Unusual Electronic Effects of Ancillary Ligands on the Perfluoroalkylation of Aryl Iodides and Bromides Mediated by Copper(I) Pentafluoroethyl Complexes of Substituted Bipyridines

Article abstract of DOI:10.1021/jacs.9b10540

Several perfluoroalkylcopper compounds have been reported previously that serve as reagents or catalysts for the perfluoroalkylation of aryl halides. However, the relationships between the reactivity of such complexes and the electronic properties of the ancillary ligands are unknown, and such relationships are not well-known in general for copper complexes that mediate or catalyze cross coupling. We report the synthesis and characterization of a series of pentafluoroethylcopper(I) complexes ligated by bipyridine ligands possessing varied electronic properties. In contrast to the limited existing data on the reactivity of L₂Cu(I)-X complexes bearing amine and pyridine-type ligands in Ullmann-type aminations with aryl halides, the reactions of aryl halides with pentafluoroethylcopper(I) complexes bearing systematically varied bipyridine ligands were faster for complexes bearing less electron-donating bipyridines than for complexes bearing more electron-donating bipyridines. Analysis of the rates of reaction and the relative populations of the neutral complexes [(R₂bpy)CuC₂F₅] and ionic complexes [(R₂bpy)₂Cu][Cu(C₂F₅)₂] formed by these reagents in solution suggests that this effect of electronics on the reaction rate results from an unusual trend of faster oxidative addition of aryl halides to [(R₂bpy)CuC₂F₅] complexes containing less electron-donating R₂bpy ligands than to those containing more electron-donating R₂bpy ligands.

Full text of DOI:10.1021/jacs.9b10540

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Article
Unusual Electronic Effects of Ancillary Ligands on the
Perfluoroalkylation of Aryl Iodides and Bromides Mediated by
Copper(I) Pentafluoro-ethyl Complexes of Substituted Bipyridines
Eric D Kalkman, Michael G Mormino, and John F. Hartwig
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b10540 • Publication Date (Web): 14 Nov 2019
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Unusual Electronic Effects of Ancillary Ligands on the Perfluoroalkyla-
tion of Aryl Iodides and Bromides Mediated by Copper(I) Pentafluoro-
ethyl Complexes of Substituted Bipyridines
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Eric D. Kalkman, Michael G. Mormino, and John F. Hartwig*
Department of Chemistry, University of California, Berkeley, California 94720, United States.
ABSTRACT: Several perfluoroalkylcopper compounds have been reported previously that serve as reagents or catalysts for the
perfluoroalkylation of aryl halides. However, the relationships between the reactivity of such complexes and the electronic properties
of the ancillary ligands are unknown, and such relationships are not well-known in general for copper complexes that mediate or
catalyze cross coupling. We report the synthesis and characterization of a series of pentafluoroethylcopper(I) complexes ligated by
2
bipyridine ligands possessing varied electronic properties. In contrast to the limited existing data on the reactivity of L Cu(I)–X
complexes bearing amine and pyridine-type ligands in Ullmann-type aminations with aryl halides, the reactions of aryl halides with
pentafluoroethylcopper(I) complexes bearing systematically varied bipyridine ligands were faster for complexes bearing less elec-
tron-donating bipyridines than for complexes bearing more electron-donating bipyridines. Analysis of the rates of reaction and the
relative populations of the neutral complexes [(R
gents in solution suggests that this effect of electronics on the reaction rate results from an unusual trend of faster oxidative addition
of aryl halides to [(R bpy)CuC ] complexes containing less electron-donating R bpy ligands than to those containing more electron-
donating R bpy ligands.
2 2 5 2 2 2 5 2
bpy)CuC F ] and ionic complexes [(R bpy) Cu][Cu(C F ) ] formed by these rea-
2
2
F
5
2
2
Introduction
Consistent with this trend, Taillefer showed that the yields of a
Cu-catalyzed arylation of phenols correlated positively with the
electron-donating properties of ancillary bipyridine ligands,
but many factors besides reaction rate can affect the yield of a
catalytic reaction. Thus, the scope of the trend of the electronic
properties on the reactivity of Cu(I) complexes has been as-
sessed in only a cursory fashion and has not been studied for
reactions of perfluoroalkyl copper complexes.
The relationship between the properties of the ancillary lig-
ands and the reactivity of complexes of these ligands in elemen-
tary reactions must be understood to select or design catalysts
for valuable transformations. While the properties of phos-
phines and their influence on reactivity have been studied ex-
2
9
1
-13
tensively, the properties of nitrogen-based ligands and their
influence on reactions of organometallic complexes are less
14-15
X
Ar–Nuc
studied.
