Reaction of N -isopropyl- N -phenyl-2,2′-bipyridin-6-amine with K2PtCl4: Selective C-H bond activation, C-N bond cleavage, and selective acylation

Article abstract of DOI:10.1021/om400540y

The selective C-H bond activation of N-isopropyl-N-phenyl-2,2′- bipyridin-6-amine promoted by Pt(II) was complicated by the low selectivity of sp² C-H bond activation in acetonitrile and low yield of sp ³ C-H activation in acetic acid. The use of a base was found to effectively suppress the competing sp³ C-H bond activation in acetonitrile, improving the selectivity of sp² C-H bond activation from 70% to 99%. In the reaction in acetic acid, the low yield was due to the competing C-N bond cleavage. The use of a base reduced the C-N bond cleavage, but not completely. The reaction of N-tert-butyl-N-phenyl-2,2′-bipyridin- 6-amine with K₂PtCl₄ in acetic acid produced the cyclometalated complex with complete C-N bond cleavage and its acylated derivative. These results indicated that the C-N bond cleavage might proceed via heterolytic C-N bond dissociation. The acylation following the C-N cleavage in the reaction in acetic acid is regioselective. Further experiments showed that the reaction of N-phenyl-2,2′-bipyridin-6-amine with K ₂PtCl₄ in acetic acid produced the cyclometalated complex, while the reaction in a mixture of acetic anhydride and acetic acid produced the acylated cyclometalated complex. An X-ray crystal structure study revealed strong intramolecular H bonding in the acylated complexes. The regioselectivity was explained in terms of H bonding and the electron distribution predicted by the DFT calculations.

Full text of DOI:10.1021/om400540y

Article
pubs.acs.org/Organometallics
Reaction of N‑Isopropyl‑N‑phenyl-2,2′-bipyridin-6-amine with
K2PtCl4: Selective C−H Bond Activation, C−N Bond Cleavage, and
Selective Acylation
Jeffrey Carroll,† Joshua P. Gagnier,† Alexander W. Garner,†,§ Justin G. Moots,†,∥ Robert D. Pike,‡
Yumin Li,† and Shouquan Huo*,†
†Department of Chemistry, East Carolina University, Greenville, North Carolina 27858, United States
‡Department of Chemistry, College of William and Mary, Williamsburg, Virginia 23185, United States
S
* Supporting Information
ABSTRACT: The selective C−H bond activation of N-
isopropyl-N-phenyl-2,2′-bipyridin-6-amine promoted by Pt(II)
was complicated by the low selectivity of sp2 C−H bond
activation in acetonitrile and low yield of sp3 C−H activation in
acetic acid. The use of a base was found to effectively suppress the
competing sp3 C−H bond activation in acetonitrile, improving
the selectivity of sp2 C−H bond activation from 70% to 99%. In
the reaction in acetic acid, the low yield was due to the competing
C−N bond cleavage. The use of a base reduced the C−N bond
cleavage, but not completely. The reaction of N-tert-butyl-N-
phenyl-2,2′-bipyridin-6-amine with K2PtCl4 in acetic acid
produced the cyclometalated complex with complete C−N
bond cleavage and its acylated derivative. These results indicated
that the C−N bond cleavage might proceed via heterolytic C−N bond dissociation. The acylation following the C−N cleavage in
the reaction in acetic acid is regioselective. Further experiments showed that the reaction of N-phenyl-2,2′-bipyridin-6-amine with
K2PtCl4 in acetic acid produced the cyclometalated complex, while the reaction in a mixture of acetic anhydride and acetic acid
produced the acylated cyclometalated complex. An X-ray crystal structure study revealed strong intramolecular H bonding in the
acylated complexes. The regioselectivity was explained in terms of H bonding and the electron distribution predicted by the DFT
calculations.
compound out of two or more possibilities. For cyclo-
palladation and cycloplatination, it is generally observed that
the activation of an sp2 C−H bond is preferred over the
activation of an sp3 C−H bond; however, there have been many
reports on the competing sp2/sp3 C−H activation in cyclo-
palladation.8 Remarkably, in some cases either of the two C−H
activations can be achieved through a switch of the selectivity.
One of the earliest examples of the selectivity switch was
reported by Yoshida et al.,8a who observed that the cyclo-
palladation of N-thiobenzoylpyrrolidine with PdCl2 in meth-
anol occurred at the phenyl ring while a similar reaction in
hexamethylphosphoramide (HMPA) resulted in metalation at
the CH2 group next to the nitrogen of the pyrrolidine ring. The
preference in the competing sp2/sp3 C−H bond activation can
also be influenced by other factors such as the ring size of the
metallacycle,8b,f,i metal precursors,8d,f−h and the reaction
temperature.8b,f In contrast, reports on the competing sp2/sp3
C−H bond activation via cycloplatination are very rare.3 In the
cycloplatination of 6-(1-methylbenzyl)-2,2′-bipyridine and 6-
INTRODUCTION
■
Cyclometalation is an important process in organometallic
chemistry, which involves chelation-assisted carbon−metal
bond formation.1 Cyclometalation via the cleavage of a C−H
bond to form the metallacycle is the intramolecular version of
organometallic C−H bond activation.2 Since the chelation can
significantly improve the thermal stability of organometallic
compounds, many of the cyclometalated complexes are very
stable and can be isolated and fully characterized. This makes
them extremely important in studying fundamental mechanistic
issues associated with C−H bond activation.1,2 Some key active
intermediates involved in or proposed for the C−H bond
activation may be stabilized by the chelation so that they can be
fully studied. For example, an agostic interaction is conceived to
be an important process in intermolecular C−H bond
