Unusual cleavage of the enolsilane C-O bond: Transformation of 2-silyloxy-1,3-dienes into 1,3-dienyl-2-zirconium compounds and their cross-coupling reactions

Article abstract of DOI:10.1055/s-2001-9709

Aryl enolsilanes and 2-silyloxy-1,3-dienes react with zirconocene to give alkenylzirconium, and novel 1-methylene-2-propenylzirconium compounds which can be used as 2-dienylation reagents. Thus, one-pot coupling of 4-phenyl-1,3-butadienyl-2-zirconocene (2d) with a range of electrophiles including aryl, alkynyl, allyl halides, bromine, iodine and a Michael acceptor occurs regioselectively at the C-2 position in the presence of Pd or Cu catalysts.

Full text of DOI:10.1055/s-2001-9709

LETTER
123
Unusual Cleavage of the Enolsilane C-O Bond: Transformation of 2-Silyloxy-
,3-dienes into 1,3-Dienyl-2-zirconium Compounds and their Cross-Coupling
1
Reactions
Benjamin Ganchegui, Philippe Bertus, Jan Szymoniak*
Réactions Sélectives et Applications, CNRS and Université de Reims, 51687 Reims Cedex 2, France
Fax +33-3-26913166; E-mail: jan.szymoniak@univ-reims.fr
Received 6 October 2000
While studying the reactivity of zirconocene-coordinated
silyloxy azadienes, we performed some reactions with
the nitrogen-free analogs and simpler enolsilanes. We
were surprised to find that oxidative addition reactions oc-
Abstract: Aryl enolsilanes and 2-silyloxy-1,3-dienes react with zir-
conocene to give alkenylzirconium, and novel 1-methylene-2-pro-
penylzirconium compounds which can be used as 2-dienylation
reagents. Thus, one-pot coupling of 4-phenyl-1,3-butadienyl-2-zir-
7
conocene (2d) with a range of electrophiles including aryl, alkynyl, curred starting from aryl enolsilanes, to provide the corre-
allyl halides, bromine, iodine and a Michael acceptor occurs regi- sponding vinylzirconium compounds. Thus, treatment of
oselectively at the C-2 position in the presence of Pd or Cu catalysts.
naphthyl enolsilane 1a with the zirconocene equivalent
8
Key words: silyloxydienes, zirconium, dienylzirconocenes, cross- (‘Cp Zr') in THF at room temperature for 20 h, followed
coupling
2
by quenching with D O, afforded the deuterated alkene
2
9
3
a. By heating at reflux, the reaction was completed
within only 2 h. As can be seen from Table (entry 1) sim-
The cross-coupling reactions of vinylmetals with electro- ilar yields (68% and 72%) were obtained in these cases.
philes constitutes henceforth a powerful strategy for car- The formation of the vinylzirconium intermediate 2a was
1
bon-carbon bond formation.
In this matter, further evidenced by a one-pot Pd(0)-catalyzed reaction
alkenylzirconocenes are very attractive reagents, since with 4-iodotoluene to produce the cross-coupling product
they can be readily generated in situ from alkynes via hy-
2
drozirconation, and further transmetallated to various
3
metals such as Al, B, Cu, Pd, Ni, Sn and Zn. This method
Table Oxidative Addition of Enolsilanes 1a-c and Silyloxy-
dienes 1d-e to Zirconocene, followed by Deuterolysis.
is primarily limited, however, by the unique regioselectiv-
ity of hydrozirconation, invariably placing Zr at the least
hindered (terminal) position. The oxidative addition reac-
a,b
tion of alkenyl derivatives to zirconocene (‘Cp Zr') ap-
2
pears as an interesting alternative. Yet, in contrast to the
well known oxidative addition of allyl ethers or acetals to
4
'
Cp Zr', the reactions that involve alkenyl substrates are
2
restricted to a few recent examples including vinyl chlo-
rides, vinyl bromides, tosylates and/or fluorides and phe-
nyl vinyl ether (Scheme 1).5
ZrCp2
'
Cp2Zr'
R
X
R
X
R
ZrCp2X
X = Cl, Br, F, OTs, OPh
Scheme 1
6
Among numerous reactions of enolsilanes, the most com-
mon involve the O-Si bond breaking to generate specific
enolate anions. Here we report an unprecedented, zirconi-
um induced, C-O bond breaking in some enol- and dienol-
silanes to provide alkenyl- and novel 1-methylene-2-pro-
penyl-zirconocene derivatives. 1-Zirconated 1,3-dienes
thus produced in situ, were further demonstrated to be ef-
fective components in several different cross-coupling re-
actions.
