Synthesis of 1,2-dimetallic compounds via direct insertion of zinc powder in the presence of InCl3: Synthesis of ortho-bis-functionalized aromatics

Article abstract of DOI:10.1055/s-0033-1340106

A variety of functionalized 1,2-dizinc reagents can be prepared by an indium-catalyzed insertion of zinc powder into aromatic 1,2-dibromides or 1-bromo-2-triflates. These 1,2-dimetallics undergo Cu- or Pd-catalyzed acylations, allylations, or cross couplings. Georg Thieme Verlag KG Stuttgart · New York.

Full text of DOI:10.1055/s-0033-1340106

290▌
PRACTICAL SYNTHETIC PROCEDURES
practical synthetic procedures
Synthesis of 1,2-Dimetallic Compounds via Direct Insertion of Zinc Powder in
the Presence of InCl3: Synthesis of ortho-Bis-functionalized Aromatics
ortho-Bis-functionalized Aromatics
Thomas Klatt, Tobias D. Blümke, Maximilian A. Ganiek, Paul Knochel*
Ludwigs-Maximilians-Universität München, Department Chemie, Butenandtstraße 5-13, Haus F,
81377 München, Germany
Fax +49(89)218077680; E-mail: Paul.Knochel@cup.uni-muenchen.de
PSP
Received: 10.10.2013; Accepted: 14.10.2013
No265
Abstract: A variety of functionalized 1,2-dizinc reagents can be prepared by an indium-catalyzed insertion of zinc powder into aromatic
1,2-dibromides or 1-bromo-2-triflates. These 1,2-dimetallics undergo Cu- or Pd-catalyzed acylations, allylations, or cross couplings.
Key words: zinc, 1,2-dimetallic, insertion, cross-coupling, indium(III) chloride
procedure 1
Zn (3.0 equiv)
TMSCl (3 mol%)
InCl3 (7.5 mol%)
EtO2C
Br
EtO2C
ZnX
ZnX
DMPU, 50 °C, 2 h
OTf
1a
1b
2a: 75% (20 mmol scale)
X = Br, Cl, OTf
procedure 1
Zn (3.0 equiv)
TMSCl (3 mol%)
InCl3 (7.5 mol%)
MeO
Br
Br
MeO
ZnX
DMPU, 50 °C, 2 h
ZnX
2b: 59% (16 mmol scale)
X = Br, Cl
procedure 2
CHO
Br
CHO
3a
EtO2C
EtO2C
ZnX
PEPPSI-i-Pr (1.4 mol%)
THF–DMPU, 50 °C, 12 h
O
ZnX
2a
CHO
4a: 70% (20 mmol scale)
69% (2 mmol scale)
X = Br, Cl, OTf
procedure 3
Cl
OEt
3b
MeO
ZnX
MeO
CO2Et
CO2Et
Pd(PPh3)4 (10 mol%)
THF–DMPU
–30 °C to 25 °C, 14 h
ZnX
2b
X = Br, Cl
4b: 69% (16 mmol scale)
72% (2 mmol scale)
Scheme 1 Typical procedures for the preparation of 1,2-dizinc reagents via indium(III) chloride catalyzed insertion of zinc powder into
aromatic 1,2-dibromides or 1-bromo-2-triflates (Procedure 1) and subsequent functionalization via cross coupling (Procedure 2) or acylation
(Procedure 3)
available zinc dust (activated with 1,2-dibromoethane and
chlorotrimethylsilane) into the corresponding aryl bro-
Introduction
Organozinc reagents are important for organic synthesis mides or benzyl chlorides in the presence of lithium chlo-
and the interest in their unique chemical properties has in- ride.4,5 However, this method fails for the preparation of
creased during the past two decades.1 Due to their high aryl dimetallics. The works of Takai6 and others7 have
functional group tolerance and reactivity in transition- shown that activation of the metal surface is important for
metal-catalyzed reactions, such as Negishi cross cou- insertion reactions into different unsaturated aryl halides.