activation but could not be fully characterized. With chelation
stabilization, complexes displaying an agostic interaction have
been isolated and fully characterized.3,4 On the other hand,
cyclometalated complexes have found applications in a wide
range of areas, from catalysis5,6 to advanced materials.7
One of the fundamental issues in chemical transformations is
the selectivity. A selective reaction allows the formation of one
Received: June 11, 2013
Published: August 26, 2013
© 2013 American Chemical Society
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(1,1-dimethylbenzyl)-2,2′-bipyridine, metalation was reported
to occur at the phenyl ring rather than the methyl groups,9a,b
although forced sp3 C−H bond activation in cycloplatination of
6-alkyl-2,2′-bipyridine is known.9c,d Recently, we have discov-
ered a solvent-controlled switch of selectivity between intra-
molecular sp2 and sp3 C−H bond activation mediated by
platinum,10 in which the reaction of N-alkyl-N-phenyl-2,2′-
bipyridin-6-amine (alkyl = Me (1), Et (2), i-Pr (3)) with
K2PtCl4 in acetic acid produced predominantly sp3 C−H
activation products, while interestingly, the reaction in
acetonitrile gave selectively sp2 C−H activation products
(Scheme 1). Further experiments suggested that the switch of
On the other hand, since the formation of 4 was considered
to be kinetically controlled,10 the isomerization of 4 to 5 is at
least one major contributor to the poor selectivity. The
isomerization likely proceeds via the cleavage of the sp2 C−Pt
bond of 4 by the HCl generated from the cyclometalation
reaction; therefore, the use of a base may terminate this
pathway by neutralizing the HCl. We may not be able to alter
the intrinsic reactivity of the C−H bond but certainly can use
an HCl scavenger in the reaction. The choice of the HCl
scavenger could be critical; a strong base may coordinate
strongly to the platinum and inhibit the reaction, while a weak
base may not bind to the proton strongly enough to block the
protonolysis of the sp2 C−Pt bond. A strong but bulky base
may be the best choice. Nonetheless, a few different bases were
examined in the reaction of 3 with K2PtCl4 in acetonitrile, and
the results are summarized in Table 1.
Scheme 1. Reaction of N-Alkyl-N-phenyl-2,2′-bipyridin-6-
amine with K2PtCl4
Table 1. Effect of Bases on the Selectivity of Reaction of 3
a
with K2PtCl4 in Acetonitrile
selectivity could be attributed to kinetic or thermodynamic
control under different conditions. The degree of the selectivity
control is quite remarkable except for the reaction of N-
isopropyl-N-phenyl-2,2′-bipyridin-6-amine (3), which pro-
duced a 70:30 mixture of both sp2 and sp3 activation products
4 and 5 in acetonitrile (Table 1). In addition, the reaction in
acetic acid gave the product 5 resulted from exclusive sp3 C−H
bond activation, but only in 38% yield. In this paper, we report
our effort to improve the control of the selectivity of this
reaction and to gain an insight into the competing side
reactions associated with the C−N bond cleavage.
amt,
time
yield
b
selectivity
entry
base
equiv
(days)
(%)
(4:5)
c
1
none
3
10
5
36
70:30
99:1
98:2
94:6
98:2
97:3
90:10
2
sodium acetate
triethylamine
triethylamine
1
1
1
1
1
2
1
1
1
64
52
3
4
10
10
2
91
e
5
DABCO
58
d
6
TMP
41
7
TMP
Na2CO3
NaOH
3
3
34
8
5
trace
trace
83
9
5
10
1
99:1
RESULTS AND DISCUSSION
■
a
Reactions were run with equimolar amounts of the ligand 3 and
K2PtCl4 in acetonitrile at reflux. Isolated yield of the mixture of 4 and
There are two issues with the C−H bond activation of N-
isopropyl-N-phenyl-2,2′-bipyridin-6-amine (3) by platinum:
namely the low selectivity of 4 formation in the reaction in
acetonitrile and the low yield of 5 in the reaction in acetic acid.
This is in sharp contrast with the reaction of N-methyl-N-
phenyl-2,2′-bipyridin-6-amine (1), which exhibited both high
selectivity and high yield of sp2 C−H bond activation in
acetonitrile and sp3 C−H bond activation in acetic acid.10
Selectivity of sp2 C−H Bond Activation. The cause of
the lower selectivity may be complicated; however, two factors
may be significant. First of all, the intrinsic reactivity of different
types of sp3 C−H bonds is different. Generally, the bond
strength decreases in the order methane, primary, secondary,
and tertiary C−H bonds,11 which indicates that the secondary
C−H bond in N-isopropyl-N-phenyl-2,2′-bipyridin-6-amine (3)
may be more reactive than the methyl C−H bond in N-methyl-
N-phenyl-2,2′-bipyridin-6-amine. Therefore, the sp3 C−H
activation becomes relatively more competitive under kineti-
cally controlled conditions of the reaction of 3 in acetonitrile,
resulting in lower selectivity of 4. It should be mentioned that
the relative reactivity of primary, secondary, and tertiary C−H
bonds depends on the nature of the reactions and other factors
and a reverse order is known in cyclometalation chemistry.1a
b
c
5 by flash column chromatography. Isolated yield of pure 4 by
recrystallization.10 2,2′-6,6′-Tetramethylpiperidine. 1,4-Diazabicyclo-
d
e
[2.2.2]octane.
It can be seen from Table 1 that the effect of the bases is
quite remarkable. Excellent to perfect selectivity was achieved
with the use of a proper base. When sodium acetate was used,
the reaction appeared to be much slower in comparison with
the reaction in the absence of a base. A reasonable conversion
could be attained after 10 days of reaction at reflux; however, a
nearly perfect control of selective sp2 C−H bond activation
(99%) was achieved. The slower reaction rate may be attributed
to the acetate ion, which may bind to the platinum and
deactivate the platinum complex toward C−H bond activation.