One class of reactions that is sensitive to the elec-
Ar–X
[
CuI] Nuc
[CuIII
]
Ar
[CuI] X (1)
tronic properties of ancillary ligands is oxidative addition. Of-
ten, oxidative addition is faster for more electron-rich com-
oxidative addition
(slow)
reductive elimination
(fast)
Nuc
4
-7, 14, 16-17
plexes,
but exceptions to this trend are known, and one
such exception of the reactions of three-coordinate Ir(I)–X com-
plexes with C–H and N–H bonds was recently studied in de-
tail.
Chart 1. Discrete perfluoroalkylcopper(I) complexes
18
Previous studies of the mechanism of Cu(I)-based cross cou-
pling reactions of aryl halides have provided strong evidence
that oxidative addition of an aryl halide to a Cu(I) complex to
form a formal Cu(III) intermediate is rate-limiting and is fol-
lowed by facile reductive elimination to furnish the coupled
1
7, 19-27
product (eq 1).
Thus, one would expect electron-donating
ancillary ligands to increase the electron density at the metal
Several discrete perfluoroalkyl complexes of copper(I) bear-
2
8
center and facilitate the rate-limiting oxidative addition. Con-
sistent with this hypothesis, our group previously reported that
reactions of aryl halides with copper(I) amidate and imidate
complexes were faster for complexes ligated by more electron-
donating bis-amine ligands than those ligated by less electron-
donating ligands based on phenanthroline or pyridine structures
ing ancillary dative ligands, including N-heterocyclic car-
2
1, 30
31-35
benes,
1,10-phenanthrolines (phen),
2,2’-bipyridine
3
2
31-32
(bpy), and triphenylphosphine
have been isolated (Chart
1), but studies on the effects of the properties of these ligands
on Cu-mediated perfluoroalkylation of aryl halides have not
been reported. Several groups have reported that the inclusion
of an ancillary ligand (usually phen) in the Cu-catalyzed per-
fluoroalkylation of aryl iodides is vital to achieve catalytic turn-
17
or ligandless systems. These data suggest that complexes of
more electron-rich ligands react faster, but amine and pyridine
ligands differ in more than just their electron-donating abilities.
2
8, 35-38
over at moderate reaction temperatures,
but others have
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developed Cu-catalyzed reactions that occur in the absence of
electron-donating R
2
bpy than for those of less electron-donat-
3
9-40
1
2
3
4
5
6
7
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ancillary ligands at similar temperatures.
Grushin and co-workers even reported that addition of phenan-
throline, pyridine, and phosphine ligands to “ligandless” CuCF
In addition,
ing R bpy ligands (K, Table 1). Indeed, at 65 °C, the system
2
containing the most electron-donating ligand 4,4’-di(1-piperidi-
nyl)-2,2’-bipyridine (1e) comprised a 9:91 ratio of the ionic to
neutral forms, while the system containing the least electron-
donating ligand 4,4’-dichloro-2,2’-bipyridine (1f) comprised a
30:70 ratio. A similar trend was observed at 90 °C. A Hammett
plot of the equilibrium constants governing the relative popula-
3
4
1
diminished the reactivity of this reagent. Thus, the effect of
the ancillary ligand on the reactivity of Cu-based perfluoroal-
kylation reactions is particularly ambiguous.
We report the synthesis of a series of pentafluoroethyl-
copper(I) complexes containing bipyridine (bpy) ligands with
varied electronic properties that react with aryl halides to form
perfluoroalkyl arenes. We have measured the effects of the elec-
tronic properties of these bpy ligands on the speciation of per-
fluoroalkylcopper(I) complexes between neutral and ionic
forms, the effect of these properties on the rates of the reaction
of these complexes with aryl halides, and the effect of the ligand
on selectivity. Combined, these data suggest that the reactions
of the aryl halides with perfluoroalkylcopper complexes bear-
ing less electron-donating bpy ligands are faster than with those
containing more electron-donating bpy ligands. These data con-
trast the common effects of electronic properties on the rate of
oxidative addition of aryl halides.
tions of the two forms displayed a loose positive correlation (ρ
44
≈
1.3) with the σ
p
values of the substituents on the bpy ligands
(Figure 1).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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9
0
1
2
3
4
5
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9
0
1
2
3
4
5
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9
0
K
+
]– (2)
2 (R
2
bpy)CuCF
2
CF
3
2
[(R bpy)
2
Cu] [Cu(CF
2 3
CF )
2
6
5 °C
Results and Discussion
Our investigations began with the preparation of a series of
pentafluoroethylcopper(I) complexes ligated by 4,4’-ho-
modisubstituted derivatives of 2,2’-bipyridine (1a–f, Scheme
1). Complexes 2a–e containing electron-donating groups on the
bpy scaffold were prepared in good yields by treating a mixture
of ligands 1a–e and anhydrous copper(I) acetate in THF or tol-
uene with trimethyl(pentafluoroethyl)silane (TMS–C F ).