plings, they have found numerous synthetic applica- It was shown that salts such as indium(III) chloride can
tions.2,3 Aryl- and benzylzinc reagents are accessible activate several metals very effectively for direct insertion
under mild conditions by direct insertion of commercially reactions,7c and we have envisioned a novel catalyzed in-
sertion of aluminum or zinc into substituted arenes to af-
ford 1,2-dimetallics. We found that the insertion of zinc
dust is greatly enhanced by catalytic amounts of indi-
um(III) chloride (7.5 mol%), allowing the generation of
SYNTHESIS 2014, 46, 0290–0294
Advanced online publication: 13.11.2013
DOI: 10.1055/s-0033-1340106; Art ID: SS-2013-T0671-PSP
© Georg Thieme Verlag Stuttgart · New York
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X

PRACTICAL SYNTHETIC PROCEDURES
ortho-Bis-functionalized Aromatics
291
1,2-dimetallics from inexpensive unsaturated 1,2-dibro- thermore, the electron-rich aromatic dibromide 1b under-
mides, as well as readily available 1-bromo-2-triflates.8 went a smooth insertion, furnishing the 1,2-dizinc reagent
These organometallic zinc reagents showed high func- 2b in 59% yield. 1,2-Dimetallics bearing electron-donat-
tional group tolerance and were prepared in good yields. ing groups are generally more difficult to obtain than re-
Herein, we wish to report typical practical procedures il- agents bearing electron-withdrawing substituents. After
lustrating the preparation and reactivity of functionalized subsequent palladium-catalyzed acylation with ethyl
aromatic 1,2-dizinc reagents, as well as their scaleup.
chloroformate (3b), the diester 4b was obtained in 72%
yield on a 2 mmol scale, and in 69% yield on a 16 mmol
scale (Scheme 1, procedure 3). Interestingly, only palladi-
um-catalyzed acylations were found to proceed smoothly
and attempts to perform acylation reactions in the pres-
Scope and Limitations
1,2-Dimetallic zinc reagents are accessible under mild ence of copper(I) salts resulted in extensive decomposi-
conditions starting from 1,2-dibromides or 1-bromo-2-tri- tion. Similarly, the dizinc reagent 2a underwent a smooth
flates. Thus, the addition of the ester-functionalized tri- acylation reaction with 4-chlorobenzoyl chloride (3c) in
flate 1a to zinc dust (3 equiv) and indium(III) chloride the presence of 10 mol% tetrakis(triphenylphosphine)pal-
(7.5 mol%) in 1,3-dimethyl-3,4,5,6-tetrahydropyrimidin- ladium(0) (Table 1, entry 1). It should be noted that other
2(1H)-one (DMPU) furnished the expected dizinc reagent palladium-catalysts are not as efficient for performing
2a in 75% yield.9 The described insertion reaction was such acylation reactions. A cross coupling with 4-bromo-
performed on a 20 mmol scale and proceeds smoothly in benzonitrile (3d) led to the corresponding ortho-substitut-
two hours at 50 °C (Scheme 1, procedure 1). The resulting ed product 4d in 63–64% yield (entry 2). Even a sensitive
dizinc reagent 2a reacted well with 4-bromobenzaldehyde methyl ester 1c was tolerated under these reaction condi-
(3a) in a twofold palladium-catalyzed cross-coupling tions, and Negishi cross coupling of the corresponding
reaction in the presence of 1.4 mol% [1,3-bis(2,6-diiso- 1,2-dimetallic reagent 2b with ethyl 3-bromobenzoate
propylphenyl)imidazol-2-ylidene](3-chloropyridyl)palla- (3e) furnished the triester 4e in 61–63% yield (entry 3).
dium(II) dichloride (PEPPSI-iPr),10 affording the ester 4a Remarkably, using these synthetic procedures, a nitrile
in 69% yield on a 2 mmol scale. Upscaling was easily pos- functionality (1d) could be tolerated, and the correspond-
sible leading to a similar yield of 70%, on a larger scale ing dimetallic reagent 2c was obtained in 57% yield. After
(Scheme 1, procedure 2). The unprotected aldehyde was subsequent palladium-catalyzed acylation, the diketone 4f
perfectly compatible with the reaction conditions. Fur- was isolated in 63–68% yield (entry 4).
