Reactivity of alkynylzirconates towards allyl bromides: Selective formation of β-allyl-zirconacyclopentadienes

Article abstract of DOI:10.1039/c0cc02997a

Reaction of alkynylzirconates with allyl bromides afforded β-allyl-zirconacyclopentadienes with high selectivity in unique reaction site. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0cc02997a

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Reactivity of alkynylzirconates towards allyl bromides: selective
formation of b-allyl-zirconacyclopentadieneswz
Xiaoyu Yan,a Yiqing Zhoua and Chanjuan Xi*ab
Received 3rd August 2010, Accepted 6th September 2010
DOI: 10.1039/c0cc02997a
Reaction of alkynylzirconates with allyl bromides afforded
b-allyl-zirconacyclopentadienes with high selectivity in unique
reaction site.
Initially, to a THF solution of zirconate 1a, generated by
the reaction of nBuLi with bis(phenylethynyl)zirconocene at
room temperature,1t,2 was added one equivalent of allyl
bromide. The reaction mixture was stirred at room tempera-
ture for 3 h and quenched by HCl; 1,3-diene 4a was formed
exclusively. Deuteriolysis of the reaction mixture prepared
from 1a with DCl/D2O afforded the dideuterated product
5a with >98% of deuterium incorporation in 60% yield
(Scheme 2). This result indicated that zirconacyclopentadiene
6a was formed as intermediate before hydrolysis. The zircona-
cyclopentadienes have been demonstrated to be very useful
precursors for various substituted benzene derivatives in the
presence of CuCl or Ni(PPh3)2Cl2.3 In light of these methods,
treatment of zirconacyclopentadiene 6a with dimethyl acetylene-
dicarboxylate (DMAD) in the presence of two equiv. of CuCl
afforded benzene derivative 7a in 47% yield. This reaction
further verified the formation of zirconacyclopentadiene 6a as
an intermediate.
Organozirconate complexes with 18-electron configuration
have received increasing attentions in the organic synthesis.
Representative reactions of zirconates are intramolecular
rearrangements via 1,2-migration to form various organic
compounds.1 Recently, we reported that zirconate 1 can act
as a multifaceted reagent in varies reactive sites with electro-
philes. For example, (1) when chloroformates or esters were
employed as electrophiles, the reaction proceeded well to
afford functionalized allenes 2, in which C-1 atom was
attacked by electrophiles; (2) when aldehydes were employed
as electrophiles, the reaction proceeded well to afford dienols
3, in which C-4 atom was attacked by electrophiles.2 Herein we
would like to report that allyl bromides were used as electro-
philes and the reaction proceeded well to afford 2-allyl-1,3-
dienes 4, in which C-3 atom was attacked by electrophiles
(Scheme 1).
Scheme 1 Diverse reactivity of zirconates.
a Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical
Biology (Ministry of Education), Department of Chemistry,
Tsinghua University, Beijing 100084, China.
Scheme 2 Reaction of zirconate with allyl bromide.
E-mail: cjxi@tsinghua.edu.cn; Tel: +86-10-62782695
b State Key Laboratory of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071, China
w Electronic supplementary information (ESI) available: Full experi-
mental details, including 1H and 13C NMR data for new compounds.
CCDC number 785907. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0cc02997a
z Dedicated to Professor Uwe Rosenthal on the occasion of his 60th
birthday.
A variety of allyl bromides were subjected to this novel
reaction. The representative results are summarized in
Table 1. Reaction of zirconate 1 with allyl bromide afforded
the corresponding 2-allyl-1,3-dienes in 62–67% isolated
yields (entries 1–4). Using substituted allyl bromides such
as 3-bromo-2-methylprop-1-ene, 3-bromocyclohex-1-ene,
2,3-dibromoprop-1-ene instead of allyl bromide afforded the
c
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Chem. Commun., 2010, 46, 7801–7803 7801

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Table 1 Formation of 2-allyl-1,3-dienes via the reaction of zirconates
with allyl bromides
Entry Zirconate
Allyl bromide 1,3-Diene
Yielda
1
67%
Scheme 3 Diels–Alder reaction of 4 with TCNE.
2
3
4
64%
71%
62%
21%
Fig. 1 Structure of 8c.
tetracyanoethylene (TCNE) and compound 8 was obtained
(Scheme 3). To our delight, the crystal of 8c was suitable for
single crystal analysis, and its structure was fully characterized
by the X-ray diffraction analysis (Fig. 1).y This result indirectly
confirmed that the allyl group occupied C-3 position of the
zirconacyclopentadienes 6 and zirconate 1.
5
6
1a
1a
Based on the above results, we propose the following
mechanism for this reaction (Scheme 4). Firstly, double
bond of allyl bromide coordinates to zirconate 1 and
induces coupling of alkynyl and zirconacyclopropene to give
intermediate 9,1d,4 which is in equilibrium with 10 in this
reaction. C-3 attacks allyl bromide via SN20 to afford zircona-
cyclopentadiene 65 with elimination of LiBr. The different
reactivity of allyl bromide and carbonyl compound may
due to the different coordination mode. When aldehyde
was used, carbonyl interacted with zirconium center in
Z1-coordination mode, and the C-4 atom favors to attack
carbonyl compound.2 When allyl bromide was used, its double
bond interacted with zirconium center in Z2-coordination
mode, and the C-3 atom favors to attach to allyl
bromide.
44%
32%
7
1a
a
Isolated yield.
desired 2-allyl-1,3-dienes in moderate yields (entries 5–7). It is
noteworthy that these allylic compounds induced coupling
between alkynyl group and sp2 carbon with high regio-
selectivity. This is due to higher reactivity of the carbon
attached to alkyl group than aryl group.2 When benzyl
bromide was used as electrophile, the reaction afforded messy
result. When simple alkyl halides, such as EtI and nBuBr, were
used, the reactions did not proceed. These results revealed that
the double bond was essentially important to the special
reactivity and selectivity in this reaction.
In summary, we have demonstrated a novel coupling
reaction of alkynylzirconate complexes with allyl bromide.
Differently from a previous report, a unique reaction site with
C-3 was observed. Further investigations on reactivity of
alknylzirconates are now in progress.
This work has been supported by the National Natural
Science Foundation of China (20872076, 20972085, 21032004),
and by Specialized Research Fund for the Doctoral Program of
Higher Education (200800030072). We gratefully thank Prof.
Tamotsu Takahashi for his kindly discussion.
The 1,3-diene is an important precursor for Diels–
Alder reaction. We tried the reaction of product 4 with
c
7802 Chem. Commun., 2010, 46, 7801–7803
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Notes and references
y Crystal data for 8a: C26H20N4, M = 388.46, colorless, monoclinic,
P21/n, a = 11.132(2) A, b = 9.6628(19) A, c = 20.142(4) A, b =
92.96(3)1, crystal size/mm: 0.32 ꢀ 0.21 ꢀ 0.20, T = 173(2) K,
V = 2163.7(7) A3, Z = 4, Dc = 1.192 Mg mꢁ3, m = 0.072 mmꢁ1
,
Mo-Ka, 17 162 independent reflections, Rint = R1 = 0.1343, final
R1 [I > 2s(I)] = 0.1249. (CCDC: 785907).
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c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7801–7803 7803