Preparation and reactions of highly functionalized bis-arylzinc reagents using a Li(acac)-catalyzed iodine-zinc exchange

Article abstract of DOI:10.1055/s-2005-872097

Polyfunctional bis-arylzinc reagents are accessible via a new lithium acetylacetonate-catalyzed iodine-zinc exchange. This protocol tolerates sensitive functional groups (ketones, aldehydes or isothiocyanates) in the aryl iodides and allows the formation of highly functionalized zinc reagents. These zinc reagents can be used in a variety of catalyzed and uncatalyzed transformations. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-872097

PRACTICAL SYNTHETIC PROCEDURES
2625
Preparation and Reactions of Highly Functionalized Bis-Arylzinc Reagents
Using a Li(acac)-Catalyzed Iodine–Zinc Exchange
H
ighly Functio
l
nalize
o
Bis-Arylzin
r
c
R
eage
i
nts by
a
Iodine–Zin
n
c
Exchange F. Kneisel, Helena Leuser, Paul Knochel*
Ludwig-Maximilians-Universität München, Department Chemie und Biochemie,
Butenandtstrasse 5-13, Haus F, 81377 München, Germany
Fax +49(89)218077680; E-mail: paul.knochel@cup.uni-muenchen.de
PSP
Received 16 June 2005
No56
Abstract: Polyfunctional bis-arylzinc reagents are accessible via a new lithium acetylacetonate-catalyzed iodine-zinc exchange. This pro-
tocol tolerates sensitive functional groups (ketones, aldehydes or isothiocyanates) in the aryl iodides and allows the formation of highly
functionalized zinc reagents. These zinc reagents can be used in a variety of catalyzed and uncatalyzed transformations.
Key words: functionalized zinc reagents, halogen–metal exchange, cross-coupling, nucleophilic catalysis
Br
MeO
OHC
OMe
MeO
OHC
OMe
MeO
OHC
OMe
I
CuCN·2LiCl
(10 mol%)
s-Bu2Zn (0.6 equiv)
Li(acac) (10 mol%)
Procedure 1
Procedure 2
Procedure 3
Zn
NMP/Et2O
0 °C to r.t., 2 h
0 °C to r.t., 2 h
I
2
3: 77%
1
2
CHO
OHC
OAc
OAc
OAc
Zn
Pd(PPh3)4
(3 mol%)
s-Bu2Zn (0.6 equiv)
Li(acac) (10 mol%)
I
I
I
I
NMP/Et2O
0 °C to r.t., 12 h
6: 83%
r.t., 12 h
CO2Et
CO2Et
CO2Et
2
4
5
O
Me
Me
CN
O
Me
CN
Me
Cl
Zn
i-Pr2Zn (0.6 equiv)
Li(acac) (10 mol%)
I
Pd(tfp)2 (3 mol%)
Me
9: 87%
NMP/Et2O
0 °C to r.t., 10 h
r.t., 4 h
CN
7
2
8
Scheme 1
agents was unprecedented.7 Recently, we have found an
iodine–zinc exchange reaction catalyzed by Li(acac) for
Introduction
The use of polyfunctional organometallic reagents for the the preparation of polyfunctional bis(aryl)zinc reagents.8
synthesis of complex polyfunctional target molecules rep- Herein, we wish to report three typical practical proce-
resents a versatile method for the selective introduction of dures illustrating this method.
well-defined and complex building blocks under mild
conditions.1 Various halogen–metal exchange reactions
are well established and allow the preparation of function- Scope and Limitations
alized organolithium,2 organomagnesium3 and organo-
copper4 reagents. Especially organozincs are known to The mild reaction conditions of this procedure for per-
tolerate a wide range of functional groups.5 Although the forming an iodine–zinc exchange are compatible with a
preparation of functionalized bis(alkyl)zincs or mixed surprisingly wide variety of functionalities. Thus, the
functionalized arylzinc compounds6 is well established, a treatment of 3-iodo-4,5-dimethoxybenzaldehyde (1) with
preparation protocol for polyfunctional bis(aryl)zinc re- s-Bu2Zn (0.6 equiv) in the presence of a catalytic amount
of Li(acac) (10 mol%) in a N-methylpyrrolidinone
(NMP)–Et2O mixture furnished after two hours at room
temperature the desired bis(aryl)zinc reagent
(Scheme 1).
SYNTHESIS 2005, No. 15, pp 2625–2629
x
x.
x
x
.