Table 1 Indium(III) Chloride Catalyzed Zinc Insertion into Substrates 1 and Subsequent Functionalization Leading to Products 4a
Entry
Substrate
Yieldb (%) Electrophilec
Productd
Scale (mmol) Yield (%)
C6H4-4-Cl
O
EtO2C
Br
EtO2C
O
O
Cl
10
2
65
54
1e
75
OTf
Cl
C6H4-4-Cl
1a
1a
3c
4c
CN
CN
CN
EtO2C
20
2
64
63
2f
Br
3d
4d
MeO2C
Br
MeO2C
CO2Et
CO2Et
20
2
63
61
3f
70
Br
CO2Et
OTf
3e
1c
4e
Ph
Ph
O
NC
NC
Br
O
O
Cl
40
2
68
63
4e
57
OTf
1d
3f
4f
© Georg Thieme Verlag Stuttgart · New York
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292
T. Klatt et al.
PRACTICAL SYNTHETIC PROCEDURES
Table 1 Indium(III) Chloride Catalyzed Zinc Insertion into Substrates 1 and Subsequent Functionalization Leading to Products 4a (continued)
Entry
Substrate
Yieldb (%) Electrophilec
Productd
Scale (mmol) Yield (%)
CO2Et
CO2Et
NC
20
2
53
55
5f
1d
I
3g
CO2Et
4g
O
C6H4-4-Cl
O
O
O
O
O
O
Br
Cl
20
2
61
62
6e
60
OTf
Cl
C6H4-4-Cl
1e
1e
3c
4h
OHC
CO2Et
15
2
58g
54g
7f
CO2Et
Br
CO2Et
3e
4i
COMe
O
OHC
15
2
59g
60g
8f
1e
Br
COMe
OMe
3h
4j
EtO2C
Br
OMe
EtO2C
EtO2C
10
2
64
69
9f
45
59
59
EtO2C
OTf
I
3i
1f
OMe
4k
CO2Et
CO2Et
Br
CO2Et
20
2
63
61
Br
10h
Br
3j
1g
4l
CO2Me
CO2Me
MeO
MeO
1b
Br
CO2Me
15
2
71
68
11f
Br
I
3k
4m
a Reaction temperature: 50 °C; reaction time: 2 h.
b Yield determined by GC analysis of iodolyzed reaction aliquots.
c 2.0 equiv of electrophile were used.
d Yield of isolated analytically pure compounds.
e 10 mol% [Pd(PPh3)4] was used.
f 1.4 mol% PEPPSI-iPr.
g Yield including a deprotection step.
h 10 mol% CuCN·2LiCl was added.
Synthesis 2014, 46, 290–294
© Georg Thieme Verlag Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
ortho-Bis-functionalized Aromatics
293
Preparation of Aromatic 1,2-Dizinc Reagents from Aromatic
1,2-Dibromides or ortho-Bromo Triflates; Typical Procedure 1
InCl3 (7.5 mol%) was placed in an argon-flushed Schlenk flask and
dried for 5 min by heating with a heat gun (450 °C) under high vac-
uum. Zinc powder (3 equiv) was added under argon, and the drying
Cross coupling of the organometallic reagent 2c with eth-
yl 4-iodobenzoate (3g) afforded the desired product 4g in
53–55% yield (entry 5). An aldehyde was not compatible
with the insertion reaction conditions; however, after pro-
tection as an acetal, the bromo triflate 1e reacted well with process was repeated for a further 5 min. The flask was evacuated
and backfilled with argon (3 ×) and then DMPU (1–2 mL/mmol)
was added, along with the internal standard (heptadecane). TMSCl
(3 mol%) was added, and the mixture was heated with a heat gun
until it began to boil. The mixture was cooled to r.t. (20 °C), the tri-
zinc powder and led to the 1,2-dizinc reagent 2d in 60%
yield. This dimetallic reagent was smoothly acylated with
4-chlorobenzoyl chloride (3c), affording the diketone 4h
in 61–62% yield (entry 6). Furthermore, Negishi cross
flate or bromide (1 equiv) was added in one portion, and the mixture
coupling of this dizinc reagent with ethyl 3-bromobenzo-
ate (3e) or the methyl ketone 3h, followed by acetal cleav-
age, furnished the expected products 4i,j in 54–60% yield
(entries 7 and 8). Also, a dimetallic species bearing two
ester groups 2e could be generated, and after cross cou-
pling with 4-iodoanisole (3i), the highly functionalized
terphenyl 4k was obtained in 64–69% yields (entry 9).
