Copper-mediated coupling reaction of kinetically stabilized 1-methyl-2-phosphaethenyllithiums involving an isomerization to a 3-phospha-2-propenyllithium derivative

Article abstract of DOI:10.1246/cl.2002.708

Copper-mediated coupling reaction of 1-methyl-2-(2,4,6-tri-t-butylphenyl)-2-phosphaethenyllithiums afforded 1,4-dipho-sphabuta-1,3-diene or 1,6-diphosphahexa-1,5-diene derivatives, depending upon non-coordinated or coordinated to W(CO)₅, which facilitates an isomerization to a 3-phospha-2-propenyl-lithium.

Full text of DOI:10.1246/cl.2002.708

708
Chemistry Letters 2002
Copper-mediated Coupling Reaction of Kinetically Stabilized 1-Methyl-2-phosphaethenyllithiums
Involving an Isomerization to a 3-Phospha-2-propenyllithium Derivative1
Shigekazu Ito, Shigeo Kimura, and Masaaki Yoshifujiꢀ
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578
(Received April 11, 2002; CL-020307)
Copper-mediated coupling reaction of 1-methyl-2-(2,4,6-tri-
t-butylphenyl)-2-phosphaethenyllithiums afforded 1,4-dipho-
sphabuta-1,3-diene or 1,6-diphosphahexa-1,5-diene derivatives,
depending upon non-coordinated or coordinated to W(CO)5,
which facilitates an isomerization to a 3-phospha-2-propenyl-
lithium.
with a torsion angleof 174.6(1) ꢁ inthe P=C–C=P system (Figure
ꢀ
1). The P=C bond lengths [1.703(2) and 1.699(2) A] and the
ꢀ
(P)C–C(P) bond length [1.465(3) A] are slightly longer than the
8
ꢀ
corresponding distances for 2 (X=Cl) [P=C 1.691(4) A; C–C
3a
ꢀ
1.445(7) A], probably due to an electron donating effect of the
methyl groups.
We and others have reported various types of stable multiple
bonds of phosphorus by utilizing the kinetic stabilization
method.2 Phosphaethenyllithiums 1 bearing the 2,4,6-tri-t-
butylphenyl (hereafter abbreviated to Mesꢀ) group are suitable
reagents for synthesis of some low-coordinated phosphorus
compounds, and we have studied copper-mediated coupling
reactions of 1, where X¼Cl, Br, SPh, and STol, i. e. lithium
phosphanylidene carbenoids, affording 1,4-diphosphabuta-1,3-
diene derivatives 2 as a conjugated system involving the P=C
moieties.2;3 Recently, a considerable attention has been paid to
coordination chemistry of multiple-bonded phosphorus com-
pounds to develop a novel class of synthetic catalyst,4 and we
expected to utilize 2 for a ligand of metal complexes as an analog
of 3,4-diphosphanylidenecyclobutene derivatives 3.4c{f,5
Although 2 displayed quite low reactivity to the transition metal
such as tungsten or chromium, or undesirable reactivity to some
palladium or platinum reagents, an alternative method was
employed to synthesize phosphaethenyllithiums in a transition-
metal complex form exemplified such as 4.6 In this paper we
report copper-mediated coupling reaction of a bulky 1-methyl-2-
phosphaethenyllithium in a free form (5) or a coordinated form on
the tungsten metal (5w).
Figure 1. Molecular structure of EE-7.