2
0
0
5
2
Advanced online publication: 22.07.2005
DOI: 10.1055/s-2005-872097; Art ID: T08305SS
© Georg Thieme Verlag Stuttgart · New York

2626
F. F. Kneisel et al.
PRACTICAL SYNTHETIC PROCEDURES
MeO
OMe
SnMe3
Me3SnCl
r.t., 12 h
OHC
MeO
OHC
MeO
OHC
OMe
OMe
I
s-Bu2Zn (0.6 equiv)
Li(acac) (10 mol%)
10: 60%
Zn
CHO
NMP/Et2O
0 °C to r.t., 2 h
I
2
OMe
OMe
Br
Br
Pd(PPh3)4 (3 mol%)
r.t., 12 h
11: 76%
Scheme 2
Allylation of 2 with allyl bromide (1.5 equiv) at room
temperature in the presence of CuCN·2LiCl9 for two
hours provided 3-allyl-4,5-dimethoxybenzaldehyde (3) in
77% yield (Procedure 1). Another application for the
arylzinc reagents prepared by this method is a palladium-
catalyzed cross-coupling reaction.10 Bis(aryl)zinc reagent
5, which was prepared from the aryl iodide 4, provided
ethyl-6-(acetyloxy)-4¢-formyl-5-iodobiphenyl-3-carboxyl-
ate (6) in 83% yield in the presence of Pd(PPh3)4 (3 mol%)
after ten hours at room temperature (Procedure 2).
S
S
C
S
C
C
N
N
N
Zn
Me3SnCl
r.t., 4 h
s-Bu2Zn (0.6 equiv)
Li(acac) (10 mol%)
I
SnMe3
NMP/Et2O
0 °C to r.t., 2 h
CO2Et
CO2Et
CO2Et
2
12: 66%
Scheme 3
With heterocyclic aromatic iodides, the exchange reaction
proceeds also well. The corresponding bis(heteroaryl)zinc
species could be applied in a Negishi cross-coupling reac-
tion with methyl p-iodobenzoate resulting in the forma-
tion of the expected biaryl 13 in 52% yield. Another
reaction pathway for the conversion of heteroaromatic
bis(organo)zincs is a copper-catalyzed allylation which
yields the expected functionalized heteroarene 14 in 84%
yield (Scheme 4).
Finally, the reaction of bis(aryl)zinc reagent 8, which
was prepared from the aryl iodide 7 by treatment with
i-Pr2Zn,11 reacted with acetyl chloride resulting after
transmetalation with CuCN·2LiCl in the formation of 3-
acetyl-4-methylbenzonitrile (9) in 87% yield (Procedure
3).
Furthermore, an aryl iodide bearing an aldehyde function-
ality can be converted into the corresponding bis(aryl)zinc
reagent and be directly trapped with Me3SnCl affording
the corresponding formyl-substituted aryltin compound
10. It can also be used in a Negishi cross-coupling reac-
tion to provide the highly functionalized biphenyl 11
(Scheme 2).12
The bis(aryl)zinc species containing a m-cyano group re-
acted in a cross-coupling reaction with 2-nitro-iodoben-
zene and yielded biaryl 15 in 84% yield. Reaction with 4-
chloronicotinoyl chloride in the presence of CuCN·2LiCl
(10 mol%) furnished the functionalized mixed diaryl ke-
tone 16 with 77% yield (Scheme 5).
An isothiocyanate functionality is also tolerated in this io-
dine–zinc exchange reaction. Trapping of an isothiocyan-
ate-substituted zinc compound with Me3SnCl (4 h, r.t.)