2 5
While we were unable to prepare complexes containing bpy lig-
ands with strongly electron-withdrawing groups by this proce-
Figure 1. Hammett plot of the equilibrium constants governing the
relative populations of neutral and ionic forms of 2a–e at 65 °C
45
To assess the effect of the electronic properties of the bpy
ligand on perfluoroalkylation reactions with aryl halides, com-
plexes 2a–f were treated with excess 1-butyl-4-iodobenzene
(3a) or 1-bromo-4-butylbenzene (4) in DMF at 65 °C or 90 °C,
respectively (eq 3). The formation of pentafluoroethyl arene 5a
2 2 5
dure, we did prepare (Cl bpy)CuC F by treating CuOtBu gen-
erated in situ with dichlorobipyridine 1f, followed by benzoyl
42
fluoride and then TMS–C
2
F
5
.
1
9
was followed by F NMR spectroscopy, and rate constants for
these reactions were determined from first-order exponential
Scheme 1. Preparation of (R
2
bpy)CuC
2
F
5
complexes
I
Br
1
2
. R2bpy (1a–e, 1.1 equiv),
THF or toluene, rt, 45 min
fits (Table 1, kobs ) or initial rates (kinit ) of the formation of 5a.
These data provide the surprising result that reactions of aryl
iodide 3a with complexes containing the less electron-donating
bpy ligands were generally faster than those containing the
more electron-donating bpy ligands, with dichlorobipyridine
complex 2f reacting ~4 times faster than di(piperidinyl)bipyri-
R =
2a: H
b: CH3
c: C(CH3)3
d: OCH3
2e: 1-piperidinyl 85%
2f: Cl
41%
CuOAc
CuCl
R
96%
74%
75%
77%
. TMS–C2F5 (1.2 equiv)
2
2
2
1
8 h
N
or
Cu C2F5
1. NaOtBu (1.0 equiv), THF, 1 h
N
2
. Cl2bpy (1f, 1.0 equiv),1.5 h
R
3. PhC(O)F (1.2 equiv), 10 min
. TMS–C2F5 (1.2 equiv), 18 h
dine complex 2e. A Hammett plot of these rate constants vs σ
revealed a small but positive correlation (ρ ≈ 0.5, Figure 2a).
p
4
Consistent with previous reports of perfluoroalkylcopper(I)
31-35, 42-43
19
R
complexes,
DMF solution all consisted of four resonances, two correspond-
ing to the ethyl group in the neutral [(R bpy)CuC ] form and
two to those in the ionic [(R bpy) Cu][Cu(C ] form (eq 2).
the F NMR spectra of complexes 2a–f in
N
N
X
2 5
C F
CuC
2
F
5
+
(3)
2
2 5
F
DMF/DMF-d
25–45 min
7
,
Bu
Bu
2
2
2 5 2
F )
(20 equiv)
5a
65 °C (X=I) or 90 °C (X=Br)
The equilibrium between the neutral and ionic species was
reached within 5 min at 65 °C and remained unchanged at dif-
ferent total concentrations of perfluoroalkylcopper species, sug-
gesting that the ionic components exist as solvent-separated
R
2
a–f
3a (X = I) or 4 (X = Br)
Reactions of 2a–f with aryl bromide 4 at 90 °C proceeded
much more slowly than reactions with aryl iodides 3 and in
lower yields, complicating analysis of the full reaction profiles.
However, the initial rates of reaction, determined from linear
fits of the first 5–10% formation of 5a (Table 1, kinit ), followed
the same trend of electronic effects on reaction rates as was ob-
1
7
ions as opposed to a tight ion pair (see the supporting infor-
mation). While more electron-donating ligands might be ex-
+
pected to stabilize the cationic [(R
2
bpy)
2
Cu] species more ef-
fectively than would more electron-poor ligands, the equilibria
of compounds 2a–f instead generally favored the neutral
Br
2 2 5
[(R bpy)CuC F ] form more strongly for complexes of more
served for reactions with aryl iodides 3 (ρ ≈ 0.4, Figure 2b).
