LiCl-mediated preparation of functionalized benzylic indium(III) halides and highly chemoselective palladium-catalyzed cross-coupling in a protic cosolvent

Article abstract of DOI:10.1002/anie.200805588

(Chemical Equation Presented) Sensitive functional groups such as COR, CHO, or CH₂OH can be present in benzylic indium reagents prepared by the direct insertion of indium in the presence of LiCl. These reagents undergo palladium-catalyzed cross

Full text of DOI:10.1002/anie.200805588

Communications
DOI: 10.1002/anie.200805588
Indium Reagents
LiCl-Mediated Preparation of Functionalized Benzylic Indium(III)
Halides and Highly Chemoselective Palladium-Catalyzed Cross-
Coupling in a Protic Cosolvent**
Yi-Hung Chen, Mai Sun, and Paul Knochel*
The development of palladium-catalyzed cross-coupling reac-
tions has revolutionized the formation of carbon–carbon
bonds.[1] These coupling reactions have found many applica-
tions in natural product synthesis,[2] material science,[3] and
medicinal chemistry.[4] The Suzuki reaction, which involves
organoboron compounds, has been widely used because of
the ready availability of boronic esters and their excellent
compatibility with many functional groups during the cross-
coupling reaction.[5] Typically, functional groups bearing
acidic protons, ketones, and aldehydes are compatible with
these coupling reactions. However, the low reactivity of
Scheme 1. Preparation of benzylic indium reagents 2 by the direct
insertion of In0 in the presence of LiCl followed by activation with
iPrMgCl·LiCl and palladium-catalyzed cross-coupling. FG=functional
group, SPhos=2-dicyclohexylphosphino-2’,6’-dimethoxybiphenyl.
boronic acids may require harsh reaction conditions or
sophisticated ligand systems. Furthermore, some classes of
boronic acid derivatives such as functionalized benzylic
boronic acids are more difficult to prepare.[6]
Indium organometallic reagents have attracted consider-
able attention because of their unique compatibility with
aqueous media.[7] Palladium-catalyzed cross-coupling reac-
tions of organoindium reagents with aryl halides and triflates
were pioneered by Sarandeses and co-workers.[8] The stan-
dard method for the preparation of organoindium reagents
involves a Li/In or Mg/In transmetalation. Recently, the
preparation of arylindium(III) reagents by direct metal
insertion in the presence of LiCl was reported by us and
Papoian and Minehan.[9] Herein, we report a general way to
prepare benzylic InIII reagents[10] by the direct insertion of In0
into benzylic chlorides and bromides, as well as the use of
these reagents in highly chemoselective cross-coupling reac-
tions in protic cosolvents.
20–30 min for the benzylic bromides but required 408C and 6–
15 h for benzylic chlorides. A broad range of sensitive
functional groups such as CN, CO2Et, COR, CHO, and
CH2OH were tolerated (see Table 1).
The palladium-catalyzed cross-coupling reactions of the
In reagents 2 with aryl iodides were very sluggish and not
preparatively useful. However, the addition of a protic
solvent (such as ethanol) to these In reagents prior to the
cross-coupling dramatically increased their reactivity. This
compatibility of organoindium species with aqueous media
has been reported previously.[7] Palladium-catalyzed cross-
coupling reactions of indium organometallic reagents with
aryl halides in aqueous media was first reported by Oshima
and co-workers.[8k] The reaction in a protic cosolvent still
required high temperature (reflux) to afford the cross-
coupling product. However, we found that a transmetalation
of 2 with iPrMgCl·LiCl[11] (1.1 equiv, À608C, 30 min) provided
a more-reactive InIII reagent 3. These mixed InIII reagents
underwent smooth cross-coupling with various aryl iodides 4
at 25–308C and with aryl bromides at 408C to give products 5,
with selective transfer of the aryl group.
Various functionalized benzylic chlorides and bromides 1
were treated with In powder (1.2–2.5 equiv) in THF in the
presence of LiCl (1.2–2.5 equiv), which allowed the smooth
formation of the corresponding benzylic InIII reagents 2
(Scheme 1). The insertion reactions proceeded at 08C within
[*] Dr. Y.-H. Chen, M. Sun, Prof. Dr. P. Knochel
Department Chemie & Biochemie
The treatment of ethyl (3-chloromethyl)benzoate (1a)
with activated In powder (99.99% from Chempur, 2.5 equiv)
and LiCl (2.5 equiv) in THF at 408C for 12 h provided the
corresponding InIII reagent 2a. After the addition of
iPrMgCl·LiCl (1.1 equiv, À608C, 30 min), ethanol or water
was added to give a 6:1 (THF/cosolvent) ratio. Ethyl 4-
bromobenzoate (4a; 0.7 equiv) was then added and the
resulting reaction mixture was warmed to 25–308C. The cross-
coupling was carried out in the presence of Pd(OAc)2
(2 mol%) and SPhos[12,13] (4 mol%) at 408C for 4 h, which
afforded the desired product 5a (84% yield with ethanol as
Ludwig Maximilians-Universitꢀt Mꢁnchen
Butenandtstrasse 5–13, Haus F, 81377 Mꢁnchen (Germany)
Fax: (+49)89-2180-77680
E-mail: paul.knochel@cup.uni-muenchen.de
[**] Y.-H.C. thanks the Humboldt Foundation for financial support. We
thank Dr. Sheng-Kai Wang (The Scripps Research Institute) for
valuable suggestions. We thank the Fonds der Chemischen
Industrie, the DFG, and the European Research Council (ERC) for
financial support. We thank Chemetall for the generous gift of
chemicals.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805588.
