Preparations and properties of polymers containing 3,4-bis[(2,4,6-tri-t- butylphenyl)phosphinidene]-1,2-di(2-thienyl)cyclobutene moieties

Article abstract of DOI:10.1016/j.tetlet.2004.08.108

A polymer containing diphosphinidenecyclobutene units was prepared and its properties were studied.

Full text of DOI:10.1016/j.tetlet.2004.08.108

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 7609–7612
Preparations and properties of polymers containing
3,4-bis[(2,4,6-tri-t-butylphenyl)phosphinidene]-
1,2-di(2-thienyl)cyclobutene moieties
Kozo Toyota, Junichi Ujita, Subaru Kawasaki, Keita Abe, Naoki Yamada and
Masaaki Yoshifuji*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan
Received 29 June 2004; revised 12 August 2004; accepted 19 August 2004
Abstract—Reaction of 1,8-bis[5-{(2,4,6-tri-t-butylphenyl)phosphinoethynyl}-2-thienyl]octane with butyllithium followed by treat-
ment with 1,2-dibromoethane afforded a new polymer containing 3,4-bis[(2,4,6-tri-t-butylphenyl)phosphinidene]-1,2-di(2-thien-
yl)cyclobutene units. The polymer was allowed to react with bis(benzonitrile)dichloropalladium to give the polymer complex.
A Sonogashira coupling reaction between ethynyltrimethylsilane and 4-bromonitrobenzene proceeded in DMF at 100°C to give
4-nitro(trimethylsilylethynyl)benzene in the presence of the polymer complex, CuI, and triethylamine.
Ó 2004 Elsevier Ltd. All rights reserved.
Sterically protected 3,4-diphosphinidenecyclobutenes
(abbreviated as DPCB),1 bearing an extremely bulky
2,4,6-tri-t-butylphenyl group2 (hereafter abbreviated as
Mes*), are unique ligands of interest,3 because of their
relatively rigid framework containing the phosphorus–
carbon p-bond.4 Various transition metal complexes of
DPCB have been prepared3 and some of them have been
used as homogeneous catalysts.5,6 In the course of our
studies on DPCB derivatives, we have recently reported
the preparations of DPCB derivatives 1a,b as well as a
DPCB polymer 2.1f,g,h In contrast to successful applica-
tion of phosphine complexes to polymer-supported
catalysts such as polystyrene-supported triarylphos-
phine–palladium complex,7 polymers containing dou-
ble-bonded phosphorus1f,8 are rare and their catalytic
activities have remained unexplored. Preparations and
applications of polymeric (or oligomeric) DPCB deriva-
tives and their transition metal complexes are thus of
interest, from the viewpoint of synthetic organic chemis-
try as well as materials science, because DPCB–metal
complexes have unique catalytic activities, due to their
remarkable p-back-donation character.9 In addition,
the DPCB-polymer metal complexes are expected to
act as heterogeneous catalysts having the advantage of
easy separation from the reaction product.10 We now
report here the preparation and properties of the first
DPCB linear polymer and its dichloropalladium com-
plex.
Mes*
P
Mes*
P
S
S
P
Mes*
P
Mes*
1a
1b
Mes* = 2,4,6-t-Bu3C6H2
Mes*
P
(CH2)4
(CH2)4
P
Mes*
n
2
Keywords: Polymers; Steric and strain effects; Phosphaalkenes.
*
At first, in the preparation of 1a,1g we found that a small
amount (ca. 5% yield) of oligomeric DPCB derivative 3
Corresponding author. Tel.: +81 22 217 6558; fax: +81 22 217
6562; e-mail: yoshifj@mail.tains.tohoku.ac.jp
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.08.108

7610
K. Toyota et al. / Tetrahedron Letters 45 (2004) 7609–7612
i) n-BuLi
ii) BrCH2CH2Br
Mes*P(H)
Mes*P(H)
CH2
(CH2)4
CH2
i) n-BuLi
ii) BrCH2CH2Br
4A
Mes*
P
P Mes*
P
Mes* Mes*
P
1a
+
(CH2)6
S
S
n
n
3
CH2
(CH2)7
9
n
Scheme 2.