Reaction conditions: Enolsilane (1 eq) and ’Cp Zr’[Cp ZrCl (1 eq.)
2
2
2
a
and n-BuLi (2 eq.)],THF, -78 °C, 1h, followed by (a) or (b): reflux
b
c
d
in THF, 2 h, D O; r.t., 20 h, D O.
Isolated yield.
NMR yield.
2
2
Np = 2-naphthyl.
Synlett 2001 No. 1, 123–125 ISSN 0936-5214 © Thieme Stuttgart · New York

1
24
B. Ganchegui et al.
LETTER
4
(Scheme 2). The oxidative addition also took place start- Whereas 1-zirconated 1,3-dienyl compounds could be
ing from acetophenone-derived enolsilane 1b (Table, en- readily obtained by hydrozirconation of the conjugated
12
try 2). In contrast to the reactions employing aryl enynes, there is no report on the 2-zirconated 1,3-dienyl
enolsilanes (1a,b), no reaction occurred from the alkyl compounds, to the best of our knowledge. We thought that
enolsilane 1c (entry 3), the substrate was entirely recov- the dienylzirconium compounds such as 2d or 2e might be
ered. Neither prolonged reaction time nor warming of the useful reagents to perform dienylation reactions, specifi-
reaction mixture produced the expected vinylzirconation cally at the C-2 position.13 We were pleased to find that
compound. The presence of a neighbouring aryl group these compounds underwent various cross-coupling reac-
must have allowed the -elimination of OSiMe from the tions. Representative examples involving 2d are shown in
3
intermediary zirconacyclopropane.
Scheme 3. The reactions were carried out in one pot via
transmetallation, by analogy with the reactions involving
alkenylzirconium compounds.3 A complete regioselec-
,14
OSiMe3
Cp2Zr
tivity at C-2 and moderate to good yields were observed.9
'
Cp2Zr'
THF
Pd(PPh3)4 0.05 eq.
ZnCl2 1.3 eq.
1a
+ Ar
I
Starting from the preformed 2d, the reaction with p-tolyl
iodide proceeded smoothly in THF at reflux for 3 h in the
presence of the palladium (0) catalyst (5% mol) and zinc
chloride to afford 5 (60% isolated yield from 1d).9 The
coupling reaction with 1-iodoheptyne was performed in a
similar fashion to produce the dienyne 6. The Cu-cata-
lyzed allylation led to the triene 7 as expected. Reactions
Np
Ar
Np
2a
4
,10
(
Np = 2-naphthyl, Ar = p-tolyl)
Scheme 2
of 2d with N-bromosuccinimide and I provided 2-bromo-
and 2-iodo-1,3-dienes, 8 and 9, respectively. It is worthy
Thereafter, we used silyloxydienes 1d,e as substrates, and
noticed that, in this case, the oxidative addition reactions
did not proceed at room temperature, but could be
achieved at reflux within 2 hours. The formation of die-
nyl-2-zircononium compounds 2d,e was evidenced by
deuterolysis. When the reaction mixtures were treated
2
to note that methods for preparing 2-halogeno-1,3-dienes
1
5
are rare. Finally, the Cu(I)-mediated conjugated addition
of 2d to methyl vinyl ketone was achieved to give the di-
enylketone 10.
with D O, 2-deuterated dienes 3d,e (>95% D incorpora-
2
In summary, the oxidative addition reactions of aryl enol-
silanes and 2-silyloxydienes to zirconocene produced the
corresponding alkenyl- and dienyl-2-zirconocenes. The
novel dienylzirconium compounds, which are not avail-
able by hydrozirconation, have been demonstrated to be
effective 2-dienylation reagents. Their successive C-C
bond-forming reactions with a variety of coupling compo-
nents were achieved in one-pot through transmetallation.
