Structure-reactivity relationships in negishi cross-coupling reactions

Article abstract of DOI:10.1002/chem.200902132

Competition experiments have been performed to determine the relative reactivities of substituted bromobenzenes and of different arylzinc reagents in the [Pd(PPh₃)₄]-catalyzed Negishi cross-coupling reaction in THF at 25 °C. The crosscoupling reactions are accelerated by electron acceptors in the bromobenzenes, the effect of which increases in the order ortho a larger effect than substituent variations in the arylzinc halides (ρ = -0.98).

Full text of DOI:10.1002/chem.200902132

DOI: 10.1002/chem.200902132
Structure–Reactivity Relationships in Negishi Cross-Coupling Reactions
Zhi-Bing Dong,[a] Georg Manolikakes,[b] Lei Shi,[b] Paul Knochel,[b] and Herbert Mayr*[b]
Dedicated to Professor Mieczysław Ma˛kosza on the occasion of his 75th birthday
Abstract: Competition experiments have been performed to determine the relative
reactivities of substituted bromobenzenes and of different arylzinc reagents in the
ꢀ
[Pd
(PPh3)4]-catalyzed Negishi cross-coupling reaction in THF at 258C. The cross-
Keywords: C C bond formation ·
Hammett equation · kinetics · reac-
tion mechanisms · substituent ef-
fects
coupling reactions are accelerated by electron acceptors in the bromobenzenes,
the effect of which increases in the order ortho <meta <para. On the other hand,
electron acceptors in the arylzinc halides diminish the reaction rates. Hammett cor-
relations show that substituent variations in the bromobenzenes (1=+2.5) have a
larger effect than substituent variations in the arylzinc halides (1=ꢀ0.98).
Introduction
well as for carbon–heteroatom bond formations. The general
catalytic cycle, consisting of an oxidative addition, a trans-
metalation, and a reductive elimination step, furnishes the
coupling product and regenerates the active catalyst
(Scheme 1).
Pd-catalyzed cross-coupling reactions of unsaturated halides
with organometallic reagents belong to the best methods for
the formation of CACHTUNGTRENNUNG ACHUTNGTRENNUGN
(sp2)–C(sp2) bonds.[1] Among many orga-
nometallics used in these cross-couplings, organozinc re-
agents have proven to be particularly useful, as they react
under very mild conditions,[2,3] and are compatible with
many functionalities.[4] Therefore, they can be used for the
synthesis of polyfunctional molecules without the need of
protecting groups.
So far most mechanistic studies on cross-coupling reac-
tions have focused on other organometallic reagents, espe-
cially organostannanes.[5] The conclusions that arise from
studies on Stille reactions have been considered to be repre-
sentative also for palladium- and nickel-catalyzed cross-cou-
pling reactions with other organometallic nucleophiles as
[a] Dr. Z.-B. Dong
School of Chemistry and Chemical Engineering
Jiangsu University
301 Xuefu Road, Zhenjiang 212013, Jiangsu (China)
Scheme 1. Catalytic cycle of the palladium-catalyzed Negishi reaction.
[b] Dr. G. Manolikakes, Dr. L. Shi, Prof. Dr. P. Knochel,
Prof. Dr. H. Mayr
We were interested in gaining a more detailed insight into
the mechanism of the Negishi cross-coupling reaction by
studying the influence of the substitution pattern of both the
zinc reagent and the aryl halide on the rates of the various
steps.[6]
Department Chemie und Biochemie
Ludwig-Maximilians-Universitꢀt Mꢁnchen
Butenandtstrasse 5–13 (Haus F), 81377 Mꢁnchen (Germany)
Fax : (+49)89-2180-77717
E-mail: herbert.mayr@cup.lmu.de
Supporting information for this article (determination of the relative
reaction rates k) is available on the WWW under http://dx.doi.org/
10.1002/chem.200902132.
248
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Chem. Eur. J. 2010, 16, 248 – 253