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Table 1. Equilibrium constants for complexes 2a–f in DMF and rate constants for the pentafluoroethylation of aryl halides
3a and 4 by reaction with complexes 2a–2f
I
kinitBr
Br
k
corr
(± 0.5×10–5
k
corr
I
k
obs
Complex
R
K (65 °C)a
(± 0.3×10–3
K (90 °C)a
–6
(
± 0.2×10
± 0.2×10–3 s–1)b
(
–1
–1 b,d
–1 c
–1 –1 c,e
M ·s )
6.3
M·s )
M ·s )
2a
2b
H
2.8×10–1
1.6×10–1
1.6×10–1
5.1×10–2
7.7×10–3
1.8×10–1
3.2
2.6
2.2
2.5
1.2
4.7
1.9×10–1
1.4×10–1
1.4×10–1
4.8×10–2
8.1×10–3
7.8×10–2
2.9
2.4
2.6
2.7
1.5
5.4
11
8.3
9.1
7.7
3.6
17
CH
3
4.8
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2c
tBu
4.0
2d
OCH
3
3.7
2e
1-piperidinyl
Cl
1.4
2f
8.8
a
b
– 2
]2; relative concentrations determined by 19F NMR spectroscopy in DMF at the specified temperature.
K = [[Cu(C
2
F
5
)
2
] ] /[(R
2
bpy)CuC
2
F
5
I
Reaction conditions: 2 (0.0500 M) and aryl iodide 3a (1.00 M) in 5:1 DMF:DMF-d
7
at 65 °C for 45 min; kobs derived from exponential fit
c
Br
of formation of 5a. Reaction conditions: 2 (0.0500 M) and aryl bromide 4 (1.00 M) in 5:1 DMF:DMF-d
mined from linear fit of formation of 5a. dꢀꢁꢂꢃꢃ = ꢀꢂꢄꢅ(1 + 2ꢆ√ꢇ)/[ꢈꢉ]ꢊ eꢀꢁꢂꢃꢃ = ꢀꢂꢄꢅ(1 + 2ꢆ√ꢇ)/[ꢋ] [ꢌ] ꢆ (see the supporting infor-
7
at 90 °C for 30 min; kinit deter-
ꢊ
ꢊ
mation for derivations)
electron-rich iodides, and is consistent with commonly-ob-
served effects of the electronic nature of the aryl halide on rate-
2
6, 46
limiting oxidative addition.
However, the effect of the bpy
ligand on the reactions of complexes 2a and 2e with aryl iodides
3b and 3c remained the same as those for reactions with 3a:
reactions of the more electron-poor complex 2a proceeded more
rapidly than reactions involving more electron-rich complex 2e.
Thus, the unusual electronic effects of the ancillary ligand on
the rate of the reaction between the perfluoroalkyl copper com-
plex and the aryl halide are not compensated by a balancing
trend in electronic effects of the aryl halide.
R
N
N
I
2 5
C F
CuC
2
F
5
+
(4)
7
DMF/DMF-d ,
T, 45 min
X
X
(20 equiv)
5b–c
R
2a or 2e
3b (X = CN) or
c (X = OCH
3
3
)
Table 2. Rate constants for the reaction of electron-neutral
and electron-rich complexes 2a and 2e with electronically
varied aryl iodides (eq 4).
