Synthesis of biaryl compounds through three-component assembly: Ambidentate effect of the tert-butyldimethylsilyl group for regioselective diels-alder and hiyama coupling reactions

Article abstract of DOI:10.1002/anie.200803011

Two for the price of one: A method has been developed for the regiocontrolled synthesis of multisubstituted biaryl derivatives. This protocol involves the use of the tert-butyldimethylsilyl (TBDMS) group to direct the regioselective Diels-Alder reaction of a 3-TBDMS-benzyne with a furan derivative and a subsequent Hiyama cross-coupling reaction of the TBDMS group with aryl iodides (see scheme).

Full text of DOI:10.1002/anie.200803011

Angewandte
Chemie
DOI: 10.1002/anie.200803011
Synthetic Methods
Synthesis of Biaryl Compounds through Three-Component Assembly:
Ambidentate Effect of the tert-Butyldimethylsilyl Group for
Regioselective Diels–Alder and Hiyama Coupling Reactions**
Shuji Akai,* Takashi Ikawa, Sho-ichi Takayanagi, Yuki Morikawa, Shinya Mohri,
Masaya Tsubakiyama, Masahiro Egi, Yasufumi Wada, and Yasuyuki Kita
The biaryl moiety is ubiquitous in natural products,[1a–c]
pharmaceuticals,[1d,e] polymers,[1f] sensors,[1g] and in ligands
for transition-metal catalysts.[1h,i] A number of useful methods
for the preparation of these compounds have been developed
based on the transition-metal-catalyzed cross-coupling of aryl
halides with aryl metal species through Stille, Suzuki, Negishi,
and Hiyama reactions.[2]
Multicomponent assembly protocols have been recog-
nized as a powerful means for the preparation of molecular
complexity and diversity, and are thus particularly suited for
combinatorial chemistry and diversity-oriented synthesis.[3]
As an approach towards this goal, we now report the novel
preparation of multisubstituted unsymmetrical biaryl com-
pounds through the assembly of the following three compo-
nents: a silylated benzyne, a furan derivative, and an aryl
iodide [Eq. (1)]. Thus, this methodology involves the con-
struction of silylnaphthalene derivatives by a regioselective
Diels–Alder reaction of 3-tert-butyldimethylsilylbenzynes
(3-TBDMS-benzynes) with furan derivatives followed by a
Hiyama cross-coupling reaction of the generated TBDMS-
substituted cycloadducts with aryl iodides.
We initially examined the Diels–Alder reaction of
3-phenylbenzyne (3a, R1 = Ph, R2 = H), generated in situ by
treatment of 2-bromo-6-phenylphenyl triflate (1a) with
nBuLi at À788C, and 2-tert-butylfuran (2a) to synthesize a
multisubstituted biaryl compound. However, the regioselec-
tivity of the reaction was quite low and afforded a 1:1.3
mixture of the head-to-tail (anti) and the head-to-head (syn)
cycloaddition products 4a (Table 1, entry 1).[4–6] We devised
Table 1: Diels–Alder reactions of 3-substituted benzynes 3, generated
from 1a–d, and 2a.[a]
Entry
R1
R2
1, 3
anti-4:syn-4[b]
4
Yield [%][c]
1
2
3
4
Ph
tBu
TMS
H
1a, 3a
1b, 3b
1c, 3c
1d, 3d
1:1.3
1.7:1
39:1
4a
4b
4c
4d
70
53
63
69
tBu
Me
Me
TBDMS
>50:1
[a] Reaction conditions: 1.0 equiv 1, 3.0–10 equiv 2a, 2.0 equiv nBuLi in
toluene (0.2m) at À788C under nitrogen. [b] Determined by 500 MHz
1H NMR spectroscopic analysis of a crude reaction mixture and also by
the yield of the isolated product. [c] Total yield of isolated anti- and syn-4.
Tf =triflate.