When triethylamine was used, again, the reaction was slow but
the selectivity was high. It was also found that the selectivity
decreased slightly as the conversion increased with prolonged
heating (entry 4). 1,4-Diazabicyclo[2.2.2]octane (DABCO)
displayed a similar effect (entry 5). For comparison, a sterically
hindered secondary amine, 2,2,6,6-tetramethylpiperidine
(TMP), was tested. When 1 equiv of TMP was used, after 2
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Scheme 2. Reaction of 3 with K2PtCl4 in Acetic Acid10
days, the product 4 was isolated in moderate yield (41%) with
high selectivity (97%). When 2 equiv of TMP was used, the
reaction was much slower. After 3 days of reaction, a mixture of
4 and 5 was isolated in 34% yield with a ratio of 90:10. It
appears that the tertiary amines are a better choice, perhaps
because of the stronger basicity and greater bulkiness of the
tertiary amine. It should be pointed out that when the amount
of other bases used in the reaction was increased, the reaction
became very sluggish. When sodium carbonate and sodium
hydroxide were used, only a trace of cyclometalation product(s)
could be detected by the TLC analysis and the reaction did not
proceed further to produce more of the product(s) after the
first day of the reaction. The formation of black precipitates was
observed, likely due to the reduction of platinum(II) to
platinum(0). Finally, the use of another 1 equiv of the ligand 3
resulted in nearly exclusive formation of 4 without a low
reaction rate (entry 10).
Scheme 3. Preparation of 9 and Its Reaction with K2PtCl4 in
Acetic Acid
C−N Bond Cleavage. The second issue with the C−H
bond activation of N-isopropyl-N-phenyl-2,2′-bipyridin-6-
amine (3) by platinum is the low yield of 5, although there
was no presence of 4 in the products of the reaction.10 A careful
analysis of the reaction mixture by TLC showed that there were
two other compounds accompanying the major desired product
5. The two byproducts are more polar and moved much more
slowly than 5 on TLC. The proton NMR spectra of the two
compounds did not show any signals that could be assigned to
the isopropyl group, suggesting that the C−N bond was
cleaved. In particular, one of the two byproducts, which is less
polar and moved more quickly on TLC, showed a singlet
proton signal appearing at significantly low field (DMSO-d6,
13.70 ppm) and a singlet at 2.76 ppm, which could be assigned
to a methyl group according to the integration. We proposed
that the two byproducts were the cyclometalated compound 6
with the C−N bond cleavage and its acylated derivatives 7, as
shown in Scheme 2. Complex 6 was formed in a very small
amount and could not be recovered from column chromato-
graphic separation.
To gain further information on the C−N bond cleavage, the
ligand N-tert-butyl-N-phenyl-2,2′-bipyridin-6-amine (9) with a
tert-butyl group was prepared by consecutive palladium-
catalyzed C−N12 and C−C7b,13 bond cross-coupling reactions,
as shown in Scheme 3. The cross coupling of N-tert-butylaniline
with an excess amount of 2,6-dibromopyridine produced
intermediate 8. Palladium-catalyzed Negishi coupling of 8
with 2-pyridylzinc bromide produced the ligand 9. The reaction
of 9 with K2PtCl4 in acetic acid resulted in exclusive C−N bond
cleavage, producing 6 and 7 in 45% and 38% isolated yields,
respectively (Scheme 3).
with phenylacetylene in the presence of CuI and triethyl-
amine14 (Scheme 4). A crystal suitable for X-ray crystallo-
graphic analysis was obtained by diffusing hexanes into a
solution of 10 in dichloromethane.
Scheme 4. Reaction of 7 with Phenylacetylene
The crystal data are summarized in Table S1 (Supporting
Information), and the molecular structure is depicted in Figure
1. The results confirmed the proposed structure of 10, which
also substantiates the structure of 7 as well as the C−N bond
cleavage in the reaction of 3 and 9 with K2PtCl4 in acetic acid.
The platinum complex adopts a square-planar geometry with a
C(16)−Pt(1)−N(1) bite angle of 173.76°, being close to 180°,
similar to those for cyclometalated platinum complexes with a
5−6-fused metallacycle reported previously.7b,13,14b The Pt−
C(sp) bond is shorter than the Pt−C(sp2) bond, as expected.
The Pt−N bond trans to the Pt−C(sp2) bond is slightly longer
than the other Pt−N bond that is trans to the Pt−C(sp) bond,
indicating a stronger structural trans effect15 induced by an sp2
carbon donor. The phenyl ring of the phenylacetylide is slightly
twisted from the coordination plane with a dihedral angle of
24.7°. It is noteworthy that there exists a strong intramolecular
hydrogen bond between the carbonyl oxygen and the hydrogen
attached to the amino nitrogen. The hydrogen bond length is
1.81 Å, well within the range of a typical hydrogen bond of
Compounds 6 and 7 have very poor solubility. An attempt to
grow suitable crystals of compound 7 for an X-ray structure
determination was unsuccessful. Therefore, compound 7 was
converted into its phenylacetylide derivative 10 by treating 7
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with the extraordinary stability of the tert-butyl cation. The fact
that the reactions of 1 and 2 with K2PtCl4 in acetic acid were
not accompanied by C−N bond cleavage is also supportive of a
heterolytic cleavage of the C−N bond in the reactions of 3 and
9 with K2PtCl4, because the methyl and ethyl cations that
would be formed from the heterolytic C−N dissociation of 1
and 2, respectively, are too unstable to be formed. Another fact
that the C−N cleavage occurred readily in acetic acid but not in
acetonitrile provides additional support of the heterolytic C−N
bond dissociation process, which is favored in protic solvents. It
should be noted that transition-metal-assisted C−N bond
cleavage through oxidative addition17 and β-hydrogen
abstraction18 has been proposed. In the former case, the
cleavage of the methyl C−N bond,17d,e allyllic C−N bonds,17a
benzylic C−N bonds,17b and strained aziridines17c appears
more common. A β-hydrogen is required in the latter case.18
The role of acid in the C−N bond cleavage has also been
examined. When ligand 3 alone was heated at reflux in acetic
acid, no C−N bond cleavage was observed. The cyclo-
metalation produced HCl as the side product, thus, the HCl
might facilitate the C−N bond dissociation. However, even
with addition of a few drops of concentrated hydrochloric acid
to the reaction of 3 in acetic acid, C−N bond cleavage was not
observed. These results indicated that the presence of the
platinum salt and/or the cyclometalation process may play a
role in the C−N bond cleavage. Complexation of platinum to
the bipyridyl motif would further stabilize the amide anion, thus
promoting the C−N dissociation as shown in Scheme 5.