I
X (in
ArI)
T
(°C)
k
obs
Complex
R
(± 0.2×10–3 s–1)
I
2a
2a
2a
2e
H
CN
Bu
27
65
65
27
65
65
3.1
3.2
3.1
1.8
1.2
1.3
p
Figure 2. Hammett plots (vs σ ) of a) kobs for the pentafluoroeth-
ylation of aryl iodide 3a by reaction with complexes 2a-f and b)
init for the pentafluoroethylation of aryl bromide 4 by reaction
H
Br
k
H
OCH
CN
3
45
with complexes 2a-f
1-piperidinyl
1-piperidinyl
1-piperidinyl
The results in Table 2 show the effect of varying the elec-
tronic properties of the aryl iodide on the rate of the reaction
with pentafluoroethylcopper complexes 2a and 2e (eq 4). Reac-
tions of electron-neutral and electron-rich complexes 2a and 2e
with electron-poor aryl iodide 4-iodobenzonitrile (3b) occurred
more readily than those with more electron-rich 4-butyl iodo-
benzene (3a) or 4-iodoanisole (3c), proceeding at 27 °C at rates
that were similar to those of reactions of the same complexes
with 3a or 3c at the higher temperature of 65 °C. This trend is
in agreement with the trend in the rates of reaction of electron-
2
2
e
e
Bu
OCH
3
Reaction conditions: 2 (0.0500 M) and 3b or 3c (1.00 M) in 5:1
DMF:DMF-d
7
at the specified temperature for 45 min
I
2 5
C F
2
a
+
(n equiv)
(5)
DMF/DMF-d
5 °C, 45 min
7
,
Bu
Bu
6
26
3a
5a
ically varied aryl halides with “ligandless” CuCF
3
,
wherein
more electron-poor aryl iodides reacted more quickly than more
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quantities of free CuC
ligands. Moreover, Mikami and Grushin have both shown that
solutions of “ligandless” CuC in DMF react with aryl iodides
and bromides at temperatures similar to or lower than those in
2 5
F than complexes of more electron-poor
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2 5
F
43, 48
this work to form pentafluoroethyl arenes.
less” complex, instead of [(R bpy)CuC ], could be the species
that reacts with aryl halides during our studies.
Thus, a “ligand-
2
2 5
F
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3. Dependence of the rate of the reaction of 2a with 3a on
the initial concentration of 3a ([3a] ). Reaction conditions: 2a
0.0500 M) and aryl iodide 3a (0.500–2.00 M, n = 10–40) in 5:1
DMF:DMF-d at 65 °C for 45 min
0
(
7
The rates of reaction of 2a with varying concentrations of aryl
iodide 3a revealed the reaction order in aryl halide. The rates of
the reactions of 2a in the presence of 0.500, 1.00, 1.50, and 2.00
M 3a (eq 5) displayed a strong linear correlation with the initial
0
concentration of 3a ([3a] ), implying that the reaction is first-
order in in aryl iodide (Figure 3). Combined with the first-order
behavior in the concentration of complexes 2 (revealed by the
first-order exponential fits of product formation in reactions
with a large excess of 3a), these findings are consistent with
rate-limiting oxidative addition of aryl halide to complexes 2.
Because the neutral form of perfluoroalkylcopper species, not
the ionic form, is known to be the component that reacts with
2
1, 47
aryl halides,
between these forms of the CuC
effect of the bpy ligands on the rate of reaction of
(R bpy)CuC ] with aryl halides 3a and 4. Given the first-or-
der dependence of the rate on both 2 and the aryl halide, the rate
it is necessary to account for the equilibrium
2 5
F
reagents to determine the
[
2
2 5
F
Figure 4. Hammett plots (vs σ
p
) of equilibrium-adjusted rate con-
I
Br
constants kcorr obtained after correcting kobs and kinit for the
relative equilibrium population of the [(R bpy)CuC ] species
stants kcorr for the pentafluoroethylation of a) 3a and b) 4 by reac-
4
5
2
2
F
5
tion with complexes 2a-f
are given by eq 6 and eq 7, respectively (see the supporting in-
formation for detailed derivations). The values for kcorr are
given in Table 1.
ArX
CuX
[(R
2
bpy)CuC
2
F
5
]
"CuC
2
F
5
"
ArC
2
F
5
(8)
R
2
bpy
6
ꢎꢏꢐꢑ√ꢒꢓꢔꢘ
ꢎꢏꢐꢑ√ꢒꢓꢔ
ꢛꢜꢛꢝ
ꢞꢟ
ꢍ
ꢁ
ꢕꢖꢗ
ꢚꢃ
To evaluate this hypothesis, a solution of “ligandless”
CuC (6) in DMF was prepared by treatment of a solution of
K(DMF)[Cu(OtBu)
nate, followed by neutralization with 3HF·NEt
ꢀ
=
(6)
ꢀ =
(7)
ꢂꢃꢃ
ꢁꢂꢃꢃ
[ꢈ]ꢙ
[ꢋ] [ꢌ]
ꢙ ꢙ
2 5
F
4
9
Hammett plots of kcorr vs σ
p
for the reactions with aryl iodide
2
] in DMF with ethyl pentafluoropropio-
48
3a and bromide 4 (Figure 4) provided positive correlations be-
tween the rate constants and σ that were more linear than those
3
.
