Completion of a catalytic cycle of zirconium-catalyzed alkylation of silanes by addition of organic halides

Article abstract of DOI:10.1039/b002516j

A catalytic cycle in zirconium-catalyzed alkylation of silanes with secondary Grignard reagents was completed by addition of organic halides which were not incorporated in the products.

Full text of DOI:10.1039/b002516j

Completion of a catalytic cycle of zirconium-catalyzed alkylation of silanes by
addition of organic halides
Yasuyuki Ura, Ryuichiro Hara and Tamotsu Takahashi*
Catalysis Research Center and Graduate School of Pharmaceutical Sciences, Hokkaido University; and CREST,
Science and Technology Corporation (JST) Sapporo 060-0811, Japan. E-mail: tamotsu@cat.hokudai.ac.jp
Received (in Cambridge, UK) 28th March 2000, Accepted 11th April 2000
A catalytic cycle in zirconium-catalyzed alkylation of silanes
with secondary Grignard reagents was completed by addi-
tion of organic halides which were not incorporated in the
products.
oxidation of the ‘(C5H5)2Zr’ species via oxidative addition7–9
and tolerate the reaction conditions.
Under catalytic conditions, typically 10 mol% of
(C5H5)2ZrCl2 and 1 equiv. of an organic halide were employed
[eqn. (4)]. The results of a comparison study using various
We have found and reported catalytic cycles containing a Zr(II
)
species and zirconacycles in carbon–carbon bond formation by
the reaction of olefins and/or acetylenes with Grignard
reagents.1–5 One principle of the Zr(II)-mediated or -catalyzed
reactions is the use of facile interchange of its oxidation states,
II and IV. The zirconium(II) species is known to be of good p-
electron accepting ability and oxidatively couples two un-
saturated organic molecules. During the coupling reaction, the
Zr(II) species is converted into Zr(IV). On these bases,
zirconium(II)-catalyzed reactions have been developed involv-
ing olefins1–4 and, in some cases, acetylenes.5 We also reported
not only unsaturated substrates but also dihydrogen2 or silanes3
as partners of unsaturated compounds such as olefins. During
our investigations of zirconium-mediated alkylation reactions
of silanes, we found that addition of organic halides led to a
catalytic cycle in which the organic halides were not incorpo-
rated in the products. Herein, we report a novel type of catalytic
cycle in alkylation of silanes with secondary Grignard reagents
using zirconium.
(4)
organic halides as additives are shown in Table 1 As a control
experiment, when no organic halide was added, the reaction
gave only 22% yield of PrnSiHPh2 1a. When 1 equiv. of bromo-
or iodo-propane was added, the desired product was obtained in
67 and 91% yield, respectively. This remarkable improvement
clearly showed that a catalytic cycle was achieved. The best
result of 99% yield was obtained when 1,3-dibromopropane
was used as the additive. Though bromobenzene, bromopro-
pane and iodopropane showed fairly good performances,
iodobenzene showed a very poor effect on this reaction, which
may be due to the readily occurring deiodination.9 1,3-Di-
bromopropane was the best choice by far as additive and the
reaction proceeded with as little as 2 mol% of the catalyst. In the
absence of (C5H5)2ZrCl2 1a was not formed.
It is well known that when H2SiPh2 is treated with PrnMgBr
in THF at room temperature, a nearly quantitative yield of
PrnSi(H)Ph2 1 is obtained [eqn. (1)].6 It is interesting that
Similarly, when H2SiPh2 was treated under the same
conditions with 2 equiv. of BusMgCl, BunSiHPh2 1b was
obtained in 89% yield. In spite of our attempts, tertiary Grignard
reagents such as ButMgCl or secondary Grignard reagents such
as cyclohexylmagnesium bromide did not successfully promote
this reaction owing, probably, to the instability of the corre-
sponding disubstituted-olefin zirconium complexes.
(1)
H2SiPh2 does not react at all with the more sterically hindered
PriMgBr under the same conditions [eqn. (2)].
To understand the role and the superiority of 1,3-di-
bromopropane, 2-benzyl-1,3-dibromopropane 3 was used as an
(2)
Table 1 Reactions of PriMgBr with H2SiPh2 in the presence of a catalytic
amount of (C5H5)2ZrCl2 and various organic halidesa,b
However, we found that when a stoichiometric amount of
(C5H5)2ZrCl2 was added in the presence of a two-fold amount
of PriMgBr, the reaction of H2SiPh2 with PriMgBr proceeded to
Entry
RAX
Cp2ZrCl2/ Time/h
eq
Yield (%)c
give 1a, instead of the expected product PriSi(H)Ph2
2
[eqn. (3)]. A stoichiometric amount of (C5H5)2ZrCl2 is required
1
2
3
4
5
6
7
8
9
—
0.1
0.1
0.1
0.1
0.02
0.01
0.1
0.1
0.1
0.1
24
6
6
22
67
91
99
93
41
3
78
2d
73
1-Bromopropane
1-Iodopropane
1,3-Dibromopropane
1,3-Dibromopropane
1,3-Dibromopropane
1,3-Diiodopropane
Bromobenzene
Iodobenzene
(3)
3
24
18
3
6
1
since the outcome of the reaction is explained by the total
conversion to 1a, ‘(C5H5)2Zr,’ n-C3H8 and 2 equiv. of MgX2.
Since the Pri group was changed to an Prn group in the product,
it is reasonable to consider that the Pri group is converted into
propene on zirconium and then reacts with silane to afford
PrnSi(H)Ph2 and the Zr(II) species.
In order to extend this stoichiometric reaction to a catalytic
reaction, oxidation of the built-up Zr(II) species was investi-
gated. We found that certain organic halides were suitable for
10
2-Bromopropene
24
a R = Me, corresponding to eqn. (4). b Typical reaction conditions: H2SiPh2
(0.5 mmol), (C5H5)2ZrCl2 (as indicated), PriMgBr (1.5 mmol), RAX (0.5
mmol); room temp.c Yields were determined by GC.d PhI was consumed in
1 h and further reaction did not proceed.
DOI: 10.1039/b002516j
Chem. Commun., 2000, 875–876
This journal is © The Royal Society of Chemistry 2000
875

