Synthesis of pentacarbonyltungsten(0) complexes of bulky 1,2-diphosphabut-1-en-3-ynes as a heavier enyne congener

Article abstract of DOI:10.1016/S0040-4039(02)01008-0

Pentacarbonyltungsten(0) complexes of kinetically stabilized 1,2-diphosphabut-1-en-3-ynes were synthesized as a heavier conjugated enyne system featuring a phosphorus-phosphorus double bond, and characterized by spectroscopic and crystallographic analyses

Full text of DOI:10.1016/S0040-4039(02)01008-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5075–5078
Synthesis of pentacarbonyltungsten(0) complexes of bulky
1,2-diphosphabut-1-en-3-ynes as a heavier enyne congener
Shigekazu Ito, Katsunori Nishide and Masaaki Yoshifuji*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan
Received 17 April 2002; accepted 24 May 2002
Abstract—Pentacarbonyltungsten(0) complexes of kinetically stabilized 1,2-diphosphabut-1-en-3-ynes were synthesized as a
heavier conjugated enyne system featuring a phosphorusꢀphosphorus double bond, and characterized by spectroscopic and
crystallographic analyses. © 2002 Elsevier Science Ltd. All rights reserved.
Considerable attention is being paid to ethynylphosphi-
nes [ꢁPꢀCꢂCꢀ] because of the electronic interaction
between the phosphorus atom and the p-orbital of the
acetylene moiety. It has been known that ethynylphos-
phines show certain changes in spectroscopic and ther-
modynamic properties as well as interconversion of the
skeleton,1–5 and they have been utilized as a source of
some organometallic and cyclic compounds.6–9 On the
other hand, in 1981, we reported the first successful
example of a stable diphosphene 1 (Chart 1) by use of
a kinetic stabilization method with a bulky 2,4,6-tri-t-
butylphenyl group (hereafter abbreviated to Mes*),10
and until now, a number of stable diphosphenes have
been derived as described in the reviews.11–14 Although
the diphosphene moiety can be incorporated with any
conjugated system, the number of conjugated systems
involving the PꢃP unit is quite limited if those with
directly bound aromatic rings are excluded.11,15–17 Tak-
ing these results into consideration, substitution of the
acetylene group on the low-coordinated phosphorus
atom would affect the intriguing properties of the
diphosphene moiety, which would be utilized as a mate-
rial for novel conjugated systems including low-coordi-
nated phosphorus atoms, since conjugated enyne
compounds indicate versatile properties for organic
synthesis18 and antibiotics.19 Here we report the prepa-
ration, structure and some properties of kinetically
stabilized 1,2-diphosphabut-1-en-3-yne compounds (2),
ligating on the tungsten(0) pentacarbonyl.
Taking into account the expected large protecting effect
of the Mes* group, we first chose 1,3,5-tri-t-butyl-2-
ethynylbenzene (3a)20 as the starting acetylene for
ethynylphosphines 4a–6a, and then ethynylbenzene (3b)
for the series of 4b–6b, as shown in Scheme 1.7 The
acetylenes 3 were derived to acetylides, followed by
treatment with chlorobis(dimethylamino)phosphine to
give the corresponding diaminophosphines 4 almost
quantitatively.21 Compounds
4 were treated with
hydrogen chloride in ether to give dichlorophosphines
5, and 5 was allowed to react with lithium aluminum
hydride to afford primary phosphines 6 almost quanti-
tatively.21 Although bulky phosphine 6a was an air-
and moisture-sensitive colorless solid, its polymeriza-
tion was avoided probably due to the bulky sub-
stituents. Phosphine 6b was isolated without
decomposition by extraction with ether.22
Chart 1.
Scheme 1. Preparations of ethtynylphosphines. Reagents and
conditions: (a) (1) n-BuLi/THF/−78°C, (2) ClP(NMe2)2; (b)
HCl/Et2O/−78 to 15°C; (c) LiAlH4/Et2O/−78°C; (d)
W(CO)5(THF)/THF.
Keywords: phosphorus compounds; diphosphenes; alkynes; com-
plexes; steric effects.
* Corresponding author. Fax: (+81) 22 217 6562; e-mail: yoshifj@
mail.cc.tohoku.ac.jp
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01008-0