Complexation of 9 with the platinum (9 to 11) not only
improves the planar geometry of the bipyridine for better
electron delocalization of the amide anion 12 but also makes
the bipyridine more electron deficient, both of which would
lead to the stabilization of 12. The protonation of the amino
nitrogen could also precede the C−N bond cleavage to form 13
and the heterolytic C−N bond cleavage of 13 would be
facilitated by the electron delocalization of the developing lone
pair at the nitrogen. C−N bond cleavage may also occur
following the cycloplatination, producing the intermediates 14
and 15 rather than the coordination giving 11. The C−N bond
cleavage likely results from a cooperative action of stabilization
of the amide anion or delocalization of the lone pair at the
Figure 1. Perspective drawing of the molecular structure of 10.
Selected bond lengths (Å): Pt(1)−C(19) = 1.961(4), Pt(1)−C(16) =
2.006(4), Pt(1)−N(2) = 2.029(3), Pt(1)−N(1) = 2.084(3). Selected
bond angles (deg): C(19)−Pt(1)−C(16) = 92.08(16), C(19)−
Pt(1)−N(2) = 173.38(15), C(16)−Pt(1)−N(2) = 94.32(14),
C(19)−Pt(1)−N(1)
= 93.89(15), C(16)−Pt(1)−N(1) =
173.76(14), N(2)−Pt(1)−N(1) = 79.77(13).
1.6−2.0 Å. This intramolecular hydrogen bond may explain the
unusual downfield proton signal observed in the NMR
spectrum of 7. This proton in 10 appeared at 13.89 ppm
(CDCl3). In contrast, the NH proton in 6 appeared at 10.69
ppm (DMSO-d6).
The C−N cleavage may be attributed to the heterolytic
dissociation of the alkyl C−N bond. In general, C−N bond
dissociation is an unfavorable process, because the amide is a
strong base and thus a poor leaving group. However, with
stabilization of the anion and the cation, C−N bond
dissociation is possible. In a classical Hoffman elimination, a
quaternary ammonium salt has to be formed to create a
reasonably good leaving group. Under strongly acidic
conditions, elimination of tertiary alkyl groups from alkylani-
lines by hydrolysis was reported.16 Therefore, the C−N
cleavage in the reaction of 3 and 9 may benefit from several
factors, including stabilization of the amide anion by the
bipyridyl ring, the increasing stability of the isopropyl and tert-
butyl carbocation, and the protonation of the amino nitrogen
by the HCl generated from the C−H bond activation. The
complete C−N bond cleavage in the reaction of 9 is consistent
Scheme 5. Proposed Pathways for C−N Bond Cleavage in the Reaction of 9 with K2PtCl4 in Acetic Acid
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a
nitrogen, protonation of the amino nitrogen, and stabilization
of the carbocation (Scheme 5).
Scheme 6. Cycloplatination of 16 and Related Reactions
To suppress this unwanted process in the formation of 5, the
use of a base to neutralize the HCl generated from the C−H
bond activation may be necessary. Therefore, the use of NaOAc
in the reaction of 3 with K2PtCl4 in acetic acid was examined. It
turned out that the addition of 1 equiv of NaOAc slowed the
reaction and suppressed the C−N bond cleavage to some
extent, and an isolated yield of 50% for 5 was achieved. When
an extra 1 equiv of NaOAc was used, the reaction was even
slower, and the C−N bond cleavage still could not be
suppressed completely. Apparently the presence of HCl was
not a necessary factor in the C−N bond cleavage but might
accelerate the process. Furthermore, the use of excess base led
to the decomposition of the platinum complexes, as a black
precipitate was observed. An attempt to use 2-ethoxyethanol as
the solvent and sodium bicarbonate as the base for the reaction
resulted in mainly decomposition of the complexes to black
precipitates, which is likely the reduced platinum metal. Finally,
4 was completely isomerized to 5 when it was heated in acetic
acid at reflux for 3 h, producing 5 in 72% isolated yield, but was
accompanied by the decomposition to black platinum metal.
The formation of 6 was also detected by TLC, and the ratio of
6 and 5 was determined to be 5:95 from the 1H NMR spectrum
of the reaction mixture, which further indicates that C−N bond
cleavage could be effected under the reaction conditions
without the presence of HCl. The isomerization of 4 to 5 is an
intramolecular process. It should be noted that the proton-
assisted intermolecular ligand exchange in cyclopalladation and
cycloplatination has been reported.19
a
Reagents and conditions: (a) aniline (2 equiv), Pd(dba)2 (2%),
DPPF (2%), NaOtBu (1.2 equiv), toluene, reflux; (b) K2PtCl4 (1
equiv), AcOH/Ac2O (1/1), reflux.; (c) K2PtCl4 (1 equiv), AcOH,
reflux; (d) AcOH/Ac2O (1/1), reflux.
