An efficient synthetic method of 1-(2,4,6-Tri-t-butylphenyl)-3-phenyl-1- phosphaallenes [Mes*P=C=CR-Ph; R = H, SiMe3] from Dibromophosphaethene [Mes*P=CBr2] (Mes* = 2,4,6-t-Bu 3C6H2)

Article abstract of DOI:10.1246/bcsj.78.1142

2-Bromo-3-phenyl-1-(2,4,6-tri-t-butylphenyl)-3-trimethylsiloxy-1-phospha-1- propene reacted with t-butyllithium and chlorotrimethylsilane to afford 3-phenyl-1-(2,4,6-tri-t-butylphenyl)-1-phosphaallene and/or 3-phenyl-3- trimethylsilyl-1-(2,4,6-tri-t-butyl

Full text of DOI:10.1246/bcsj.78.1142

1142 Short Articles
Bull. Chem. Soc. Jpn., 78, 1142–1144 (2005)
R
R
Br
Li
R
R
Br
R
R
H
1) R'CHO
t-BuLi
C C
C C
C C C
An Efficient Synthetic Method of
1-(2,4,6-Tri-t-butylphenyl)-3-phenyl-
2) ClSiMe3
CHR'
R'
Me3SiO
ꢀ
1-phosphaallenes [Mes P=C=CR–
Scheme 1.
Ph; R ¼ H, SiMe3] from Dibromo-
Mes*
Br
ꢀ
(Mes = 2,4,6-t-Bu3C6H2)
Mes*
Br
C
Br
phosphaethene [Mes P=CBr2]
ꢀ
1) PhCHO
n-BuLi
P C
CHPh
Me3SiO
2
P
2) ClSiMe3
4
5
Shigekazu Ito, Satoshi Sekiguchi,
Mes*
SiMe3
1) t-BuLi
ꢀ
and/or
P C C
1
Matthias Freytag, and Masaaki Yoshifuji
5
2) ClSiMe3
Ph
6
Department of Chemistry, Graduate School of Science,
Tohoku University, Aoba, Sendai 980-8578
Mes* P
Ph
P Mes*
Ph
Mes*
P C C Ph
Received January 13, 2005; E-mail: yoshifj@mail.tains.tohoku.
ac.jp
Me3Si
7
8
Scheme 2.
2-Bromo-3-phenyl-1-(2,4,6-tri-t-butylphenyl)-3-trimeth-
ylsiloxy-1-phospha-1-propene reacted with t-butyllithium
and chlorotrimethylsilane to afford 3-phenyl-1-(2,4,6-tri-t-
butylphenyl)-1-phosphaallene and/or 3-phenyl-3-trimethyl-
silyl-1-(2,4,6-tri-t-butylphenyl)-1-phosphaallene through the
elimination of lithium trimethylsiloxide or hexamethyldi-
siloxane. The molecular structure of the latter phosphaallene
was confirmed by X-ray crystallography.
bond-formation and the elimination are separated, which is ex-
pected to be a promising method to prepare a number of phos-
phaallene derivatives. Indeed, these procedures correspond to
the Peterson olefination, which is one of the most convenient
syntheses of olefins.2c On the other hand, Seebach and co-
workers reported on the preparation of allenes from geminal
dibromoethenes and aldehydes (Scheme 1).7 Because novel
synthetic method of 1 is close to the Seebach method, we ex-
amined the effects on the reaction conditions. Additionally, the
structural determination of a novel phosphaallene (6) is de-
scribed.
Dibromophosphaethene 48 was allowed to react with butyl-
lithium to generate 2, which was then allowed to react with
benzaldehyde and chlorotrimethylsilane to afford 2-bromo-
3-phenyl-1-(2,4,6-tri-t-butylphenyl)-3-trimethylsiloxy-1-phos-
pha-1-propene (5). Compound 5 was stable to air and moisture
at room temperature, and was purified by silica-gel column
chromatography (Scheme 2). We next examined reactions
of 5 with t-butyllithium and chlorotrimethylsilane: compound
5 was allowed to react with t-butyllithium9 at ꢁ78 ꢂC, and
chlorotrimethylsilane was added to the reaction mixture
(Scheme 2). Equimolar amounts of t-butyllithium and chloro-
trimethylsilane were allowed to react with 5 to afford 1 in
moderate yield. On the other hand, 5 was allowed to react with
two molar amounts of t-butyllithium and two molar amounts
of chlorotrimethylsilane; 3-phenyl-3-trimethylsilyl-1-phospha-
allene 6 was obtained. Compound 6 was characterized by spec-
troscopic data, showing a higher field 31P chemical shift com-
pared with that of 1. The structure of 6 was unambiguously
confirmed by X-ray crystallography, as displayed in Fig. 1.