Mes*
CH2
P
P
Mes*
a mixture containing polymer 9, probably via 10 and
11, a cyclic DPCB derivative 12 (dP = 164.1), and phos-
(CH2)5
3
phaallene 13 [dP = 80.6 (d, JPH = 26.6Hz)].15
Scheme 1.
Table 1 shows the results of the polymerization reac-
tions. The reaction in DME (entry 6) gave the best
result, taking the distribution of molecular weight of
the polymer into account. The crude product obtained
in entry 6 was then separated by GPC. From an early
fraction a highest-molecular-weight-polymer 90 was
obtained (ca. 9% isolated yield, Mn = 58,000, Mw/
Mn = 7.8) and used for evaluation of properties.16
[Mn = 2300, Mw = 2700]11 was formed as by-product
(Scheme 1). In this reaction, intramolecular coupling
dominated, probably because formation of eight-mem-
bered rings are energetically favored, to give 1a (8% iso-
lated yield) with the normal reaction mechanism for
DPCB formation, via a di(phosphaallene) intermediate.1
We then planned an polymer formation, in order to in-
crease the yield and average molecular weight of the poly-
mers: introduction of rigid moieties, such as aromatic
rings, into methylenes may limit the conformational
flexibility of the alkyl chain and consequently prevent
intramolecular coupling. We thus investigated coupling
reactions of a,x-bis[5-{(2,4,6-tri-t-butylphenyl)phosphino-
ethynyl}-2-thienyl]alkanes 4 as follows: a,x-di(2-thi-
enyl)alkanes 5A–C12 were allowed to react with N-iodo-
succinimide or n-BuLi/I2 to give 6A–C. The diiodo
derivatives 6A–C were converted to 7A–C by Sono-
gashira coupling, and then to 8A–C by desilylation
reaction of 7A–C with KOH. Metallation of 8A–C with
ethylmagnesium bromide followed by reaction with
Mes*
S
Mes*
S
P P
CH2
(CH2)7
n
n
10
Mes*
P
P Mes*
chloro-(2,4,6-tri-t-butylphenyl)phosphine13
afforded
S
S
CH2
(CH2)7
a,x-bis[5-{(2,4,6-tri-t-butylphenyl)phosphinoethynyl}-
2-thienyl]alkane 4A–C. The compound 4A14 was oily
and soluble in chloroform and EtOAc, while the solids
4B,C, once formed, were sparingly soluble in common
solvents, such as chloroform and DME. Attempted
polymerizations of 4B,C thus failed.
11
Mes*
P
Mes*
P
CH2
CH2
(CH2)6
CH2
CH
CH
S
S
S
S
(CH2)6
CH2
P
Mes*
P
Mes*
R
(CH2)n
R
S
S
12
13
4A: n=8, R=Mes*PH; 4B: n=6, R=Mes*PH;
4C: n=4, R=Mes*PH
7A: n=8, R=Tms; 7B: n=6, R=Tms; 7C: n=4, R=Tms
8A: n=8, R=H; 8B: n=6, R=H; 8C: n=4, R=H
PdCl2
P
Mes*
P
Mes*
PdCl2
R
(CH2)n
R
S
S
S
S
5A: n=8, R=H; 5B: n=6, R=H; 5C: n=4, R=H
6A: n=8, R=I; 6B: n=6, R=I; 6C: n=4, R=I
CH2
(CH2)7
n
14
In contrast, polymers of 4A were successfully obtained
as follows (Scheme 2): Lithiation of the diphosphane
4A with butyllithium (2molar ratio) followed by treat-
ment with 1,2-dibromoethane (1molar ratio) afforded
The 31P NMR(162MHz, CDCl 3) spectrum of 90 (Fig. 1)
indicated that most of the DPCB units in the polymer

K. Toyota et al. / Tetrahedron Letters 45 (2004) 7609–7612
Table 1. Result of polymerization reaction of 4A
7611
Entry
Solvent
Concn (moldmꢀ3
)
Ratio of producta
9
12
13
4A (Recov.)