[*] Prof. Dr. S. Akai, Dr. T. Ikawa, S. Takayanagi, Y. Morikawa, S. Mohri,
M. Tsubakiyama, Dr. M. Egi
School of Pharmaceutical Sciences, University of Shizuoka
52-1, Yada, Suruga-ku, Shizuoka, Shizuoka 422-8526 (Japan)
Fax: (+81)54-264-5672
E-mail: akai@u-shizuoka-ken.ac.jp
Y. Wada, Prof. Dr. Y. Kita[+]
an alternative method for the synthesis of the biaryl
compounds which involved the Diels–Alder reaction of
3-silylbenzyne 3c or 3d[7,8] followed by the substitution of
the silyl group of the cycloadducts with an aryl group. In this
approach the Diels–Alder reaction of 3-(trimethylsilyl)ben-
zyne (3-TMS-benzyne) 3c with 2a took place to give anti-4c
with high regioselectivity (Table 1, entry 3). Moreover, the
reaction of 3-TBDMS-benzyne 3d and 2a exhibited even
higher regioselectivity (Table 1, entry 4): syn-4d was not
detected by 1H NMR analysis (500 MHz) of the crude
reaction mixture. The exclusive anti selectivity of the reaction
cannot be explained by the simple steric hindrance of the silyl
group, because the Diels–Alder reaction of 3-tert-butylben-
Graduate School of Pharmaceutical Sciences, Osaka University
1-6, Yamadaoka, Suita, Osaka 565-0871 (Japan)
[+] Current address:
School of Pharmaceutical Sciences, Ritsumeikan University
1-1-1 Noji Higashi, Kusatsu, Shiga 525-8577 (Japan)
[**] This work was supported by Grants-in-Aid for Scientific Research
and the global COE program from the Ministry of Education,
Culture, Sports, Science, and Technology (Japan). We thank the
Uehara Memorial Foundation for financial support.
Supportinginformation for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803011.
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Communications
zyne 3b, generated from 1b with 2a, gave 4b with poor
regioselectivity (Table 1, entry 2).
(Table 2, entry 3), the TMS group on the cycloadduct 4g
was not stable enough during the subsequent transformation
(see Table 3, entry 2 and Ref. [7]). Substituents R1, such as the
fluoro (Table 2, entries 11–13) and silyloxy (Table 2, entry 17)
groups, on 3 were tolerated under the stated reaction
conditions.
Next, we achieved the regioselective cleavage of the
epoxy ring of anti-4 f by using p-toluenesulfonic acid
(pTsOH·H2O) to give 1-naphthol 5a almost quantitatively,[9]
while the TBDMS group remained intact (Table 3, entry 1).
Similar reactions of the 3-TBDMS-benzynes 3d–i gener-
ated from the reactions of 1d–i and furans 2a–g were found to
give the corresponding anti-4 products (Table 2). The follow-
ing results are noteworthy: The formation of the anti-4
product invariably predominated and the regioselectivity
increased in the order: R2 = Me < nBu < tBu ꢀ SiMe3 ꢀ
SnBu3 (see Table 2). The formation of syn-4 was below the
1H NMR detection limit (500 MHz) for the reactions with
2-tert-butylfuran (2a; Table 2, entries 4, 10, 12, 15, and 17),
2-TMS-furan (2e; Table 2, entries 6 and 13), and 2-(tributyl-
stannyl)furan (2 f; Table 2, entry 7). However, no regioselec-
tivity was observed in the reaction involving 2-phenylfuran
(2d; Table 2, entry 5). The use of 2 f provides a solution to this
problem [see Eq. (3)]. The reaction of 2,3-dimethylfuran (2g)
provided anti-4k (Table 2, entry 8) with a regioselectivity that
was much higher than that of 2-methylfuran (2b). Although a
trimethylsilyl (TMS) group on benzyne 3c was also as
Table 3: Regioselective cleavage of anti-4.[a]
Entry
R1
R2
4
t [h]
5
Yield [%][b]
1
Me
Me
Me
Me
H
F
Ph
OMe
nBu
nBu
tBu
Ph
nBu
nBu
nBu
nBu
anti-4 f
anti-4g
anti-4d
anti-4h
anti-4l
anti-4n
anti-4q
anti-4s
18
23
48
19
19
48
60
24
5a
5b
5c
5d
5e
5 f
94
59
61
87
87
91
97
97
effective as
a directing group as the TBDMS group
2[c]
3
4
5
6
7
Table 2: Regioselective Diels–Alder reactions of 3-silylbenzynes 3c–i,
generated from 1c–i, and 2.[a]
5g
5h
8
[a] Reaction conditions: 1.0 equiv anti-4, 1.5–3.0 equiv pTsOH·H2O in
THF (0.15m) at RT under nitrogen. [b] Yield of isolated 5. [c] With the 3-
TMS-substituted Diels–Alder adduct anti-4g instead of the 3-TBDMS
derivative.