The method thus provides a two-step conversion of
tion) were isolated as the only products (Table, entries
1
10
4
,5). Moreover, the H NMR spectrum of 2d in C D
6 6
showed singlet signals at 5.90 (10 H) and 0.10 (9 H), as-
13
signable to Cp and OSiMe protons, respectively. Its
C
3
NMR spectrum revealed the Cp carbons at 111.5 and a
signal at 191.2, assignable to an alkenyl carbon attached
5
a,11
to zirconium.
Thus, similarly to the aryl, also the alk-
enyl group on enolsilane allowed the reaction to occur,
even if much more drastic conditions were needed.
,
-enones into various 2-dienyl derivatives. We current-
ly explore the synthetic potential of these new transforma-
tions.
p-Tol
1d
O
Ph
References and Notes
5
a
e
[
Zr]
(
1) Diederich, F.; Stang, P. J. Metal-Catalyzed Cross-Coupling
Reactions, Wiley-VCH, Weinheim, 1998.
Ph
Ph
Ph
10
(
2) (a) Schwartz, J.; Labinger, J. A. Angew. Chem. Int. Ed. Engl.
1976, 15, 333. (b) Labinger, J. A. Comprehensive Organic
Synthesis, Vol. 8, Trost, B. M.; Fleming, I Eds., Pergamon
Press, Oxford, 1991.
C5H11
2
d
d
b
X
c
(
(
3) Wipf, P.; Heike, J. Tetrahedron 1996, 52, 12853.
4) (a) Ito, H.; Motoki, Y.; Taguchi, T.; Hanzawa, Y. J. Am.
Chem. Soc. 1993, 115, 8835. (b) Negishi, E.; Takahashi, T.
Bull. Chem. Soc. Jpn. 1998, 71, 755 and references therein.
Ph
8
(X=Br)
6
9 (X=I)
Ph
(5) (a) Takahashi, T.; Kotora, M.; Fischer, R.; Nishihara, Y.;
7
Nakajima, K. J. Am. Chem. Soc. 1995, 117, 11039.
(
b) Ichikawa, J.; Fujiwara, M.; Nawata, H.; Okauchi, T.;
[
Zr] = ZrCp OSiMe . Isolated yields based on 1d are given.
2 3
Minami, T. Tetrahedron Lett. 1996, 37, 8799. (c) Fujiwara,
M.; Ichikawa, J.; Okauchi, T.; Minami, T. Tetrahedron Lett.
1999, 40, 7261.
a) 5 mol% Pd(PPh ) , ZnCl , p-iodotoluene, 65 °C, 3 h, 60%. b) 5
mol% Pd(PPh ) , ZnCl , 1-iodo-1-heptyne, 65 °C, 3h, 42%. c) 10
3
4
2
3
4
2
mol% CuCl, allyl chloride, 3 h, 53%. d) NBS, 15 min, 66% or I2,
5 min, 60%. e) CuCl, LiCl, 3-buten-2-one, 2 h, 40%.
(
6) Colvin, E. W. Silicon in Organic Synthesis, Krieger, R. E.
Publishing Company, Malabar, Florida, 1985.
1
Scheme 3
Synlett 2001, No. 1, 123–125 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Unusual Cleavage of the Enolsilane C-O Bond
125
(
(
(
7) (a) Gandon, V.; Bertus, P.; Szymoniak, J. Tetrahedron Lett.
(11) (a) Takahashi, T.; Suzuki, N.; Kageyama, M.; Kondakov, D.
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1995, 117, 5871.
(12) Fryzuk, M. D.; Bates, G. S.; Stone, C. Tetrahedron Lett. 1986,
27, 1537.
(13) A few 2-metallated buta-1,3-dienes (M = Mg, Li, Sn, Zn)
were employed as nucleophilic reagents, often giving low
yields and/or regioselectivities, see Mg: (a) Nunomoto, S.;
Yamashita. Y. J. Org. Chem. 1979, 44, 4788. (b) Shea, K. J.;
Pham. P. Q. Tetrahedron Lett. 1983, 24, 1003. Li: (c) Wada,
E.; Kanemasa, S.; Fujiwara, I.; Tsuge. O. Bull. Chem. Soc.