FULL PAPER
Results and Discussion
(3) (mass balance) yields Equation (4), which calculates the
competition constant k from the gas chromatographically
determined ratios [P1]t/[[R1]t and [P2]t/[[R2]t.
Relative reactivities of bromoarenes in the oxidative addi-
tion step: The influence of substituents on the reactivities of
aryl bromides in Negishi cross-coupling reactions has been
determined by competition experiments. For this purpose,
mixtures of two differently substituted aryl bromides were
combined with less than one equivalent of 4-tolylzinc
iodide/lithium chloride in the presence of catalytic amounts
of a palladium complex. Commercially available aryl bro-
k1 logð½R1ꢁ0=½R1ꢁtÞ
ð1Þ
k ¼
¼
k2
logð½R2ꢁ0=½R2ꢁtÞ
½R1ꢁ0 ¼ ½R1ꢁt þ ½P1ꢁt
½R2ꢁ0 ¼ ½R2ꢁt þ ½P2ꢁt
ð2Þ
ð3Þ
mides were chosen as electrophiles, and [PdACTHUNTRGNE(UNG PPh3)4], a
logð1 þ ½P1ꢁt=½R1ꢁtÞ
k ¼
ð4Þ
common and widely used catalyst, as palladium source. The
yield of the resulting biaryls was derived by gas chromato-
graphic determination of the product ratio [P1]/[P2] ob-
tained after quenching with aqueous NH4Cl (Scheme 2). All
reactions were run in THF at 258C.
logð1 þ ½P2ꢁt=½R2ꢁtÞ
The competition constants k are independent of the con-
centrations of the reactants. Because the oxidative insertion
is an irreversible step,[8] the competition constants k reflect
the relative reactivities of the bromoarenes towards the Pd0
complexes. Each of the 18 aryl bromides listed in Figure 1
was subjected to competition experiments with several other
bromobenzene derivatives to give the 28 competition con-
stants k listed in Figure 1. Solving the resulting overdeter-
mined set of linear equations [Eq. (5)] by least squares mini-
mization yielded the krel values listed in Figure 1.
log k1 ꢀlog k2 ¼ log k
ð5Þ
Figure 1 shows a reactivity range of 103, from 4-methoxy-
bromobenzene, the least reactive compound, to 4-bromo-
benzonitrile, the most reactive compound of this series. In
agreement with earlier studies on oxidative additions of pal-
ladium complexes into carbon–halide bonds,[9] electron ac-
ceptors in the aromatic ring accelerate the reaction while
electron donors retard the cross-coupling reaction.
Scheme 2. Determination of the relative reactivities of bromoarenes.
Figure 2 shows that the accelerating effects of strong elec-
tron acceptors CN, CO2Et, and CF3 decrease in the order
para> meta> ortho, opposite the reactivity order in Br–Mg
exchange reactions with iPrMgCl·LiCl (ortho> meta>
para), which are dominated by inductive effects.[10]
As illustrated in Figure 3, p-bromobenzonitrile is 300–500
times more reactive than the parent bromobenzene towards
the palladium complex as well as towards iPrMgCl·LiCl.
Whereas this ratio decreases to 13 for the oxidative addition
The relative reactivities of the aryl bromides R1 and R2
with 4-tolylzinc iodide have been calculated by Equa-
tion (1),[7] which also holds under conditions where the ratio
[R1]/[R2] varies during the course of the reaction. Substitu-
tion of [R1]0 and [R2]0 by the terms in Equations (2) and
of [PdACHTNUTGRNEUG(N PPh3)4] into o-bromobenzonitrile, it increases to
10000 for the reaction of iPrMgCl·LiCl with the correspond-
ing ortho-isomer.
Abstract in German: Durch Konkurrenzexperimente wurden
die relativen Reaktivitꢀten von substituierten Brombenzolen
sowie von verschiedenen Arylzink-Reagenzien in [Pd-
The low reactivities of the ortho-substituted compounds
in the oxidative additions of palladium complexes can be ra-
tionalized by steric effects and shall not be discussed in
detail. Unlike the substituent effects in the Br–Mg exchange
reactions,[10] the p- and m-substituent effects of the aryl bro-
mides correlate with Hammettꢃs substituent constants sꢀ
(Figure 4),[11,12] suggesting a three-centered transition state 1
resembling that of a nucleophilic aromatic substitution,[9e]
while the transition state of the bromine–magnesium ex-
change reaction resembles the ate complex 2 (Figure 5).[10,13]
The Hammett reaction constant 1=+2.5 is consistent
with previously reported 1 values for the amination of aryl
A
Negishi-Kreuzkupplungsreaktionen
bestimmt (THF, 258C). Die Kreuzkupplungsreaktionen
werden durch Elektronenakzeptoren in den Brombenzolen
beschleunigt, deren Effekt in der Reihe ortho <meta <para
zunimmt. Andererseits verringern Elektronenakzeptoren in
den Arylzinkhalogeniden die Reaktionsgeschwindigkeiten.
Hammett-Beziehungen zeigen, dass sich Substituentenvaria-
tionen bei den Brombenzolen (1=+2.5) stꢀrker auswirken
als bei den Arylzinkhalogeniden (1=ꢀ0.98).
Chem. Eur. J. 2010, 16, 248 – 253
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www.chemeurj.org
249