Scheme 7. Proposed Mechanism for the Selective Acylation
of 6
Regioselective Acylation. It was also found that the
presence of HCl is responsible for the formation of the
acylation product 7 in the reaction of 3 with K2PtCl4 in acetic
acid, because there was no acylation detected when NaOAc was
used in the reaction to neutralize the HCl and the isomerization
of 4 to 5 in acetic acid was accompanied by 6 but not 7. We
speculate that a classical Friedel−Crafts acylation might be the
mechanism of this reaction, because hydrogen chloride
generated in the cyclometalation can catalyze the Friedel−
Crafts acylation by promoting the formation of the acyl cation
from acetic acid. To gain more information on the acylation
reaction, compound 16 was synthesized and subject to a series
of reactions as shown in Scheme 6. The reaction of 16 with
K2PtCl4 in acetic acid produced cyclometalation product 6 in
89% yield. As acetic anhydride is a good reagent for the
Friedel−Crafts reaction, the cyclometalation of 16 with K2PtCl4
in pure acetic anhydride was attempted. However, no reaction
proceeded, as the platinum salt remained undissolved in the
solvent even after 24 h of refluxing. Surprisingly, when water
was added to the reaction mixture, the reaction proceeded to
form 7 cleanly, which suggests that acetic acid is needed for this
reaction. Indeed, when a mixture of acetic acid and acetic
anhydride was used as the solvent, the acylation product 7 was
the only product detected by TLC and there was no 6 present.
Furthermore, when 6 was heated in the mixed solvent of acetic
acid and acetic anhydride, complete acylation of 6 was
observed. The lack of 7 in the reaction of 16 in acetic acid
may be attributed to the poor solubility of 6 in acetic acid.
Once precipitated, 6 was essentially insoluble in acetic acid.
It is also noteworthy that the acylation is regioselective.
There are two competitive sites on the phenyl ring of 6 that
could be acylated, labeled A and B, as shown in Scheme 7. The
electronic effects of the substituents are expected to be similar
for both positions, so why did the acylation occur exclusively at
the position A that is ortho to the amino nitrogen? A possible
explanation could be that the regioselective acylation is directed
by the hydrogen bond between the incoming carbonyl oxygen
and the proton attached to the nitrogen (Scheme 7). The
hydrogen bond not only stabilizes the product 7 and proposed
intermediate 17 but may also stabilize the transition state
leading to the intermediate in the rate-determining step.
To gain information on the electron distribution of the
compound 6, particularly in the phenyl ring, DFT (density
functional theory) calculations were performed on 6 and the
optimized geometry is depicted in Figure 2. The optimized
structure of 6 displays a nearly perfect square-planar geometry,
similar to that of 7 revealed by X-ray crystallography (Figure 1).
Notably, all atoms of the molecule are coplanar. The C−Pt
bond is 2.015 Å. The Pt−N(1) bond (2.132 Å), which is trans
to the carbon donor, is significantly longer than the Pt−N(2)
bond (2.038 Å) trans to the chlorine, indicating a much
stronger structural trans effect induced by a carbon donor.15
The valence angle is 174.32°. At the optimized geometry, the
atomic charges were calculated at the CCSD level of theory.
Net charges on the carbons of the metalated phenyl ring are
shown in Figure 2. There is net negative charge on both A and
B carbons, indicating that both sites are activated toward the
electrophilic aromatic substitution reaction; however, the
negative charge on carbon A (−0.056) ortho to N3 is much
larger than that on carbon B (−0.025) para to the N3, which
further explains the preferential substitution at the A carbon. It
should be mentioned that regioselective aromatic substitution
of cyclometalated iridium(III) complexes has been re-
ported,20,21 but the carbon of the cyclometalated phenyl ring
that was substituted is para to the iridium.
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EXPERIMENTAL SECTION
■
Synthesis. All reactions involving moisture- and/or oxygen-
sensitive organometallic complexes were carried out under a nitrogen
or argon atmosphere and anhydrous conditions. Tetrahydrofuran
(THF) and diethyl ether were distilled from sodium and
benzophenone under nitrogen before use. All other anhydrous
solvents were purchased from Aldrich Chemical Co. and were used
as received. N-tert-Butylaniline23 and 6-bromo-2,2′-bipyridine24 were
prepared according to the literature procedures. All other reagents
were purchased from chemical companies and were used as received.
NMR spectra were measured on a Bruker 400 or a Varian 500
spectrometer. Spectra were taken in CDCl3 or CD2Cl2 using
1
tetramethylsilane as the standard for H NMR chemical shifts and
the solvent peak (CDCl3, 77.0 ppm; CD2Cl2, 53.8 ppm) as the
standard for 13C NMR chemical shifts. Coupling constants (J) are
reported in Hz. Elemental analyses were performed at Atlantic
Microlab, Inc., Norcross, GA.
Figure 2. Optimized geometry of 6 in the ground state and net charges
on the carbons of the phenyl ring. Selected bond lengths (Å): Pt−Cl =
2.345, Pt(1)−C = 2.015, Pt−N(1) = 2.132, Pt−N(2) = 2.038. Selected
bond angles (deg): Cl−Pt−C = 94.04, Cl−Pt−N(2) = 170.76, C−Pt−
N(2) = 95.2, Cl−Pt−N(1) = 91.64, C−Pt−N(1) = 174.32, N(2)−
Pt(1)−N(1) = 79.12.
Reaction of 3 with K2PtCl4 in the Presence of a Base. General
Procedure. In a dry 50 mL three-necked flask were added 3 (36 mg,
0.125 mmol), K2PtCl4 (52 mg, 0.125 mmol), and acetonitrile (5 mL).
A few crystals of tetrabutylammonium chloride were added to the
mixture. The reaction mixture was refluxed for 10 days and then
cooled to room temperature. The solvent was removed by rotary
evaporation. The crude product was analyzed by 1H NMR
spectroscopy, which showed that the ratio of 4 and 5 was 99:1. The
crude product was purified by flash column chromatography on silica
gel first with dichloromethane then with a mixture of dichloromethane
and ethyl acetate (v/v, 50/1) to give 4: orange solid, 38 mg, 64%. It
should be mentioned that, under the column chromatographic
conditions, there was no enrichment in either 4 or 5.
An alternative mechanism involving oxidative addition of
acetic anhydride to the platinacycle 6 could also be proposed
for the selective acylation, as shown in Scheme 8. Reductive
elimination of the intermediate 18, which is a Pt(IV) species,
would be expected to proceed readily, leading to the acylated
intermediate 19. A subsequent cycloplatination of 19 would
occur to produce 7. Although not reported in the cyclo-
platinated complexes, regioselective acylation22 and acetox-
ylation8f of palladacycles have been observed and an oxidative
addition mechanism was proposed for the reactions.22a Since
the selective acylation of 6 is an isolated special case, further
investigations would be necessary to gain a deeper under-
standing of the reaction mechanism.