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Table 1: Direct insertion of indium into benzylic halides 1 and cross-coupling with electrophiles 4 to give biphenylmethanes 5.
Entry Benzyl halide[a]
Electrophile
Product
Entry Benzyl halide[a] Electrophile
Product
yield [%][b]
yield [%][b]
1
8
1a (2.5 equiv,
408C, 12 h)
4a
5a: 84%[c] (86%)[d]
1g (1.2 equiv,
08C, 20 min)
4h
5h: 75%[c]
2
9
1a
4b
4c
5b: 77%[c]
5c: 92%[c]
1g
4i
4j
5i: 75%[c]
5j: 82%[c]
3
10
1b (2.5 equiv,
408C, 6 h)
1h (2.5 equiv,
408C, 15 h)
4
11
12
1c (2.5 equiv,
408C, 6 h)
4d
4e
4 f
5d: 94%[d]
5e: 78%[d]
5 f: 72%[c]
1i (1.2 equiv,
4k
5k: 75%[c]
5l: 62%[c]
5m: 37%[f]
08C, 20 min)[e]
5
1d (1.2 equiv,
08C, 20 min)[e]
1j (1.2 equiv,
4l
08C, 20 min)[e]
6
13
14
1e (2.5 equiv,
408C, 8 h)
1k (2.5 equiv,
408C, 15 h)[e]
4m
7
1 f (2.5 equiv,
408C, 7 h)
4g
5g: 77%[c]
1l (1.2 equiv,
4n
5n: 56%[f]
08C, 20 min)[e]
[a] Reaction conditions for the formation of the benzylic indium reagents (equivalents of In powder and LiCl, reaction temperature, and reaction time).
[b] Yields of pure and isolated material. Bn=benzyl, Ts=tosyl. [c] THF/EtOH (6:1). [d] THF/H2O (6:1). [e] The indium reagent was used without
activation with iPrMgCl·LiCl. [f] The cross-coupling reaction was performed in THF without any cosolvent.
cosolvent and 86% yield with water as cosolvent; see Table 1,
entry 1).[14] These cross-coupling reactions displayed a unique
chemoselectivity and a range of functional groups were
tolerated: the cross-coupling of 3a with 4-bromobenzyl
alcohol (4b; 0.7 equiv) bearing a free hydroxy group occurred
smoothly without any protecting groups, and afforded the
benzylic alcohol 5b in 77% yield (Table 1, entry 2). Similarly,
benzylic organometallic compounds bearing a ketone or an
aldehyde group were readily tolerated. Thus, the keto-
substituted benzylic chlorides 1b,c were cleanly converted
into the corresponding benzylic InIII reagents 2b,c. These
reagents displayed a substantial thermostability and could be
stirred at 408C for at least 15 h without appreciable decom-
position. After their convertion into the isopropyl-indium-
(III) intermediates 3b,c, cross-coupling reactions with an aryl
iodide 4c, which bears a secondary alcohol, and 4-bromo-
benzaldehyde (4d[15]
provided the expected polyfunc-
)
tionalized diarylmethanes 5c and 5d in 92 and 94% yield,
respectively (Table 1, entries 3 and 4). Similarly, (3-bromo-
methyl)benzaldehyde (1d) was smoothly converted into the
corresponding InIII reagent 2d. The activation of 2d with
iPrMgCl·LiCl was problematic in this case because of the
presence of the formyl group. Nevertheless, its cross-coupling
with iodothiophene 4e in THF/water (5:1) at reflux was
complete in 3.5 h, and afforded the desired product 5e in 78%
yield (Table 1, entry 5). A range of electron-withdrawing
substituents such as a trifluoromethyl group (1e), a fluorine
atom (1 f), or a cyanide group (1g) were tolerated during the
formation of the corresponding benzylic In reagents. The
subsequent cross-coupling reactions proceeded as expected
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(after conversion into their isopropyl derivatives 3) with a
range of aryl halides bearing acidic hydrogen atoms, for
example, with indole 4 f, amide 4g, benzylic alcohol 4h, and
sulfonamide 4i, with the cross-coupling products 5 f–i
obtained in each case in 72–77% yield (Table 1, entries 6–
9). Electron-donating substituents such as a methoxy group
(1h) or even a hydroxymethyl group (1i,j) led in THF to the
corresponding In reagents, despite the presence of the
unprotected alcohol in 1i and 1j. Their cross-coupling
reactions provided the diarylmethanes 5j–l in 62–82% yield
(Table 1, entries 10–12). Benzylic In reagents having an
electron-withdrawing group in the para position (2k,l)
showed moderate stability and were used directly for the
cross-coupling step without activation with iPrMgCl·LiCl.