provided the aromatic stannane 12 in 66% yield
(Scheme 3).13
The iodine–zinc exchange was also performed on a readi-
ly available alkynyl iodide14 with i-Pr2Zn in the presence
of Li(acac) (10 mol%) resulting in the bis(aryl)zinc re-
agent. Subsequently, a smooth intramolecular carbomet-
I
CO2Me
i-Pr2Zn
cat. Li(acac)
Me
Pd(tfp)2 (3 mol%)
S
Me
Me
Zn
O
S
I
S
CO2Me
NMP/Et2O
0 °C, 5 h
r.t., 5 h
O
O
2
13: 52%
Br
Me
Me
Me
Me
i-Pr2Zn (0.6 equiv)
Li(acac) (10 mol%)
Me
Me
N
N
N
CuCN·2LiCl (10 mol%)
Zn
0 °C to r.t., 2 h
N
N
Ph
N
Ph
Ph
I
NMP/Et2O
0 °C, 10 h
O
O
O
2
14: 84%
Scheme 4
Synthesis 2005, No. 15, 2625–2629 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Bis-Arylzinc Reagents by Iodine–Zinc Exchange
2627
NO2
CN
I
NO2
Pd(tfp)2 (3 mol%)
r.t., 6 h
CN
CN
i-Pr2Zn (0.6 equiv)
Li(acac) (10 mol%)
15: 84%
NMP/Et2O
0 °C to r.t., 10 h
I
Zn
O
2
NC
O
N
Cl
Cl
N
16: 77%
Cl
CuCN·2LiCl (10 mol%)
0 °C to r.t., 2 h
Scheme 5
EtO2C
I
1) i-Pr2Zn (0.6 equiv)
Li(acac) (10 mol%)
NMP, r.t., 12 h
Cu
Ph
Ph
CO2Et
Br
2) CuCN·2LiCl (1.1 equiv)
60 °C, 8 h
0 °C,
then r.t., 5 h
Ph
18: 54%
(E/Z > 99/1)
17
Scheme 6
alation reaction15 occurred in the presence of CuCN·2LiCl
(1.1 equiv) at 60 °C giving the expected copper reagent
17. Intermediate 17 was trapped with ethyl (2-bromo-
methyl)acrylate16 to provide the tetrasubstituted E-alkene
18 (E/Z = >99:1) in 54% overall yield (Scheme 6).
s-Dibutylzinc (s-Bu2Zn)
s-BuLi (100 mmol, 71 mL, 1.4 M in cyclohexane, 1.0 equiv) was
purified by filtration through Celite®. The concentration of the
alkyllithium reagent was determined thoroughly according to the
literature.17,18 The solution was cooled to 0 °C and the volume was
reduced in vacuo to approximately 7 mL. The orange, highly vis-
18
cous solution was cooled to –78 °C. An ethereal solution of ZnCl2
Crucial for the formation of the bis(aryl)zinc species 21
from the bis(alkyl)zinc precursor 19 is the addition of
Li(acac) and the use of NMP. The acetylacetonate ligand
coordinates to the mixed zinc species resulting in the for-
mation of the zincate 20 which undergoes a rapid ex-
change reaction with another equivalent of ArI leading to
the diarylzinc 21 (Scheme 7).
(6.82 g, 50 mmol, 1.0 M, 0.5 equiv) was slowly added. The solution
was allowed to warm up slowly to r.t. and stirred overnight under
strict light exclusion by protection with aluminum foil. The precip-
itated LiCl was removed from the solution either by decantation, fil-
tration or centrifugation.19 The resulting clear solution of s-Bu2Zn is
light sensitive and is approximately 0.60–0.80 M in concentration
(determined by iodometric titration).20
In summary, the Li(acac)-catalyzed iodine–zinc exchange
reaction allows the preparation of a range of new poly-
functional arylzinc reagents which undergo typical reac-
tions with various electrophiles like allylic halides,
aromatic iodides, acid chlorides and trialkyltin chloride.
3-Allyl-4,5-dimethoxybenzaldehyde (3); Procedure 1
A dry and argon-flushed 10-mL round-bottom flask equipped with
a magnetic stirrer was charged with 4,5-dimethoxy-3-iodobenz-
aldehyde (1; 584 mg, 2.0 mmol, 1.0 equiv) and Li(acac) (21 mg,
0.2 mmol, 0.1 equiv) in anhyd NMP (1.5 mL) and cooled to 0 °C.
The previously prepared s-Bu2Zn solution (1.4 mL of a 0.8 M solu-
tion, 1.1 mmol, 0.6 equiv) was added. The reaction mixture was
stirred at r.t. for 3 h and the complete formation of the zinc reagent
2 was checked by GC analysis using tetradecane as internal stan-
dard. Allyl bromide (363 mg, 3.0 mmol) was added to the zinc re-
agent 2, followed by the dropwise addition of CuCN·2LiCl (0.2 mL,
0.2 mmol, 0.1 equiv, 1.0 M in NMP). After 5 h, the conversion was
complete. The consumption of the zinc reagent was checked by GC
analysis of a reaction aliquot. Aq sat. NH4Cl solution (5 mL) was
added to the mixture. After addition of aq NH4Cl (30 mL), the aque-
ous phase was extracted with Et2O (3 × 80 mL) and the combined
organic layers were dried (MgSO4). The solvent was evaporated
and the product was purified by flash chromatography (SiO2, pen-
tane–Et2O, 80:20) to give the aldehyde 3 (317 mg, 77%) as a color-
less oil.