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S. Ito et al. / Tetrahedron Letters 43 (2002) 5075–5078
10
28
29
,
,
,
Next, attempts were made to prepare diphosphene 2a
by a similar preparative method for unsymmetrical
diphosphene 7,23 but either from combination of 5a and
8–10 or 6a and 11 (Chart 2), the attempts failed to give
2a, and the decomposition of 5a and 6a was observed.
Accordingly, we tried to prepare 2a in a complexed
form. Thus, 6a was allowed to react with an equivalent
amount of W(CO)5(THF) to afford the corresponding
complex 6aW21 in 35% isolated yield after SiO2 column
chromatography as an air-stable orange solid (Scheme
2). According to the method for complex 7W,24 com-
plex 6aW was allowed to react with dichlorophosphine
11 in the presence of DBU (diazabicyclo[5.4.0]undec-7-
ene) to give the corresponding carbonyltungsten com-
plex 2aW25 as an air-stable orange solid. Complex 2aW
was isolated after SiO2 column chromatography (hex-
ane) followed by gel permeation chromatography (chlo-
roform) in 20% yield from 3a (Scheme 2). Moreover,
complex 6bW, prepared in a similar manner, afforded
the corresponding diphosphene complex 2bW25 in 10%
isolated yield from 3b (Scheme 2). Complex 2bW can be
handled in air, although it decomposes on an SiO2
column. It seems likely that the bulky Mes* group as a
substituent on the acetylene is not essential for stabi-
lization of either 2W or 6W. On the other hand, the
reaction of 5 with phosphine complex 8W did not give
the diphosphene complex, probably due to the steric
encumbrance.
[2.034(2) A], 12 [2.025(3) A], 13 [2.041(4) A], and
,
the present 2aW (2.037(4) A). The Mes* ring on the
phosphorus atom is almost perpendicular to the
CMes*ꢀPꢃP(W)ꢀCꢂCꢀCAr plane (Ar=Mes* or Ph)
[CMes*ꢀP1ꢀP2(W)ꢀC1ꢀC2ꢀCAr versus Mes*(P): 85.4(3)°
for 2aW and 89.6(4)° for 2bW], suggesting its sterically
effective protection of the PꢃP skeleton. On the con-
trary, the Ar ring on the acetylene unit is considerably
planar
to
the
CMes*ꢀPꢃP(W)ꢀCꢂCꢀCAr
plane
[CMes*ꢀPꢃP(W)ꢀCꢂCꢀCAr versus Ar(C): 19.7(3)° for
2aW and 12.2(2)° for 2bW], in contrast to the average
torsion angle of 64° observed for 1 in the solid state.10,11
Steric congestion around the P2 atom in 2aW or 2bW is
released due to an acetylene spacer, and the aromatic
ring on the acetylene side apparently tends to take
co-planarity with the PꢃPꢀCꢂC group.
31P NMR spectra of 2W display a typical AB pattern,
which is similar to diphosphene complex 7W,24 indicat-
ing an E-configuration from its chemical shifts and an
end-on coordination on W(CO)5 from its JPW values.26
UV–vis spectra of 2W show longer umax values [457 nm
(2aW), 462 nm (2bW)] than that for 7W (436 nm),24
probably due to the effect of the acetylene unit on the
extended conjugation.
Figure 1. An ORTEP drawing of the molecular structure of
2aW. Hydrogen atoms are omitted for clarity. Selected bond
,
lengths (A) and angles (°): P2ꢀW 2.475(3), P1ꢀP2 2.038(4),
P1ꢀC(Mes*) 1.829(10), P2ꢀC1 1.745(10), C1ꢀC2 1.19(1),
C2ꢀC(Mes*) 1.43(1), C(Mes*)–P1–P2 97.3(3), P1–P2–C1
101.3(4), P1–P2–W 142.5(1), C1–P2–W 116.0(4), P2–C1–C2
175.8(9), C1–C2–C(Mes*) 177.3(10).
The structures of 2aW and 2bW were confirmed by
X-ray crystallographic analyses, as shown in Figs. 1
and 2.27 The CMes*ꢀPꢃP(W)ꢀCꢂCꢀCAr systems are pla-
nar. The PꢀW distances of 2aW and 2bW are both
,
2.475 A, which are comparable to those of 12 [2.456(2)
28
29
,
,
A] and 13 (Chart 3) [2.484(3) and 2.491(3) A]. The
complex 2bW showed the longest PꢃP bond length
,
[2.049(3) A] ever reported for diphosphenes, i.e. 1
Figure 2. An ORTEP drawing of the molecular structure of
2bW. Hydrogen atoms are omitted for clarity. The p-t-butyl
group in the Mes* group is disordered and the atoms with a
predominant occupancy factor (0.57), which is refined
Chart 2.
,
isotropically, are shown. Selected bond lengths (A) and angles
(°): P2ꢀW 2.475(2), P1ꢀP2 2.049(3), P1ꢀC(Mes*) 1.855(7),
P2ꢀC1 1.739(7), C1ꢀC2 1.21(1), C2ꢀC(Mes*) 1.448(10),
C(Mes*)ꢀP1ꢀP2 102.9(2), P1ꢀP2ꢀC1 98.8(3), P1ꢀP2ꢀW
143.4(1),
C1ꢀC2ꢀC(Mes*) 175.5(8).
C1ꢀP2ꢀW
116.8(3),
P2ꢀC1ꢀC2
175.6(7),
Scheme 2. Preparation of diphosphene complexes 2W.