1
2
3
4
5
6
7
8
9
THF
THF
THF
0.1
0.5
1.0
0.5
0.5
0.5
0.5
0.5
0.5
48
24
17
9
29
0
0
0
83b
43
48
3
0
THF + HMPA
2-Methyltetrahydrofuran
DME
35
10
17
18
11
1
52
6
55c
53d
42
22
26
47
15
85
3
1,4-Dioxane
Et2O
Toluene
0
30
14
54
1
0
a Estimated based on 31P NMRpeak heights.
b Mn = 1400, Mw/Mn = 6.4, based on GPC (gel permeation chromatography) analysis (CHCl3, polystyrene standard).
c Mn = 21,000, Mw/Mn = 23, based on GPC analysis.
d Mn = 27,400, Mw/Mn = 33, based on GPC analysis.
Cl4P2Pd2S2)n: C, 54.60; H, 6.38; Cl, 11.51; S, 5.21%]
showed relatively good agreement with the calculated
value of ideal polymer composition 14 containing DPCB
moieties and PdCl2 moieties in 1:2 ratio. The result of
elemental analysis also indicates that nitrogen atoms
derived from benzonitrile in the starting PdCl2(PhCN)2
did not remain in the polymer complex. Although no
information about the coordination state in the polymer
has been obtained due to the insolubility, we assume
incorporation of PdCl2 in addition to the normal chelate
coordination of PdCl2 at the sp2-phosphorus atoms.
For the purpose of testing the catalytic activity of the
DPCB-polymer palladium complex (14), the coupling
reaction
between
4-bromonitrobenzene
(61mg,
0.30mmol) and ethynyltrimethylsilane (0.60mol) was
tried, in the presence of 14 (5mg), CuI (0.018mmol),
and triethylamine (0.13mL) in DMF (1.5mL) at
100°C for 24h to give 4-nitro(trimethylsilylethynyl)benz-
ene in 83% yield (average of two runs, Scheme 3). It
should be noted that 14 acted essentially as a hetero-
geneous catalyst because it is insoluble in DMF at
100°C, while its activity was comparable to that of the
homogeneous monomeric 1b–PdCl2 catalyst [71% yield
of 4-nitro(trimethylsilylethynyl)benzene under similar
conditions].18 The complex 14 is easily removed from
reaction media in work-up, which merits application
of 14.
Figure 1. P{1H} NMRspectrum of 90 in CDCl3.
31
adopt (E,E)-configuration: The spectrum showed a major
signal due to phosphorus atoms in the (E,E)-DPCB
units (dP 165), two weak signals due to the (E,Z)-DPCB
units (dP 174 and 191, JPP is not clear because of broad-
ening), and some high-field signals (ꢀ44, ꢀ51, ꢀ57,
ꢀ72, ꢀ100) probably due to the phosphorus atoms in
the terminal moieties. It is likely that the partial E/Z-
isomerization of the DPCB units in 90 occurred after
formation of the polymer by light or a trace of Lewis
acid,17 because the reactions of ethynylphosphines with
butyllithium and 1,2-dibromoethane generally proceed
stereospecifically to give only the (E,E)-isomers.1b
In summary, we have prepared a new DPCB polymer
and its dichloropalladium complex. The catalytic
activities of the DPCB-polymer Pd-complex have
been preliminarily shown. Further investigations on
preparations, structures, and applications of DPCB-poly-
mer Pd-complexes are now in progress.
We then investigated the preparation of a DPCB-poly-
mer transition-metal complex and its catalytic activity.
Dichloropalladium complexes of some monomeric
DPCB derivatives have been known to catalyze the
Sonogashira reaction of trimethylsilylacetylene and 4-
bromonitrobenzene.5a Thus we prepared a DPCB-poly-
mer palladium complex and applied it to the Sono-
gashira reaction, as follows: The polymer 90 was allowed
to react with PdCl2(PhCN)2, in a 1:1 mixture of THF
and dichloromethane at ambient temperature for
1day, to give a brown solid insoluble in common sol-
vents. Elemental analysis of this solid [Found: C,
47.92; H, 5.69; Cl, 12.74; S, 5.50%. Calcd for (C56H78-
Br
NO2
Me3Si
H
+
14, CuI, Et3N
Me3Si
NO2
DMF
Scheme 3.