Similarly, 1,2-dizinc reagents can be derived from aro-
matic 1,2-dibromides. Thus, 1,2-dibromobenzene (1g) or
the electron-rich 1,2-dibromoanisole (1b) were converted
into the corresponding 1,2-dizinc reagents 2e,f both in
59% yield. The hereby obtained reagents were allylated
(10 mol% CuCN·2LiCl)11 with ethyl 2-(bromometh-
yl)acrylate12 (3j) or underwent a Negishi cross coupling
with an aryl bromide 3k, providing the bis(acrylate) 4l and
the terphenyl 4m in 61–71% yields (entries 10 and 11).
was stirred at 50 °C. The progress of the insertion reaction was mon-
itored by GC analysis of hydrolyzed reaction aliquots quenched
with 2 M HCl or sat. aq NH4Cl solution until a conversion of >95%
was reached (usually 2 h). The zinc powder was allowed to settle
down, and the remaining solution containing the zinc reagent was
used for further reactions.
Ethyl 4,4′′-Diformyl-[1,1′:2′,1′′-terphenyl]-4′-carboxylate
(4a);8 Typical Procedure 2
The zinc reagent 2a was prepared according to typical procedure 1
from ethyl 3-bromo-4-[(trifluoromethylsulfonyl)oxy]benzoate (1a,
7.54 g, 20 mmol), Zn powder (3.92 g, 60 mmol), and InCl3 (0.33 g,
1.5 mmol). The reaction was carried out in DMPU (20 mL) at 50 °C
for 2 h. Iodolysis indicated 75% yield (15.0 mmol) of bimetallic re-
agent. The solution containing the zinc reagent was separated from
the remaining zinc powder and transferred to a new flask containing
a solution of 4-bromobenzaldehyde (3a, 7.40 g, 40 mmol) and
PEPPSI-iPr (0.19 g, 0.28 mmol) in THF (20 mL). The mixture was
stirred at 50 °C for 12 h and then it was quenched with 2 M HCl
(50 mL). Flash column chromatographical purification (silica gel,
isohexane–Et2O, 5:1) afforded 4a as a white solid (3.76 g,
10.5 mmol, 70%); mp 123–126 °C.
Conclusion
IR (diamond-ATR, neat): 2955, 2926, 2849, 2748, 1716, 1696,
1602, 1574, 1568, 1517, 1473, 1424, 1400, 1387, 1366, 1302, 1286,
1266, 1235, 1211, 1169, 1140, 1106, 1043, 1020, 1010, 1003, 918,
895, 838, 828, 767, 748, 735, 723, 702, 666 cm–1.
1H NMR (400 MHz, C6D6): δ = 9.58 (s, 1 H), 9.58 (s, 1 H), 8.27–
8.24 (m, 1 H), 8.18 (dd, J = 8.0, 1.8 Hz, 1 H), 7.38–7.32 (m, 4 H),
7.12 (dd, J = 8.0, 0.4 Hz, 1 H), 6.88–6.84 (m, 4 H), 4.21 (q, J = 7.2
Hz, 2 H), 1.08 (t, J = 7.1 Hz, 3 H).
In summary, a straightforward and efficient insertion of
zinc powder promoted by catalytic amounts of indium(III)
chloride was demonstrated. The preparation of various
functionalized 1,2-dizinc species using commercially
available metal powder and aromatic dibromides or read-
ily available 1-bromo-2-triflates under mild reaction con-
ditions was performed. The chemical properties of these
organometallic reagents and the topological proximity of
the two metals allow a novel approach to a variety of or-
tho-bis-functionalized aromatics. We have extended our
previous work and have reported herein readily scalable
experimental procedures (12–40 mmol scale) using stan-
dard laboratory glassware and usual laboratory tech-
niques. Further synthetic extensions of this method are
currently underway in our laboratories.
13C NMR (100 MHz, C6D6): δ = 190.6, 190.6, 165.7, 146.0, 145.9,
143.7, 140.0, 135.8, 135.6, 132.0, 131.2, 131.0, 130.5, 130.4, 129.6,
129.5, 129.4, 61.3, 14.3.
MS (EI, 70 eV): m/z (%) = 359 (20), 358 (100), 313 (40), 285 (10),
229 (40), 228 (33), 227 (11), 226 (11).
HRMS (EI): m/z [M]+ calcd for C23H18O4: 358.1205; found:
358.1196.
