Coupling reactions of zirconate complexes induced by carbonyl compounds

Article abstract of DOI:10.1002/anie.200904255

Multifaceted organozirconate: The novel coupling reaction of organozirconates 1 and carbonyl compounds (through intermediate 2) selectively affords highly substituted allene 3 and dienol 4 derivatives.

Full text of DOI:10.1002/anie.200904255

Communications
DOI: 10.1002/anie.200904255
Organozirconate Chemistry
Coupling Reactions of Zirconate Complexes Induced by Carbonyl
Compounds**
Chanjuan Xi,* Xiaoyu Yan, Wei You, and Tamotsu Takahashi*
During the past few decades the chemistry of 16-electron
organozirconocene complexes has been extensively explored
and a tremendous number of applications in synthetic
chemistry have been found.[1] However, organozirconate
complexes, which usually have an 18-electron configuration
À
with two cyclopentadienyl (Cp) rings, three Zr C bonds, and
alkali counter ions, have rarely been explored in spite of being
the supposed intermediates in a number of stoichiometric and
À
catalytic reactions, and able to form C C bonds through a
successful 1,2-migration of organozirconates.[2–4] Our research
group has an interest in the above mentioned subjects, and we
have recently reported[5] that by adding quinone as an oxidant
Scheme 1. Diverse reactivity of zirconates.
to a mixture of alkynylzirconates then zirconocenendiyne
compounds are afforded: these compounds can be converted
into various endiynes through coupling reaction with electro-
philes. These results prompted us to further study the
chemistry of organozirconate complexes. Herein, we report
the coupling reactions of three-membered alkynylzirconates
induced by carbonyl compounds. The unique features of this
reaction involve the carbonyl compound acting as an electro-
phile, participating in the reaction with zirconates, and also
inducing the coupling of an alkynyl group and an sp2-
hybridized carbon atom of zirconacyclopropene. To the best
of our knowledge, this type of reaction has not been reported.
When acyl compounds were used as electrophiles, then
functionalized allenes were formed. When aldehydes were
used as electrophiles, then dienols were formed. These
outcomes are illustrated in Scheme 1.
the reaction mixture with DCl in D2O provided allene 5a in a
comparable yield with 91% deuterium incorporation. This
result indicates that the zirconium-containing complex 6a is
formed (Scheme 2).
Oxazirconacycle 6a was formed in 74% yield (based on
NMR spectroscopy). The 1H NMR spectrum of 6a showed a
singlet resonance at d = 5.96 ppm, which was assigned to the
protons on the Cp ring. In the 13C NMR spectrum, the signal
The reaction of zirconate 1a, which was generated by the
treatment of nBuLi with bis(phenylethynyl)zirconocene at
room temperature,[6] with chloroformate at 08C for five hours
afforded allene 3a in 73% yield after hydrolysis. Quenching
Scheme 2. The reaction with chloroformate.
[*] Prof. C. Xi, X. Yan, W. You
Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical
Biology (Ministry of Education), Department of Chemistry
Tsinghua University, Beijing 100084 (China)
and
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (China)
Fax: (+86)10-6277-1194
for the carbon atoms in the Cp ring appeared at d =
104.5 ppm, and the signal for the allenic-type sp-hybridized
carbon atom appeared at d = 198.5 ppm. The carbon signals in
À
À
Zr C(Ph) and C(OEt)O appeared at d = 149.5 and
169.6 ppm, respectively.
E-mail: cjxi@tsinghua.edu.cn
The allene group is a versatile functionality because it is
useful as either a nucleophile or an electrophile as well as a
substrate for cycloaddition reactions. This multiple reactivity
make allenes as excellent candidates for synthetic manipu-
lation.[7] The reaction described here allowed the efficient
synthesis of various substituted allenes. A variety of acyl
compounds were subjected to this novel reaction and all
reactions afforded analogous products in good to high yields
after hydrolysis. Notably, the acyl compounds induced
Prof. T. Takahashi
Catalysis Research Center and Graduate School of Life Science
Hokkaido University, Sapporo 001-0021 (Japan)
[**] We thank the National Natural Science Foundation of China
(20872076), and the Doctoral Program of Higher Education
(200800030072) for financial support. We also thank Dr. Haifang Li
for her kind help with measurement of HRMS data.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904255.
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coupling between an alkynyl group and an sp2-hybridized
carbon atom with high regioselectivity. This result is due to
the higher reactivity of the carbon atom attached to the alkyl
group as compared to the carbon atom attached to the aryl
group. The representative results are summarized in Table 1.
Chloroformates afforded the corresponding allenes in 47–
61% yields (Table 1, entries 1–5). The addition of esters
similarly gave allenes in good yields (Table 1, entries 6–8).
The diethylcarbamyl chloride afforded the corresponding
product in 55% yield (Table 1, entry 9). When acyl chloride
and anhydride were treated with zirconates under similar
reaction conditions the desired products were not observed.
Scheme 3. The reaction of oxazirconacycle 6 with allyl bromide (yield
of isolated product given in parentheses).
compound. The intermediate 2 reacts with an acyl compound
through nucleophilic substitution to form 8 with elimination
of a lithium salt and then undergoes cyclic oxidation/
rearrangement (path a) to give 6. Alternatively, intermediate
8 undergoes oxidative coupling of the carbonyl and alkenyl
groups to give oxazirconacycle 9 with an alkynyl moiety at the
a position (path b),[9] which subsequently rearranges to 6.