Coordination chemistry of 7 is of interest, because compound
3, including the 1,4-diphosphabuta-1,3-diene skeleton, has been
utilized as ligands for synthetic catalysts.4 But the coordinating
property of 7 on transition metals turned out to be very poor by the
fact that 7 did not coordinate on the transition-metals such as
group 6 or group 10 metals. Thereupon we attempted to couple 5
on a transition metal. Compound 6 was allowed to react with
pentacarbonyl(tetrahydrofuran)tungsten(0) to afford the corre-
sponding complex 6w in a moderate yield (60%).10 In an attempt
to obtain complex 7w, 6w was allowed to react with butyllithium
and copper(II) chloride, but the product was not 7w but 8 in 30%
yield as purple crystals,11 indicating a [1,2]-hydrogen rearrange-
ment to leading to the 1,6-diphosphahexa-1,5-diene skeleton
through an intermediate (50w) (Scheme 1). Oxygen was not used
in this coupling reaction to prevent decomplexation of 6w. The
structure of 8 was confirmed by X-ray crystallography as shown
in Figure 2,11 displaying a ZZ configuration. The P=C length
(Z)-2-Bromo-1-(2,4,6-tri-t-butylphenyl)-1-phosphapropene
(6) was allowed to react with butyllithium to afford 2-lithio-1-
phosphapropene 5,7 which was treated with copper(II) chloride
and oxygen at À78 ꢁC to give 2,3-dimethyl-1,4-diphosphabuta-
1,3-diene 7 as a mixture of three isomers (EE-, ZZ-, and EZ-7)
almost quantitatively (Scheme 1). The ratio of the isomers was
EE : ZZ : EZ ¼ 1 : 1 : 3, which was determined by 31P NMR
spectroscopy.8 The mixture 7 was irradiated by a medium-
pressure mercury lamp through a Pyrex filter (ꢀ > 300 nm) or
heated in refluxing toluene to afford the single isomer (EE-7),
whereas (EZ)-1,4-bis(2,4,6-tri-t-butylphenyl)-1,4-diphosphabu-
ta-1,3-diene was reported to afford the ZZ-isomer by irradiation or
heating by Appel et al.9 The structure of EE-7 was confirmed by
X-ray crystallography revealing a planar s-trans conformation
Scheme 1. Copper-mediated coupling reaction of 1-methyl-2-
phosphaethenyllithiums. Reagents and conditions: a) n-BuLi,
THF, À78 ꢁC; b) CuCl2, O2, À78 ꢁC; c) Medium pressure Hg lamp
(100 W), benzene, 0 ꢁC; d) toluene, reflux; e) W(CO)5(thf), rt; f)
CuCl2.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
709
Yoshifuji, Chem. Lett., 2000, 1390.
4
a) B. Breit, J. Mol. Catal. A: Chem., 143, 143 (1999). b) R. Shintani,
M. M.-C. Lo, and G. C. Fu, Org. Lett., 2, 3695 (2000). c) K. Toyota, K.
Masaki, T. Abe, and M. Yoshifuji, Chem. Lett., 1995, 221. d) S. Ikeda,
F. Ohhata, M. Miyoshi, R. Tanaka, T. Minami, F. Ozawa, and M.
Yoshifuji, Angew. Chem. Int. Ed., 39, 4512(2000). e) F. Ozawa, S.
Yamamoto, S. Kawagishi, M. Hiraoka, S. Ikeda, T. Minami, S. Ito, and
M. Yoshifuji, Chem. Lett., 2001, 972. f) T. Minami, H. Okamoto, S.
Ikeda, R. Tanaka, F. Ozawa, and M. Yoshifuji, Angew. Chem. Int. Ed.,
40, 4501 (2001). g) L. Weber, Angew. Chem. Int. Ed., 41, 563 (2002).
N. Yamada, K. Abe, K. Toyota, and M. Yoshifuji, Org. Lett., 4, 569
(2002).
5
6
Figure 2. Molecular structure of 8.
[1.660(7) A] is comparable to the corresponding value of 9
a) P. Le Floch, D. Carmichael, and F. Mathey, Organometallics, 10,
2432 (1991). b) H. Teunissen and F. Bickelhaupt, Organometallics,
15, 794 (1996).