Jpn. 1985, 58, 1942. (d) Bloch, R.; Chaptal-Gradoz, N.;
J. Org. Chem. 1994, 59, 4162. Sn: (e) Luo, M.; Iwabuchi, Y.;
Hatakeyama, S. Synlett 1999, 7, 1109. Zn: (f) Matsubara, S.;
Ukai, K.; Toda, N.; Utimoto, K.; Oshima, K. Synlett 2000, 7,
995.
2000, 41, 3053. (b) Gandon, V.; Bertus, P.; Szymoniak, J.
Tetrahedron 2000, 56, 4467.
8) For reviews of "Cp Zr" chemistry, see ref. 4b and (a) Negishi,
2
E.; Kondakov, D. Y. Chem. Soc. Rev. 1996, 26, 417.
(
b) Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124.
1
13
9) All products were fully characterized by H, C NMR and MS
spectroscopy.
(
10) Typical Procedure (1d Å 2d Å 5): To a solution of Cp ZrCl
2 2
(1 mmol) in THF (4 mL) was added n-BuLi (2 mmol, 2.5 M
in hexanes) at -78 °C. After stirring for 1 h at -78 °C the
silyloxydiene 1d (1 mmol) in THF (2 mL) was added via
syringe. After warming up to r.t. (15 min) the reaction mixture
was refluxed for 2 h. At this stage, isolation of the
dienylzirconocene 2d (I) or a direct cross-coupling step (II)
might follow:
(I) After evaporation of the solvent, toluene (3 mL) was added
and the inorganic salts were filtered off. Washing with pentane
(14) (a) Negishi, E.; Okukado, N.; King. A. O.; Van Horn, D. E.;
Spiegel, B. I. J. Am. Chem. Soc. 1978, 100, 2254. (b) Lipshutz,
B. H.; Sengupta, S. Org. React. 1992, 41, 135. (c) Takahashi,
T.; Kotora, M. Kasai, K.; Suzuki, N. Tetrahedron Lett. 1994,
35, 5685. (d) Hara, R.; Liu, Y.; Sun, W-H.; Takahashi, T.
Tetrahedron Lett. 1997, 38, 4103.
1
(
2
2 mL) afforded 2d, 95% pure by H NMR. Spectral data for
1
d: H NMR (250 MHz, C D ) 0.10 (s, 9H), 5.50 (d,
6 6
J = 3.8Hz, 1H), 5.90 (s, 10H), 6.12 (d, J = 3.8Hz, 1H), 6.22 (d,
1
3
J = 15.6Hz, 1H), 6.90-7.50 (m, 6H); C NMR (62.5 MHz,
C D ): 2.1 (CH ), 111.5 (CH), 124.5 (CH ), 126.2 (CH),
6
6
3
2
1
1
26.6 (CH), 128.0 (CH), 128.6 (CH), 139.4 (C), 143.8 (CH),
91.2 (C).
(15) (a) Mitani, M.; Kobayashi, Y.; Koyama, K. J. Chem. Soc.,
Perkin Trans. 1 1995, 653. (b) Okamoto, S.; Sato. H.; Sato, F.
Tetrahedron Lett. 1996, 37, 8865. (c) Brandsma, L.;
Vasilevsky, S. F.; Verkruijsse, H. D. Application of Transition
Metal Catalysts in Organic Synthesis, Springer, Berlin, 1998,
p. 42.
(II) Pd(PPh ) (0.05 mmol), ZnCl (1.3 mmol) and
3 4 2
p-iodotoluene (1 mmol) were added at r.t. and the reaction
mixture refluxed for 3 h. The mixture was treated with 1 M
HCl (2mL) for 30 min and extracted with ether (3 15 mL).
The organic extracts were dried, filtered and concentrated, and
the residue was purified by flash chromatography (petroleum
ether) to give 5 (70%) as colorless syrup.
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