H. Mayr et al.
Figure 2. Substituent effects on the reactivities of aryl bromides in the
Negishi reaction.
Figure 3. Substituent effects on the reactivities of CN-substituted bromo-
benzene in the Negishi reaction and the Br–Mg exchange reaction (log
k
rel =0 for bromobenzene).
chlorides, Heck and Suzuki reactions with aryl bromides,
and the directly measured reaction of [Pd
halides (Table 1).
ACHTUNGTRENN(NUG PPh3)4] with aryl
Relative reactivities of arylzinc iodides in the transmetala-
tion step: Competition experiments analogous to those de-
scribed above have been performed for determining the rel-
ative reactivities of different p-substituted phenylzinc io-
dides against 4-bromobenzonitrile/[PdACHTNURTGNE(UNG PPh3)4] (Scheme 3).
As shown in Figure 6, the relative reactivities increase
moderately with increasing electron donating abilities of the
p-substituents of the arylzinc iodide.
An even smaller substituent dependence has been ob-
served in couplings with aryl bromides bearing weaker elec-
tron acceptor substituents, as illustrated for the selectivities
kMe/kCOOEt in Scheme 4. While the p-cyano substituted phe-
Figure 1. Relative reactivities of substituted bromobenzenes in [Pd-
ACHTUNGTRENNUNG(PPh3)4]-catalyzed cross-couplings with 4-tolylzinc iodide/lithium chloride
in THF at 258C.
250
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Chem. Eur. J. 2010, 16, 248 – 253

Negishi Cross-Coupling Reactions
FULL PAPER
Scheme 3. Competition experiment for determing the relative reactivities
of arylzinc iodides.
Figure 4. Hammett correlation for [PdACHTNUGTRNEUNG(PPh3)4]-catalyzed cross-coupling
reactions of substituted bromobenzenes with 4-tolylzinc iodide/lithium
chloride in THF at 258C.
Figure 6. Relative reactivities of arylzinc iodides towards p-bromobenzo-
Figure 5. Comparison of the transition states of the oxidative addition of
nitrile/[PdACHTNUGTRNEGNU(PPh3)4] in THF at 258C.
PdL2 and of the Br–Mg exchange with iPrMgCl·LiCl.
Table 1. Hammett reaction constants 1 for the oxidative addition step in
various palladium-catalyzed coupling reactions.
ꢀ
ꢀ
Ar Br
Ar X
Substrate
[Pd(PPh3)4]/p-tolyl-
ZnI·LiCl
[Pd
(dipp)2][a]
XphosPd0
Conditions
1
Ref.
this
G
THF, 258C
+2.50
work
[14]
ꢀ
Ar Cl
E
dioxane, 588C +2.4
[15]
[16]
[16]
[17]
[17]
[9a]
[9d]
[9e]
ꢀ
Ar Cl
toluene, 258C
+2.3
Ar Br
H2C=CH-CO2Bu/Pd0 [b] DMA, 1508C
+(2.4–2.5)
+(1.7–1.8)
+2.3
+0.65
+2.55
+2.3
ꢀ
Ar I
H2C=CH-CO2Bu/Pd0 [b] DMA, 808C
ꢀ
Ar Br
PhB(OH)2/Pd0 [c]
PhB(OH)2/Pd0 [c]
DMF, 808C
DMF, 808C
DMF, 208C
toluene, 258C
THF, 258C
ꢀ
ꢀ
Ar I
ꢀ
Ar OTf [Pd
A
Ar I
[Pd
[Pd
(PPh3)4][e]
ꢀ
Ar I
(PPh3)4][e]
+2
ꢀ
[a] 31P NMR spectroscopic monitoring of the oxidative addition.
[b] Various five-membered palladacycles with N, P, and
S ligands.
[c] Five-membered S-palladacycle. [d] Conductimetric monitoring of the
formation of the cationic arylpalladium complexes. [e] Electrochemical
monitoring of the consumption of [PdACHTNUGTRNEUNG(PPh3)4] in the oxidative addition.
nylpalladium(II) complex 3 reacts 4.1 times faster with the
p-methyl-substituted phenylzinc iodide than with the p-
ethoxycarbonyl-substituted phenylzinc reagent, this reactivi-
Scheme 4. Relative reactivities of arylzinc iodides towards substituted
aryl bromides and [Pd(PPh3)4].
AHCTUNGTRENNUNG
Chem. Eur. J. 2010, 16, 248 – 253
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251