7612
K. Toyota et al. / Tetrahedron Letters 45 (2004) 7609–7612
Nishide, K.; Ito, S.; Yoshifuji, M. Tetrahedron Lett. 2003,
44, 8297.
Acknowledgements
7. (a) Terasawa, M.; Kaneda, K.; Imanaka, T.; Teranishi, S.
J. Organomet. Chem. 1978, 162, 404; (b) Uozumi, Y.;
Danjo, H.; Hayashi, T. J. Org. Chem. 1999, 64, 3384; (c)
Parrish, C. A.; Buchwald, S. L. J. Org. Chem. 2001, 66,
3820, and references therein.
This work was supported in part by the Grants-in-Aid
for Scientific Research (Nos 13304049, 14044012,
16033207, and 50217569) from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology, Japan.
8. (a) Smith, R. C.; Chen, X.; Protasiewicz, J. D. Inorg.
Chem. 2003, 42, 5468; (b) Smith, R. C.; Protasiewicz, J. D.
J. Am. Chem. Soc. 2004, 126, 2268.
Supplementary data
9. Ozawa, F.; Kawagishi, S.; Ishiyama, T.; Yoshifuji, M.
Organometallics 2004, 23, 1325.
10. For a review, see: Shuttleworth, S. J.; Allin, S. M.; Wilson,
R. D.; Nasturica, D. Synthesis 2000, 1035.
11. Molecular weight was determined by GPC (solvent:
CHCl3) using a polystyrene standard.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2004.08.108. Experimental procedures and spectro-
scopic data for compounds (E,Z)-1b, 4A–C, 6A–C,
7A–C, 8A–C, 90, 12, and 14.
12. (a) Kossmehl, G.; Hoppe, F. D.; Hirsch, B. Z. Natur-
forsch. B. 1993, 48, 826; (b) Schuetz, R. D.; Baldwin, R. A.
J. Org. Chem. 1962, 27, 2841; (c) Benoit, P.; Collignon, N.
Bull. Soc. Chim. Fr. 1975, 1302.
References and notes
1. (a) Appel, R.; Winkhaus, V.; Knoch, F. Chem. Ber. 1987,
120, 243; (b) Yoshifuji, M.; Toyota, K.; Murayama, M.;
Yoshimura, H.; Okamoto, A.; Hirotsu, K.; Nagase, S.
Chem. Lett. 1990, 2195; (c) Toyota, K.; Tashiro, K.;
Yoshifuji, M.; Nagase, S. Bull. Chem. Soc. Jpn. 1992, 65,
2297; (d) Toyota, K.; Tashiro, K.; Abe, T.; Yoshifuji, M.
Heteroat. Chem. 1994, 5, 549; (e) Ma¨rkl, G.; Hennig, R.
Justus Liebigs Ann. Chem. 1996, 2059; (f) Yamada, N.;
Toyota, K.; Yoshifuji, M. Chem. Lett. 2001, 248; (g)
Yamada, N.; Abe, K.; Toyota, K.; Yoshifuji, M. Org.
Lett. 2002, 4, 569; (h) Toyota, K.; Abe, K.; Horikawa, K.;
Yoshifuji, M. Bull. Chem. Soc. Jpn. 2004, 77, 1377.
2. (a) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.;
Higuchi, T. J. Am. Chem. Soc. 1981, 103, 4587; (b)
Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.;
Higuchi, T. J. Am. Chem. Soc. 1982, 104, 6167; (c)
Yoshifuji, M. J. Organomet. Chem. 2000, 611, 210.
3. (a) Toyota, K.; Tashiro, K.; Yoshifuji, M. Chem. Lett.