The for-
p
mation of pentafluoroethyl arene 5a by reaction of 6 with excess
I
Br
19
obtained from the raw rate constants kobs and kinit . The substit-
uent effects on kcorr were also slightly stronger than on kobs and
init (ρ ≈ 0.7 for kcorr and ρ ≈ 0.5 for kcorr ). Assuming rate-
limiting oxidative addition of the aryl halides to the neutral
(R bpy)CuC ] complex (vide supra), this result suggests that
aryl iodide 3a at 65 °C was monitored by F NMR spectros-
I
–4
–
copy and was found to occur with a rate constant of 9.7×10 s
Br
I
Br
1
k
. While this value is similar to the rate constant of reactions of
I
–3 –1
3a with piperidinyl-substituted 2e (kobs = 1.4×10 s ), which
is the slowest-reacting of complexes 2a–f, reactions of 2a–d
[
2
2 5
F
oxidative addition is faster to the complexes of less electron-
donating bpy ligands than to complexes of more electron-do-
nating bpy ligands. This trend is the opposite of what is ob-
served most commonly for oxidative addition to a metal cen-
and 2f with 3a proceeded at least 2.5 times faster than the reac-
I
tions of “ligandless” CuC
2
F
5
reagent 6 with 3a (Table 1, kobs ).
2 5
This result shows that “ligandless” CuC F is not kinetically
competent to be an intermediate in the reactions of complexes
2a–d and 2f with aryl halides.
4
-7, 14, 16
ter.
An alternative hypothesis to account for this trend of ligand
electronic effects on the rate of the reaction of aryl halides
To assess the potential reactivity through the “ligandless”
complex 6 further, we conducted a competition experiment,
whereby complexes 2a, 2e, or 6 were allowed to react with an
equimolar mixture of 2- and 4-iodotoluene (3d and 3e, Scheme
2). The ratio of pentafluoroethyl toluene products 5d:5e formed
from the reactions with 2a and 2e (44:56 and 39:61, respec-
tively) were different from that with ligandless 6 (68:32), im-
plying that the steric environment of the Cu center that reacts
with[(R
2 2 5
bpy)CuC F ] is that the bpy ligand initially dissociates
from the Cu center to generate a small amount of free “CuC
2 5
F ”
(6), which then reacts more rapidly with the aryl halide than the
ligated species (eq 8). Because more electron-rich σ-donor lig-
ands generally bind more strongly to metal centers, complexes
of more electron-rich bpy ligands would generate smaller
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Journal of the American Chemical Society
with the aryl halide starting from 2a and 2e is different from calculated previously for other reactions of copper complexes
5
0
17, 23
‡
1
2
3
4
5
6
7
8
9
that starting from 6, and is, therefore, not the “ligandless” cop-
per complex.
with aryl halides.
not in ΔGcalc likely results from the large uncertainties intro-
duced in implicit solvation models and calculated entropies.
The observation of a trend in ΔHcalc but
‡
5
1
Scheme 2. Competition reactions of pentafluoroethyl com-
These calculations, therefore, support our hypothesis that the
effect of the ancillary ligand on the experimental rates is due to
modulation of the barrier to the rate-limiting elementary step of
oxidative addition.
plexes with aryl iodides 7a and 7b
The observed trends in the rates of reaction of complexes 2
‡
with aryl halides and in ΔHcalc could be explained by an accu-
mulation of negative charge around the Cu center during the
oxidative addition step, which would be stabilized by more
electron-poor ligands. Indeed, the natural atomic charges on the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
52
Cu centers determined through natural population analysis of
the optimized transition state structures TS
R
in all cases de-
Finally, the possibility of reactions of 2a–f through the
ligandless complex was assessed by measuring the rate of reac-
tion of 2a with 3a in the presence of 2 equiv added bpy. The
rates of reaction in the presence and absence of added ligand
creased relative to those of the GS structures (Δq = –0.02 –
R
Cu
–0.04), although all of the net charges were positive (q = +0.44
Cu
– +0.56; see Tables S5–S6 in the supporting information for a
list of natural atomic charges of specific atoms and fragments
of the computed structures). Such a decrease in charge could
result from the aryl halide attacking an electrophilic Cu center
during the oxidative addition step. In such a pathway, more
electron-rich aryl halides would be expected to react more
quickly with compounds 2 than electron-poor aryl iodides due
to better stabilization of a partial positive charge on the arene.
I
–3 –1
were indistinguishable (kobs = 3.0×10
s
with 2 equiv of
–
3
–1
added bpy vs 3.2×10
s without), implying that the overall
effect of a pathway involving reaction with the ligandless spe-
cies must be minor and that the electronic effect of the ligand
on the rate of reaction is not due to dissociation of R
(R bpy)CuC ] prior to reaction with the aryl halide.