Preparation of 6-Bromo-N-tert-butyl-N-phenylpyridin-2-
amine (8). In a dry, nitrogen-flushed, 100 mL three-necked round-
bottom flask were charged 2,6-dibromopyridine (2.49 g, 10.5 mmol),
N-tert-butylaniline (1.05 g, 7 mmol), NaOtBu (0.81 g, 8.4 mmol),
toluene (14 mL), bis(dibenzylideneacetone)palladium (Pd(dba)2, 0.16
g, 0.28 mmol), and 1,1′-bis(diphenylphosphino)ferrocene (DPPF,
0.16 g, 0.28 mmol). The mixture was stirred and heated at reflux for 26
h. After it was cooled to room temperature, the mixture was diluted
with 10 mL of ethyl acetate. The organic layer was washed with 20 mL
of water and dried over MgSO4. After filtration, the organic solvents
were removed by rotary evaporation and the crude product was
purified by column chromatography on silica gel with hexanes and
dichloromethane (v/v 1/1) as the mobile phase: colorless crystalline
CONCLUSION
■
The use of a proper base was found to improve the selectivity
of the sp2 C−H bond activation of N-isopropyl-N-phenyl-2,2′-
bipyridin-6-amine by platinum(II) in acetonitrile, from 70% up
to 99%. However, the selective sp3 C−H bond activation in
acetic acid was complicated by the cleavage of the
C(isopropyl)−N bond. The use of a base retarded the C−N
bond cleavage but did not block it completely. The fact that the
reaction of N-tert-butyl-N-phenyl-2,2′-bipyridin-6-amine with
K2PtCl4 in acetic acid resulted in completely C−N bond
cleavage suggested that the C−N bond cleavage proceeded via
heterolytic C−N bond dissociation. Further investigations
showed that the acylation of platinacycle 6 occurred
regioselectively at the N-phenyl ring to form complex 7,
possibly via a Friedel−Crafts reaction facilitated by intra-
molecular H bonding, although other mechanisms are also
possible.
1
solid, 0.66 g, 31%. H NMR (400 MHz, CDCl3): δ 7.35 (t, J = 7.2, 2
H), 7.27 (t, J = 7.3, 1 H), 7.04 (d, J = 7.1, 2 H), 6.86 (t, J = 7.9, 1 H),
6.55 (d, J = 7.4, 1 H), 5.59 (d, J = 8.4, 1 H), 1.43 (s, 9H). 13C NMR
(75 MHz, CDCl3): δ 159.4, 144.0, 138.3, 137.7, 131.2 (2C), 129.7
(2C), 127.5, 114.4, 109.6, 57.9, 31.0 (3C). Anal. Calcd for
C15H17BrN2: C, 59.03; H, 5.61; N, 9.18. Found: C, 58.54; H, 5.71;
N, 8.77.
Preparation of N-tert-butyl-N-phenyl-2,2′-bipyridin-6-amine
(9). Under nitrogen flushing, a 100 mL three-necked round-bottom
flask equipped with a condenser was dried with a heat gun. A solution
of nBuLi in hexanes (1.6 M, 3 mL, 3.8 mmol) was added to the flask
via a syringe, and the flask was cooled to −78 °C with a dry ice/
acetone bath. Under nitrogen, in a 25 mL dried flask was prepared a
solution of 2-bromopyridine (0.58 g, 4 mmol) in diethyl ether (5 mL).
Scheme 8. Alternative Mechanism for the Selective Acylation of 6
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Organometallics
Article
The solution was cooled with a dry ice/acetone bath. The cold
solution was added dropwise into the solution of nBuLi with stirring.
After 10 min, the reaction mixture was warmed to 0 °C, and then a
solution of ZnCl2 in diethyl ether (1 M, 4 mL) was added and the
mixture was warmed to room temperature. To the in situ generated 2-
pyridylzinc reagent were added 6-bromo-N-tert-butyl-N-phenylpyridin-
2-amine (0.59 g, 2 mmol), Pd(PPh3)4 (119 mg, 0.1 mmol), and THF
(15 mL). The mixture was stirred at reflux for 20 h and then cooled to
room temperature. Ethylenediaminetetraacetic acid (EDTA, 2.3 g) was
used to facilitate aqueous workup. The crude product was purified on a
silica gel packed column with dichloromethane and ethyl acetate (v/v
Preparation of N-Phenyl-2,2′-bipyridin-6-amine (16). In a
dry, argon-flushed, 50 mL three-necked round-bottom flask were
charged 6-bromo-2,2′-bipyridine (0.94 g, 4 mmol), aniline (0.73 mL, 8
mmol), NaOtBu (0.46 g, 4.8 mmol), toluene (15 mL), Pd(dba)2 (92
mg, 0.16 mmol), and DPPF (89 mg, 0.16 mmol). The mixture was
stirred and heated at reflux for 19 h. After it was cooled to room
temperature, the mixture was diluted with 10 mL of ethyl acetate and
filtered through a disk of Celite. The filtrate was extracted with ethyl
acetate, and the organic phase was washed with brine and dried over
MgSO4. After filtration, the organic solvents were removed and the
crude product was purified by column chromatography on silica gel
first with dichloromethane and hexanes (v/v 5/1) then with hexanes
and ethyl acetate (v/v 3/1): off-white crystalline solid, 0.83 g, 85%. 1H
NMR (400 MHz, CDCl3): δ 8.60 (d, J = 4.8, 1 H), 8.26 (d, J = 8.0, 1
H), 7.77 (d, J = 7.5, 1 H), 7.74 (td, J = 7.7, 1.8, 1 H), 7.57 (t, J = 8.0, 1
H), 7.37 (d, J = 8.5, 2 H), 7.29 (t, J = 7.4, 2 H), 7.22 (t, J = 6.2, 1 H),
6.99 (t, J = 7.3, 1 H), 6.81 (d, J = 8.2, 1 H), 6.59 (s, 1 H). 13C NMR
(75 MHz, CDCl3): δ 156.3, 155.4, 154.7, 149.1, 140.6, 138.6, 136.9,
129.2 (2 C), 123.5, 122.6, 121.0, 120.1 (2 C), 112.6, 108.9. Anal. Calcd
for C16H13N3: C, 77.71; H, 5.30; N, 16.99. Found: C, 77.47; H, 5.31;
N, 16.86.