However, a similar reaction of the TMS analogue anti-4g
suffered partial cleavage of its TMS group, thus resulting in a
significant decrease in the yield of the 1-naphthol 5b (Table 3,
entry 2). To study the generality of this reaction several anti-4
derivatives were isomerized to 1-naphthols 5 by using
pTsOH·H2O (Table 3). The presence of electron-withdrawing
and electron-donating R1 groups on the benzene ring and the
presence of substituents R2 at the ring junction of anti-4 did
not affect the cleavage reactions, with 5 being obtained in
good to excellent yields (Table 3, entries 4–8). The presence
of a bulky tBu substituent as R2 resulted in the reaction being
slower and only a moderate yield of the product (Table 3,
entry 3). In all cases, the epoxy ring was opened regioselec-
tively.
Cleavage of the epoxy ring of anti-4i and anti-4j took
place with similar perfect regioselectivy, although complete
protodesilylation and protodestannylation occurred simulta-
neously. These reactions are useful as a selective method for
the synthesis of 8-TBDMS-1-naphthols such as 5i [Eq. (2)].
Ruthenium-catalyzed isomerization[10] was found to be
effective as an alternative method for the chemoselective
cleavage of the epoxy ring of anti-4 derivatives with acid-
sensitive functional groups. Thus, anti-4j was selectively
converted into 8-TBDMS-4-stannyl-1-naphthol 5j in the
presence of [Cp*RuCl(cod)] (0.15 equiv; Cp* = C5Me5,
cod = 1,5-cyclooctadiene). The tributylstannyl group, which
can be replaced with various aryl groups by Stille coupling
Entry
R1 (1)
R2
R3
2
anti-4:syn-4[b] Yield
[%][c]
1[d]
2[e]
3[f]
4[g]
5
6
7
8
9
10
11[e]
12[e]
13[e]
14
15
16[h]
17
Me (1d)
Me (1d)
Me (1c)
Me (1d)
Me (1d)
Me (1d)
Me (1d)
Me (1d)
H (1e)
H (1e)
F (1 f)
F (1 f)
F (1 f)
Me
nBu
nBu
tBu
H
H
H
H
H
H
H
2b
2c
2c
3.9:1
5.9:1
5.0:1
4e
4 f
4g
4d
4h
4i
68
71
65
69
53
40
64
62
56
63
58
63
49
62
53
43
64
2a >50:1
2d 1.2:1
2e >50:1
2 f >50:1
Ph
SiMe3
SnBu3
Me
nBu
tBu
nBu
tBu
SiMe3
nBu
tBu
4j
Me 2g
12:1
7.0:1
4k
4l
H
H
H
H
H
H
H
H
H
2c
2a >50:1 4m
2c
10 :1
4n
4o
4p
4q
4r
2a >50:1
2e >50:1
2c
2a >50:1
2c 6.3:1
2a >50:1
Ph (1g)
Ph (1g)
OMe (1h)
CH2OTBDMS (1i)
6.2:1
nBu
tBu
4s
4t
[a] Reaction conditions: 1.0 equiv 1, 3.0 equiv 2, 2.0 equiv nBuLi in
toluene (0.2m) at À788C under nitrogen. [b] Determined by 500 MHz
1H NMR spectroscopic analysis of a crude reaction mixture and also by
the yield of the isolated product. [c] Total yield of the isolated anti- and
syn-4. [d] With 15 equiv 2 and 1.5 equiv nBuLi. [e] With 10 equiv 2.
[f] With the 3-TMS-benzyne precursor 1c instead of the 3-TBDMS
derivative 1d. [g] From entry 4 in Table 1. [h] With the corresponding
mesylate instead of the triflate.