Diethyl 4-Methoxyphthalate (4b);8 Typical Procedure 3
The zinc reagent 2b was prepared according to typical procedure 1
from 1,2-dibromo-4-methoxybenzene (1b, 4.25 g, 16 mmol), Zn
powder (3.14 g, 36 mmol), and InCl3 (0.2 g, 1.2 mmol). The reac-
tion was carried out in DMPU (16 mL) at 50 °C for 2 h. Iodolysis
indicated that the bimetallic reagent was obtained in 59% yield
(9.44 mmol). The solution containing the zinc reagent was separat-
ed from the remaining zinc powder and transferred to a new flask
and cooled to –30 °C. Pd(PPh3)4 (1.85 g, 1.6 mmol) was added at
–30 °C, followed by subsequent addition of ethyl chloroformate
(3b, 3.47 g, 32 mmol). The mixture was stirred at –30 °C and slowly
warmed to r.t. and was quenched after 14 h with 2 M HCl (16 mL).
Flash column chromatographical purification (silica gel, isohex-
ane–EtOAc, 4:1) afforded 4b as colorless oil (1.64 mg, 6.51 mmol,
69%).
All reactions were carried out under argon atmosphere in dried
glassware. Commercially available starting materials were pur-
chased from commercial suppliers and used without further purifi-
cation unless otherwise stated. THF was continuously refluxed and
freshly distilled from Na/benzophenone ketyl under N2. Yields refer
to isolated compounds estimated to be >95% pure as determined by
1H NMR and capillary GC analyses.
Preparation of a 1 M CuCN·2LiCl Solution in THF
A Schlenk flask was charged with LiCl (16.96 g, 0.40 mol) and
CuCN (17.95 g, 0.20 mol) and dried for 6 h at 150 °C under high
vacuum (5·10–2 mbar; dry stirring). Careful addition of THF (200
mL) and stirring overnight furnished a slightly yellowish to green-
ish solution that was stored over 4 Å molecular sieves.
IR (diamond-ATR, neat): 2982, 2939, 1714, 1602, 1573, 1500,
1464, 1446, 1423, 1390, 1366, 1324, 1274, 1229, 1182, 1173, 1115,
1067, 1030, 930, 924, 849, 803, 779, 702, 672 cm–1.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 290–294

294
T. Klatt et al.
PRACTICAL SYNTHETIC PROCEDURES
1H NMR (400 MHz, C6D6): δ = 7.76 (d, J = 8.6 Hz, 1 H), 7.12 (d,
J = 2.5 Hz, 1 H), 6.57 (dd, J = 8.6, 2.7 Hz, 1 H), 4.26 (q, J = 7.0 Hz,
2 H), 4.14 (q, J = 7.2 Hz, 2 H), 3.05 (s, 3 H), 1.10 (t, J = 7.1 Hz, 3
H), 1.03 (t, J = 7.1 Hz, 3 H).
13C NMR (100 MHz, C6D6): δ = 168.1, 166.3, 162.2, 137.0, 131.7,
123.5, 115.7, 113.8, 61.5, 61.0, 54.9, 14.1, 14.1.
(3) (a) Milne, J. E.; Buchwald, S. L. J. Am. Chem. Soc. 2004,
126, 13028. (b) Han, C.; Buchwald, S. L. J. Am. Chem. Soc.
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MS (EI, 70 eV): m/z (%) = 252 (29), 207 (25), 180 (10), 179 (100).
HRMS (EI): m/z [M]+ calcd for C13H16O5: 252.0998; found:
252.0999.
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1107.
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Acknowledgment
(7) (a) Blümke, T. D.; Chen, Y.-H.; Peng, Z.; Knochel, P. Nat.
Chem. 2010, 2, 313. (b) Blümke, T. D.; Peng, Z.; Mayer, P.;
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We thank the European Research Council (ERC) under the
European Community’s Seventh Framework Programme
(FP7/2007-2013) ERC Grant Agreement No. 227763 for financial
support. We also thank BASF SE (Ludwigshafen); W. C. Heraeus
(Hanau), and Chemetall GmbH (Frankfurt) for the generous gift of
chemicals.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.SnuIomprifgtooatrinSugpIoi
n
nfomartit
(8) Blümke, T. D.; Klatt, T.; Koszinowski, K.; Knochel, P.
Angew. Chem. Int. Ed. 2012, 51, 9926.
(9) The yield was determined by GC analysis of iodolyzed
reaction aliquots in THF
(10) (a) Organ, M. G.; Calimsiz, S.; Sayah, M.; Hoi, K. H.;
Lough, A. J. Angew. Chem. Int. Ed. 2009, 48, 2383.
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Synthesis 2014, 46, 290–294
© Georg Thieme Verlag Stuttgart · New York