Li(acac) (10 mol%)
2 ArI
+
(s-Bu)2Zn
Ar2Zn
+
2 s-BuI
NMP/Et2O, 25 °C
19
21
ArI
s-BuI
Li(acac)
ArI
s-BuI
ArZn(s-Bu)
Ar Zn(acac)
20
s-Bu
Li
Scheme 7
Synthesis 2005, No. 15, 2625–2629 © Thieme Stuttgart · New York

2628
F. F. Kneisel et al.
PRACTICAL SYNTHETIC PROCEDURES
IR (film): 3079 (w), 3005 (w), 2978 (m), 2941 (m), 2837 (m), 2739
(w), 1694 (s), 1639 (m), 1586 (s), 1487 (s), 1464 (s), 1427 (s), 1388
(s), 1333 (m), 1299 (s), 1232 (m), 1140 (s), 1074 (m), 1003 (s), 915
(m), 858 cm–1 (m).
1H NMR (300 MHz, CDCl3): d = 7.69 (d, J = 7.8 Hz, 1 H), 7.51–
7.58 (m, 2 H), 2.58 (s, 3 H), 2.51 (s, 3 H).
13C NMR (75 MHz, CDCl3): d = 200.8, 141.6, 139.0, 135.3, 129.6,
129.1, 118.1, 114.8, 29.8, 21.0.
1H NMR (300 MHz, CDCl3): d = 9.86 (s, 1 H), 7.32 (d, J = 1.5 Hz,
1 H), 7.30 (d, J = 2.1 Hz, 1 H), 5.87–6.03 (m, 1 H), 5.01–5.14 (m,
2 H), 3.91 (s, 3 H), 3.89 (s, 3 H), 3.43–3.47 (m, 2 H).
MS (EI, 70 eV): m/z (%) = 159 (M+, 29), 144 (100), 128 (1), 116
(31), 103 (1), 89 (11), 75 (1), 63 (5).
HRMS: m/z calcd for C10H9NO [M]+: 159.0684; found 159.0698.
13C NMR (75 MHz, CDCl3): d = 191.3, 153.3, 152.6, 136.4, 134.4,
132.3, 126.6, 116.4, 109.1, 60.8, 55.9, 34.0.
MS (EI, 70 eV): m/z (%) = 206 (M+, 100), 191 (4), 177 (14), 163
(9), 147 (7), 135 (11), 131 (13), 103 (22), 91 (14).
Acknowledgment
We thank the Fonds der chemischen Industrie and the DFG for fi-
nancial support. We thank Chemetall GmbH (Frankfurt) and BASF
AG (Ludwigshafen) for the generous gift of chemicals.
HRMS: m/z calcd for C12H14O3 [M]+: 206.0943; found: 206.0944.
Ethyl 6-(Acetyloxy)-4¢-formyl-5-iodobiphenyl-3-carboxylate
(6); Procedure 2
Ethyl 4-(acetyloxy)-3,5-diiodobenzoate21 (4; 920 mg, 2.0 mmol,
1.0 equiv) and Li(acac) (21 mg, 0.2 mmol, 0.1 equiv) were dis-
solved in anhyd NMP (1.5 mL). The iodine–zinc exchange with
s-Bu2Zn (1.8 mL, 1.1 mmol, 0.6 M in Et2O) at 0 °C was complete
after 10 h according to GC analysis. Pd(PPh3)4 (55 mg, 0.05 mmol,
3 mol%) in anhyd THF (2 mL) was stirred with 4-iodobenzalde-
hyde (371 mg, 1.6 mmol, 0.8 equiv) at r.t. for 5 min, and the freshly
prepared solution of bis(aryl)zinc reagent 5 was added at 0 °C. After
stirring for 12 h at r.t., aqueous workup with aq NH4Cl solution
(80 mL), extraction with Et2O (3 × 80 mL) and purification by flash
chromatography (SiO2, pentane–Et2O, 70:30) yielded aldehyde 6
(583 mg, 83%) as colorless crystals; mp 125–126 °C.
References
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IR (KBr): 3423 (w), 1768 (s), 1721 (s), 1698 (s), 1606 (m), 1367
(m), 1301 (s), 1241 (s), 1171 (s), 1048 (m), 841 (m), 766 cm–1 (m).
1H NMR (600 MHz, CDCl3): d = 10.07 (s, 1 H), 8.53 (d,
J = 1.72 Hz, 1 H), 8.05 (d, J = 1.72 Hz, 1 H), 7.95 (d, J = 8.17 Hz,
2 H), 7.59 (d, J = 7.95 Hz, 2 H), 4.40 (q, J = 7.09 Hz, 2 H), 2.13 (s,
3 H), 1.40 (t, J = 7.09 Hz, 3 H).
13C NMR (150 MHz, CDCl3): d = 191.7, 167.4, 164.2, 151.9, 142.7,
140.5, 135.9, 135.3, 132.0, 130.3, 129.8 (2 C), 129.5 (2 C), 92.3,
61.7, 21.0, 14.3.