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Table 4: Hiyama cross-couplingreactions of 8-TBDMS-1-naphthols 5.[a]
Entry
R1
R2
5
Ar
t [h]
7
Yield [%][b]
reactions, remained intact.[11] Although the Diels–Alder
reaction of 2-phenylfuran (2d) exhibited little selectivity
(Table 2, entry 5), the series of a regioselective Diels–Alder
reaction with 2-stannylfuran (2 f), a ring-opening reaction,
and a Stille coupling is a highly reliable method for the
regioselective synthesis of 8-TBDMS-4-aryl-1-naphthols 5d’
[Eq. (3)].
1
2
3
4
5
6
7
8
9
10
Me
Me
H
Me
Me
Me
Me
Me
Me
Me
nBu
H
5a
5i
Ph
Ph
Ph
2
7a
7b
7c
7d
7e
7 f
7g
7h
7i
77
72
2.5
12
2
8
2
2
2
7
1.5
nBu
nBu
nBu
nBu
nBu
nBu
nBu
nBu
5e
5a
5a
5a
5a
5a
5a
5a
61
2-MeOC6H4
4-MeOC6H4
4-EtOCOC6H4
4-NCC6H4
4-MeCOC6H4
4-O2NC6H4
1-naphthyl
55[c]
60
80[c]
81
68
55
46
7j
[a] Reaction conditions: 1.0 equiv ArI, 1.2 equiv 5, 0.06 equiv
[{(allyl)PdCl}2], 0.24 equiv AsPh3, 1.5 equiv Cs2CO3 in DME (0.1m) at
608C under argon. [b] Yield of isolated 7. [c] Yield determined by NMR
spectroscopy. Pure product was isolated after desilylation.
Another highly intriguing, but difficult task was the
substitution of the TBDMS group of 5 with an aryl group.
Although Hiyama cross-coupling reactions of aryl silanes are
widely used,[2,12] there is no precedent, to the best of our
knowledge, of such a reaction with the robust TBDMS group.
After intensive screening of various palladium sources,
ligands, bases, and solvents, we finally found that the coupling
reaction between 5a and iodobenzene could be effected by
the use of [{(allyl)PdCl}2], AsPh3, and Cs2CO3 in 1,2-
dimethoxyethane (DME) to provide the 8-phenyl-1-naphthyl
TBDMS ether (7a) in 77% yield (Table 4, entry 1). Moderate
to good yields of the biaryl products 7 were obtained from
electron-rich (Table 4, entries 4 and 5) and electron-poor aryl
iodides (Table 4, entries 6–9) as well as from 1-naphthyl
iodide (Table 4, entry 10). This conversion protocol was found
to be tolerant of a number of functional groups, including
ester (Table 4, entry 6), nitrile (Table 4, entry 7), carbonyl
(Table 4, entry 8), and nitro groups (Table 4, entry 9).
More noteworthy is that this reaction was achieved
without using any fluoride activator. The fact that the
TBDMS group was transferred to the neighboring oxygen
atom during the reaction indicated the formation of the
pentacoordinate silicate 6 as an intermediate.[13] Although
fluoride-free coupling reactions of aryl silanes through intra-
molecular activation have been reported, all the examples are
limited to the use of flexible aliphatic alcohols.[14,15] Our
Figure 1. Contrastingcleavage sites of two types of intermediates.
In summary, we have developed a new method for the
synthesis of biaryl compounds by assembling three compo-
nents, namely, silylated benzyne precursors 1, furan deriva-
tives 2, and aryl iodides. This method features the use of the
robust tert-butyldimethylsilyl (TBDMS) group for two differ-
ent purposes, namely, the silicon-directing Diels–Alder reac-
tions of 3-TBDMS-benzynes and the Hiyama coupling of the
TBDMS-substituted cycloadducts. The Diels–Alder reactions
of the 3-TBDMS-benzynes and the 2-substituted furans
possibly take place through a nonsynchronous concerted
mechanism[5a] that reflects the regioselectivity of the reaction.
Further investigations into the detailed mechanism and
expansion of the scope of the reaction are nowunderway.