S. Ito et al. / Tetrahedron Letters 43 (2002) 5075–5078
14. Yoshifuji, M. J. Organomet. Chem. 2000, 611, 210.
5077
15. Appel, R.; Niemann, B.; Schuhn, W.; Knoch, F.
Angew. Chem., Int. Ed. Engl. 1986, 25, 932.
16. Appel, R.; Niemann, B.; Nieger, M. Angew. Chem., Int.
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19. Maier, M. E. Synlett 1995, 13.
Chart 3.
20. Zimmermann, H. E.; Dodd, J. R. J. Am. Chem. Soc.
1970, 92, 6507.
21. NMR data. 4a: 31P{1H} NMR (81 MHz, CD2Cl2) l
76.2; 1H NMR (200 MHz, CD2Cl2) l 1.34 (9H, s, p-t-
Diphosphene complex 2aW was isomerized by photo-
irradiation. Complex 2aW in benzene-d6 was irradiated
with a medium-pressure mercury lamp through a Pyrex
filter to afford an E/Z mixture in a 4:1 ratio after 3
days, as shown in Scheme 2. The Z-isomer (2aW%)
displays an AB pattern with a higher 31P chemical shift
4
Bu), 1.58 (18H, s, o-t-Bu), 2.85 (12H, d, JHH 12 Hz,
NMe2), 7.37 (2H, s, arom); 13C{1H} NMR (50 MHz,
1
2
CD2Cl2) l 103.5 (d, JPC 16 Hz, PCꢂC), 107.6 (d, JPC
7 Hz, PCꢂC). 5a: 31P{1H} NMR (81 MHz, CD2Cl2) l
123.0; 1H NMR (200 MHz, CD2Cl2) l 1.35 (9H, s,
p-t-Bu), 1.58 (18H, s, o-t-Bu), 7.24 (2H, s, arom);
1
and a larger JPP value than those for the E-isomer
1
[2aW%: lP 268, 388 (1JPW 254 Hz), JPP 549 Hz].30
1
13C{1H} NMR (100 MHz, CD2Cl2) l 99.3 (d, JPC 75
Attempts to obtain free diphosphene 2a by decomplex-
ation of 2aW are now in progress.
2
Hz, PCꢂC), 118.1 (d, JPC 5 Hz, PCꢂC). 6a: 31P NMR
1
1
(81 MHz, CD2Cl2) l −176.0 (t, JPH 214 Hz); H NMR
(400 MHz, CD2Cl2) l 1.34 (9H, s, p-t-Bu), 1.55 (18H,
1
s, o-t-Bu), 3.95 (2H, d, JPH 214 Hz, PH2), 7.36 (2H, s,
Acknowledgements
arom); 13C{1H} NMR (50 MHz, CD2Cl2) l 90.2 (d,
2
1JPC 9 Hz, PCꢂC), 106.1 (d, JPC 2 Hz, PCꢂC). 6aW:
1
31P{1H} NMR (81 MHz, CD2Cl2) l −139.0 (t, JPH 373
This work was supported in part by a Grant-in-Aid for
Scientific Research (No. 1334049) from the Ministry of
Education, Culture, Sports, Science and Technology,
Japan.
1
Hz; JPW 228 Hz); 1H NMR (200 MHz, CD2Cl2) l
1.35 (9H, s, p-t-Bu), 1.57 (18H, s, o-t-Bu), 5.40 (2H, d,
1JPH 373 Hz, PH2), 7.40 (2H, s, arom); 13C{1H} NMR
1
(50 MHz, CD2Cl2) l 86.3 (d, JPC 86 Hz, PCꢂC), 109.1
2
(d, JPC 13 Hz, PCꢂC).
22. Guillemin, J.-C.; Savignac, P.; Denis, J.-M. Inorg.
Chem. 1991, 30, 2170.
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(1JPW 252 Hz), JPP 467 Hz; 1H NMR (400 MHz,
CDCl3) l 1.34 (9H, s, p-t-Bu), 1.54 (18H, s, o-t-Bu),
7.53 (2H, s, Mes*), 7.2–7.7 (5H, m, Ph); UV–vis
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(
,
P1 (c2), a=14.530(4), b=15.24(1), c=11.712(4) A,
h=106.26(5), i=111.15(2), k=100.64(4)°, V=2200(2)
3
A , Z=2, zcalcd=1.36 g cm−1, v(MoKa)=2.74 mm−1
,
,
T=140 K, 6780 total reflections, 6490 unique reflec-

5078
S. Ito et al. / Tetrahedron Letters 43 (2002) 5075–5078
tions [I>2.0|(I)], 2q max=50.1°, R1=0.072, RW=0.115
(all data), S=1.58 for 464 parameters (CCDC 176662).
for 350 parameters (CCDC 178461).
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(
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2bW: C31H34O5P2W, M=732.40, triclinic, P1 (c2), a=
,
11.750(2), b=15.57(1), c=9.672(3) A, h=98.52(3), i=
3
,
113.04(2),
k=76.33(3)°,
V=1578(1)
A ,
Z=2,
zcalcd=1.54 g cm−1, v(MoKa)=3.80 mm−1, T=140 K,
4880 total reflections, 4642 unique reflections [I>2.0|(I)],
2q max=50.1°, R1=0.050, RW=0.129 (all data), S=1.27