MS (EI, 70 eV): m/z (%) = 30 (M+, <1), 315 (100), 300 (28), 285
(10), 270 (7), 225 (3).
(4) Piazza, C.; Knochel, P. Angew. Chem. Int. Ed. 2002, 41,
3263.
HRMS: m/z calcd for C18H15IO5 [M]+: 437.9964; found: 437.9968.
(5) (a) Knochel, P. Science of Synthesis, Vol. 3; O’Neil, I., Ed.;
Thieme: Stuttgart, 2004, 5. (b) Knochel, P.; Millot, N.;
Rodriguez, A. L.; Tucker, C. A. Org. React. 2001, 58, 417.
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3-Acetyl-4-methylbenzonitrile (9); Procedure 3
3-Iodo-4-methylbenzonitrile (7; 486 mg, 2.0 mmol) and Li(acac)
(21 mg, 0.2 mmol) were dissolved in anhyd NMP (1.5 mL). The io-
dine–zinc exchange was performed at r.t. using i-Pr2Zn11 (0.2 mL,
1.1 mmol, 0.6 equiv). The iodine–zinc exchange was completed af-
ter 10 h. To a preformed palladium–phosphine complex, generated
21
by dissolving Pd(dba)2 (30 mg, 2.5 mol%) and tfp (tri-2-fu-
rylphosphine)12a (25 mg, 5.0 mol%) in THF (2 mL), was added
AcCl (237 mg, 3.0 mmol) at r.t. and the solution was stirred for
10 min. Subsequently, the freshly prepared solution of the zinc re-
agent 8 was added dropwise at r.t. The mixture was stirred for 4 h.
After addition of aq NH4Cl (30 mL), the aqueous phase was extract-
ed with Et2O (3 × 80 mL) and the combined organic layers were
dried (MgSO4). Flash chromatographic purification (SiO2, pentane–
Et2O, 70:30) yielded ketone 9 (277 mg, 87%) as a yellow solid; mp
101–102 °C.
(10) (a) Negishi, E.; Valente, L. F.; Kobayashi, M. J. Am. Chem.
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IR (film): 3115 (w), 3080 (w), 3045 (w), 2971 (w), 2931 (w), 2228
(s), 1687 (s), 1557 (m), 1494 (w), 1443 (s), 1379 (s), 1362 (s), 1291
(m), 1250 (s), 1141 (w), 1060 (w), 1034 (w), 972 (m), 960 (w), 918
(w), 897 (m), 827 (s), 656 (m), 615 (m), 545 (m), 516 (m), 446
cm–1 (m).
Synthesis 2005, No. 15, 2625–2629 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Bis-Arylzinc Reagents by Iodine–Zinc Exchange
2629
(12) (a) Klement, I.; Rottländer, M.; Tucker, C. E.; Majid, T. N.;
Knochel, P.; Venegas, P.; Cahiez, G. Tetrahedron 1996, 52,
7201. (b) Staubitz, A.; Dohle, W.; Knochel, P. Synthesis
2003, 233.
(13) Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc.
1995, 117, 704.
(14) The alkynyl iodide 6 was obtained in two steps from
commercially available 1-bromomethyl-2-iodobenzene in
accordance to a literature procedure: Bouyssi, D.; Balme, G.
Synlett 2001, 1191.
(15) Rao, S. A.; Knochel, P. J. Am. Chem. Soc. 1991, 113, 5735.
(16) Villiéras, J.; Rambaud, M. Synthesis 1982, 924.
(17) Lin, H.-S.; Paquette, L. A. Synth. Commun. 1994, 24, 2503.
(18) The exact determination of all the concentrations of the
solutions used (s-BuLi, ZnCl2, s-Bu2Zn) is decisive for the
success of this procedure. Threefold titration and averaging
have been typically done.
(19) The complete absence of LiCl is decisive for the success of
the iodine–zinc exchange procedure.
(20) Threefold titration has been performed.
(21) Prepared by acylation of 4-hydroxy-3,5-diiodobenzoate with
Ac2O (1.3 equiv) in pyridine in the presence of DMAP (0.1
equiv).
Synthesis 2005, No. 15, 2625–2629 © Thieme Stuttgart · New York