Potassium tert-butoxide-mediated regioselective silaboration of aromatic alkenes

Article abstract of DOI:10.1039/c2cc32778c

The regio- and diastereoselective silaboration of aromatic alkenes with a silylboron compound proceeds in the presence of a catalytic amount of potassium tert-butoxide, providing a complementary method to the corresponding transition metal-catalyzed reactions.

Full text of DOI:10.1039/c2cc32778c

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COMMUNICATION
Potassium tert-butoxide-mediated regioselective silaboration of aromatic
alkenesw
Hajime Ito,* Yuko Horita and Eiji Yamamoto
Received 19th April 2012, Accepted 18th June 2012
DOI: 10.1039/c2cc32778c
The regio- and diastereoselective silaboration of aromatic
alkenes with a silylboron compound proceeds in the presence
of a catalytic amount of potassium tert-butoxide, providing a
complementary method to the corresponding transition metal-
catalyzed reactions.
anions and other silylation reactions.28,29 Diboron compounds
were found to be activated by alkoxide bases, phosphines or
N-heterocyclic carbenes (NHC) and underwent conjugate
addition to cyclic and acyclic a,b-unsaturated carbonyls.30–37
Fernandez and co-workers recently reported a base-catalyzed
´
diboration of alkenes and allenes.32 Hoveyda and co-workers
also reported an asymmetric boration of a,b-unsaturated
compounds with diboron, which was promoted by a chiral
NHC.34 Other transition-metal-free procedures with diboron
were also developed.35–37 In contrast, base-mediated reaction
of silylboron is still rare. Kawachi, Tamao and co-workers
reported that silyl nucleophilic species could be generated
through reactions between silylboron and a stoichiometric
nucleophile, such as RLi, MeMgBr, and K(O-t-Bu).28 Hoveyda
and co-workers recently reported that a chiral NHC can also
activate silylboron compounds, promoting enantioselective
silylation of a,b-unsaturated compounds.38 In this case, only
the silyl group remained in the product and to the best of our
knowledge, there have been no reported silaboration reaction
conducted under transition-metal-free conditions where both
the silyl and boryl groups are incorporated into the product.
Herein, we report the first example of a transition-metal-free,
base-mediated silaboration of aromatic alkenes.
Regio- and stereoselective silaborations of unsaturated compounds
are synthetically useful because the silaboration products can
be converted to multi-functionalized compounds through the
stepwise derivatization of the Si–C and B–C bonds.1,2 The
silaboration requires an appropriate catalyst, and research
efforts in this regard have focused almost exclusively on Si–B
bond activation with transition-metal catalysts, including Pt,3,4
Pd,5–13 Ni,14–16 Rh,17–20 and Cu21–26 metals. Consideration of
the processing costs associated with heavy metals and the
requirement for their effective removal from product streams
can limit the use of the silaboration reaction on an industrial
scale, and a transition-metal-free alternative would be desirable.
Recently, base-mediated activation of B–B and Si–B bonds
has been focused as transition-metal-free procedures for silyl-
ation and borylation of unsaturated compounds (Fig. 1).27–38
Heterolytic cleavage of Si–Si bonds with nucleophiles has been
known for several decades and utilized in generation of silyl
Our initial efforts focused on the reaction of styrene (1a) with
silylboron 2 under various reaction conditions (Table 1).39 In
the absence of a catalyst, no reaction occurred (entry 1). In
contrast, the addition of a base catalyst in THF promoted the
reaction. For example, the addition of 10 mol% of K(O-t-Bu)
in THF at room temperature gave complete consumption of 1a
(0.3 mmol) and 2 (1.2 equiv., 0.36 mmol) within 1 h, affording
the silaboration product 3a in excellent yield (entry 2).
The reaction also proceeded in a highly regioselective manner,
with only a single regioisomer being detected. A 10 mol% loading
of the base catalyst gave the best results, with lower loadings
(5 and 1 mol%) in lower yields with the remaining starting
materials (entries 3 and 4, respectively). The use of a 50 mol%
of catalyst gave a comparable yield to that of the 10 mol% loading
(entry 5), whereas the use of a 100 mol% of the base catalyst
resulted in a lower yield (entry 6). Furthermore, a reduction in the
amount of 2 (1.0 equiv.), coupled with a 100 mol% loading of the
base catalyst, led to a significant reduction in the yield (entry 7),
likely because a reduction in the amount of the Lewis acidic 2
resulted in enhanced basicity, promoting decomposition of the
product. Other alkoxy bases, such as KOMe and Li(O-t-Bu), gave
Fig. 1 Recent developments of nucleophile-mediated reactions of
diboron and silyboron compounds.
Division of Chemical Process Engineering, Faculty of Engineering,
Hokkaido University, Sapporo 060-8628, Japan.
E-mail: hajito@eng.hokudai.ac.jp; Fax: +81-(0)11-706-6561
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc32778c
c
8006 Chem. Commun., 2012, 48, 8006–8008
This journal is The Royal Society of Chemistry 2012