Reaction of 16 with K2PtCl4 in Acetic Anhydride−Acetic
Acid. A mixture of 16 (62 mg, 0.25 mmol) and K2PtCl4 (104 mg, 0.25
mmol) in acetic anhydride (5 mL) and acetic acid (5 mL) was refluxed
for 24 h. After removal of the solvents, the residue was dissolved in
dichloromethane and purified by flash column chromatography on
silica gel with dichloromethane and ethyl acetate (v/v 5/1) to give 7:
bright orange solid, 60 mg, 50%.
Reaction of 16 with K2PtCl4 in Acetic Acid. A mixture of 16 (40
mg, 0.16 mmol) and K2PtCl4 (66 mg, 0.16 mmol) in acetic acid (6
mL) was refluxed for 24 h. After the mixture was cooled to room
temperature, the precipitates were collected by filtration, washed with
acetic acid, water, methanol, and ethyl acetate, and dried in air to give a
yellow solid of 6: 67 mg, 89%.
Reaction of 6 with Acetic Anhydride in Acetic Acid. Complex
6 (30 mg, 0.063 mmol) was suspended in a mixture of acetic
anhydride (4 mL) and acetic acid (4 mL). The mixture was heated at
reflux for 24 h, and then the solvents were removed by rotary
evaporation. The residue was dissolved in dichloromethane and
purified by flash column chromatography on silica gel with
dichloromethane and ethyl acetate (v/v 5/1) to give 7: bright orange
solid, 30 mg, 92%.
X-ray Crystallography. All crystals were grown by diffusing
hexanes into dichloromethane solutions of the complexes. A suitable
crystal was selected and mounted on a glass fiber. All measurements
were made using graphite-monochromated Cu Kα radiation (1.54178
Å) on a Bruker-AXS three-circle diffractometer, equipped with a
SMART Apex II CCD detector. In each case, initial space group
determination was based on a matrix consisting of 120 frames. The
data were reduced using SAINT+,25 and empirical absorption
correction was applied using SADABS.26 Structures were solved
using direct methods. Least-squares refinement for all structures was
carried out on F2. All non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were placed in calculated positions and allowed
to be refined isotropically as riding models. Structure solution,
refinement, and calculation of derived results were performed using
the SHELXTL package of computer programs.27
1
30/1) as eluting solvents: colorless crystalline solid, 0.51 g, 84%. H
NMR (400 MHz, CDCl3): δ 8.58 (d, J = 4.8, 1 H), 8.35 (d, J = 8.0, 1
H), 7.75 (td, J = 7.7, 1.8, 1 H), 7.61 (d, J = 7.4, 1 H), 7.35 (t, J = 7.3, 2
H), 7.27 (t, J = 7.4, 1 H), 7.20 (t, J = 8.5, 1 H), 7.19 (t, J = 7.5, 1 H),
7.11 (d, J = 8.3, 2 H), 5.80 (d, J = 8.5, 1 H), 1.54 (s, 9H). 13C NMR
(75 MHz, CDCl3): δ 159.6, 157.4, 152.7, 148.9, 145.2, 136.8, 131.8
(2C), 129.6 (2C), 127.1, 123.0, 120.9, 109.2, 57.3, 29.6 (3C). Anal.
Calcd for C20H21N3: C, 79.17; H, 6.98; N, 13.85. Found: C, 78.72; H,
6.98; N, 13.51.
Reaction of 9 with K2PtCl4 in Acetic Acid. A mixture of 9 (153
mg, 0.5 mmol) and K2PtCl4 (207 mg, 0.5 mmol) in acetic acid (20
mL) was refluxed for 22 h. The solvent was removed by rotary
evaporation, and the residue was dissolved in dichloromethane. The
solution was run through a silica gel column with dichloromethane and
ethyl acetate (v/v 15/1) as eluting solvents. Two major bands were
resolved and collected. After removal of the solvents, the first band
gave 0.1 g of 7 (38%). 1H NMR (400 MHz, DMSO): δ 13.70 (s, 1 H),
9.71 (d, J = 5.6, 1 H), 8.91 (d, J = 7.9, 1 H), 8.67 (d, J = 8.2, 1 H), 8.33
(t, J = 8.0, 1 H), 8.24 (t, J = 7.5, 1 H), 8.17 (d, J = 7.7, 1 H), 7.93 (d, J
= 7.8, 1 H), 7.88 (t, J = 6.4, 1 H), 7.49 (t, J = 8.3, 1 H), 6.93 (t, J = 7.7,
1H), 2.73 (s, 3 H). Anal. Calcd for C18H14ClN3OPt: C, 41.67; H, 2.72;
N, 8.10. Found: C, 41.46; H, 2.78; N, 7.94. The second band produced
0.12 g of 6 (45%). 1H NMR (400 MHz, DMSO-d6): δ 10.69 (s, 1 H),
9.74 (d, J = 5.5, 1 H), 8.64 (d, J = 8.3, 1 H), 8.60 (d, J = 7.7, 1 H), 8.32
(t, J = 7.3, 1 H), 8.14 (t, J = 7.4, 1 H), 8.05 (d, J = 7.4, 1 H), 7.86 (t, J
= 6.9, 1 H), 7.49 (d, J = 8.5, 1 H), 7.09 (t, J = 12.4, 1 H), 6.78 (t, J =
7.2, 1 H). 13C NMR (75 MHz, DMSO-d6): δ 155.4, 152.5, 147.8,
144.7, 139.6, 139.2, 135.6, 134.9, 126.5, 124.5, 123.0, 120.3, 118.2,
115.2, 115.0, 113.6. Anal. Calcd for C16H12ClN3Pt: C, 40.30; H, 2.54;
N, 8.81. Found: C, 40.49; H, 2.52; N, 8.79.