The bond lengths and angles do not largely deviate from the
previously reported phosphaallenes.1,5
Although phosphaallenes [–P=C=C ] are essentially high-
ly reactive, we and other groups have succeeded in the synthe-
sis, isolation, and characterization of such as 3-phenyl-1-
(2,4,6-tri-t-butylphenyl)-1-phosphaallene (1) so far by using
kinetic stabilization with the bulky 2,4,6-tri-t-butylphenyl
ꢀ
(hereafter abbreviated to Mes ) group.1,2 Recently, we report-
ed on a concise preparation method of phosphaallenes by using
1-bromo-2-(2,4,6-tri-t-butylphenyl)-2-phosphaethenyllithium
(2);3 moreover, we succeeded in regioselective solid state
[2 þ 2] dimerizations4 and the formation of 1,4-diphosphaful-
vene 3 by the formal [3 þ 2] dimerization of 1 (Chart 1).5
Thus, phosphaallenes are promising compounds for the syn-
thesis of organophosphorus compounds, since allenes have
become important for organic synthesis.6
We report here on an alternative synthetic procedure of 1 by
utilizing 2, benzaldehyde, and chlorotrimethylsilane, which in-
cludes the preparation and isolation of a 1-phospha-1-propene
derivative 5 as a precursor. In this novel procedure, the C–C
Ph
Mes*
H
Mes*
Br
Li
Mes*
Mes*
P C C
P C
P
P
Ph
Ph
When no chlorotrimethylsilane was employed in the reac-
tion of 5 and t-butyllithium, a mixture of 1, 3, and 3,4-diphos-
phanylidenecyclobutene 7 was observed in a 10:7:1 ratio (by
31P NMR spectroscopy). Additionally, 5 was allowed to react
3
1
2
Mes* = 2,4,6-t-Bu3C6H2
Chart 1.
Published on the web June 6, 2005; DOI 10.1246/bcsj.78.1142

Bull. Chem. Soc. Jpn., 78, No. 6 (2005)
Ó 2005 The Chemical Society of Japan 1143
butylphenyl)-1-phospha-1-propene with t-butyllithium.5 Thus,
the reaction of 5 with one molar amount of t-butyllithium in-
cluded formal dimerizations of 1, and the elimination of lithi-
um alkoxides, which might play important roles in affording 3
and 7. Anion 9 reacted with chlorotrimethylsilane to afford 1
by the elimination of hexamethyldisiloxane, which is a modi-
fied pathway of the Peterson olefination. When two molar
amounts of t-butyllithium reacted with 5, probably a dilithium
intermediate 10 was generated, and chlorotrimethylsilane re-
acted with 10 to form another intermediate 11. The elimination
of hexamethyldisiloxane follows to form anions 12 and 13,
which give 1, 6, and 8 upon protonation or silylation.10 Con-
cerning the equilibrium between 12 and 13, we previously
reported that 1 reacted with t-butyllithium to generate only
135 and, indeed, reaction of 1 with t-butyllithium and iodo-
Fig. 1. Molecular structure of 6 with 50% probability ellip-
soids. Hydrogen atoms are omitted for clarity. Selected
ꢀ
bond lengths (A) and angles ( ): P–C1 1.629(2), P–
ꢀ
methane gave ethynylphosphine [Mes P(Me)CꢃCPh]11 al-
ꢂ
most quantitatively. At the same time, 11 reacted with chloro-
trimethylsilane to afford 6 through the elimination of hexa-
methyldisiloxane. In this reaction, the formation of 6 after
the generation of 1 did not seem to proceed mainly, since 1 re-
acted with one molar amount of t-butyllithium and one molar
amount of chlorotrimethylsilane to afford a mixture of 1, 6,
and 8 in a ratio of 3:1:4.