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Table 1 Base-mediated silaboration of styrene 1a with silylboron 2a
Table 2 Potassium tert-butoxide-catalyzed silaboration of aromatic
alkenesa
Entry
Base (mol%)
Solvent (mL)
Time (h)
Yieldb (%)
1
2
3
4
None
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
DMF
toluene
1,4-dioxane
20
1
4
4
1
1
4
4
4
4
4
8
21
4
0
Time Yieldb
K(O-t-Bu), 10
K(O-t-Bu), 5
K(O-t-Bu), 1
K(O-t-Bu), 50
K(O-t-Bu), 100
K(O-t-Bu), 100
KOMe, 10
Li(O-t-Bu), 10
LiOMe, 10
Mg(OEt)2, 10
TBAF, 10
95 (91)
74
13
93
77
22
72
14
0
0
0
0
0
Entry Substrate
Product
(h)
(%)
5
6
7c
8
9
1
1b (X = OMe)
1c (X = Cl)
1d (X = CF3)
3b
3c
3d
4
1
42
87
67
0
2c,d
3e,f
10
11
12c
13d
14
15
16
Sm(O-i-Pr)3, 10
K(O-t-Bu), 10
K(O-t-Bu), 10
K(O-t-Bu), 10
4c,d
1
66
22
1
81
99
a
Conditions: 1a (0.3 mmol), 2 (0.36 mmol), K(O-t-Bu) (0.05 M,
b
0.6 mL, 0.03 mmol). Yield was determined by GC analysis. Isolated
85
(anti/syn
92 : 8)
5
17
14
c
yield is given in parentheses. 1.0 equiv. (0.30 mmol) of 2 was used.
d
The reaction was carried out at rt for 1 h then 50 1C for 7 h. The
reaction was carried out at 50 1C.
82
(anti/syn
84 : 16)
6e
inferior results (entries
8 and 9, respectively). Li(O-t-Bu),
Mg(OEt)2, TBAF, and Sm(O-i-Pr)3 did not afford the product
(entries 10–13). DMF performed poorly as a solvent for the
transformation and the reaction proceeded at a slower rate in
toluene (entries 14 and 15, respectively). The use of 1,4-dioxane
provided similar results to that of THF (entry 16).
7c,f
1
1
41
55
8c,f
With the optimized reaction conditions in hand, we examined
the scope of this novel catalysis with other aromatic alkenes
(Table 2). The styrene type substrate with an electron donating
methoxy substituent at the para-position (1b) underwent
silaboration effectively (entry 1). 4-Chlorostyrene (1c) gave a
low yield with formation of oligomeric side products (entry 2).
4-Trifluoromethylstyrene (1d) resulted in no reaction (entry 3).
The reaction of 2-phenylpropene (1e) proceeded smoothly,
affording the desired product 3e in good yield (entry 4), with
the boryl group being introduced at the sterically congested
carbon alpha to the phenyl group. The (E)- and (Z)-isomers of
1f were converted to the silaboration product 3f with high levels
of diastereoselectivity, with the major product having anti
stereochemistry in both cases (entries 5 and 6, respectively).40
Silaboration of the 1- and 2-vinylnaphthalenes also proceeded
with moderate yields (entries 7 and 8, respectively). (E)-Stilbene
(1i) reacted with 2 to afford the desired product 3i in excellent
yield (entry 9), although a low level of diastereoselectivity was
observed (57 : 43 mixture). 1,1-Diphenylethene also performed
well in the reaction, giving the corresponding silaboration
product 3j in good yield (entry 10). A good yield and stereo-
selectivity were observed in the reaction of 1,2-dihydronaphthalene
(1k) (entry 11). In contrast, indene (1l) gave none of the desired
products after quenching of the reaction mixture (entry 12). In all
of the reactions shown in Table 2, only a single regioisomer was
detected. It is worthy of note that non-aromatic alkenes, including
1-hexene and methyl acrylate, did not react under similar reaction
conditions. Kawachi and co-workers reported that silyl anion
9d
1
1
95 (57 : 43)
10
85
72
(trans/cis
94 : 6)
11e,g
1
12e,f
Indene 1l
—
42
0
a
Conditions: 1 (0.3 mmol), 2 (0.36 mmol), K(O-t-Bu) (1.0 M, 0.03 mL,
b
0.03 mmol), additional THF (0.57 mL). Isolated yield. The stereo-
chemical ratio was determined by 1H NMR. c 1H NMR yield deter-
mined after the crude samples were passed through a short silica gel
d
column. 1.1 equiv. of 2 was used. The reaction was conducted at
e
f
g
50 1C. 1.0 equiv. of 2 was used. 15 mol% of K(O-t-Bu) was used.
species were generated through the reaction between silylboron
compounds and n-BuLi. They carried out 29Si NMR observa-
tions of the anionic species and trapping experiments with
electrophiles (Scheme 1).28 In contrast, as reported by O’Brien
and Hoveyda, an NHC adduct of the silylboron compound is the
key intermediate in the silylation reaction of a,b-unsaturated
carbonyls.38 Related diboron adducts with an alkoxide and NHC
were also isolated by Marder and co-workers.41,42
c
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Chem. Commun., 2012, 48, 8006–8008 8007