ꢀ
12
ꢀ
[1.635(4) A], and the P–W bond length [2.539(2) A] is similar to
ꢀ
7
8
a) S. Ito, K. Toyota, and M. Yoshifuji, Chem. Commun., 1997, 1637. b)
G. Markl and W. Bauer, Angew. Chem., Int. Ed. Engl., 28, 1695
that of the pentacarbonyltungsten(0) complex of 3,3-diphenyl-1-
13
¨
(1989).
ꢀ
(2,4,6-tri-t-butylphenyl)-1-phosphaallene [2.531(2) A].
The
ꢀ
a) To a solution of 6 (0.13 mmol) in THF (6 mL) was added
butyllithium (0.15 mmol), and in 5 min copper(II) chloride
(0.25 mmol) was added at À78 ꢁC. The reaction mixture was stirred
at À78 ꢁC for 1 h, and oxygen was bubbled for 5 min (ca. 32mmol).
After treatment with aqueous sodium sulfite and warming up to room
temperature, the mixture was treated with ammonia (10% NH3 in
saturated aqueous NH4Cl) and extracted with ether. The organic layer
was concentrated in vacuo, and the residue was purified by silica-gel
column chromatography (hexane/toluene 5 : 1) to afford 37 mg of 7
(97% yield) as yellow solids. b) EE-7: Yellow crystals, mp 233–
234 ꢁC; 1H NMR (200 MHz, CDCl3) ꢂ ¼ 1:35 (18H, s, p-tBu), 1.50
(36H, s, o-tBu), 1.55 (m, 6H, Me), 7.41 (4H, brs, arom); 13C{1Hg NMR
(50 MHz, CDCl3) ꢂ ¼ 183:9 (dd, 1JPC ¼ 13 Hz, 2JPC ¼ 3 Hz, P=C);
31P{1Hg NMR (81 MHz, CDCl3) ꢂ ¼ 232; HRMScalcd for C40H64P2,
606.4483; found, 606.4488. c) ZZ-7; ꢂP ¼ 237; EZ-7; ꢂP ¼ 240, 2 88
ꢀ
distances are 1.478(9) and 1.54(1) A,
C1–C2and C–2C2
respectively.
To verify the [1,2]-hydrogen shift of 5w, we examined the
reaction of 6w as follows: 6w was allowed to react with 1 eq of
butyllithium and quenched with methanol-d4 at À78 ꢁC to give
(Z)-3-deuterio-1-phosphapropene complex Z-10 in an 80%-D
ratio, (E)-1-phosphapropene complex E-11, and 1-phosphaallene
complex 12 in a 6 : 3 : 1 ratio almost quantitatively.10 Formation
of Z-10 indicates the [1,2]-rearrangement leading to a 3-phospha-
2-propenyllithium intermediate (50w). Compound E-11 might
have been generated by proton abstraction of 5w from 6w,
indicating slow rate of the halogen-metal exchange to generate
5w, and 12 might havebeen formed by anelimination of hydrogen
bromide from 6w. It indicates a facile [1,2]-hydrogen shift of 5w
proceeds to afford the 3-phospha-2-propenyllithium derivative
(Z-50w).
3
(ABq, Jpp ¼ 154 Hz). d) Crystal data for EE-7; C40H64P2 M ¼
ꢁ
606:89, triclinic, P1 (no. 2), a ¼ 18:524ð3Þ, b ¼ 18:564ð2Þ, c ¼
6:237ð6Þ A, ꢃ ¼ 98:23ð2Þ, ꢄ ¼ 93:72ð2Þ, ꢅ ¼ 64:40ð1Þ ꢁ, V ¼
ꢀ
ꢀ 3
1914ð1Þ A , Z ¼ 2, ꢆ ¼ 1:053 g cmÀ3, ꢇðMo KꢃÞ ¼ 0:138 mmÀ1
,
T ¼ 150 K, 5428 reflections measured (2ꢈmax ¼ 50:0 ꢁ), 5426
observed [I > 1:0ꢉðIÞ], R1 ¼ 0:047 [I > 2:0ꢉðIÞ]. Rw ¼ 0:108 (all
data). CCDC-181946.