In conclusion, we have established an alternative synthetic
procedure of 1 by using 2-bromo-1-phosphapropene 5. The
efficiency was comparable to that of previously reported
methods.2 The introduction of a trimethylsilyl group into the
P=C=C skeleton was successful, and the structure of 6 was
confirmed by X-ray analysis.
ꢀ
CMes 1.869(7), C1–C2 1.313(3), C2–Si 1.897(2), C2–
CPh 1.495(3), C1–P–CMes 102.81(9), P–C1–C2 170.9(2),
ꢀ
C1–C2–Si 121.2(2), Si–C2–CPh 119.8(1), C1–C2–CPh
119.0(2).
Mes*
Li
ClSiMe
3
t-BuLi
P C
1
5
−(Me Si) O
3
2
CHPh
Me3SiO
9
t-BuLi
Mes*
Li
P C
C(Li)Ph
Me3SiO
Experimental
Preparation of 5. According to our previous report,8 4 was
prepared from a reaction of (2,4,6-tri-t-butylphenyl)phosphonous
dichloride and bromoform in the presence of lithium diisopropyl-
amide (LDA) at ꢁ100 ꢂC. To a solution of 4 (1.0 g, 2.2 mmol) in
THF (20 mL) was added butyllithium (2.2 mmol, 1.6 M solution
in hexane, 1 M = 1 mol dmꢁ3) at ꢁ100 ꢂC and stirred for 10 min.
The mixture was treated with benzaldehyde (2.2 mmol) and
allowed to warm to room temperature. To the reaction mixture
was added chlorotrimethylsilane (2.2 mmol) and stirred for 15
min. The solvent and volatile materials were removed in vacuo,
and the residue was extracted with hexane. Silica-gel column
chromatography (hexane) of the hexane extracts afforded 0.85 g
of 5 (70% yield). Colorless needles (EtOH), mp 118–119 ꢂC;
1H NMR (400 MHz, CDCl3) ꢀ 0.23 (9H, s, SiMe3), 1.37 (9H, s,
p-tBu), 1.47 (9H, s, o-tBu), 1.55 (9H, s, o-tBu), 5.76 (1H, d,
3JPH ¼ 13 Hz, CH), 7.2–7.7 (7H, m, arom.); 13C{1H} NMR
(151 MHz, CDCl3) ꢀ 0.7 (s, SiMe3), 31.8 (s, p-CMe3), 33.0 (d,
10
ClSiMe
3
Mes*
SiMe3
ClSiMe
3
6
P C
−(Me Si) O
3
2
C(Li)Ph
Me3SiO
11
−(Me Si) O
3
2
Mes*
Li
Mes*
P C C Ph
P C C
Li
Ph
13
12
+
H
or ClSiMe
3
+
+
1
6
8
4
4JPC ¼ 7 Hz, o-CMe3), 33.2 (d, JPC ¼ 7 Hz, o-CMe3), 35.5 (s,
2
Scheme 3.
p-CMe3), 38.3 (s, o-CMe3), 38.4 (s, o-CMe3), 80.6 (d, JPC
ꢀ
¼
37 Hz, CH), 122.6 (s, m-Mes ), 127.3 (s, m-Ph), 127.9 (s, p-
ꢀ
with two molar amounts of t-butyllithium and one molar
amount of chlorotrimethylsilane to afford a mixture of 1, 6,
and phosphinoacetylene 8 in a 1:2:6 ratio.
By taking the results into consideration, we suggest a plau-
sible reaction mechanism, as displayed in Scheme 3. Com-
pound 5 reacted with t-butyllithium to give 9; the elimination
of LiOSiMe3 gave a mixture of 1, 3, and 7. This reaction is
similar to the condition described in Scheme 1, and is close
to the reaction of 2-bromo-3-methoxy-3-phenyl-1-(2,4,6-tri-t-
1
Ph), 128.2 (s, o-Ph), 137.4 (d, JPC ¼ 54 Hz, ipso-Mes ), 142.2
ꢀ
ꢀ
(d, 3JPC ¼ 10 Hz, ipso-Ph), 151.1 (s, p-Mes ), 153.6 (s, o-Mes ),
ꢀ
1
154.0 (s, o-Mes ), 168.4 (d, JPC ¼ 64 Hz, P=C); 31P NMR (162
3
MHz, CDCl3) ꢀ 253 (d, JPH ¼ 13 Hz). Anal. Calcd for
C29H44BrOPSi: C, 63.59; H, 8.10%. Found: C, 63.61; H, 8.08%.