Table 3. Computed energies of activation for the oxidative
2
bpy from
[
2
2 5
F
addition of iodobenzene to [(R
2
bpy)CuC
2
F
5
]
ΔGcalc‡
ΔHcalc‡
(kcal/mol)
9.5
ΔScalc‡
(cal/mol·K)
–41
R
σ
p
(
kcal/mol)
N(CH
3
)
2
–0.83
–0.27
0
24.5
OCH
3
(2d)
23.9
9.4
–40
H (2a)
Cl (2f)
CN
23.9
8.8
–42
0.23
0.66
25.0
8.3
–46
26.1
7.5
–52
See the supporting information for computational details
These experimental results were consistent with DFT calcu-
lations (see the supporting information for computational de-
tails). The computed free energy barriers of concerted oxidative
addition of unsubstituted iodobenzene (3f) to the neutral forms
–) of the rates of reaction of 2a with
p
para-substituted iodobenzenes relative to unsubstituted iodoben-
zene as determined by competition experiments. See the supporting
information for experimental details.
Figure 5. Hammett plot (vs σ
‡
of 2a, 2d, and 2f (ΔGcalc = 24–25 kcal/mol; Table 3; eq 9) were
similar in magnitude to those determined experimentally from
I
‡
k
obs (ΔGexpt = 23–24 kcal/mol). However, no clear trend was
observed between the computed free energies of activation and
the σ values of the substituents on the bpy ligand (Table 3).
Nevertheless, a clear trend was observed between the enthalpies
This prediction is inconsistent with the measured rates of re-
action of aryl iodides 3a–c with 2a and 2e (vide supra). To gain
a more detailed view into this electronic effect, we subjected 2a
to reactions with equimolar mixtures of para-substituted aryl
iodides 3b, 3c, or 3e–3h and unsubstituted iodobenzene (3i) at
p
‡
p
of activation ΔHcalc and the σ values of the substituents on the
bpy ligand, and this trend was consistent with the trend ob-
served experimentally; the enthalpies of activation for oxidative
addition of iodobenzene to complexes of the more electron-poor
bpy ligands were computed be lower than those to complexes
of the more electron-rich ligands. These calculated values for
6
5 °C (eq 10). The rates of reaction of the substituted iodoben-
zenes with 2a were determined relative to 3i with 2a by the final
ratio of substituted pentafluoroethylarene products 5 to pen-
tafluoroethylbenzene (5i; see the supporting information for de-
tails). A Hammett plot of these relative rates revealed a strong
‡
‡
ΔHcalc are significantly lower than those for ΔGcalc , highlight-
–44
‡
‡
positive correlation with σp (ρ = 0.8, Figure 5), the opposite
ing a large entropic contribution to ΔGcalc (ΔScalc = –40 – –52
cal/mol·K), which has been measured experimentally and
4
6
of what would be expected for a mechanism in which the copper
acts as an electrophile. The positive ρ value suggests a buildup
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Journal of the American Chemical Society
Page 6 of 8
of negative charge on the arene, which is reflected by a decrease
in the summed natural atomic charges of the aryl fragment in
all TS (Δqaryl = –0.06 – –0.07). This result is consistent with
the results of Grushin and co-workers, who found a ρ value of
0.91 for reactions involving substituted aryl iodides with a
are supported in part by NIH S10OD024998. The Molecular
Graphics and Computational Facility is supported by NIH
S10OD023532.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
R
REFERENCES
+
–
“
ligandless” CuCF
eters of the substituent on the arene.
Despite the aforementioned decrease in positive charge on
the Cu center in TS , relative to GS , the aggregate natural
atomic charges of the R bpy fragments of the computed struc-
tures in all cases were slightly more positive in TS than in GS
ΔqR2bpy = +0.02 – +0.03). Such an observation is inconsistent
3
system when fitting against the σ
p
param-
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R
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53-56
Cu(III) complex Cu(CF
3
)
4
revealed no clear trends that
might explain the reactivities of complexes 2 with aryl halides.
A more detailed theoretical treatment than that presented in this
work is needed to elucidate the underlying effect of the bpy lig-
and on the reactivities of complexes 2 with aryl halides.
7
.