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Notes and references
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We carried out an in situ 11B{1H} NMR experiment to obtain
a key intermediate of our reaction. Kawachi et al. did not provide
spectroscopic results for the K(O-t-Bu)/Si–B case.28 The NMR
experiment revealed that an intermediate generated by the
reaction between 2 and K(O-t-Bu) is the adduct product with a
sp3-B structure (Scheme 2). Treatment of 2 with an equimolar
amount of K(O-t-Bu) in a THF/THF-d8 (90 : 10) solution led to
a new 11B signal with a high-field shift (d 3.9), resulting from the
tetrahedral configuration of the B atom, whereas t-BuOB(pin)
(d 22.0) was not detected (ESIw).42 These results are in contrast
with Kawachi’s report including heterolytic cleavage and gen-
eration of silyl anion species.28 The high yield that was observed
in a stoichiometric reaction (Table 1, entry 6) indicates that this
species could be the key intermediate.
23 M. Tobisu, H. Fujihara, K. Koh and N. Chatani, J. Org. Chem.,
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We at this stage cannot present a clear reaction mechanism
that can explain all results we observed. We first envisaged
that the free silyl anion nucleophile was generated and under-
went nucleophilic attack to styrene substrates. However, the
NMR experiment represents the complexation of the silyl-
boron and the alkoxide rather than the free silyl anion
generation. The low reactivity of substrates with an electron
deficient substituent (4-CF3, entry 3, Table 2), which is more
electrophilic than parent styrene, does not match with that of
the silyl nucleophile model. For related base-mediated dibora-
24 A. Welle, J. Petrignet, B. Tinant, J. Wouters and O. Riant,
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´ ´ ´
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´ ´ ´
35 C. Sole, H. Gulyas and E. Fernandez, Chem. Commun., 2012,
s and E. Fernandez, Angew. Chem., Int. Ed.,
´ ´
tion, Fernandez proposed a novel concerted mechanism based
´
on the DFT calculations.32,33 This mechanism nicely explains
the reactivity including the stereospecific diboration of cis- and
trans- alkenes. However, this mechanism cannot be simply
´
s
s
and
and
´
´
´
extrapolated to our silylboration reaction because Fernandez’s
´
mechanism cannot account for the lack of stereospecificity we
found in entries 5 and 6. Further detailed studies on the
reaction mechanism are required.
48, 3769.
36 I. Ibrahem, P. Breistein and A. Cordova, Chem.–Eur. J., 2012, 18, 5175.
37 H. Li, L. Wang, Y. Zhang and J. Wang, Angew. Chem., Int. Ed.,
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38 J. M. O’Brien and A. H. Hoveyda, J. Am. Chem. Soc., 2011,
133, 7712.
We have reported the first example of the base-catalyzed
silaboration of aromatic alkenes. The key intermediate for this
catalysis would be the alkoxide adduct of silylboron reagents
rather than silyl anion species, which we first postulated. This
reaction performs well with sterically congested substrates
with good diastereoselectivity under transition-metal-free
conditions, providing a useful complementary method to
transition-metal-catalyzed silaboration.
39 V. Liepins and J. E. Backvall, Chem. Commun., 2001, 265.
¨
40 The stereoselectivities of 3f, 3j and 3l were determined by 1H NMR
spectra of the crude products. The stereochemical assignment is
given in ESIw.
41 C. Kleeberg, A. G. Crawford, A. S. Batsanov, P. Hodgkinson,
D. C. Apperley, M. S. Cheung, Z. Lin and T. B. Marder, J. Org.
Chem., 2012, 77, 785.
42 C. Kleeberg, L. Dang, Z. Y. Lin and T. B. Marder, Angew. Chem.,
Int. Ed., 2009, 48, 5350.
We gratefully acknowledge the Funding Program for Next
Generation World-Leading Researchers (NEXT Program,
No. GR002) for financial support.
c
8008 Chem. Commun., 2012, 48, 8006–8008
This journal is The Royal Society of Chemistry 2012