Experimental Section
General procedure for the regioselective Diels–Alder reactions of
3-TBDMS-benzynes 3d–i (Table 2): An oven-dried pear-shaped flask
was charged with 1d–i (1.0 equiv), capped with an inlet adapter with a
three-way stopcock, and then evacuated and back-filled with nitrogen
(this process was repeated twice). An azeotropic dehydration using
toluene was then carried out (this process was repeated twice).
Anhydrous toluene (0.2m) and furan 2a–g (3.0 equiv) were added by
syringe, followed by the slow addition of a 1.6m solution of nBuLi in
hexane (2.0 equiv) over 10 min at À788C. After 10–30 min, a
saturated aqueous NH4Cl solution was added and the reaction
mixture then extracted with EtOAc (this process was repeated three
times). The combined organic layers were dried over MgSO4 and
À
examples present the possible additional activation of the Si
C bond by an intramolecular phenolic hydroxy group for
cross-coupling reactions. Furthermore, our results are of
particular
interest
because
the
equatorial
À
Si C bond of 6 was cleaved (Figure 1), whereas in the
previously reported cases the axial substituents of 8 were
exclusively cleaved, as observed in common palladium-
catalyzed cross-coupling reactions.
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Communications
concentrated under reduced pressure. The crude product was purified
by flash column chromatography on silica gel.
[4] For recent reviews on benzynes, see: a) H. Pellissier, M. Santelli,
Tetrahedron 2003, 59, 701 – 730; b) H. H. Wenk, M. Winkler, W.
Sander, Angew. Chem. 2003, 115, 518 – 546; Angew. Chem. Int.
Ed. 2003, 42, 502 – 528; c) A. M. Dyke, A. J. Hester, G. C. Lloyd-
Jones, Synthesis 2006, 4093 – 4112.
[5] Regioselective Diels–Alder reactions of benzynes controlled by
their C3 substituents have been limited to the 3-fluoro-[5a] and 3-
alkoxybenzynes,[5b,c] which produced cycloadducts with a good to
excellent syn regioselectivity: a) G. W. Gribble, D. J. Keavy, S. E.
Branz, W. J. Kelly, M. A. Pals, Tetrahedron Lett. 1988, 29, 6227 –
6230; b) T. Matsumoto, T. Hosoya, M. Katsuki, K. Suzuki,
Tetrahedron Lett. 1991, 32, 6735 – 6736; c) R. G. F. Giles, A. B.
Hughes, M. V. Sargent, J. Chem. Soc. Perkin Trans. 1 1991, 1581 –
1587.
General procedure for the regioselective cleavage of anti-4
(Table 3): A round-bottom flask was charged with anti-4 (1.0 equiv)
and pTsOH·H2O (0.5 equiv), then evacuated, and back-filled with
nitrogen. Anhydrous THF (0.15m) was added by syringe and the
mixture stirred at room temperature. After a fewhours, a second
portion of pTsOH·H2O (0.5–2.0 equiv) was added and the reaction
mixture stirred for several hours (this portion-wise addition was
repeated until anti-4 was consumed, as judged by TLC analysis). The
reaction mixture was extracted with EtOAc (this process was
repeated three times). The combined organic layers were dried over
MgSO4 and concentrated under reduced pressure. The crude product
was purified by flash column chromatography on silica gel.
General procedure for the Hiyama coupling reactions of 5 with
aryl iodides (Table 4): An oven-dried reaction tube was charged with
5 (1.2 equiv), AsPh3 (0.24 equiv), and Cs2CO3 (1.5 equiv). The
reaction tube was capped with a rubber septum and then evacuated
and back-filled with argon (this process was repeated twice).
Anhydrous DME (0.1m) was added by syringe, through the septum,
followed by the addition of the aryl iodide (1.0 equiv; solid aryl
halides were added with other reagents before the evacuation) and
[{(allyl)PdCl}2] (0.06 equiv). The reaction mixture was heated to 608C
until 5 was completely consumed, as determined by TLC analysis. At
this point the reaction mixture was cooled to room temperature,
filtered through a thin pad of celite (eluting with EtOAc), and the
eluent was concentrated under reduced pressure. The crude product
purified by flash column chromatography on silica gel.