Stradiotto, M.; Lundgren, R. J., Application of Sterically
Demanding Phosphine Ligands in Palladium-Catalyzed Cross-
2
2
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Conclusions
In summary, we have prepared a series of L
2
Cu–C
2
F
5
com-
plexes in which L are bpy ligands possessing varied electronic
2
properties and investigated the kinetics of the reactions of these
complexes with aryl halides to form pentafluoroethyl arenes.
The complexes comprised an equilibrium between the neutral
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palladium-catalyzed amination. Angew. Chem. Int. Ed. Engl. 2008, 47,
6
338-6361.
0. Lundgren, R. J.; Stradiotto, M., Addressing challenges in
2 2 5 2 2 2 2
[(R bpy)CuC F ] and ionic [(R bpy) Cu][Cu(C F5) ] forms in
1
DMF solution, which favored the neutral form more heavily for
the complexes of more electron-donating bpy ligands than for
complexes of less electron-donating bpy ligands. A combina-
tion of equilibrium data on this interconversion and kinetic data
on the reactions with aryl halides implies that the rate-limiting
oxidative addition of aryl halides to the neutral form of these
complexes is, in contrast to common effects of ancillary ligands,
faster for complexes of less electron-donating ligands than for
those of more electron-donating ligands. This effect contrasts
with the trend of the electronic effects of ligands on the rate of
oxidative addition of aryl halides to amidate complexes ligated
by more electron-rich alkylamines vs less electron-rich phenan-
palladium-catalyzed cross-coupling reactions through ligand design.
Chemistry 2012, 18, 9758-9769.
11. Chow, K. K.; Levason, W.; McAuliffe, C. A., Transition Metal
Complexes Containing Phosphine Ligands. In Transition Metal
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trimethylphosphine ligands. Organometallics 2001, 20, 2669-2678.
13. Kaddouri, H.; Vicente, V.; Ouali, A.; Ouazzani, F.; Taillefer, M.,
Copper-Catalyzed
Arylation
of
Nucleophiles
by
Using
Butadienylphosphines as Ligands: Mechanistic Insight. Angew. Chem.
Int. Ed. Engl. 2009, 48, 333-336.
17
throlines and highlights the unusual properties of complexes
containing perfluoroalkyl ligands.
1
4. Rendina, L. M.; Puddephatt, R. J., Oxidative Addition Reactions
of Organoplatinum(II) Complexes with Nitrogen-Donor Ligands.
Chem. Rev. 1997, 97, 1735-1754.
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Chemistry and Homogeneous Catalysis. Angew. Chem. Int. Ed. Engl.
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ASSOCIATED CONTENT
Supporting Information. Experimental procedures, spectral data,
computational procedures, and molecular coordinates. This mate-
rial is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
1
8. Wang, D. Y.; Choliy, Y.; Haibach, M. C.; Hartwig, J. F.; Krogh-
* jhartwig@berkeley.edu
Jespersen, K.; Goldman, A. S., Assessment of the Electronic Factors
Determining the Thermodynamics of "Oxidative Addition" of C–H and
N–H Bonds to Ir(I) Complexes. J. Am. Chem. Soc. 2016, 138, 149-163.
19. Cristau, H.-J.; Cellier, P. P.; Spindler, J.-F.; Taillefer, M., Highly
Efficient and Mild Copper-Catalyzed N- and C-Arylations with Aryl
Bromides and Iodides. Chem. Eur. J. 2004, 10, 5607-5622.
ACKNOWLEDGMENT
We thank the NSF (CHE-1565886) for support of this work. We
also thank the College of Chemistry’s NMR and Molecular
Graphics and Computational facilities for resources provided and
the staff for their assistance. Instruments in the CoC-NMR facility
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0. Zhang, S.-L.; Liu, L.; Fu, Y.; Guo, Q.-X., Theoretical Study on
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3d and 3e in the presence of 2 equiv of added bpy 1a to ensure ligation
2
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3
complexes. J. Am. Chem. Soc. 2008, 130, 8600-8601.
3
1. Tomashenko, O. A.; Escudero-Adan, E. C.; Belmonte, M. M.;
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Engl. 2011, 50, 7655-7659.
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Belmonte, M.; Benet-Buchholz, J.; Grushin, V. V., Well-defined
CuC complexes and pentafluoroethylation of acid chlorides. Angew.
3
3
of any free CuC
(40:60 of 5d:5e) than the ratio without added bpy (44:56) suggesting
that the reaction of free CuC is an active but minor pathway. The
same reaction with 15 equiv of added bpy gave a 39:61 ratio of 5d:5e,
2 5
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