EtOAc in pentane) and thereafter the two products (2 and Mes∗P = PMes∗)
ꢀR
were separated by preparative HPLC (Phenomenex Gemini 110A C18, starting mixture
methanol:THF = 95:5 and then a gradient, flow 2 mL/min). Yield: 0.154 g (0.329 mmol,
23%). Analytical data are in accordance to those reported earlier.2c
REFERENCES
1. (a) Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon Copy. From Organophos-
phorus to Phospha-organic Chemistry; Wiley: New York, 1998. (b) Quin, L. D. A Guide to
Organophosphorus Chemistry; Wiley: New York, 2000.
Scheme 2 Synthesis of C,C-diacetylenic phosphaalkene 2 via a Grignard route.

SYNTHESIS OF A C,C-DIACETYLENIC PHOSPHAALKENE
167
2. (a) Geng, X.-L.; Hu, Q.; Schaa¨fer, B.; Ott, S. Org. Lett. 2010, 12, 692-695. (b) Geng, X.-L.;
¨
Ott, S. Chem. Commun. 2009. (c) Oberg, E.; Schaa¨fer, B.; Geng, X.-L.; Pettersson, J.; Hu, Q.;
¨
Kritikos, M.; Rasmussen, T.; Ott, S. J. Org. Chem. 2009, 74, 9265-9273. (d) Scha¨fer, B.; Oberg,
E.; Kritikos, M.; Ott, S. Angew. Chem., Int. Ed. Engl. 2008, 47, 8228-8231. (e) Geng, X.-L.; Ott,
¨
S. Chem. Eur. J. 2011, 17, 12153-12162. (f) Oberg, E.; Geng, X.-L.; Santoni, M.-P.; Ott, S. Org.
Biomol. Chem. 2011, 9, 6246-6255.
3. (a) Bates, J. I.; Dugal-Tessier, J.; Gates, D. P. Dalton Trans. 2010, 39. (b) Gudimetla, V. B.;
Ma, L.; Washington, M. P.; Payton, J. L.; Cather Simpson, M.; Protasiewicz, J. D. Eur. J. Inorg.
Chem. 2010, 2010, 854-865. (c) Smith, Rhett C.; Protasiewicz, John D. Eur. J. Inorg. Chem. 2004,
2004, 998-1006. (d) Smith, R. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 2004, 126, 2268-2269.
(e) Washington, M. P.; Gudimetla, V. B.; Laughlin, F. L.; Deligonul, N.; He, S.; Payton, J. L.;
Simpson, M. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 2010, 132, 4566-4567. (f) Wright, V. A.;
Gates, D. P. Angew. Chem., Int. Ed. Engl. 2002, 41, 2389-2392. (g) Wright, V. A.; Patrick, B. O.;
Schneider, C.; Gates, D. P. J. Am. Chem. Soc. 2006, 128, 8836-8844.
4. Jones, G. B.; Wright, J. M.; Plourde, G. W.; Hynd, G.; Huber, R. S.; Mathews, J. E. J. Am. Chem.
Soc. 2000, 122, 1937-1944.
5. (a) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J. Am. Chem. Soc. 1981, 103,
4587-4589. (b) Arkhypchuk, A. I.; Ott, S. Phosphorus, Sulfur, Silicon Relat. Elem. 2011, 186,
664-665.
6. (a) Reich, H. J.; Holladay, J. E. Angew. Chem., Int. Ed. Engl. 1996, 35, 2365-2367. (b) Reich,
H. J.; Holladay, J. E.; Walker, T. G.; Thompson, J. L. J. Am. Chem. Soc. 1999, 121, 9769-9780.
(c) Furuta, K.; Ishiguro, M.; Haruta, R.; Ikeda, N.; Yamamoto, H. Bull. Chem. Soc. Jpn. 1984,
57, 2768-2776.