oxidation reagent and the catalytic reaction was carried out
[eqn. (5)]. A mixture of three products, benzylcyclopropane 4
reacts with an alkyl bromide (RX) to give 9,7 followed by ligand
exchange with PriMgBr and b-hydride elimination to regenerate
(C5H5)2Zr(C3H6) 7. Alternatively, oxidation of ‘(C5H5)2Zr’
with alkyl bromides might afford (C5H5)2ZrX2 which is also
converted into 7 by the reaction with PriMgBr. An alkyl
bromide plays an important role in reoxidizing the zirconium(II
)
species to zirconium(IV). In its absence, regeneration of the
catalyst does not occur and ‘(C5H5)2Zr’ reacts with H2SiPh2
leading to the m-hydride bridged dimer 10.3a
One-pot reaction of H3SiPh with 3 PriMgBr without a
catalyst for 1 h followed by the addition of 10 mol% of
(C5H5)2ZrCl2 and 1,3-dibromopropane selectively afforded
HSi(Pri)(Prn)Ph 11 in 66% yield [eqn. (6)]. An Pri group was
(5)
and reduction products 5 and 6 were obtained with 5 observed
at an early stage. After completion of the reaction, two products,
4 and 6 were obtained in 44 and 24% yields, respectively. The
formation of the major product 4 is explained by an oxidative
(6)
addition of one carbon–bromine bond of 3 to the zirconium(II
)
species, followed by nucleophilic ring closure forming a
cyclopropane ring and (C5H5)2ZrBr2.
On the basis of these observations, the mechanism of Scheme
1 is proposed. Zirconium–olefin complex 7 is formed and reacts
with H2SiPh2 (7 ? 8). Reductive elimination of the silylated
compound gives PrnSiHPh2 1. The so-formed ‘(C5H5)2Zr’
incorporated directly from the Grignard reagent in the absence
of the catalyst, and an Prn group was incorporated after the
addition of the zirconium catalyst with 1,3-dibromopropane.
We conclude that a zirconium(II) species is efficiently
oxidized by organic halides, which assists regeneration of the
active catalyst. We believe this is the first example of a
zirconium(II)-catalyzed reaction in which an oxidant is explic-
itly added to complete the catalytic cycle. Further studies to
clarify the mechanism are in progress.
Notes and references
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Scheme 1
876
Chem. Commun., 2000, 875–876