Hydrolysis of 6 affords the product 3.
À
The remaining Zr C bond of oxazirconacycle 6 was
further treated with allyl bromide in the presence of one
equivalent of CuCl[8] and afforded fully substituted allenes 7a
and 7b in moderate yields, respectively (Scheme 3).
Based on the above results, we propose the following
mechanism for this reaction (Scheme 4). The zirconate
3
1
2
ꢀ
=
[LiCp2Zr(C CR )(R C CR )] 1 undergoes a coupling reac-
tion to give intermediate 2 in the presence of an acyl
In light of the unusual reactivity of the organozirconate,
we further investigated the reac-
Table 1: Formation of functionalized allenes by the reaction of zirconates with acyl compounds.
tion of zirconate 1a with benzalde-
hyde (Scheme 5). Interestingly,
dienol 4a was formed in good
yield after hydrolysis. Deuteriolysis
instead of hydrolysis afforded the
dideuterated compound 10a in
66% yield with 93% and 97%
deuterium incorporation. The
structure of product 4a was con-
firmed by 1H NMR, 13C NMR,
HSQC, HMBC, and NOESY
experiments.[10]
Entry
Zirconate
Acyl
compound
Product[a]
Yield [%][b]
1
2
3
3a
3b
3c
73(57)
65(47)
76(61)
1a
Intermediate 11a was also
characterized by NMR spectrosco-
1
py. The H NMR spectrum of 11a
showed two signals at d = 5.72 and
6.00 ppm which are assigned to the
protons of two Cp rings, and a
resonance at d = 5.39 ppm is
assigned to the proton in
4
5
3d
3e
69(53)
67(52)
OCHPh. In the 13C NMR spec-
À
trum, the signals for the carbon
atoms of the Cp ring appear at d =
111.0 and 111.6 ppm, and the res-
onance for the carbon atoms of the
diene appear at d = 146.9, 150.4,
154.8, and 183.4 ppm.
6
7
1a
1a
3 f
46(32)
44(28)
3g
A variety of aromatic alde-
hydes were subjected to the reac-
tion. The results were summarized
in Table 2. Aromatic aldehydes
bearing a chlorine, fluorine, or
phenyl group were treated with
zirconates and led to the corre-
sponding products in good yields
(Table 2, entries 3–6, and 8). The p-
methylbenzaldehyde was treated
with zirconates and led to the
8
9
3h
3i
40(31)
67(55)
1c
[a] Single isomer, the stereoselectivity was not defined. [b] Yield is based on 1H NMR spectroscopy, yield
of isolated product given in parentheses. Hex=hexyl, Tol=tolyl.
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Table 2: Formation of dienols by the reaction of zirconates with
aldehydes.
Entry Ziconate Aldehyde
Product
Yield
[%][a]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1c
1c
1e
1e
PhCHO
4a
66(54)
51(45)
69(57)
66(50)
64(52)
70(59)
46(37)
65(50)
p-MeC6H4CHO 4b
p-FC6H4CHO
4c
Scheme 4. Proposed mechanism for the reaction with acyl com-
pounds.
p-ClC6H4CHO 4d
p-PhC6H4CHO 4e
p-ClC6H4CHO 4 f
p-MeC6H4CHO 4g
p-ClC6H4CHO 4h
Scheme 5. The reaction with benzaldehyde.
corresponding products in moderate yields (Table 2, entries 2
and 7). When aliphatic aldehydes were treated with zirconates
under similar reaction conditions, the desired products were
not observed, but aldol products were obtained.
[a] Yield is based on 1H NMR spectroscopy; yield of isolated product
given in parentheses.
These results indicate that aldehydes behaved differently
to acyl compounds. A plausible reaction mechanism is shown
in Scheme 6. Firstly, the oxygen atom of the carbonyl group
coordinates to zirconate 1 and induces coupling of the alkynyl
and an sp2-hybridized carbon atom of zirconacyclopropene to
give intermediate 12, which forms an equilibrium between 13.
In this case, the reaction pathway differs from the reaction
with acyl compound. A possible explanation is that the
nucleophilicity of the carbon atom attached to zirconium is
not high enough to react with an aldehyde. On the other hand,
oxidative coupling of an aldehyde and an alkyne on the
zirconocene(II) is preferred.[11] The intermediate 13 under-
goes oxidative coupling to form 11. Hydrolysis of 11 affords
the product 4.[12]
In summary, we have demonstrated a novel coupling
reaction of alkynylzirconate complexes with carbonyl com-
pounds. A unique feature of this reaction is that carbonyl
compounds not only participated in the reaction with
zirconates but also induced coupling of an alkynyl and
alkenyl group of zirconoacyclopropene with high regioselec-
tivity. In this reaction, two alkyne groups, introduced as to
alkynyllithiums and an organolithium, and carbonyl com-
pounds were selectively coupled in one-pot by the reaction of
[Cp2ZrCl2] to afford highly substituted allene and dienol
derivatives. Investigation of the reaction mechanism and
further application of this chemistry are in progress.
Experimental Section
Typical procedure for the reaction of alkynylzirconates with acyl
compounds: nBuLi (4.5 mmol, 1.6m solution in hexane) was added to
a solution of phenylacetylene (3.0 mmol) at À788C and stirred for 1 h.
Then [Cp2ZrCl2] (1.5 mmol) was added and the solution was stirred at
À788C for 1 h then warmed to room temperature and kept at this
Scheme 6. Plausible mechanism for the formation of dienols.
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Angewandte
Chemie
temperature for 12 h. After cooling the reaction mixture to 08C, ethyl
chloroformate (1.0 mmol) was added and the reaction mixture was
stirred for 5 h. The reaction mixture was quenched with 6n HCl and
extracted with ethyl acetate. The organic extract was dried over
MgSO4, and the filtrate was concentrated under reduced pressure.
Purification of the residue by column chromatography on silica gel
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Received: July 31, 2009
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Published online: September 23, 2009
Keywords: acyl compounds · aldehydes · allenes · dienols ·
.
organozirconates
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