Zr-promoted linear coupling of alkynes to generate bis(allene)s

Article abstract of DOI:10.1039/b912175g

Zirconium promoted linear coupling of alkynes to generate bis(allene)s with high yields in the presence of ⁿBuLi and chlorophosphines or trimethylsilyl trifluoromethanesulfonate.

Full text of DOI:10.1039/b912175g

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Zr-promoted linear coupling of alkynes to generate bis(allene)swz
Xiaoyu Yan,a Chunbo Laia and Chanjuan Xi*ab
Received (in Cambridge, UK) 22nd June 2009, Accepted 6th August 2009
First published as an Advance Article on the web 20th August 2009
DOI: 10.1039/b912175g
Zirconium promoted linear coupling of alkynes to generate
bis(allene)s with high yields in the presence of nBuLi and
chlorophosphines or trimethylsilyl trifluoromethanesulfonate.
ð1Þ
Transition metal-mediated trimerization reactions of alkynes
have been known for years.1 Among them, the most common
Initially, reaction of Li[Cp2Zr(CRCPh)3] (1 mmol), generated
in situ by the reaction of Cp2ZrCl2 with 3 equiv. of acetylide
LiCRCPh,7 with chlorodiphenylphosphine (2 mmol) was
firstly examined and 1,4,6-triphenyl-1,3,6-tris(diphenyl-
phosphino)hexa-1,2,4,5-tetra-ene 1a was isolated in 38% yield.
In this reaction, the product 1a contained three C–P bonds.
Meanwhile, some 1,4-diphenylbut-1-en-3-yne was detected by
GC-MS. When 3 equiv. of chlorodiphenylphosphine were
used, the reaction proceeded smoothly, and 1a was obtained
in 97% NMR yield (eqn (2)). The product 1a was characterized
by NMR spectroscopy. In its 13C NMR spectrum, the
two allenic-type sp-hybridized carbon signals appeared at
209.0 and 209.9 ppm and four sp2 carbon signals at 103.3,
105.5, 107.2, and 108.6 ppm, respectively. Its 31P NMR
spectrum showed three signals at ꢀ6.9, ꢀ7.9, ꢀ9.8 ppm,
respectively. Under similar conditions, various terminal
alkynes were examined. When aromatic 1-alkynes were
used, the desired products were formed in high yields. The
representative results are shown in eqn (2). However, the
aliphatic 1-alkynes led to the formation of a complex, inseparable
mixture. Nevertheless, a trace amount of desired product was
detected by 31P NMR and ESI-MS.
coupling procedure is the cycloaddition reaction of three
alkynes to give arenes and fulvenes.2,3 Recently, it has been
reported that the transition metal-induced trimerization of
alkynes to acyclic dieneynes,4 endiynes,5 and trienes after
hydrolysis.6 However, the transition metal-mediated coupling
of alkynes to afford bis(allene) has not been reported
(Scheme 1), to the best of our knowledge.
Scheme 1 Trimerization of alkynes.
Negishi and co-workers reported that Li[Cp2Zr(CRCR)3]
underwent 1,2-migration insertion to give an eneyne after
hydrolysis,7 in which two alkynes coupled. We recently
reported that treatment of Li[Cp2Zr(CRCR)3] with
quinone afforded geminal endiynes.5 In this case, three
alkynes coupled together. During the course of our further
investigation of this reaction with other reagents, we found
three alkynes coupled together to afford bis(allene) as shown
in eqn (1).
(2)
a Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical
Biology (Ministry of Education), Department of Chemistry,
Tsinghua University, Beijing 100084, China.
E-mail: cjxi@tsinghua.edu.cn; Tel: +86-10-62782695
b State Key Laboratory of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071, China
w Dedicated to Professor John M. Birmingham on the occasion of his
80th birthday.
z Electronic supplementary information (ESI) available: Full
experimental details, including 1H and 13C NMR data for new
compounds. CIF file for compound 2a. CCDC 736782. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b912175g
To
extend
the
reaction,
we
tried
reacting
Li[Cp2Zr(CRCPh)3] (1 mmol) with trimethylsilyl trifluoro-
methanesulfonate (TMSOTf) (4 mmol) and bis(allene) 1f was
obtained in 57% isolated yield (eqn (3)). It is noteworthy that
in this reaction two diastereomers were observed in 1 : 1 ratio,
which was in contrast to the reaction with chlorophosphine.
ꢁc
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-
ð3Þ
To confirm the structure of bis(allene) product 1, we
conducted the crystallization of compound 1a several times
but failed. When 1a was kept in aerobic conditions for about
three months, compound 2a was obtained. Colorless crystals
of 2a suitable for X-ray analysis were obtained by diffusing
n-hexane into the ethyl acetate solution at room temperature.
The structure of compound 2a is shown in Fig. 1.y This result
indirectly indicated that the bis(allene) had formed in the
reaction.
Scheme 2 Proposed mechanism.
Notes and references
Fig. 1 X-Ray crystal structure of 2a. Thermal ellipsoids are shown at
the 30% probability level; hydrogen atoms have been omitted for
clarity.
y Crystal data for 2a: C60H45O3P3, M = 906.87, colorless, 0.32 ꢂ
3
ꢀ
0.23 ꢂ 0.15 mm , triclinic, P1, a = 9.8957(4), b = 14.1576(6),
c = 18.0625(8) A, a = 82.699(3), b = 75.037(2), g = 81.991(2)1,
T = 293(2) K, V = 2409.89(18) A3, Z = 2, Dc = 1.250 Mg mꢀ3
,
m
=
0.170 mmꢀ1
Rint = 0.0813, final R1 [I 4 2s(I)] = 0.0897. (CCDC: 736782).
,
Mo-Ka, 29 875 independent reflections,
On the basis of the above observations, a plausible
reaction mechanism was proposed in Scheme 2. First, the
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zirconate complex Li[Cp2Zr(CRCR)3] 3 undergoes a
1,2-migratory insertion reaction to give intermediate 4.7
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Intermediate 4 is in equilibrium with zirconacyclocumulene
5.8,9 In the presence of an electrophile,
5 undergoes
electrophilic attack to give intermediate 610,11 with the
elimination of lithium salt, in which the alkynyl group
attacks the sp2-carbon to form a new carbon–carbon bond.
When chlorophosphine was used as an electrophile, the
intermediate 6 rearranged to 7, which coupled with 2 equiv.
of chlorophosphine10–12 to give compounds 1a–1e with
high stereoselectivity through isomerization of compound 8.
When TMSOTf was used as an electrophile, the intermediate 6
attacked
2 equiv. of TMSOTf to give compound 1f
as a mixture of two diastereomers. TMSOTf as an electro-
phile behaved differently to chlorophosphine. This may
be attributed to the bulky TMS group affecting the formation
of 7.
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This work has been supported by the National
Natural Science Foundation of China (20372041, 20872076),
and by the Specialized Research Fund for the Doctoral
Program of Higher Education (200800030072). We
gratefully thank Prof. Tamotsu Takahashi for his kindly
discussion.
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6028 | Chem. Commun., 2009, 6026–6028