[6] The Diels–Alder reactions of the 3-alkyl- or 3-aryl benzynes
provided mixtures of the syn and anti adducts in various ratios,
the regioselectivity of which could not be predicted, see: a) J. E.
Anderson, R. W. Franck, W. L. Mandella, J. Am. Chem. Soc.
1972, 94, 4608 – 4614; b) M. S. Newman, R. Kannan, J. Org.
Chem. 1976, 41, 3356 – 3359.
[7] The Diels–Alder reaction between 3-fluoro-6-(trimethylsilyl)-
benzyne and 2-(trimethylsilyl)furan (2e) has been reported to
give exclusively the 1,5-bis(trimethylsilyl)-1,4-epoxy-8-fluoro-
1,4-dihydronaphthalene. However, the Diels–Alder reaction of
the simple 3-(trimethylsilyl)benzyne was not examined. It was
also reported that the trimethylsilyl group was cleaved during
the acid-catalyzed opening of the 1,4-epoxide of another Diels–
Alder product, see: E. Masson, M. Schlosser, Eur. J. Org. Chem.
2005, 4401 – 4405.
[8] For the regioselective dipolar [2+3] cycloaddition reactions of
3-silylated benzynes with nitrones, see: T. Matsumoto, T. Sohma,
S. Hatazaki, K. Suzuki, Synlett 1993, 843 – 846.
Received: June 24, 2008
Published online: September 5, 2008
[9] For some examples of acid-catalyzed opening of 1,4-epoxy-1,4-
dihydronaphthalenes, see: a) D. G. Batt, D. G. Jones, S.
La Greca, J. Org. Chem. 1991, 56, 6704 – 6708; b) M. Schlosser,
E. Castagnetti, Eur. J. Org. Chem. 2001, 3991 – 3997; c) D. E.
Kaelin, S. M. Sparks, H. R. Plake, S. F. Martin, J. Am. Chem. Soc.
2003, 125, 12994 – 12995; d) S. Sörgel, C. Azap, H.-U. Reißig,
Eur. J. Org. Chem. 2006, 4405 – 4418.
[10] K. Villeneuve, W. Tam, J. Am. Chem. Soc. 2006, 128, 3514 – 3515.
[11] For examples, see: G. Bringmann, R. Götz, S. Harmsen, J.
Holenz, R. Walter, Liebigs Ann. 1996, 2045 – 2058.
[12] a) T. Hiyama, J. Organomet. Chem. 2002, 653, 58 – 61; b) S. E.
Denmark, M. H. Ober, Aldrichimica Acta 2003, 36, 75 – 85.
[13] No reaction took place with the methyl ether of 5a under the
same reaction conditions, which strongly suggests that the
formation of the silicate 6 is crucial for this coupling reaction.
[14] a) H. Taguchi, K. Ghoroku, M. Tadaki, A. Tsubouchi, T. Takeda,
J. Org. Chem. 2002, 67, 8450 – 8456; b) Y. Nakao, H. Imanaka,
A. K. Sahoo, A. Yada, T. Hiyama, J. Am. Chem. Soc. 2005, 127,
6952 – 6953; c) Y. Nakao, J. Chen, M. Tanaka, T. Hiyama, J. Am.
Chem. Soc. 2007, 129, 11694 – 11695; for the activation of
vinylsilanes by an intramolecular carboxyl group, see: d) M.
Shindo, K. Matsumoto, K. Shishido, Synlett 2005, 176 – 178.
[15] For examples of intermolecular fluoride-free Hiyama reactions,
see: E. Alacid, C. Nꢀjera, Adv. Synth. Catal. 2006, 348, 2085 –
2091.
À
Keywords: arynes · biaryls · C C coupling· cycloaddition ·
synthetic methods
.
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Am. Chem. Soc. 2006, 128, 1464 – 1465; b) S.-X. Wang, M.-X.
Wang, D.-X. Wang, J. Zhu, Angew. Chem. 2008, 120, 394 – 397;
Angew. Chem. Int. Ed. 2008, 47, 388 – 391.
7676 www.angewandte.org
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Angew. Chem. Int. Ed. 2008, 47, 7673 –7676