9
R. Appel, J. Hunerbein, and N. Siabalis, Angew. Chem., Int. Ed. Engl.,
¨
26, 779 (1987).
10 31P{1Hg NMR (162MHz, CDCl 3) 6w: ꢂ ¼ 210 (1JPW ¼ 270 Hz); Z-
10: ꢂ ¼ 208 (1JPW ¼ 262 Hz); E-11: ꢂ ¼ 199 (1JPW ¼ 259 Hz); 12:
ꢂ ¼ 42 (1JPW ¼ 259 Hz).
As for 5w, coordination of the pentacarbonyltungsten(0)
group might increase the acidity of the methyl group, which
resembles in the property of (ꢁ6-arene)transition metal complex-
es.14 Moreover, we assume that steric congestion around the
‘‘solvated’’ lithium atom might facilitate the [1,2]-rearrange-
ment. Although the detailed reaction mechanism for the facile
[1,2]-rearrangement of 5w has not been clarified, it is interesting
to note that the reactivity of 5 changes upon coordination.
11 a) To a solution of 6w (0.14 mmol) in THF (10 mL) at À78 ꢁC was
added butyllithium (0.15 mmol) and in 10 min copper(II) chloride
(0.29 mmol). The reaction mixture was stirred for 1 h and warmed to
room temperature. The reaction mixture was treated with ammonia as
described in the case of 7, and extracted with ether. The organic layer
was concentrated in vacuo, and the residue was purified by silica gel
column chromatography (hexane/toluene 5 : 1) from recrystallization
in hexane to afford 26 mg of 8 (30% yield) together with E=Z-11 and
their decomplexed products. 8: mp 228–231 ꢁC; 1H NMR (400 MHz,
CDCl3) ꢂ ¼ 1:03 (4H, m, CH2), 1.33 (18H, s, p-tBu), 1.58 (36H, s, o-
tBu), 6.80 (2H, dt, 2JPH ¼ 20 Hz, 3JHH ¼ 7 Hz, P=CH), 7.42(4H, d,
4JPH ¼ 3 Hz, arom); 31P NMR (162MHz, CDCl 3) ꢂ ¼ 202 (dt,
This work was supported in part by a Grant-in-Aid for
Scientific Research (No. 13304049) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
3
1
2JPH ¼ 20 Hz, JPH ¼ 20 Hz; JPW ¼ 259 Hz); IR (KBr) ꢊ ¼ 2073,
1990, 1932cm À1; FAB-MS m=z 1254 (Mþ). Anal. Calcd for
C50H64O10P2W2: C, 47.84; H, 5.14. Found: C, 48.11; H, 5.14. b)
Crystal data for 8ꢂ1=2C7H8: C53:5H68O10P2W2, M ¼ 1300:73,
monoclinic, P21/c (no. 14), a ¼ 13:320ð8Þ, b ¼ 15:838ð2Þ, c ¼
References and Notes
1
This paper is dedicated to Prof. Dr. Dieter Seebach at Eidgenossische
¨
ꢀ
ꢀ 3
Technische Hochschule Zurich on the occasion of his 65th birthday.
2a) M. Yoshifuji,
¨
J. Chem. Soc. Dalton Trans., 1999, 3343. b) M.
14:112ð3Þ A, ꢄ ¼ 102:65ð2Þ, V ¼ 2904ð1Þ A , Z ¼ 2, ꢆ ¼
1:540 g cmÀ3, ꢇðMo KꢃÞ ¼ 4:070 mmÀ1, T ¼ 150 K, 4936 reflec-
tions measured (2ꢈmax ¼ 50:0 ꢁ), 4714 observed [I > 2:0ꢉðIÞ],
R1 ¼ 0:050 [I > 2:0ꢉðIÞ]. Rw ¼ 0:064 (all data). CCDC-181947.
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