Preparation of Platinum Complex 10. In a 25 mL dry, argon-
flushed flask was charged complex 7 (40 mg, 0.08 mmol),
phenylacetylene (25 mg, 0.24 mmol), CuI (1.2 mg, 0.006 mmol),
Et3N (0.7 mL), and dichloromethane (20 mL). The mixture was
stirred under argon at room temperature for 24 h. After removal of the
solvents, the crude material was purified by flash chromatography on
silica gel with dichloromethane and ethyl acetate (v/v 50/1) to give a
1
bright orange solid: 32 mg, 70%. H NMR (400 MHz, CDCl3): δ
3
13.89 (s, 1H), 10.01 (d, J = 5.6, 1 H), 9.35 (dd, J = 3.8, 1.5, JPt−H
=
41.3, 1 H), 8.03 (d, J = 8.03, 1 H), 7.97 (t, J = 7.4, 1 H), 7.87 (t, J =
8.0, 1 H), 7.79 (d, J = 7.8, 1 H), 7.56−7.42 (m, 3 H), 7.47 (t, J = 6.4, 1
H), 7.29−7.21 (m, 3 H), 7.14 (t, J = 7.5, 1 H), 6.90 (t, J = 7.7, 1 H),
2.70 (s, 3 H). 13C NMR (75 MHz, CDCl3): δ 204.4, 155.5, 153.1,
153.0, 151.3, 145.0, 137.2, 136.8, 135.1, 131.6 (2 C), 129.6, 129.0,
128.0 (2 C). 125.9, 125.2, 121.5, 120.4, 120.2, 118.5, 118.2, 113.6,
103.8, 100.7, 28.7. Anal. Calcd for C26H19N3OPt: C, 53.42; H, 3.28; N,
7.19. Found: C, 53.13; H, 3.26; N, 7.24.
Isomerization of 4 in Acetic Acid. A mixture of 4 (92 mg, 0.18
mmol) in acetic acid (8 mL) was refluxed for 3 h. 1H NMR analysis of
the reaction mixture showed that 4 was completely isomerized to 5
with the presence of a small amount of 6. The ratio of 5 and 6 was
determined to be 95:5. After removal of acetic acid by rotary
evaporation, the residue was dissolved in dichloromethane (30 mL)
and the solution was washed with water and brine, dried over Na2SO4,
and filtered. The filtrate was concentrated, and the crude product was
purified by flash column chromatography on silica gel with
dichloromethane and ethyl acetate (v/v 80/1) to give 5 as an orange
solid: 66 mg, 72%.
DFT and CCSD Calculations. Geometry optimizations were
performed for the ground state using density functional theory (DFT)
with Gaussian 0328 and the B3LYP exchange-correlation functional.29
The DEF2_TZVP basis set30 was used for platinum, while the cc-pvdz
basis set31 was used for all other atoms. At the optimized geometry
with the same basis sets, the atomic charges were calculated at the
CCSD (coupled-cluster with single and double excitations) level of
theory.32
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Organometallics
Article
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ASSOCIATED CONTENT
* Supporting Information
■
S
A CIF file giving crystallographic data for complex 10, tables
giving crystal data and refinement details for 10 and Cartesian
coordinates of the optimized geometry of complex 6, and
figures giving an 1H NMR analysis of ratio of 4 and 5 formed in
the reaction of 3 with K2PtCl4 in acetonitrile in the presence of
a base and NMR spectra of new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.H.: huos@ecu.edu.
■
(9) (a) Minghetti, G.; Cinellu, M. A.; Chelucci, G.; Gladiali, S. J.
Organomet. Chem. 1986, 307, 107−114. (b) Sana, G.; Minghetti, G.;
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Foy, A. G.; Henling, L. M.; Day, M.; Labinger, J. A.; Bercaw, J. E. J.
Mol. Catal. A: Chem. 2002, 189, 3−16.
Present Addresses
§Metrics Inc., Greenville, NC 27834.
∥Campbell University, Buies Creek, NC 27506.
Notes
The authors declare no competing financial interest.
(10) Garner, A. W.; Harris, C. F.; Vezzu, D. A. K.; Pike, R. D.; Huo,
S. Chem. Commun. 2011, 47, 1902−1904.
ACKNOWLEDGMENTS
■
(11) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry: Part
A: Structure and Mechanism, 5th ed.; Springer: New York, 2007; p 258.
(12) Yang, J.-S.; Lin, Y.-H.; Yang, C.-S. Org. Lett. 2002, 4, 777−780.
(13) (a) Vezzu, D. A. K.; Ravindranathan, D.; Garner, A. W.;
Bartolotti, L.; Smith, M. E.; Boyle, P. D.; Huo, S. Inorg. Chem. 2011,
50, 8261−8273. (b) Huo, S.; Harris, C. F.; Vezzu, D. A. K.; Gagnier, J.;
Smith, M. E.; Pike, R.; Li, Y. Polyhedron 2013, 52, 1030−1040.
(14) (a) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Che, C.-M.;
Zhu, N.; Lee, S. T. J. Am. Chem. Soc. 2004, 126, 4958−4971.
(b) Ravindranathan, D.; Vezzu, D. A. K.; Bartolotti, L.; Boyle, P. D.;
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This work was supported by the ACS Petroleum Research
Fund (PRF# 51147-UR3). RDP acknowledges NSF (CHE-
0443345) and the College of William and Mary for the
purchase of the X-ray equipment.
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