These coupling reactions were performed in THF at reflux
without any cosolvent and afforded the cross-coupling
products 5m,n in 37 and 56% yield, respectively (Table 1,
entries 13 and 14).
The exceptional chemoselectivity of these benzylic InIII
reagents could be extended to 5-iodouracil (6, pKa = 14.1 in
DMSO; Scheme 2).[16] The reaction of the InIII derivative 2b
with unprotected 5-iodouracil (6; 0.5 equiv) in a 5:1 mixture
of THFand EtOH at reflux provided the 5-substituted uracil 7
in 72% yield.
Scheme 3. Coupling of benzylic indium(III) compounds 2h and 2c
with unprotected carbohydrate derivatives.
ious electrophiles bearing acidic hydrogen atoms, such as an
amide, an alcohol, a sulfonamide, an unprotected sugar, or a
uracil derivative. Furthermore, the cross-coupling is acceler-
ated by a protic cosolvent and may be well suited to
combinatorial chemistry or drug screening. Further applica-
tions of these indium reagents for other transformations are
currently underway.
Experimental Section
Scheme 2. Coupling of benzylic indium(III) 2b with 5-iodouracil (6).
Typical procedure (indium insertion)—Preparation of In reagent 2b
(Table 1, entry 3): LiCl (212 mg, 5 mmol) was placed in an argon-
flushed flask and dried with a heat gun under high vacuum (1 mbar).
Indium powder (574 mg, 5 mmol) was added, followed by THF
(2 mL). 1,2-Dibromoethane (5 mol%) was then added, followed by
trimethylsilyl chloride (2 mol%), and the resulting mixture was
heated with a heat gun to activate the indium powder. A solution of
1b (338 mg, 2 mmol) in THF (2 mL) was added dropwise at 258C and
the resulting mixture was stirred at 408C for 12 h. The completion of
the reaction was checked by GC analysis.
Typical procedure (cross-coupling)—Preparation of 5c (Table 1,
entry 3): The solution of organoindium 2b in THF was carefully
transferred to an argon-flushed flask by syringe so as to separate it
from any remaining In powder. The resulting solution was cooled to
À608C and then iPrMgCl·LiCl (2.04m solution in THF, 1.08 mL,
2.2 mmol) added. The reaction mixture was stirred at À60 to À508C
for 30 min, then EtOH (1 mL) added, and the mixture warmed to 25–
308C. 4c (347 mg, 1.40 mmol) was then added, followed by a solution
of Pd(OAc)2 (10 mg, 0.042 mmol) and SPhos (35 mg, 0.084 mmol) in
THF (1 mL). The reaction mixture was stirred at 408C for 8 h. After
work-up, the residue was purified by flash chromatography on silica
gel (diethyl ether/pentane 1:1) to afford compound 5c (325 mg, 92%).
The preparation of aromatic carbohydrates is important
due to their potential application as pharmaceuticals.[17] The
reaction of sugar derivatives with organometallic reagents
usually requires the extensive use of protecting groups; this
could be completely avoided with benzylic organoindium
reagents. Thus, biphenyl glucopyranoside 9, which was
identified as a lead compound for the treatment of type 2
diabetes,[18] could be prepared by the cross-coupling of
organoindium reagent 2h with the O-2-iodophenoxygluco-
side (8; Scheme 3).[19] The glucopyranoside 9 was generated in
82% yield without the use of any protecting groups for the
four hydroxy groups present in 8. An aromatic bromide (such
as 10) also underwent a smooth palladium-catalyzed cross-
coupling with the indium reagent 2c to give the galactopyr-
anoside 11 in 89% yield.
In summary, we have shown that a range of highly
functionalized benzylic indium compounds can be readily
prepared by the direct insertion of In powder in the presence
of LiCl. A remarkable functional group compatibility, includ-
ing the presence of a COR, CHO, or CH2OH group in the
starting benzylic halides, was observed. These benzylic
reagents also display an exceptional chemoselectivity, under-
going palladium-catalyzed cross-coupling reactions with var-
Received: November 15, 2008
Published online: February 6, 2009
Keywords: biaryls · cross-coupling · organoindium reagents ·
.
palladium
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