1991, 2079; (b) Yoshifuji, M. Synth. Org. Chem. Jpn. 2003,
61, 1116, and references cited therein.
4. Multiple Bonds and Low Coordination in Phosphorus
Chemistry; Regitz, M., Scherer, O. J., Eds.; George
Thieme: Stuttgart, 1990.
5. (a) Toyota, K.; Masaki, K.; Abe, T.; Yoshifuji, M. Chem.
Lett. 1995, 221; (b) Ikeda, S.; Ohhata, F.; Miyoshi, M.;
Tanaka, R.; Minami, T.; Ozawa, F.; Yoshifuji, M. Angew.
Chem., Int. Ed. 2000, 39, 4512; (c) Ozawa, F.; Okamoto,
H.; Kawagishi, S.; Yamamoto, S.; Minami, T.; Yoshifuji,
M. J. Am. Chem. Soc. 2002, 124, 10968.
13. Ma¨rkl, G.; Kreitmeier, P. Angew. Chem., Int. Ed. Engl.
1988, 27, 1360.
14. Selected data for 4A: brown oil; 1H NMR(400MHz,
CDCl3) d = 1.21–1.68 (12H, m, CH2), 1.34 (18H, s, p-t-
Bu), 1.68 (36H, s, o-t-Bu), 2.73 (4H, t, JHH = 7.5Hz,
3
1
CH2), 5.96 (2H, d, JPH = 249.5Hz, PH), 6.58 (2H, d,
3
3JHH = 3.6Hz, thiophene), 6.95 (2H, d, JHH = 3.6Hz,
thiophene), and 7.51 (4H, s, arom.); 13C{1H} NMR
(100MHz, CDCl3) d = 28.8, 29.0, 30.0, 31.2, 31.4, 33.6,
1
35.0, 38.4, 91.4 (d, JPC = 22.3Hz, C„C), 96.0 (s, C„C),
120–127, 132.2, 148.1, 150.5, and 155.5 (d,
2JPC = 10.6Hz); 31P NMR(162MHz, CDCl ) d = ꢀ100.1
3
1
(d, JPH = 249.5Hz). FAB-MS m/z 877 (M+ꢀ1) and 275
(Mes*P+ꢀ1). Found: m/z 878.5171. Calcd for C56H80P2S2:
M, 878.5173.
15. The coupling reaction of lithium phosphide (generated by
the reaction of 4A with n-BuLi) seemed to be rather
complicated, because the lithium (thienylethynyl)phos-
phide isomerized to phosphaallenyllithium species, which
also formed DPCB derivative by treatment with 1,2-
dibromoethane.1h This may be the reason for the large
Mw/Mn ratio.
16. Selected data for 90: brown solid; UV (CH2Cl2) 247, 293,
344, and 445 (sh) nm; IR(KBr) 2951, 2158, 1589, 1462,
1394, 1360, 1238, 1207, 1174, 1120, 874, and 798cmꢀ1
.
Found: C, 73.22; H, 8.59; S, 7.33%. Calcd for
(C56H78P2S2)n : C, 76.67; H, 8.96; S, 7.31%.
17. In fact, monomeric (E,E)-1b was converted to (E,Z)-1b by
photo-irradiation or addition of a catalytic amount of
3
6. (a) For catalytic activities of compounds containing
phosphorus–carbon p-bond in a molecule, see: Weber, L.
Angew. Chem., Int. Ed. 2002, 41, 563, and references cited
therein; (b) Ionkin, A.; Marshall, W. Chem. Commun.
2003, 710; (c) Keim, W.; Appel, R.; Gruppe, S.; Knoch, F.
Angew. Chem., Int. Ed. Engl. 1987, 26, 1012; (d) Liang, H.;
iodine. (E,Z)-1b: dP 177.9 and 195.9, AB, JPP = 17.2Hz.
18. Similarly to the cases of the previously reported DPCB–
PdCl2 complexes,5a the catalytic activities of 14 and 1b-
PdCl2 are not as high as that of (Ph3P)2PdCl2 complex,
which works at room temperature in the Sonogashira
reaction.