Preparation of 1 from 5. To a solution of 5 (47 mg, 0.086
mmol) in THF (5 mL) was added butyllithium (0.086 mmol) at
ꢁ78 ꢂC and stirred for 15 min. The mixture was treated with
chlorotrimethylsilane (0.095 mmol), and allowed to warm to room

1144 S. Ito et al.
Bull. Chem. Soc. Jpn., 78, No. 6 (2005)
temperature. The solvent and volatile materials were removed in
vacuo and the residue was extracted with hexane. Silica-gel col-
umn chromatography (hexane) of the hexane extracts afforded
19 mg of 1 (59% yield).
References
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1
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stirred for 15 min. The mixture was treated with chlorotrimethyl-
silane (0.80 mmol) and allowed to warm to room temperature. The
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2
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ꢂ
1
yield). Colorless prisms (EtOH), mp 123–124 C; H NMR (400
MHz, CDCl3) ꢀ 0.44 (9H, s, SiMe3), 1.57 (9H, s, p-tBu), 1.80
(18H, s, o-tBu), 7.4–7.6 (7H, m, arom.); 13C{1H} NMR (151
3
a) S. Ito, S. Kimura, and M. Yoshifuji, Bull. Chem. Soc.
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¼
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7 Hz, o-CMe3), 35.5 (s, p-CMe3), 38.9 (s, o-CMe3), 122.3 (d,
ꢀ
3JPC ¼ 12 Hz, ipso-Ph), 122.6 (s, m-Mes ), 127.0 (s, p-Ph),
1
128.6 (brs, o-Ph), 128.7 (s, m-Ph), 130.4 (d, JPC ¼ 68 Hz, ipso-
ꢀ
2
ꢀ
Mes ), 137.5 (d, JPC ¼ 17 Hz, P=C=C), 150.0 (s, p-Mes ),
ꢀ
1
155.0 (s, o-Mes ), 238.4 (d, JPC ¼ 37 Hz, P=C=C);
31P{1H} NMR (162 MHz, CDCl3) ꢀ 37. Anal. Calcd for
C29H43PSi: C, 77.28; H, 9.62%. Found: C, 77.24; H, 9.60%.
6
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¨
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9 The reaction of 5 with n-butyllithium gave a mixture of
many unidentified compounds.
X-ray Crystallography of 6.
crystal dimension 0:30 ꢄ 0:20 ꢄ 0:20 mm3, monoclinic, P21=n
C29H43PSi, M ¼ 450:72,
ꢀ
(#14), a ¼ 10:6227ð7Þ, b ¼ 11:316ð1Þ, c ¼ 23:020ð2Þ A, ꢁ ¼
ꢂ
ꢀ 3
91:721ð6Þ , V ¼ 2765:8ð4Þ A , Z ¼ 4, 2ꢂmax ¼ 55:0ꢂ, T ¼ 130
K, ꢃðcalcdÞ ¼ 1:082 g cmꢁ3
,
ꢄ(Mo Kꢅ) ¼ 0:156 mmꢁ1
,
Fð000Þ ¼ 984, 20269 collected reflections, 5925 unique reflec-
tions (Rint ¼ 0:065), R1 ¼ 0:049 (I > 2ꢆðIÞ), RW ¼ 0:069 (all
data), S ¼ 1:07 (280 parameters) (CCDC-259388).
This work was supported in part by Grants-in-Aid for Scien-
tific Research (No. 13303039 and 14044012) from the Minis-
try of Education, Culture, Sports, Science and Technology. M.
Freytag is grateful to the Japan Society for the Promotion of
Science for Postdoctoral Fellowships for Foreign Researchers.
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Chemistry of Organic Silicon Compounds,’’ ed by Z. Rappoport
and Y. Apeloig, Wiley, Chichester (2001), Vol. 3, pp. 853–948.
11 M. Yoshifuji, H. Yoshimura, and K. Toyota, Chem. Lett.,
1990, 827.