H. Mayr et al.
ty ratio decreases to 2.3 for the p-Cl-substituted phenylpal-
ladium(II) complex 3.
decreases with decreasing electrophilicity of the arylpalladi-
um intermediate.
The Hammett plot shown in Figure 7 with a reaction con-
stant 1=ꢀ0.98 reveals that the transmetalation step de-
pends much less on the electronic nature of the para sub-
Experimental Section
Typical procedure for a competition experiment between two aryl bro-
mides: A dry and N2-flushed Schlenk flask was charged with R1 (4-bro-
mobenzonitrile, 364 mg, 2.00 mmol), R2 (4-trifluoromethylbromoben-
zene, 450 mg, 2.00 mmol), [Pd
cane (70 mg, 0.31 mmol), and THF (2 mL). The flask was thermostated
with water bath (258C). Then p-tolylzinc iodide/lithium chloride
ACHTUNGTRNE(NUNG PPh3)4] (23.1 mg, 0.02 mmol), n-hexade-
a
(4.0 mL, 0.50m in THF, 2.00 mmol) was added in one portion and the re-
action mixture was stirred at 258C. After certain times (for example
20 min), about 0.1 mL of the mixture was taken out with a syringe and
poured into saturated aqueous NH4Cl solution (1 mL). The aqueous
phase was extracted with diethyl ether (ca. 2–3 mL). The ethereal solu-
tions were dried over Na2SO4 and analyzed by GC.
Typical procedure for a competition experiment between two arylzinc re-
agents: A dry and N2-flushed Schlenk flask was charged with R1 (p-tolyl-
zinc iodide/lithium chloride, 4.0 mL, 0.50m in THF, 2.00 mmol), R2 (4-
(ethoxycarbonyl)phenylzinc iodide/lithium chloride, 3.51 mL, 0.57m in
THF, 2.00 mmol) and n-hexadecane (70 mg, 0.31 mmol). The flask was
thermostated with a water bath (258C). Then a solution of 4-bromoben-
Figure 7. Hammett correlation of the reactivities of substituted arylzinc
iodides towards p-bromobenzonitrile/[PdACTHNUTRGNE(UNG PPh3)4].
zonitrile (364 mg, 2.00 mmol) and [PdACHTNUTRGNEUNG(PPh3)4] (23.1 mg, 0.02 mmol) in
stituents than the oxidative addition. The negative slope in-
dicates an increased reactivity of arylzinc iodides, bearing
electron-donating groups. Similar Hammett reaction con-
stants have been determined for the transmetalation step in
Stille and Suzuki reactions (Table 2). In contrast, [NiCl2-
THF (2 mL) was added in one portion and the reaction mixture was
stirred at 258C. After certain times (for example 20 min), about 0.1 mL
of the mixture was taken out with a syringe and poured into saturated
aqueous NH4Cl solution (1 mL). The aqueous phase was extracted with
diethyl ether (ca. 2–3 mL). The ethereal solutions were dried over
Na2SO4 and analyzed by GC.
ACHTUNGTRENNUNG(PCy3)2]-catalyzed Suzuki cross-couplings of phenyl tosylate
with arylboronic acids were reported to be accelerated by
acceptor groups in the arylboronic acids (1=+0.80).[18]
Acknowledgements
Table 2. Hammett reaction constants 1 for the transmetalation step in
various palladium-catalyzed coupling reactions.
We thank the Fonds der Chemischen Industrie and the Deutsche For-
schungsgemeinschaft (SFB 749) for financial support. We also thank
Chemetall GmbH (Frankfurt), W. C. Heraeus (Hanau) and BASF (Lud-
wigshafen) for the generous gift of chemicals. Z. B. D. thanks the Chinese
Scholarship Council for financial support and L. S. is grateful to the
Alexander von Humboldt foundation for a research fellowship. Dr. A. R.
Ofialꢃs help for completion of this manuscript is gratefully acknowledged.
Substrate
Reaction partner
(PPh3)4]
Conditions
1
Ref.
ꢀ0.98 this
work
ArZnX·LiCl 4-NCC6H4Br/[Pd
U
THF,
258C
DMF,
808C
DMF,
808C
NMP,
258C
[17]
[17]
[19]
ArB(OH)2
ArB(OH)2
ArSnBu3
PhBr/Pd0 [a]
PhI/Pd0[a]
ꢀ0.68
ꢀ0.68
ꢀ0.89
4-(tert-butyl)cyclohexenyl-
triflate/[Pd2A(dba)3]/AsPh3
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CHTUNGTRENNUNG
[a] Five-membered S-palladacycle.
Conclusions
As in other palladium-catalyzed cross coupling reactions,
the oxidative addition of Pd0 to bromoarenes is strongly ac-
celerated by acceptor substituents. From the excellent corre-
lation of the relative reactivities of the bromoarenes with
Hammettꢃs sꢀ constants (1=2.5) one can derive a transition
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252
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Chem. Eur. J. 2010, 16, 248 – 253

Negishi Cross-Coupling Reactions
FULL PAPER
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Received: July 31, 2009
Published online: November 10, 2009
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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253